ARL-CR-0774 ● JUNE 2015

US Army Research Laboratory

Report of Research at Technische Universität 

Darmstadt on Ultrahard Materials in the 

B-C-N-Si System

edited by Ralf Riedel

Technische Universität Darmstadt

Fachbereich Material- und Geowissenschaften

Fachgebiet Disperse Feststoffe

Petersenstr. 32

D-64287 Darmstadt

Germany

and

James W McCauley (Emeritus)

US Army Research Laboratory

Weapons and Materials Research Directorate, ARL

Aberdeen Proving Ground, MD 21005-5066

under contract W911NF-09-1-0576

Approved for public release; distribution is unlimited.
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Destroy this report when it is no longer needed. Do not return it to the originator.
ARL-CR-0774 ● JUNE 2015

US Army Research Laboratory

Report of Research at Technische Universität 

Darmstadt on Ultrahard Materials in the 

B-C-N-Si System

edited by Ralf Riedel

Technische Universität Darmstadt

Fachbereich Material- und Geowissenschaften

Fachgebiet Disperse Feststoffe

Petersenstr. 32

D-64287 Darmstadt

Germany

and

James W McCauley (Emeritus)

US Army Research Laboratory

Weapons and Materials Research Directorate, ARL

Aberdeen Proving Ground, MD 21005-5066

under contract W911NF-09-1-0576

Approved for public release; distribution is unlimited.
ii

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1. REPORT DATE (DD-MM-YYYY)

June 2015

2. REPORT TYPE

Final

3. DATES COVERED (From - To)

1 August 2009–31 December 2012

4. TITLE AND SUBTITLE

Report of Research at Technische Universität Darmstadt on Ultrahard Materials 

in the B-C-N-Si System

5a. CONTRACT NUMBER

W911NF-09-1-0576

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. EDITOR(S)

Ralf Riedel and James W McCauley

5d. PROJECT NUMBER

1375-MS-01

5e. TASK NUMBER

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

US Army International Technology Center–Atlantic

8. PERFORMING ORGANIZATION REPORT NUMBER

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US Army Research Laboratory

ATTN: RDRL-WM

Aberdeen Proving Ground, MD 21005-5069

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ARL-CR-0774

12. DISTRIBUTION/AVAILABILITY STATEMENT

Approved for public release; distribution is unlimited.

13. SUPPLEMENTARY NOTES

14. ABSTRACT

There are 3 phases of this research program:

1) The synthesis of a boron carbide (B4C) ceramic powder by means of a polymer pyrolysis route. The synthesis 

involves the polycondensation of single molecules and subsequent thermal decomposition of the resulting boroncontaining polymer at 1,100 °C. The product is annealed in a second step at 2,000 °C to form polycrystalline B4C. 

The work is divided into 3 parts: i) synthesis of the educts—namely, bis(trimethylsilyl)carbodiimide and 2,4,6-

trichloroborazine; ii) synthesis of the preceramic polymer by reaction of the aforementioned educts; and iii) 

thermolysis of the polymer up to 2,000 °C in a helium atmosphere.

2) A comprehensive literature review on superhard materials and their method of fabrication, projected properties, and 

possible scale-up, with a focus on the B-C-N system.

3) Attempting to fabricate, using the multianvil press, larger samples of c-Si3N4 (transparent if possible)—a spinel 

structure material. 

15. SUBJECT TERMS

boron, carbon, nitrogen, silicon, cubic-Si3N4, boron carbide, ultrahard materials, high pressure 

16. SECURITY CLASSIFICATION OF:

17. LIMITATION

OF ABSTRACT

UU

18. NUMBER

OF PAGES

98

19a. NAME OF RESPONSIBLE PERSON

James W McCauley

a. REPORT

Unclassified

b. ABSTRACT

Unclassified

c. THIS PAGE

Unclassified

19b. TELEPHONE NUMBER (Include area code)

410-306-0711

Standard Form 298 (Rev. 8/98)

Prescribed by ANSI Std. Z39.18
iii

Contents

List of Figures v

List of Tables viii

1. Nonaqueous Sol-Gel Synthesis of Boron Carbide–Based Materials 1

1.1 Interim Report (Project Start Date: December 1, 2009) 1

1.1.1 Summary 1

1.1.2 Experimental Work 1

1.2 Final Report (Project Start Date: December 15, 2009) 3

1.2.1 Summary 3

1.2.2 Experimental Work 3

1.2.3 References 10

2. Review: Synthesis Routes to Ultrahard B-C-N Phases 11

2.1 Ternary B-C-N Compounds 12

2.1.1 Theoretical Predictions 13

2.1.2 Synthesis of B-C-N Precursors 22

2.1.3 HP-HT Experiments and Results 28

2.2 C-N Compounds 34

2.2.1 Postulated Carbon Nitrides 34

2.2.2 CNx Thin Films via Vapor Deposition 39

2.2.3 Ambient- and Low-Pressure Bulk Synthesis Approaches 40

2.2.4 HP-HT Synthesis of Dense C-N Phases 48

2.3 B-C Compounds 55

2.3.1 HP-HT Synthesis of Diamond-like B-C Phases 56

2.3.2 Potential Superhard B-C Phases 58

2.4 Summary 61

3. Experimental: Synthesis of Transparent Spinel-Type γ-Si3N4 63

3.1 High-Pressure, High-Temperature (HP-HT) Synthesis 63

3.2 Sample Characterization 64

3.2.1 Optical Observation 64

3.2.2 Raman Spectroscopy 65
iv

3.2.3 Powder X-ray Diffraction (XRD) 66

3.2.4 Energy-Dispersive X-ray Spectroscopy (EDX) 67

3.3 Conclusion 69

4. References for Sections 2 and 3 70

List of Symbols, Abbreviations, and Acronyms 85

Distribution List 87
v

List of Figures

Fig. 1.1 X-ray diffraction (XRD) of the synthesized BxCyNz ceramic 7 at 

1,100 °C .................................................................................................5

Fig. 1.2 X-ray diffraction (XRD) analysis of the sample 7 at 2,000 °C..............6

Fig. 1.3 Raman spectroscopy of 8 (at 2,000 °C): a) regions containing B4C and 

amorphous carbon and b) regions containing ß-SiC and amorphous 

carbon.....................................................................................................6

Fig. 1.4 Raman spectrum of 8’ (at 2,000 °C)......................................................7

Fig. 1.5 X-ray diffraction (XRD) analysis of 8’ (at 2,000 °C)............................8

Fig. 1.6 Scanning electron micrograph of product 8’: a) overall microstructure 

of the sample; b) porous part of the microstructure; and c) dense part 

of the microstructure ..............................................................................9

Fig. 2.1 A representative atomic arrangement in the unit cell of cubic β-BC2N 

structure................................................................................................14

Fig. 2.2 All the possible topologically different c-BC2N structures (after 

structural relaxation) starting from an 8-atom zinc-blende structured 

unit cell. The dotted bonds indicate the broken covalent bonds between 

N atoms. ...............................................................................................16

Fig. 2.3 Proposed tetragonal crystal structures of diamond-like BC2N: 

a) z*-BC2N with P-42m symmetry and b) t-BC2N with P-421m

symmetry..............................................................................................17

Fig. 2.4 a) Distribution of formation energy (eV/atom) on the upper rhombus 

of ternary B−C−N phase diagram using full spectrum. Less formation 

energy means that the corresponding composition is easier to form. b) 

BxCyNz compositions synthesized in previous experiments. Most 

compositions are located in the area with negative formation energy, 

in agreement with the theoretical prediction........................................18

Fig. 2.5 High-temperature reaction of amorphous carbon spheres. a) SEM and 

b) TEM image, with boric acid and urea yields spheres with graphitic 

structure and composition of BC4N, c) SEM, and d) TEM image. .....26

Fig. 2.6 Plot of all observed unit-cell volumes in the B–C–N system of cubic 

zinc-blende structure. The relationship of unit-cell volume vs. 

chemical composition shows a clear trend closely obeying Vegard’s 

law for ideal solid solutions, except for data of Solozhenko et al. ......30

Fig. 2.7 Optical images of diamond crystals obtained at HP-HT using Fe-Ni 

alloy as a catalyst starting from graphite a) or mixtures of graphite and 

h-BN with stoichiometries of b) C0.98(BN)0.02; c) C0.9(BN)0.1; d) 

C0.9(BN)0.1 with 1 wt% Al additive; and e) C0.5(BN)0.5. ...............34
vi

Fig. 2.8 Theoretically predicted dense carbon(IV) nitride polymorphs that have 

been proposed in literature and which were examined theoretically: 

a) β-C3N4 (P63/m), b) α-C3N4 (P31c); c) “pseudo-cubic” or “defect zinc 

blende” C3N4 (P-43m); d) “cubic” or “willemite-II” C3N4 (I-43d); 

e) spinel-type γ-C3N4 (Fd-3m). The carbon and nitrogen atoms are 

represented as gray and blue spheres, respectively..............................36

Fig. 2.9 Unit cells of the simple cubic gauche cg-N (a) and cg-CN (b)............38

Fig. 2.10 Examples for graphitic C3N4 networks with relative high symmetry 

based on the s-triazin unit (A) and the s-heptazine (tri-s-triazine) unit 

(B). Structure A was initially predicted to be the thermodynamically 

most stable C3N4 modification. However, recent DFT calculations have 

shown that B is significantly more stable than A. .................................41

Fig. 2.11 g-C3N4: s-triazine-based chemical structure and TEM image of 

nanosized hollow spheres ....................................................................42

Fig. 2.12 The 3-D resin-like structural model for C3N4 ......................................45

Fig. 2.13 The diagram presents an overview of the reported experimental routes 

to produce C3N4 phases. The bold route indicates the most promising 

synthesis pathways to dense C3N4. ......................................................48

Fig. 2.14 a) Bright field TEM image of the C2N2(NH) crystal synthesized at P 

= 41 GPa and T > 1700 °C and b) crystal structure of the defect 

wurtzite-type carbon nitride imide, dwur-C2N2(NH)...........................52

Fig. 2.15 Computed phase boundary between carbon + nitrogen and the dense 

phases of C3N4: willemite-type phase w-C3N4 (blue line200) and -

C3N4 (red line). For comparison, the phase diagram of carbon is 

shown by dashed lines .........................................................................53

Fig. 2.16 Low-magnification TEM images of -C3N4 single-crystal nanorods 

obtained via reaction of graphite nanopowder with ammonia in a highenergy ball mill followed by the thermal treatment under NH3 flow ..55

Fig. 2.17 A representative atomic arrangement in the hexagonal supercell of 

c-BC5 with carbon and boron atoms represented by gray and red 

spheres, respectively. The hexagonal supercell is compatible with a 

cubic symmetry (dashed blue line) for c/a = √6 ≈ 2.45. ......................60

Fig. 3.1 Cross section of an octahedral pressure assembly used for HP-HT 

synthesis of γ-Si3N4..............................................................................64

Fig. 3.2 The microscopic photographs of “slice 1” of the HP product in a 

platinum capsule ring taken under combined transmitted and reflected 

light (a) and in transmitted light only (b).............................................65

Fig. 3.3 The microscopic photographs of “slice 2” of the HP product in a 

platinum capsule ring taken under combined transmitted and reflected 

light (a) and in transmitted light only (b).............................................65
vii

Fig. 3.4 Typical Raman spectra measured from the translucent a) and dark b) 

areas of the HP product. The characteristic Raman shifts of the γ-Si3N4

with corresponding Raman-active phonon modes (according to Fang et 

al.) are indicated...................................................................................66

Fig. 3.5 Powder XRD pattern (red circles) of the HP product (slice 2) measured 

in transmission “STOE” geometry. The results of the full profile 

Rietveld refinement for the cubic spinel γ-Si3N4 and the corresponding 

difference curve are presented by black and blue lines, respectively. 

The calculated peak positions of the cubic spinel-type structure (space 

group Fd-3m) are denoted by green tick marks. The refinement 

agreement factors are RBragg = 7.2%, RF = 6.1%, and χ2 = 1.2. ..........67

Fig. 3.6 Typical EDX spectrum measured from translucent part of the HP 

product. C-K peak is due to the adherent carbon tape. .......................67

Fig. 3.7 a) Optical and b) SEM micrographs of the sample surface together 

with EDX elemental maps for c) nitrogen, d) oxygen, e) silicon, and f) 

platinum ...............................................................................................68
viii

List of Tables

Table 2.1 Calculated lattice parameters, bulk moduli, shear moduli, and 

estimated hardness (if available) of the predicted superhard B-C-N 

phases. The experimental values for dense carbon and BN 

modifications are given for comparison. .............................................20

Table 2.2 Predicted C3N4 phases and their calculated structural parameters, bulk 

moduli, and hardness ...........................................................................35

Table 2.3 Experimental details and main results related to the HP-HT synthesis 

of carbon nitrides .................................................................................49

Table 2.4 Structural parameters, bulk moduli, shear moduli, and estimated 

hardness (if available) of the predicted superhard diamond-like BCx

phases calculated employing GGA (LDA). The experimental values 

for diamond, c-BN, and recently discovered c-BC5 are given for 

comparison...........................................................................................59
1

1. Nonaqueous Sol-Gel Synthesis of Boron Carbide–Based 

Materials

Gabriela Mera and Ralf Riedel

1.1 Interim Report (Project Start Date: December 1, 2009)

1.1.1 Summary

The aim of the present project is the synthesis of a boron carbide (B4C) ceramic 

powder by means of a polymer pyrolysis route. The synthesis involves 

polycondensation of single molecules and subsequent thermal decomposition of the 

resulting boron-containing polymer at 1,100 °C. The product is annealed in a 

second step at 2,000 °C to form polycrystalline B4C. The work is divided into 3 

parts: i) synthesis of the educts—namely, bis(trimethylsilyl)carbodiimide and 

2,4,6-trichloroborazine; ii) synthesis of the preceramic polymer by reaction of the 

aforementioned educts; and iii) thermolysis of the polymer up to 2,000 °C in a 

helium atmosphere. 

1.1.2 Experimental Work

The synthesis of bis(trimethylsilyl)carbodiimide was performed as reported in the 

literature, starting from hexamethyldisilazane 1 (purum, ≥98.0% [gas 

chromatograph], Fluka, Germany), dicyandiamide 2 (99%, Aldrich, Germany; 

19.70 Euros for 1 kg), and ammonium sulfate as the catalyst (SigmaUltra, ≥99.0%, 

Sigma-Aldrich, Germany; 32.90 Euros for 100 g) as shown in Scheme 1. 

1 2 3

Scheme 1

The reaction mixture was heated at 164 °C for 8 h, followed by a fractional 

distillation over a Vigreux column. The yield of the product, bis(trimethylsilyl)

carbodiimide 3, was 65 wt%. 

The maximum amount of product per batch that can be produced in our lab is 

250 mL. Until now, we have produced 0.5 L of pure bis(trimethylsilyl) 

carbodiimide.
2

The synthesis of 2,4,6-trichloroborazine was achieved by an improved synthetic 

route, which has not been reported in the literature yet. The novel synthesis 

approach allows us to use BCl3·SMe2 4 instead of BCl3 gas, which is highly reactive 

and toxic. Accordingly, hexamethyldisilazane 1 (purum, ≥98.0% (GC), Fluka, 

Germany) was reacted with BCl3·SMe2 4 in dichloromethane as the solvent (2.0 M 

in methylene chloride, Aldrich, Germany) (Scheme 2). 

1 4 5

Scheme 2

Several reaction conditions were tested to produce a pure product 5 with an 

improved yield. The progress of the reactions were followed by means of liquidstate 1H, 13C{1H}, 13C DEPT (distortionless enhancement by polarization transfer), 

29Si DEPT, and 11B NMR (nuclear magnetic resonance) in C6D6.

Independently of the starting temperature or the ratio of the educts, 2,4,6-

trichloroborazine 5 was obtained as the major product, together with some traces 

of residual BCl3·SMe2. The reactions were performed at starting temperatures –25, 

0, and 25 °C, followed by refluxing at 66 °C and drying of the solid product 5 in 

vacuum. 

In the next step, the product 2,4,6-trichloroborazine 5 has to be purified by 

sublimation in vacuum at 60 °C. Subsequently, 2,4,6-trichloroborazine 5 will be 

reacted with bis(trimethylsilyl)carbodiimide 3 to form the preceramic polymer 6

(Scheme 3).

The synthesis of the educts and of the polymer will be done in several batches to 

produce the required amount of B4C.
3

 6

Scheme 3

1.2 Final Report (Project Start Date: December 15, 2009)

1.2.1 Summary

The aim of the present project is the synthesis of a boron carbide (B4C) ceramic 

powder by means of a polymer pyrolysis route. The synthesis involves 

polycondensation of single molecules and subsequent thermal decomposition of the 

resulting boron-containing polymer at 1,100 °C. The product is annealed in a 

second step at 2,000 °C to form polycrystalline B4C. The work is divided into 3

parts: i) synthesis of the educts—namely, bis(trimethylsilyl)carbodiimide and 

2,4,6-trichloroborazine; ii) synthesis of the preceramic polymer by reaction of the 

aforementioned educts; and iii) thermolysis of the polymer up to 2,000 °C in a 

helium atmosphere.

1.2.2 Experimental Work

The synthesis and purification of bis(trimethylsilyl)carbodiimide 2 was done as 

described in the interim report above.

For the synthesis of 2,4,6-trichloroborazine 5, several routes were proved. As 

described in the interim report, the product was firstly produced by the reaction of 

hexamethyldisilazane with BCl3·SMe2 4 in dichloromethane as solvent (2.0 M in 

methylene chloride, Aldrich, Germany) as shown in Scheme 1. 
4

1 4 5

Scheme 1

Several reaction conditions were tested to produce a pure product 5 with improved 

yield. The progress of the reactions were followed by means of liquid-state 1H, 13C 

{

1H}, 13C DEPT, 29Si DEPT, and 11B NMR in C6D6. Independently of the starting 

temperature or the ratio of the educts, 2,4,6-trichloroborazine 5 was obtained as the 

major product, together with some traces of residual BCl3·SMe2 4. The reactions 

were performed at starting temperatures –25, 0, and 25 °C, followed by refluxing 

at 66 °C and drying of the solid product 5 in vacuum. In the next step, the product 

2,4,6-trichloroborazine 5 has to be purified by sublimation in vacuo at 60 °C. 

Subsequently, 2,4,6-trichloroborazine 5 is reacted with bis(trimethylsilyl)

carbodiimide 3 to form the preceramic polymer 6 (Scheme 2).

 6

Scheme 2
5

The synthesis of the educts and of the polymer will be done in several batches to 

produce the required amount of boron carbide (B4C).

The polymer 6 was pyrolyzed at 1100 °C to produce an amorphous BxCyNz ceramic 

7. The pyrolysis program was as follows: 100 °C/h heating rate to 200 °C, holding 

time 2 h, heating to 1,100 °C with 100 °C/h, and dwelling time 2 h. After holding 

at the final temperature, the sample was allowed to cool to room temperature. The 

ceramic yield was ≈25 wt%. This process is summarized in Scheme 3.

The amorphous BxCyNz ceramic 7 (Fig. 1.1) was further pyrolyzed to 2000 °C in 

an Astro furnace. The yield after thermolysis of BxCyNz was ≈50 wt%. The product 

8 obtained at 2,000 °C is not pure B4C—it still contains ß-SiC and graphite as 

crystalline phases (Fig. 1.2) together with an amorphous phase. Moreover, 

contamination of sulfur was analysed (specific smell of the samples and coloration 

of the crucible).

20 40 60 80

0

500

1000

1500

2000

2500 Intensity (a.u.)

2 Theta (°), Cu K

Fig. 1.1 X-ray diffraction (XRD) of the synthesized BxCyNz ceramic 7 at 1,100 °C

Scheme 3
6

20 40 60 80

0

500

1000

1500

2000

2500 Intensity (a.u.)

2 Theta (°), Cu K

B4C Graphite SiC

Fig. 1.2 X-ray diffraction (XRD) analysis of the sample 7 at 2,000 °C

The results are also in agreement with micro-Raman spectroscopy studies (514.5-

nm laser). Figure 1.3 shows the results obtained for the regions containing B4C and 

amorphous carbon (Fig. 1.3a) and for the regions containing ß-SiC and amorphous 

carbon (Fig. 1.3b). The presence of graphite phase was not identified by Raman 

spectroscopy.

Fig. 1.3 Raman spectroscopy of 8 (at 2,000 °C): a) regions containing B4C and amorphous 

carbon and b) regions containing ß-SiC and amorphous carbon

To improve the purity of the products, 2,4,6-trichloroborazine was also synthesized 

from hexamethyldisilazane and trichloroborane 1M in hexane (Scheme 4). 

1000 2000 3000 4000 Intensity (a.u.) Raman Shift (cm-1)

B4C + amorphous C

1000 2000 3000 4000 Intensity (a.u.) Raman Shift (cm-1)

-SiC + amorphous C

a) b)
7

The trichloroborane solution violently reacts with moisture; therefore, the synthesis 

can be made only in small amounts, high dilution, and at low temperature (–65 °C). 

The highest amount of product 5’ that can be produced in one batch in our 

laboratory is 5 g. The yield of the reaction is ≈45 wt%, and the product was further 

used without purification. The quantitative polymerization of 5’ with 

bis(trimethylsilyl)carbodiimide yields polymer 6’ having the same structure as 6

but containing no sulfur impurities. 

Similar to the pyrolysis of 6, the amorphous product 7’ obtained at 1100 °C and 

crystalline 8’ synthesized at 2000 °C can be produced according to Scheme 5.

Product 8’ is a B4C-containing ceramic as proved by Raman spectroscopy (514.5 

nm) (Fig. 1.4) and by X-ray diffraction (XRD) analysis (Fig. 1.5).

400 800 1200 1600 2000 Intensity (a.u.) Raman Shift (cm-1) 480.17 533.04 721.06 823.79 998.17 1089.41 267.43 322.00 130.57 1361.83 1561.79

Fig. 1.4 Raman spectrum of 8’ (at 2,000 °C)

Scheme 4 Scheme 5
8

10 20 30 40 50 60 70 80 90 0

200

400

600

800

1000

1200

1400

1600

1800 Intensity (a.u.)

2 Theta (°), Cu(K) 71.86 60.01 37.72 43.00 35.62 35.02

Fig. 1.5 X-ray diffraction (XRD) analysis of 8’ (at 2,000 °C)

The evidence for the formation of B4C is also clearly shown by Raman 

spectroscopy. Figure 1.4 shows the micro-Raman spectra of sample 8’. Three wide 

bands at 721.06, 823.79, and 1,089.41 cm-1and a double narrow band at 

480.17/533.04 cm-1can be clearly distinguished, which are characteristic for the 

presence of rhombohedral B4C. Two other broad lines at 1,361.83 and 1,561.79 cm1

can also be found, corresponding to the D and G absorption bands of residual 

amorphous carbon, nevertheless of low intensity. From the Raman studies it is 

obvious that sample 8’ exhibits the band pattern of that of B4C with low excess 

carbon impurities.

The XRD diffractogram shows weak reflections in part related to rhombohedral 

B4C according to the PDF database (Powder Diffraction Files [PDF], International 

Center for Diffraction Data: Swarthmore, PA). The sample is not completely 

crystallized, and it looks similar to B4CN4 synthesized at 1,800 °C as reported in 

Kroke et al.1and Völger et al.2

The broad and weak (100) reflex of turbostratically disordered graphite-like BxCyNz

around 2θ = 43° is also found. To completely crystallize the sample, higher 

temperatures and/or longer annealing times are required. The experiments are 

currently under investigation.

The microstructure of 8’ was analyzed also by means of scanning electron 

microscope (SEM) analysis. Figure 1.6 presents the micrograph of the ceramic 

sample composed of 2 phases, one with a high porosity and a second one with dense 

regions.
9

a) 

b) 

c) 

Fig. 1.6 Scanning electron micrograph of product 8’: a) overall microstructure of the 

sample; b) porous part of the microstructure; and c) dense part of the microstructure
10

The elemental analysis of the product 8’ has shown that the sample still contains 

nitrogen and some oxygen impurities. The calculated molecular formula is 

B4.9C1N2.3O0.1 corresponding to 11.67 wt% C, 2.60 wt% O, 33.84 wt% N, and 51.89 

wt% B. The decomposition of BCxNy to B4C with elimination of nitrogen gas 

(reported to take place at 1,850 °C) is obviously occurring at higher temperatures 

than the one used in the present study.

Therefore, product 7’ (BCxNy, 1,100 °C) is presently synthesized in several batches 

and pyrolyzed up to 2,100 and 2,200 °C with annealing times of 4 to 10 h to achieve 

the requested amount of pure B4C ceramic. 

At this point in the project it became clear that the original goal of making large 

quantities of pure B4C powder in a time- and cost-effective manner was probably 

not possible with the available funding. The main problem was that the cost of the 

starting material, trichloroborazine, was just too high (500 g for 55,000.00 Euros) 

from the standpoint of this project or for future commercialization. It was therefore 

decided to refocus the project on the following tasks: 

• Compose a short review paper on superhard materials and their method of 

fabrication, projected properties, and possible scale up, with a focus on the 

B-C-N system.

• Try to fabricate, using the multianvil press (MAP), larger samples of 

c-Si3N4 (transparent if possible).

The reports of these 2 tasks follow in Parts 2 and 3. 

1.2.3 References

1. Kroke E, Völger KW, Klonczynski A, Riedel R. Angew Chem Int Ed. 

2001;40(9):1698–1700.

2. Völger KW, Kroke E, Gervais C, Saito T, Babonneau F, Riedel R, Iwamoto 

Y, Hirayama T. Chem Mater. 2003;15:755–764. 
11

2. Review: Synthesis Routes to Ultrahard B-C-N Phases

Dmytro Dzivenko and Ralf Riedel

Finding substances harder than diamond appears to be a real challenge. Rather 

than harder, it might be more productive to consider the possibility of novel 

superhard substances and/or materials that are more useful than diamond for 

certain applications. While diamond exhibits extreme hardness, its actual 

performance as a superabrasive or electronic material is somewhat limited. It 

easily oxidizes in air at even moderate temperatures and cannot be used as an 

abrasive for machining of ferrous alloys. Cubic BN (c-BN) exhibits greater 

thermal stability and is the superabrasive of choice for machining of steels but 

has only half of the hardness of diamond. A possible solution to improve useful 

properties is to synthesize dense binary or ternary phases containing second-row 

main group elements (B, C, N) with the goal that these materials might be more 

thermally and chemically stable than diamond, and harder than c-BN, and thus 

should be excellent materials for high-speed cutting and polishing of different 

materials including ferrous alloys. There is also potential that new dense binary 

and ternary compounds in the B-C-N system will demonstrate important 

optoelectronic and/or superconducting properties in combination with 

exceptional hardness.

Thus, the search for a superhard substance that might surpass (or at least combine) 

advanced properties of diamond and c-BN goes on, supported by continuous 

improvement of experimental techniques and growth of computational capacities; 

this search is important both for understanding the fundamental correlations 

between microscopic characteristics of interatomic interactions and macroscopic 

properties, and for pure technological applications. Taking into account the 

existence of correlations between hardness and unambiguously determined 

physical characteristics of substances (such as elastic properties, density, etc.), the 

scientific community was able to sort out “islands” of existent and hypothetical 

(super)hard substances. They can be tentatively divided into 3 classes: i) covalent 

and ionic-covalent compounds formed by light elements from the middles of the 

second and third rows of the periodic table (low-Z compounds); ii) covalently 

bonded crystalline and disordered carbon modifications; and iii) partially 

covalent compounds of transition metals with light elements, such as borides, 

carbides, nitrides, and oxides.1,2 To synthesize such compounds and substances, 

characterized by short bonds with high cohesive energy and rigid structures, the 

extreme conditions, such as high pressures (HPs) and high temperatures (HTs), 

or chemical vapor deposition (CVD) techniques are generally required. CVD 

techniques, however, do not provide bulk products and are, therefore, beyond the 

scope of the present report.
12

An alternative way to the superstrength and superhardness lies in the 

development of new materials, based on the existent (super)hard substances, via 

manipulation of their structure at the micro- and nanoscale. In other words, 

development of nanocomposites, multilayers, and superlattices via appropriate 

design and control of the material’s morphology would allow bringing its 

hardness and other mechanical characteristics closer to or even surpass the 

“ideal” values. This approach has been successfully applied, for example, to 

fabricate c-BN, Si3N4, TiN-based nanocomposites, multilayers, and 

heterostructures with hardness approaching and even surpassing that of 

diamond.3

The present review is focused on the most promising class of potential 

superhard compounds—namely, covalent binary and ternary phases in the B-CN system: diamond-like BxCyNz and BCx and dense C3N4 phases. It includes an 

overview of the theoretical works on the prediction of possible structures in the 

B-C-N system, investigation of their stability, and potentially interesting (in 

particular, elasto-mechanical) properties. It also describes the most prominent 

experimental approaches to obtain bulk superhard B-C-N compounds, including 

preparation of starting materials (precursors), experimental high-pressure hightemperature synthesis conditions, and results of characterization of the highpressure products.

2.1 Ternary B-C-N Compounds

Ternary B-C-N compounds have drawn considerable attention during the last 2

decades because of the similarity in atom sizes and structures of carbon and 

boron nitride (i.e., diamond and cubic boron nitride, or graphite and hexagonal 

graphite-like boron nitride), which suggested that it should be possible to 

synthesize phases containing all 3 elements. Indeed, the existence of many lowdensity BxCyNz phases with hexagonal or turbostratic structure has been 

reported. They can be synthesized via i) nitriding of solid-phase precursors at 

high temperatures, ii) using CVD technique, or iii) pyrolysis of inorganic 

polymers containing boron, carbon, and nitrogen. These compounds are of 

broad interest because graphitic BxCyNz can be considered for applications as 

high-temperature semiconductors, especially if they can be altered by doping or 

structural changes.4 Moreover, it is expected that the dense forms of ternary BC-N compounds or diamond-like solid solution with the general formula 

(BN)xCy can exhibit extreme hardness approaching that of diamond,4–9in 

combination with excellent thermal and chemical stability (superior to those of 

diamond)10, and would therefore be indispensable abrasive materials for highspeed machining of ferrous alloys. Even though interesting properties like 

photoluminescence11, hydrogen absorption and storage12, and exhaust gas 

absorption13 have been claimed for various B-C-N compounds (e.g., graphiticor turbostratic-BCN, hexagonal-BCN, cubic-BCN, BCN-nanotubes)14, the
13

present review will be generally focused on preparing dense ultrahard phases of 

B-C-N. This implies consideration of synthesis routes employing extreme 

pressures and temperatures or, to a lesser extent, CVD technique, which does 

not provide bulk superhard materials. The synthetic approaches resulting in 

low-density B-C-N ceramics or films (polymer routes, CVD, sputtering, etc.) 

will be also briefly considered in a sense of preparation of starting materials for 

subsequent high-pressure high-temperature (HP-HT) treatment, leading to the 

formation of hard phases. 

2.1.1 Theoretical Predictions

Theoreticians have taken the first step in the late 1980s to investigate the 

electronic structure, bonding, and properties of this interesting class of materials 

prior to thorough experimental characterization. All the theoretical predictions 

can be roughly classified in investigation of structural stability, mechanical 

properties, and electronic properties.

2.1.1.1 Structural Stability

Liu et al.15 was one of the first to calculate the electronic structure of graphitic 

BC2N compounds using the local density functional formalism. The structural 

stability of BC2N has also been studied by Nozaki et al.16 using an empirical 

molecular mechanics simulation. Both studies had 8 constituent atoms in a unit 

cell even though different structural models with different atomic arrangements 

were considered. These initial studies were focused on structural stability using 

monolayer-model and nearest neighbor environments. It has been predicted that 

BC2N is formed by maximizing C-C and B-N bonds. It has also been found that 

a structure with alternate -C-C- and -B-N- chains or rings is the most stable one. 

These predicted structures can be assigned to graphitic-BC2N type; some of the 

earlier experiments reported similar structures17,18. Quite recently the structural 

and electronic properties of boron ternary graphite-like monolayers (BCN) were 

investigated using a pseudopotential method within density functional theory 

with an emphasis on the effect of composition and atomic arrangement on the 

structural stability and electronic properties in a 32-atom unit cell.19 The 

calculations confirm that the stable structure of boron ternary monolayers 

(BCN) is formed by increasing the number of both C-C and B-N bonds and, 

independent of the unit cell size, in agreement with earlier results,15,16 obtained 

applying another approach. The structure, stability, and nature of the bonding 

of BC2N have been compared with the tetra-atomic carbon and BN molecules 

by Tapas Kar et al.20 They predicted that the most stable structure of BC2N is 

linear, whereas C4 and (BN)2 are cyclic. However, as per our knowledge,

experiments have not come up with linear-type BC2N.

Regarding the cubic B-C-N materials, some early studies of diamond/c-BN 

superlattices and alloys indicated very limited miscibility with respect to c-BN 
14

and diamond in the solid state around atmospheric pressure.21,22 In order to 

explore possible synthetic routes to the single-phase “heterodiamond” BC2N 

phase, Tateyama et al.23 investigated the effect of compression on graphitic 

BC2N (g-BC2N) using the monolayer structure proposed by Liu et al.15 and 

Nozaki et al.16 Structure optimization was done using monoclinic unit cells with 

8 atoms. The results showed that a more stable structure corresponds to that 

with no B-B or N-N bonds and more C-C and B-N bonds, so-called β-BC2N 

(Fig. 2.1). It seems that earlier predictions of the structural stability based on the 

number of B-N and C-C bonds (bond counting rule) in a monolayer structure 

hold here also. Among the 4 possible transformation structures, g-BC2N to βBC2N was the most energetically favorable. Calculations predicted that β-BC2N 

can be synthesized from g-BC2N at nearly ambient pressure while the 

superlattice BN/C2 by compression of g-BC2N to about 16 GPa23. The bulk 

modulus of dense β-BC2N is calculated to be between 374 and 409 GPa.23–26

Mattesini et al.24 predicted the metastable orthorhombic Λ-BC2N and trigonal 

Φ-BC2N, “the most stable forms of the three-dimensional BC2N system”, by 

replacing the carbon atoms with boron and nitrogen in the crystalline phase of 

fcc diamond. In new ternary B-C-N phases, the substitution was performed by 

taking the consideration of bond counting rule used by Tateyama et al.23 and 

density functional theory (DFT) and local density approximation (LDA) were 

used for the calculation. Later, Sun et al.27 confirmed that the above described 

β-BC2N has the lowest total energy among other possible atomic arrangements, 

although all the structures seem to be metastable and tend to separate into 

diamond and c-BN.

Fig. 2.1 A representative atomic arrangement in the unit cell of cubic β-BC2N structure

Yuge et al.28 examined the phase stability of c-BCN (diamond-like) with a 

composition range of (BN)(1−x)(C2)x (0 ≤ x ≤ 1) around atmospheric pressure by 

the combination of the cluster expansion technique and Monte Carlo 

simulation–based on first-principles calculations. In their work the authors 

doubted the results of a few earlier calculations that addressed the phase stability 

of c-BNC with respect to the c-BN and diamond, showing a significant 

discrepancy in the predicted phase diagrams due to the differences in the used 
15

model: One achieves complete miscibility around T = 3,546 K, which is slightly 

below the melting line between c-BN and diamond.29 Another reaches complete 

miscibility at T = 8,500 K, which is much higher than the melting line.22 In their 

work, Yuge et al.28 carefully considered the dependence of enthalpy on atomic 

arrangements, the effect of atomic orderings on configuration entropy, and the 

effect of lattice vibration, which were neglected in earlier theoretical 

predictions. The final result predicts strong preference of B-N and C-C bonding 

and disfavors B-C, C-N, B-B, and N-N bonds, which suggests phase separation 

to c-BN and diamond. Complete miscibility is achieved over T = 4,500 K, which 

is higher than the melting points of both diamond and c-BN.28 Similar trends 

are found for the miscibility in the wurtzitic w-BCN between hexagonal 

diamond (londsdaleite) and w-BN, although a significant anisotropy in 

solubility for the wurtzite-like structure is observed, which leads to a 

2-dimensional (2-D)-like solid solution in dilute composition despite its 

3-dimensional (3-D) sp3bonding.30 Furthermore, examination of pressure 

effects on the phase stability of c-BNC31 has shown that at a pressure of 10 GPa, 

the solution energies of neighboring B–N and C–C atoms into diamond and 

cubic BN are both higher than those at 0 GPa, indicating a decrease of solubility 

in c-BNC under applied pressure. This decrease can mainly be attributed to the 

increase of volume with the formation of a c-BNC solid solution. The authors 

note that the success in synthesis of c-BNC compounds under high pressure 

should relate to the kinetic reason but not to a reason of the energetic in 

thermodynamic equilibrium.31

The structural stability and mechanical and electronic properties of cubic BCxN 

(0.21 < x < 19.28) crystals within a random solid solution model were 

investigated by Zhuang et al.32 from the first principles. The authors have shown 

that, compared to c-BC2N, the BCxN solids with higher carbon content (x > 2) 

exhibit better structural stability and higher elastic moduli. Moreover, 

significant deviations of the elastic moduli and lattice parameters from the 

predictions of Vegard’s law have indicated that the BCxN solid solutions are not 

a simple mixing of diamond and cubic-BN. The computed band gaps are 

substantially lower than those of diamond and c-BN approaching the lowest 

value of 2 eV for x = 4.4.

Sun et al.33 studied BC2N in cubic structure forms using ab initio pseudopotential LDA calculations. The authors compared the recent experimental 

claim of cubic-BC2N of that time5,7 with 8 atom zinc-blende structured cubic 

unit cells (the experimental results will be discussed in the next section). 

Fortunately, out of 420 different configurations, only 7 are topologically 

different because of the high symmetry of the zinc-blende-structured lattice, 

which makes their study possible. Even though predicted structures (1) and (2) 

(Fig. 2.2) are more stable and have the lowest energies, all 7 possible structures 

are metastable and tend to separate into diamond and cubic BN. Structures (1) 
16

and (2) do not have B-B and N-N bonds, which is also consistent with earlier 

predictions. However, the full bond counting rule is not satisfied because they 

do have the same number of C-B and C-N bonds as C-C and B-N bonds. Later,

the same research group, based on first-principles total-energy, and dynamic 

phonon calculations, investigated the structural transformation to and stability 

of cubic BC2N phases under pressure.34,35 The authors showed that different 

starting material forms (graphitic BC2N with different atomic arrangement) lead 

to distinct synthesis routes, yielding end products with drastically different 

physical properties. Among all the considered structures, a high-density phase 

with no B-B or N-N bonding (phases 1 or 2 in Fig. 2.2) showed the highest 

structural stability and lowest compressibility at high pressure.

Fig. 2.2 All the possible topologically different c-BC2N structures (after structural 

relaxation) starting from an 8-atom zinc-blende structured unit cell.33 The dotted bonds 

indicate the broken covalent bonds between N atoms.

There were many other structures proposed for BC2N ternary phases, like 

pseudocubic superlattice BC2N1x1,

36 “low-density” cubic LD-BC2N,37

tetragonal z-BC2N,38 z*-BC2N and t-BC2N (Fig. 2.3),39 wurtzite-type 

w-BC2N,40 chalcopyrite-type cp-BC2N,25 body-centered bc6-BC2N,

41 etc. (see 

Table 2.1). By comparing the simulated and experimental X-ray diffraction 

(XRD) patterns of BC2N, Zhou et al.39 have shown that for nanocrystalline 

powder z-BC2N, z*-BC2N and t-BC2N reveal no significant difference and are 

in excellent agreement with the experimental results of Solozhenko et al.7

However, only tetragonal t-BC2N

39 seems to address most of the difficulties in 

explaining Raman splitting of LO and TO modes and higher experimental 

Vickers hardness values.7 Another research group employed an ab initio

evolutionary algorithm42 to resolve the crystal structure of the observed 

superhard BC2N from 2 independent experiments. Among 12 considered 

structures, the authors uncovered 2 polymorphs with rhombohedral and 

orthorhombic symmetries, with which the experimental XRD pattern is well 

reproduced. Analysis of the total energy results and the simulated energy-loss 

near-edge spectroscopy suggested that the rhombohedral structure (R3m with 2
17

BC2N units in primitive cell) is the best candidate for the superhard BC2N. The 

calculated electronic structures show that rh-BC2N is a wide-gap semiconductor 

with an indirect DFT band gap of 3.8 eV. 

Fig. 2.3 Proposed tetragonal crystal structures of diamond-like BC2N: a) z*-BC2N with 

P-42m symmetry and b) t-BC2N with P-421m symmetry38,39

Luo et al.43 studied the BC4N crystal structure using DFT with LDA. They tried 

to correlate the experimentally derived BC4N structure and properties reported 

by Zhao et al.9(see Section 2.1.3). The authors proposed 3C-BC4N 

configurations constructed with Ramsdell notation and ruled out the modeling 

BC4N (unit cell consists of 6 or 12 atoms) with a zinc blende structure, which 

has 8 atoms in its unit cell. A simulated XRD pattern of the 3C-BC4N model 

matches the experimental XRD pattern of c-BC4N. 

The same research group has also considered the B-C-N phases with other 

stoichiometries—structural, electronic, and mechanical properties as well as 

phase transitions under hydrostatic pressures have been investigated for 6 

possible B2C2N2 structures deduced from the 3C-SiC unit cell.44 The 

calculations have shown that B2C2N2 with the maximum numbers of C–C and 

B–N bonds has the lowest total energy and is a large-gap semiconductor with 

an indirect band gap of 4.10 eV. The calculated bulk-, shear moduli, and Vickers 

hardness of 398.4, 440.7, and 84.3 GPa of the B2C2N2 indicate that it is one 

potential superhard material with a hardness larger than that of cubic boron 

nitride. The pressure-induced structural transition from its low-pressure 

graphitic counterpart is estimated to occur above 11.4 GPa.44 Tang et al.45

investigated 5 nonequivalent atomic configurations of the suggested 3C-BC4N 

structure using first-principle calculations. All the configurations were found to 

be metastable. The band gap was calculated to be highly dependent on the 

atomic arrangement, thus a different 3C-BC4N configuration revealed 

insulating, semiconductor, semimetallic, or even metallic behavior.45 Two 

possible metastable high-density BC6N phases originating from the diamond 

structure were suggested by Luo et al.46 The calculations have shown that both 

BC6N phases are semiconductors with a direct generalized gradient 
18

approximation (GGA) energy gap of about 2.1 eV and have Vickers hardness 

about 79–80 GPa, higher than the values for c-BC2N and c-BN.

Weihrich et al.47 investigated a selective substitution of nitrogen with boron in 

the C3N4 for the 2- and 3-D structures using pseudo-potential and full-potential 

computations in the framework of the local DFT. This substitution led to the 

structures with the experimentally observed BC3N3 stoichiometry. The authors 

proposed a 2-D structure for the B-C-N precursor and a new ultrahard 

rhombohedral (R3m) compound, rh-BC3N3, with a bulk modulus of 

approximately 405 GPa reaching the range of the other studied BC2N structures.

Very recently, the formation ability of B-C-N ternary phases was studied by 

Jiang et al.48 using DFT based on the model of the previously synthesized 

tetrahedral amorphous carbon (ta-C).49 The “ta-C” modification was reported to 

possess not only high mass density (3.2 g/cm3) and large amounts of sp3

hybridized carbon (84%−88%) but also superior hardness (45 GPa) and 

Young’s modulus (340 GPa). Possible B-C-N ternary phases were studied and

plotted in a full-spectrum triangular phase diagram (Fig. 2.4). Formation 

energies are plotted accordingly and compared with experiential compositions 

reported to date. It is interesting that the formation energy distributes 

symmetrically along the C−BN isoelectronic line, in which the compositions in 

the range of B: 15–35 at%; C: 30–55 at%; N: 15–35 at% are relatively easier to 

form owing to their negative formation energy. Notably, most of the BxCyNz

compositions synthesized in experiments are located in this area.

Fig. 2.4 a) Distribution of formation energy (eV/atom) on the upper rhombus of 

ternary B−C−N phase diagram using full spectrum. Less formation energy means that 

the corresponding composition is easier to form. b) BxCyNz compositions synthesized in 

previous experiments. Most compositions are located in the area with negative formation 

energy, in agreement with the theoretical prediction.48

 (a) (b)
19

2.1.1.2 Mechanical and Electronic Properties

In this report, the considered mechanical properties of the B-C-N ternary phase 

are mainly restricted to bulk modulus (B), shear modulus (G), and hardness 

(Vickers microhardness, Hv). The great interest in a superdard phase has been 

aroused because solid solutions (prepared by Knittle et al.5) of carbon and boron 

nitride showed a bulk modulus value of 355 GPa. Tateyama et al.23 was the first 

to calculate a bulk modulus of 438±14 GPa for his predicted β-BC2N structure, 

which is slightly less than that of diamond. Thereafter, many theoretical 

predictions came with different bulk moduli and hardness depending on the 

considered structure predicted and the model used for calculations. The 

available theoretical values of lattice parameters, bulk and shear moduli, and 

estimated hardness of the predicted superhard BC2N structures, as well as of 

dense BCN phases with compositions different from BC2N, are summarized in 

Table 2.1. The predicted bulk moduli for all the different structures generally 

exceed the experimental ones of c-BN (369–401 GPa) with most of them 

approaching those of diamond (433–442 GPa).1,50
20

Table 2.1 Calculated lattice parameters, bulk moduli, shear moduli, and estimated 

hardness (if available) of the predicted superhard B-C-N phases. The experimental 

values for dense carbon and BN modifications are given for comparison.

Structure a

(nm)

b

(nm)

c

(nm)

α

(°)

β

(°)

γ

(°)

B

(GPa)

G

(GPa)

Hv

(GPa) Ref.

Diamond1 0.3567 0.3567 0.3567 90 90 90 442–433 534-544 60–150 [1, 50]

 C-ondsdaleite1 0.252 0.252 0.412 90 90 120 ... 382 60–70 [1, 50]

c-BN1 0.3616 0.3616 0.3616 90 90 90 369–401 409 46–80 [1, 50]

w-BN1 0.254 0.254 0.420 90 90 120 390 330 34–60 [1, 50]

c-BC2N1 0.3642 0.3642 0.3642 90 90 90 282 447 76 [7, 51]

0.3595 0.3595 0.3595 90 90 90 ... ... 62 [9]

c-BC4N1 0.3586 0.3586 0.3586 90 90 90 ... ... 68 [9]

β-BC2N 0.3577 0.3577 0.3577 89.38 90.62 90.62 438±14 ... ... [23]

0.3579 0.3579 0.3612 90 90 89.32 383.3 ... ... [52]

0.361 0.361 0.361 90 90 90 342.4 453.6 ... [37]

o-BC2N 0.2528 0.2502 0.3587 90 90 90 408.9 ... ... [24]

0.2529 0.2502 0.3591 90 90 90 403.1 446 ... [26]

Φ-BC2N 0.2496 0.2496 0.4192 90 90 120 420.1 ... ... [53]

Λ-BC2N 0.3554 0.3599 0.3553 90 90 90 459.4 481.96 ... [24, 53]

c-BC2N-1 0.3570 0.3609 0.3570 89.5 90 90 399.7 ... ... [33]

c-BC2N-2 0.3568 0.3564 0.3613 90 90 90 400.1 ... ... [33]

BC2N1x1 0.4355 0.2514 1.2483 90 90 90 419.5 ... 75.8(21) [36]

0.3579 0.3579 1.074 90 90 90 420 ... ... [54]

z-BC2N 0.3565 0.3565 0.7168 90 90 90 402.7 447.4 75.9 [38]

0.3604 0.3604 0.7247 90 90 90 385 ... 65.0 [42]

z*-BC2N 0.3577 0.3577 0.7122 90 90 90 423.3 471.7 75.6 [39]

t-BC2N 0.3579 0.3579 0.7116 90 90 90 422.1 466.9 75.3 [39]

r-BC2N 0.3602 0.3602 0.3602 n.d. ... ... 382 ... ... [54]

rh-BC2N 0.2545 0.2545 2.5068 90 90 120 395 ... 62.1 [42]

w-BC2N 0.2501 0.2501 0.4205 90 90 90 407.5 466.5 76.8 [40]

cp-BC2N 0.3613 0.3613 0.7146 90 90 90 367.4 432.0 72 [25]

0.3653 0.3653 0.7228 90 90 90 349 ... 67.5 [42]

bc6-BC2N 0.4361 0.4382 0.4405 90 90 90 305 ... 60 [41]

3C-BC4N 0.2507 0.2507 0.6196 90 90 120 418.8 521.3 84.3 [43]

BC4N2x1 0.3567 0.3567 n.d. 90 90 90 428 ... [54]

t-BC6N 0.3574 0.3574 0.3564 90 90 90 410.4 ... 79.9 [46]

r-BC6N 0.3565 0.3565 0.3565 90.15 90.15 90.15 399.9 ... 79.1 [46]

BxCyNz ... ... ... ... ... ... (650–730)2 45–50 [48]

rh-BC3N3 0.3449 0.3449 0.3449 87.5 ... ... 405.3 ... ... [47]

B2C2N2 0.2521 0.2521 0.6235 90 90 120 398.4 ... 84.3 [44]

1 Experimental values

2 Values for the Young’s modulus are given.
21

The claimed successful synthesis of cubic boron–carbonitride compounds 

BC2N with an experimentally determined extreme hardness second only to 

diamond and comparably low bulk modulus7,9 initiated a systematic theoretical 

exploration of the nature of such a phenomenon. In particular, Zhang et al.55

have examined the ideal strength of cubic BC2N using first-principles 

calculations. In contrast to the experiment, the authors revealed that, despite the 

large elastic parameters, compositional anisotropy and strain-dependent

bonding character impose limitations on their strength. Consequently, the 

hardness of the optimal BC2N structure should be even lower than that of c-BN. 

The measured extreme hardness of BC2N nanocomposites is referred to the 

nanocrystalline size effect and the bonding to the surrounding amorphous 

carbon matrix. Zhuang et al.56 have shown that the atomic arrangements (B, C, 

N) within the c-BC2N crystal lattice—namely, the different degrees of mixture 

of diamond and c-BN, may lead to distinctly different values of Vickers 

hardness and tensile strength. Similarly, Zhang et al.57 have investigated elastic 

constants for several diamond-like BC2N structures from the first principles. 

The authors have shown that, although the deviations of the lattice constants 

from those of cubic lattices are small (about 1%), the differences among the 

elastic constant components, which should be the same for cubic lattices, can 

reach 75% because of the chemical anisotropy of the BC2N structures. For 

BC2N in which exact cubic symmetry is constrained, large residual stresses of 

4–5 GPa in the structures were observed. Thus, the authors have concluded that 

any results derived by assuming exact cubic symmetry for BC2N structures may 

not be reliable.

Interest in electronic properties of B-C-N ternary phases started as early as their 

stability and structure prediction arose. Most of the theoretical calculations on 

structure and mechanical properties also predicted band gap values. However, 

predicted properties vary from semiconductor to insulator depending on the 

structure. For a layered structure, Liu et al.15 predicted a band gap (Eg) of 1.6 

eV, and for tubular and chiral structure, Miyamoto et al.58 predicted 1.25–1.75 

eV. Experimental results from Watanabe et al.59 also confirmed semiconductor 

behavior of layered BC2N. For BC2N heterodiamond, Tateyama et al.23

predicted an indirect band gap of 3.97 eV. For orthorhombic BC2N

24, a band 

gap of around 1.7–2.01 eV is predicted, whereas for c-BC2N

33 derived from a 

zinc-blende structure, a bang gap of approximately 2.0 eV is calculated. Zhou 

et al.38 have shown that despite the only difference between z*-BC2N and zBC2N is that B and N atoms are interchanged with each other, the calculated 

band structures exhibit a significant difference: z*-BC2N is an indirect 

semiconductor with a band gap of 3.6 eV, while z-BC2N is a direct one with a 

band gap of 2.7 eV. The calculated electronic structures for chalcopyrite-type 

cp-BC2N showed that this compound is a wide gap semiconductor with a direct 

band gap about 3.3 eV, displaying some potential application ultraviolet 

luminescence.25 Rhombohedral rh-BC2N

42 and wurtzite-type w-BC2N40 are 
22

predicted to have an indirect wide band gap of 3.6 and 3.99 eV, respectively. 

An even wider band gap is calculated for BC4N, to be 4.06 eV.43 In contrast, 

bc6-BC2N is an indirect semiconductor with band gap of 0.60 eV.41 Several 

reports23,24,33 also pointed out that the employment of LDA in calculation leads 

to significant underestimation of the band gap (65%–75%), thus the values 

calculated using this model cannot be considered as completely reliable.

2.1.2 Synthesis of B-C-N Precursors 

A few recent first-principles studies of structural transformation and stability of 

B-C-N phases have indicated that different starting materials may lead to 

distinct synthesis routes, yielding various dense B-C-N end products with 

drastically different physical properties.

23,34 Therefore, different precursor 

materials used for HP-HT experiments on the synthesis of ultrahard dense B-CN compounds will be considered below.

The oldest, simplest, and still widely used method is to synthesize boron 

carbonitrides from the elements at high temperatures. A typical reaction was used

by Kosolapova et al.60 in which boron and carbon powders were heated at 1,800–

2,000 °C in N2/NH3 for prolonged periods promoting the reactions below:

B(s)+2C (s)+

1

2

N2

(g)

1800-2000 °C

→ BC2N (s).

B(s)+2C (s)+ NH3

(g)

∆

→ BC2N (s) +

3

2

H2(g).

The formation of BC2N in such reactions is diffusion controlled, occurring at 

the solid interfaces (B/C or B/C3N4) by a process of nucleation and growth. At 

elevated temperatures, the reactant atoms have enough energy to diffuse toward 

the interface forming the product. However, the process slows down as the 

interface thickens, and effectively, the reactants are no longer in contact.60

Because of a complex process of diffusion of ions toward the thick interface, 

there is always some degree of inhomogeneity on the atomic level, which 

renders this method ineffective for the formation of complete solid solutions. 

Recently a stoichiometric ternary compound B4CN4 was obtained by direct 

nitridation of commercial B4C powder via heating in a stream of nitrogen at 

temperatures in the range of 1,600–1,900 °C.61 The reaction was diffusioncontrolled. The resulting powder with elemental composition B3.96-4.03C1.00N3.96-

4.07O0.01-0.08 (~B4CN4) determined by electron probe microanalysis (EPMA) had 

the turbostratic layered structure, where carbon atoms form linear C sp2–C 

sp2chains bonded to B and N within h-BN layers with a molar ratio C/BN = 1/4.

One of the first attempts of c-BN and diamond-mixed crystals by Badzian et al.4

used graphitic g-BCN prepared by a CVD route. They used gaseous mixtures

of BCl3/CCl4/N2/H2 at 1,900 °C to deposit polycrystalline BCN deposited on a 

graphite rod: 
23

BCl3 + CCl4 + N2 + H2

1900 °C

→ (BN)xC1-x + . . .

As the CVD method employed 4 different gases, controlling the reaction and 

end product stoichiometry was very crucial. In further works, Bartlett and 

coworkers 62,63 conducted the reaction of boron trichloride with acetylene and 

ammonia at relatively low temperatures, which resulted in better ordering and 

more accurate compositions (e.g., B2CN2, B0.485C0.03N0.485, B0.35C0.3N0.35):

BCl3 + y C2H2 + z NH3

700 -800 °C

→ BxNyCz.

Instead of acetylene, other hydrocarbons, such as CH4 and C3H8, have been used 

for the CVD preparation of other ternary B-C-N phases.64 For example,

Besmann65 obtained single-phase BC0.43N0.29 with a graphite-like structure from 

BCl3-NH3-CH4-H2 mixtures at 1,650 K and 3.3 kPa. If boron trichloride and 

acetonitrile (CH3CN) are used as the reactants, almost stoichiometric material 

with composition BC2N is obtained62,63:

BCl3 + CH3CN 

800 °C

→ BC2N+3 HCl .

The films produced by those methods consist of a turbostratic layered structure 

that contains sp2-hybridized B, C, and N atoms according to XRD and 

transmission electron microscopy (TEM) studies. X-ray photoelectron 

spectroscopy (XPS) investigations reveal the presence of C-C, B-N, C-N, and 

B-C bonds. The same approach was successfully applied by Watanabe et al.66

in order to produce amorphous BC2N films on polycrystalline Ni and quartz 

substrates. While using the same reactants in a hydrogen and nitrogen 

atmosphere, Kawaguchi et al.67 deposited new graphite-like films of 

composition BC0.9-1.3N0.8-0.9H0.4-0.7 and BC3.0-3.2N0.8-1.0H0.2-2, which can be 

described as BCN(H) and BC3N(H). X-ray and electron diffraction analyses 

indicate that these materials have hexagonal structures similar to that of 

graphite. 

Hegermann et al.68 investigated the influence of the carrier gases N2, argon (Ar), 

and helium (He) and the applied power on the chemical composition of boron 

carbonitride films deposited by a PA-CVD process. Si(100) wafers were used

as substrates and pyridine-borane and triazaborabicyclodecane as B-C-Nforming single-source precursors. Films that were either deposited in He using 

a low power density or in N2 using a high power density revealed similar 

chemical composition of approximately BC4N as well as comparable properties 

(in particular, outstanding hardness ~60 GPa). XRD and TEM analysis of these 

films showed their amorphous nature. It should be noted that amorphous B–C–

N films have been intensively studied during the last 2 decades since practically 

any composition within the B-C-N triangle can be synthesized by various 

conventional chemical and physical deposition methods, such as the previously
24

mentioned (PA)-CVD66,67–69 as well as ion beam–assisted deposition,70 pulsed 

laser deposition,

71 and magnetron sputtering.72 Tuning different deposition 

parameters and choosing an appropriate atmosphere allows us to control not 

only the composition but also the bond contents in the resulting B-C-N films. 

This can play a key role when choosing precursors for the high-pressure 

synthesis of ultrahard diamond-like B-C-N phases, since the nature of the 

starting material is found to have a significant influence on the structural 

features and properties of the end product. 

In contrast to the deposition routes to B-C-N materials resulting in ternary 

BxCyNz films, large amounts of similar phases can be synthesized via pyrolysis 

of polymeric B-C-N-based precursors at relatively low temperatures around 

1,000 °C. In this way, thermal treatment of B-triphenylborazine and Ntriphenylborazine in an autoclave at 1,000 °C results in boron carbonitrides with 

compositions of BC3.9N and BC3.6N, respectively73. Ceramic BxCyNz powders 

with various nonstoichiometric compositions can be also obtained via pyrolysis 

of decaborane(12)-adducts of diamines [–B10H12∙diamine–]n,

74 pyroazabole 

polymer,

75 or polyethylene-iminoborane [–CH2–CH2–N=BH2–]n.76

Riedel and coworkers17,77,78 reported the synthesis of turbostratic graphite-like 

BC2N and BC4N from piperazine borane and pyridine borane, respectively (the 

reactions A and B given below). This method can be ascribed to the so-called 

polymer-derived ceramic route, where a highly cross-linked insoluble, infusible 

polymeric intermediate is formed followed with pyrolysis under argon 

atmosphere at 1,050 °C. It is important to mention that in the latter reaction, 

utilization of nitrogen-containing atmosphere can lead to completely different 

results—namely, formation of almost pure boron nitride. In a subsequent work 

of the same research group, a crystalline ternary h-BC2N with a graphite-like 

structure was synthesized from these precursors at elevated pressures and 

temperatures.79

(A) 

(B) 
25

Amorphous B-C-N ceramics were produced by thermolysis of 

poly(borosesquicarbodiimide) of idealized formula [B2(NCN)3]n at 1,100 °C in 

an argon atmosphere.80 Wide-angle diffraction showed that the ceramics consist 

of an amorphous structure with hexagonal planar near-range ordered atomic 

arrangements similar to graphite. Nuclear magnetic resonance (NMR)

investigation of the products showed the presence of BN structures with mainly 

trigonally coordinated boron and nitrogen nuclei and the presence of amorphous 

(graphite-like) carbon within the B-C-N ceramics at the same time. The authors 

concluded that the BCN phase tends to short-range phase separation into BN 

and C regions.

Another amorphous B-C-N ceramic with composition B4CN4 was prepared via 

a nonoxide sol-gel process.81 In this approach B-trichloroborazene, B3N3H3Cl3, 

reacts with bis-(trimethylsilyl)-carbodiimide Me3Si-NCN-SiMe3 in 

tetrahydrofuran (THF) or toluene, or without any solvent, to form nonoxide 

gels. The BCN2H2-xerogels, obtained after aging and drying, are practically free 

of any chlorine or oxygen and contain only small amounts of silicon. At 1,200 

°C a novel amorphous B4CN4 phase is formed, which is stable up to 1,600 °C 

where it starts to crystallize. This novel nitrogen-rich B4CN4 phase may be a 

promising amorphous precursor for high-pressure synthesis of crystalline BCN 

phases.

A series of graphitic B-C-N compounds (g-BCxN) were prepared in good yield 

by pyrolysis of the polymers between BCl3 and commercially available nitriles 

at 1,773 K.82 Acrylonitrile, malononitrile, acetonitrile, tetracyanoethylene 

(TCNE), and polyacrylonitrile were used as the C-N sources while BCl3 was 

used as the B-source. The g-BCxN samples were prepared in 3 steps: 1) adduct

formation at ambient temperature, followed by dehydrochlorination

/polymerization at temperatures up to 773 K; 2) carbonization of the polymeric 

materials at 1273 K; and 3) graphitization at 1,773 K. The products were 

characterized by combustion elemental analysis, XPS, Fourier-transform 

infrared (FTIR) spectroscopy, and XRD. The obtained powders of g-BCxN (2 ≤ 

x ≤ 5) were subsequently used for the shock synthesis of B-C-N 

heterodiamonds, c-BCxN.82

Tian et al.83 synthesized the B-C-N precursor by mixing boric acid H3BO3 with 

melamine C3N6H6 (1:2 ratios) in an agate mortar. The mixture was heated to 
26

200 °C for 1 h and then at 300 °C for an additional 2 h. After cooling, the

pulverized powder was exposed to 1,600 °C in nitrogen atmosphere for 15 min. 

Li et al.,

84 in a similar approach, tried the mixture of boron oxide and melamine, 

subjecting it to 770 °C in vacuum for 2 h leading to B-C-N precursor. However,

by knowing boric acid coverts into boron oxide at 300 °C, both reactions look

the same, even though the starting material is different. Amorphous BCN was 

prepared by a solid-state reaction between boric acid and melamine via heat 

treatment at 1,273 K under 10-3 Pa.85 For the H3BO3 to C3N6H6 mass ratio of 

1:3, the obtained B-C-N material revealed the chemical composition of 

B0.48C0.29N0.23. Annealed for 40 min at 1,473 K under 4.0 GPa, the amorphous 

BCN crystallized into a single-phase hexagonal (h-BCN) compound with lattice 

constants of a0 = 0.2506 nm and c0 = 0.6652 nm. 

Hubacek and Sato et al.18 prepared a B-C-N precursor by simultaneous 

nitridation of boric acid and carbonization of saccharose in molten urea 

followed by annealing in nitrogen at 1,500 °C. The results showed graphite-like 

BC2N and BC4N, depending on the saccharose content, having broad XRD lines 

like turbostratic layered structures. This precursor preparation method was later 

used by Solozhenko et al.7in the synthesis of the superhard c-BC2N phase. The 

reaction of low-surface-area amorphous carbon spheres with a mixture of urea 

and boric acid at 930 °C yields a composition close to BC4N with a graphitic 

structure.86 The obtained product was characterized by electron energy loss 

spectroscopy (EELS), XPS, TEM, Raman spectroscopy, and XRD, revealing a 

porous ceramic material composed of BCN spheres (Fig. 2.5) with a layered 

structure involving random distribution of boron, carbon, and nitrogen atoms. 

In turn, the reaction of amorphous carbon nanotubes with boric acid and urea 

was shown to yield nanotubes with the approximate composition of BC4N.87

Fig. 2.5 High-temperature reaction of amorphous carbon spheres. a) SEM and b) TEM 

image, with boric acid and urea yields spheres with graphitic structure and composition 

of BC4N, c) SEM and d) TEM image.86
27

A solvothermal reaction of CH3CN·BCl3 and lithium nitride (Li3N) using 

benzene as the solvent has been successfully applied to prepare boron 

carbonitride at 300 °C and less than about 7 MPa.88 XRD and TEM investigation 

of the obtained powder revealed hexagonal ordering. The product 

predominately consisted of B, C, and N elements (B: 14.4%, C: 36.6%, N: 

20.8%) with some amount of O and H (O: 8.0%, H: 4.2%) determined by 

combustion elemental analysis. XPS and FTIR spectroscopy confirmed the 

chemical composition and atomic-level hybrid. A hexagonal graphite-like BCN 

compound was also prepared by a reaction between carbon tetrachloride (CCl4), 

boron tribromide (BBr3), Li3N, and sodium at 400 °C.89 Measurements of FTIR, 

XRD, TEM, and EELS show that 2 kinds of compounds have been formed: 

hexagonal polycrystalline BC2N phase was found to coexist with hollow spherelike amorphous C–N phase with composition close to C3N.

Another approach to the low-density ternary B-C-N compounds implies 

mechanical mixing and alloying. In this method boron-, carbon-, and nitrogencontaining materials are mixed together to form a B-C-N complex in the easiest 

possible way—for example, boron nitride powder with carbon/graphite powder 

or boron powder with carbon nitride powder. Homogenous samples with welldistributed B, C, and N are obtained via prolonged (high-energy) ball milling. 

Knittle et al.5 were the first to try mechanical mixtures of graphitic-boron nitride 

(h-BN) and graphite (g-C) in different compositions. The mixtures were ground 

together under acetone to get uniformity. The authors compared the results with 

that of chemically prepared BCN microcrystalline powders, and the performed 

high-pressure high-temperature experiments provided solid solutions of c-BN 

and diamond. Zhao et al.9tried a very similar approach to get BC2N and BC4N 

mixtures. Precursor materials were prepared by stoichiometric 2:1 and 4:1 

molar ratio mixture of graphite and hexagonal boron nitride (h-BN) to get BC2N 

and BC4N, respectively. The mixture was ball-milled for 34 h in a tungsten 

carbide vial to get good homogeneity. The idea was to break sp2bonding among 

hexagonal rings of graphite and h-BN crystal structure and to get amorphous 

BCN fine powder.9 Huang et al.90 applied a similar approach to synthesize the 

amorphous B-C-N (a-BC2N) phase. EELS and TEM studies indicated the sp2-

hybridized bonding of the a-BC2N; the mixing between the BN and the C 

species was achieved at a nanometer scale, thus a mechanical mixture rather 

than a chemical mixture is obtained. Filonenko et al.91 used a starting material 

as mixtures of boron (B) (35%–50%) and carbon nitride powders. 

Commercially available boron ranging from 1 to 10 μm mainly consisting of βphase was mechanically mixed with graphitic carbon nitride (C3N4) 

nanospheres. The authors reported that boron content in the mixture was varied 

from 20% to 80%, low B (<35%) leading to the decomposition of C3N4 and 

higher B (>50%) content leading to the synthesis of boron carbides at HP-HT 

conditions; however, the reaction mechanism is still unclear. 
28

The techniques employed to synthesize low-density B-C-N phases—such as 

high-temperature nitriding, solid phase pyrolysis, chemical and physical vapor 

deposition, and ball milling—generally result in the poorly crystallized, 

turbostratic, or amorphous products. Well-crystallized B-C-N compounds 

having hexagonal structure (h-BCN) were successfully obtained by applying 

high pressures and high temperatures. Thus, amorphous B-C-N precursors were 

transformed into h-BCN using a belt-type apparatus79 and Bridgman anvils

85 at 

pressures of 3–5 GPa and temperatures of 1,000–1,500 °C. Yamada92 produced 

a graphite-like phase by shock synthesis of a mechanical mixture of graphite 

and h-BN. The hexagonal lattice parameters of a0 = 0.2475 nm and c0 = 0.676 

nm were determined by TEM from a cluster of sheet-like particles, which 

composition corresponded to BC2N. EELS confirmed the presence of boron, 

carbon, and nitrogen—all sp2-hybridized—in a single graphite-like crystal. A 

highly crystalline hexagonal BC2N compound was prepared by the compression 

of a turbostratic B-C-N precursor (synthesized via high-temperature reaction of 

melamine and boric acid) with iron catalyst at the high temperature of 1,500 °C 

and the high pressure of 5.5 GPa.93 Yang et al.94 obtained h-BCN from a mixture 

of boron powder and a CNH compound prepared by pyrolysis of melamine 

(C3H6N6) under high temperature (1,400–1,500 °C) and high pressure (5.0–

5.5 GPa). XPS, FTIR, and Raman spectroscopy showed the presence of B-N, 

C-B-N, C-N, and B-C bonds in the product with the overall composition of 

B0.18C0.64N0.16 (near BC4N). X-ray diffraction analysis confirmed a hexagonal 

structure. Scanning and transmission electron microscopy revealed the flaky 

h-BC4N particles of about 1 μm with a thickness of 200 nm.

2.1.3 HP-HT Experiments and Results

The conditions normally employed to synthesize superhard materials require 

extreme pressures and temperatures or a chemical vapor deposition technique 

(the latter, however, does not provide bulk superhard materials). Badzian4

prepared cubic BN-C mixed crystals by the HP-HT technique (14 GPa, 

3,600 °C, Bridgman apparatus), using as the starting material hexagonal BN-C 

prepared from CVD. He believed that the material had a diamond-like structure 

and found that the material exhibited weak (200) and (420) reflections in the 

XRD pattern, suggesting the possibility of B-N pairs in the sublattices. Sasaki 

et al.

95 attempted the transformation of graphitic BC2N62 into a cubic phase 

under relatively mild HP-HT conditions (5.5 GPa, 1,400–1,600 °C, Belt Press) 

using a Co catalyst, but they obtained the phase-separated mixture of cubic 

phases (i.e., the well-crystallized diamond and c-BN crystals up to 3 μm in size). 

Expecting the catalytic promotion of the phase separation in the B-C-N system, 

Nakano et al.96 attempted the direct transformation of g-BC2N into the cubic 

phase, without additives under higher HP-HT conditions (7.7 GPa, 2,000–2,400 

°C, Belt Press). Several cubic phases with a diamond-like structure—namely,

c-BN, diamond containing minor amount of B and N, and a cubic B-C-N 
29

substance—were confirmed in the products above 2,150 °C by the powder XRD

patterns. At 2,400 °C, however, the cubic B-C-N product tended to segregate 

into c-BN and diamond. Authors concluded that not a c-BC2N but a mixture of 

c-BN and diamond is thermodynamically stable in the B-C-N system under the 

conditions employed.96 A similar result was reported recently by Huang et al.90, 

where the HP-HT treatment at 7.7 GPa and 2,300 °C of the amorphous a-BC2N 

phase resulted in the complete segregation of the carbon and BN species—

namely, c-BN, amorphous carbon, and turbostratic graphite. No mutual 

solubilities between c-BN and carbon were found at these P-T conditions.

Knittle et al.5prepared a cubic Cx(BN)(1-x) (x = 0.3–0.6) solid solution under 

high pressures (30–50 GPa) and high temperatures (2,000–2,500 K) from 

mechanical mixtures of graphitic-boron nitride and graphite using a laser-heated 

diamond anvil cell (LH-DAC). Measurements of the lattice parameters of 

samples quenched to ambient pressure showed that the solid solution series was 

nonideal, with molar volumes up to 1% larger than expected (Vegard’s law) 

based on ideal mixing between C (diamond) and cubic-BN. The bulk modulus 

of C0.3(BN)0.7 was found to be 355±19 GPa, which was lower than the ideal 

solid solution of diamond and cubic BN but consistent with the expanded unitcell volumes. The ionic bonding nature in these solid solutions was confirmed 

by Raman spectra of C0.3(BN)0.7, which exhibited LO-TO splitting of the 

phonon modes. 

Komatsu et al.6,10,82,97 converted the graphitic BC2.5N (g-BC2.5N) obtained by 

CVD technique to heterodiamond c-BC2.5N using a cylindrical shock 

compression technique. The shock pressure and temperature on the sample were 

estimated to be 35–50 GPa and 3,000–10,000 °C, respectively. They also 

studied bulk synthesis from different starting materials, g-BCxN (2 ≤ x ≤ 5) 

obtained by both CVD or solid phase pyrolysis, using the shock compression 

method and explored properties like thermal oxidation resistance, thermal 

expansion coefficient, and bulk modulus of the high-pressure products10,82. 

Heterodiamond c-BC2.5N crystals (5–20 nm in size) showed polycrystalline 

behavior. The lattice constant was measured to be 3.605 Å—a value that was 

between those of diamond (3.5667 Å) and cubic BN (3.6158 Å). The material 

revealed a thermal oxidation resistance (stable up to 700 °C) higher than that of 

diamond, a low thermal expansion coefficient (~10-6 K

-1

), and a bulk modulus 

(401 GPa) higher than that of cubic BN. 

Solozhenko et al.7prepared cubic BC2N from graphitic BC2N synthesized 

according to the method of Hubacek and Sato18 using the HP-HT conditions 

(18–25 GPa /2,000–2,200 °C in a LH-DAC and multianvil press [MAP]). The 

lattice parameter of the c-BC2N determined at ambient conditions was 

3.642±0.002 Å, much larger than those of diamond and c-BN (Fig. 2.6). For the 

large volume sample, the hardness (Hv) was measured to be 76 GPa, which is 

higher than that of c-BN. Surprisingly, the determined bulk modulus of the c-
30

BC2N was 282 GPa, which is significantly lower than the values for c-BN or cBC2N reported by other authors.5,10 Zhao et al.9

pointed out a possible reaction 

of the sample with MgO capsule material in that work as well as a large 

deviation of the lattice constant of c-BC2N from Vegard’s law. Solozhenko and 

coworkers came up with a detailed characterization of the synthesized c-BC2N 

sample.

51,98,99 Raman spectroscopy studies showed the Raman band 1326 cm-1

attributed to the LO mode of c-BC2N, whereas the TO mode was missing.98 

Atomic force microscopy images revealed the c-BC2N grains of 200 nm in size 

in contrast to initially reported 10–30 nm from TEM images.99 The authors also 

reported a Knoop hardness (HK) of 55 GPa, Young’s modulus (E) of 980 GPa, 

sheer modulus (G) of 447 GPa, and fracture toughness (KIc) of 4.5 MPa∙m1/2 for 

their c-BC2N and proclaim it the hardest-known solid after diamond.51

Subsequent Brillouin scattering measurements on the same cubic BC2N phase 

provided even lower bulk modulus and shear modulus values of 259 and 238 

GPa, respectively.100 The authors relate these results to the dependence of elastic 

properties of diamond-like B-C-N phases on the structure of the phase and on 

the synthesis condition for that specific sample. Nevertheless, the B, E, and G 

values of c-BC2N reported by Solozhenko and coworkers significantly deviate 

from the elastic moduli relationship for homogeneous isotropic materials. 

Theoretical attempts to explain lower bulk modulus, large lattice constant, and 

missing TO modes in Raman spectra of the synthesized c-BC2N and to correlate 

these issues with possible B-C-N structure33,38,39 could not give an unambiguous 

answer so far.

Fig. 2.6 Plot of all observed unit-cell volumes in the B–C–N system of cubic zinc-blende 

structure.9 The relationship of unit-cell volume vs. chemical composition shows a clear 

trend closely obeying Vegard’s law for ideal solid solutions, except for data of 

Solozhenko et al.7
31

Zhao et al.9synthesized a superhard phase of BC2N and BC4N in nanostructured 

bulks under HP-HT conditions (20 GPa, 2000 °C, MAP) from a mixture of the

graphite and h-BN powders in 2:1 and 4:1 molar ratio, respectively. The mixture 

was ball-milled in a tungsten carbide vial for 34 h, which allowed them to obtain 

homogeneous starting materials with randomly bonded B, C, and N atoms. The 

authors expected to break the sp2bonding among the hexagonal rings of the 

graphite and h-BN crystal structures and to introduce some initial amount of 

hybrid sp3bonding. XRD, SEM, and Raman spectroscopy studies of the ballmilled mixture showed that the BC2N and BC4N starting material was 

completely amorphized with a median particle size of about 2–3 nm. 

Comparative studies of synchrotron XRD, HR-TEM, EELS, and Vegard’s law 

of the unit-cell volume versus chemical compositions all indicated that the highpressure BC2N and BC4N products were single B-C-N ternary phases with 

crystallites of about 5 nm in size. Assuming a face-centered cubic zinc-blende 

structure, the authors derived a unit-cell parameter of a0 = 3.595(7) Å and a0 = 

3.586(9) Å for the synthesized BC2N and BC4N material, respectively. The 

hardness measurements showed the nominal hardness of 62 GPa for c-BC2N 

and 68 GPa for c-BC4N, which is between the hardness values of diamond and 

cubic boron nitride. A plot of unit-cell parameters as a function of chemical 

composition showed good agreement with Vegard’s law (Fig. 2.6), with the 

exception of Solozhenko et al.’s data,7 which deviate significantly from the line. 

The authors suggested that the composition of Solozhenko et al.’s sample was

not the solid solution of BC2N and clearly had a different chemistry.9 Taking 

into account the XRD and Raman spectroscopy measurements of Solozhenko 

et al.7and Zhao et al.,9theoreticians suggested 2 tetragonal (z*-BC2N and tBC2N)39 and one rhombohedral (rh-BC2N)42 as the most likely structures 

describing experimentally obtained c-BC2N. In turn, the 3C-BC4N structure 

with trigonal symmetry43 was found to give the best agreement with the 

experimental XRD pattern of c-BC4N.9 

Quite unusual boron-rich B-C-N stoichiometry, “B(CxN1-x)”, was chosen by 

Guo et al.101 for their HP-HT experiments. A B2CN precursor was prepared by 

mechanical vibration-milling process from amorphous boron, graphite, and 

h-BN powders with a mole ratio of 1:1:1 and mixed with Ca3B2N4 catalyst. The 

authors claimed that a boron-rich cubic phase is formed at 5.5 GPa and 1500 °C. 

The carbon content (detected by energy-dispersive X-ray [EDX] spectroscopy) 

was found to vary from 0 to 16 at% depending on the investigated crystal. The 

average composition is reported to be B0.58C0.16N0.26. Despite the evident 

oxygen peak in the EDX spectra, the authors gave no comment on the oxygen 

presence in the product.101 Moreover, the cubic lattice parameter of a0 = 3.618 

Å of the claimed BCN phase is similar to that of c-BN.

Nicolich et al.102 attempted to form cubic ternary crystals from turbostratic BCN 

starting materials77 at HP-HT conditions (3–5 GPa, 1,200–1,500 °C, belt-type 
32

apparatus) obtained ternary graphitic crystals. In earlier attempts, Solozhenko 

et al.103 tried to compress g-BC4N up to 7 GPa at 1900 °C in a multianvil press

(MAP); they succeeded only to densify the starting material. Tian et al.83, Li et 

al.84, and Da-Peng et al.94 studied the HP-HT behavior (5–6 GPa, 1,400–1,700 

°C, MAP) of BCN precursor prepared by reaction of melamine and boric acid 

derivatives (method is explained in Section 2.1.2). This reaction lead to the 

formation of hexagonal-BCN (h-BCN). The lattice constants a0 = 2.506 Å and 

c0 = 6.657 Å, 83 and a0 = 2.510 Å and c0 = 6.690 Å,

84 were reported. A 

completely new BC3.3N crystalline compound with a low-density orthorhombic 

structure has been synthesized using an amorphous B-C-N precursor at 6 GPa 

and 1,773 K.104 Results of energy-dispersive spectroscopy and EELS showed 

that the compound has a 1.04:3.27:1 B:C:N chemical stoichiometry. The lattice 

parameters of BC3.3N crystal were obtained to be a0 = 6.610 Å, b0= 4.977 Å, 

and c0 = 8.509 Å by XRD and select area electron diffraction (SAED). On the 

basis of XRD refinement and EELS results, one possible BC3.3N (B3C10N3) 

model with a space group Pmma (No. 51) is proposed.

Li et al.105 studied high-pressure phase transition of h-BCN83 having the 

composition of B0.47C0.23N0.30 up to 30 GPa in a DAC. Transformation from hBCN to the wurtzite BCN (w-BCN) structure was observed for pressures above 

15 GPa, and a bulk modulus of 275 GPa was derived from the equation of state 

of the w-BCN. The bulk modulus value is very close to that earlier reported by 

Solozhenko et al. for c-BC2N and is significantly smaller than those of c-BN 

and w-BN. The authors, however, neither verify the composition of the new 

high-pressure phase nor provide the data about its stability at ambient pressure. 

Filonenko et al.91 reported high-pressure synthesis of the heterodiamond phase 

using the powder mixtures of graphite-like carbon nitride g-C3N4 and crystalline 

boron (35–50 wt%) as the starting material. A “toroid”-type apparatus was 

employed to apply high pressure (6–15 GPa) and high temperature (1,000–

1,600 °C). Treatment at pressures above 8 GPa and temperatures in the range of 

1,450–1,600 °C resulted in the formation of a cubic phase with the unit cell 

parameter a0 = 3.6551 Å. It is noteworthy that the employed HP-HT conditions 

are very mild when compared to the other reports of the diamond-like B-C-N 

syntheses. The highest yield of the cubic BCN phase has been obtained from 

mixtures with approximately 50 wt% of boron. EDX analysis of the synthesized 

c-BCN crystals revealed an average atomic composition of 45.3% B, 36.5% N, 

11.6% C (roughly ~B4CN4), and a considerable amount of oxygen (6.6 at%). 

On the basis of the Rietveld structure refinement of the c-BCN, the authors 

propose that carbon atoms partially replace boron and nitrogen in their positions 

in the structure of c-BN, while oxygen takes only nitrogen positions.91 This 

statement, however, requires further experimental verification. Moreover, the 

reported unit cell parameter of 3.6551 Å exceeds that of c-BN, thus ruling out 

the possibility of matching with Vegard’s law for diamond/c-BN alloys. A 
33

detailed characterization of the synthesized c-BCN phase with respect to the 

chemical bonding and properties, as well as an unambiguous conclusion on the 

oxygen presence, are awaited.

Recently, a single-phase cubic BC4N was claimed to be formed at high pressure 

and temperature (18–20 GPa, 2,000–2,200 K, MAP) starting with either a 

mixture of diamond and c-BN or graphite and h-BN.106 The authors reported 

extreme Vickers hardness (85 ± 4 GPa) of the HP products, reaching the 

hardness of single crystal diamond. Based on the measured and calculated XRD

patterns, the authors concluded that the synthesized products are solid solutions 

of diamond and c-BN with a zinc-blende structure (F-43m) when starting from 

a mixture of C and BN but could be solid solutions based on the diamond-like 

structure (Fd-3m) if the starting materials are graphitic B-C-N compounds.106 It 

should be, however, emphasized that presented XRD data alone do not provide 

unambiguous evidence of the presence of a single B-C-N phase. In particular, 

the formation of c-BN/diamond nanocomposites cannot be excluded.

Very intriguing results were reported lately by Liu et al.,107 who succeeded in 

preparing diamond crystals doped with B and N atoms, starting from graphitic 

mixtures of C and BN with compositions of C1-x(BN)x, where x = 0.02, 0.1, or 

0.5. The BN-doped diamond crystals were grown in a large volume cubic MAP 

using iron-nickel alloy as catalysts at pressures of 5.0–7.2 GPa and temperatures 

of 1,500–2,300 K. Variation in morphology and color of the obtained crystals 

(Fig. 2.7) were attributed to the different amounts of B and N atoms 

incorporated into the crystal structure. XRD, XPS, Raman-, and FT-IR 

spectroscopy were used to verify the structure, chemical composition, and 

bonding of the crystals—in particular, the presence of all 3 elements as well as 

of the C-C, B-N, B-C, and C-N bonds. The authors have noted that the 

C0.9(BN)0.1 tends to separate into several cubic phases having different B/N 

ratios, while in the C0.5(BN)0.5 system, no phase separation was found and the 

obtained single-phase “BCN”-diamond exhibited cuboctahedral shape, light 

yellow in color, and nearly transparent.107 The derived cubic lattice parameter 

of the “BCN”-diamond of 3.575 Å is in a good agreement with Vegard’s law 

for a diamond/c-BN alloy. The exact chemical composition of the BN-doped 

diamond crystal phases was not, however, determined.
34

Fig. 2.7 Optical images of diamond crystals obtained at HP-HT using Fe-Ni alloy as a 

catalyst starting from graphite a) or mixtures of graphite and h-BN with stoichiometries 

of b) C0.98(BN)0.02; c) C0.9(BN)0.1; d) C0.9(BN)0.1 with 1 wt% Al additive; and e) 

C0.5(BN)0.5.107

2.2 C-N Compounds

For nearly 20 years, intense interest in the synthesis of new C-N materials has 

been generated by the prediction of unusual properties of saturated, i.e., carbon

sp3-hybridized, crystalline C3N4-phases.108 The basis of this prediction was 

provided by a semi-empirical model developed by Cohen.

109 The effective 

correlation between bulk modulus (BV) and bond length (d) was found to be BV

= 1,972/d

3.5

. A typical bond length of 1.47 Å for a C–N single bond was derived 

from organic nitrogen-containing molecules, while the C–C distance in 

diamond is 1.54 Å. This means that the bulk moduli of dense C3N4 phases could 

probably be higher than those of diamond. Moreover, based on the controversial 

opinion that hardness of material is primarily determined by its bulk 

modulus,109,110 the authors suggested that this hypothetical carbon nitride can 

surpass diamond in hardness. Such a daring statement excited the scientific 

community and, as a result, many theoretical investigations and experimental 

efforts to synthesize postulated carbon nitrides were made during the last 2

decades and continues up till now.

2.2.1 Postulated Carbon Nitrides

Several hypothetical carbon nitrides have been proposed and analyzed in 

literature: α-C3N4,

111 β-C3N4,108 “defect zincblende” dzb-C3N4,112

“pseudocubic” pc-C3N4 and “willemite-II” wmII-C3N4,

113 spinel γ-C3N4,114 and 

λ-C3N4.

115 Their structural data together with calculated bulk moduli and 

hardness from different theoretical works are summarized in Table 2.2.
35

Table 2.2 Predicted C3N4 phases and their calculated structural parameters, bulk 

moduli, and hardness

Modification Space Group a

(Å)

c

(Å)

α

(°)

Density 

(g/cm3)

B0

(GPa)

Hardness

(GPa) Ref.

-C3N4 P31c (159) 6.35 4.46 ... 3.78 189 [111]

6.47 4.71 ... 3.58 425 [113]

6.45 4.70 ... 3.61 n.d. [116]

6.47 4.71 ... 3.58 379 82.7 [117]

6.45 4.70 ... 3.61 367–449 84.2 [118]

-C3N4 P3 (143) 6.35 2.46 ... 3.56 250 [111]

6.40 2.40 ... 3.58 451 [113]

6.40 2.41 ... 3.58 419 85.7 [117]

P63/m (176) 6.39 2.40 ... 3.60 n.d. 116]

6.44 2.47 ... 3.58 427 [108]

6.41 2.40 ... 3.60 437 [112]

6.38 2.39 ... 3.63 409–448 84.5 [118]

dzb-C3N4 P-43m (215) 3.43 ... ... 3.79 425 [112]

3.43 ... ... 3.79 386–448 88.7 [118]

pc-C3N4 P-42m (111) 3.42 3.42 ... 3.81 448 [113]

3.43 3.43 ... 3.78 393 79.6 [117]

wmII-C3N4 I-43d (220) 5.40 ... ... 3.89 496 [113]

5.44 ... ... 3.79 n.d. [119]

5.41 ... ... 3.86 449 92.0 [117]

5.39 ... ... 3.91 441–485 90.7 [118]

γ-C3N4 Fd-3m (227) 6.87 ... ... 3.77 369 [114]

6.78 ... ... 3.92 n.d. [119]

6.71 ... ... 4.04 379 62.3 [117]

6.81 ... ... 3.87 376–432 59.4 [118]

λ-C3N4 P4322 (95) n.d. ... ... 3.0 n.d. [115]

g-C3N4 P-6m2 (187) 4.74 6.72 ... 2.33 n.d. [113]

4.37 6.69 ... 2.35 n.d. [116]

rh-C3N4 R3m (160) 4.11 ... 70.5 2.56 51 [112]

The β-C3N4 polymorph is based on the β-Si3N4 structure, with C substituted for 

Si. This structure consists of 4-fold coordinated (sp3-hybridized) carbons linked 

by 3-fold coordinated N-atoms into a network of corner-linked CN4 tetrahedra 

(Fig. 2.8a). The hexagonal unit cell contains 14 atoms (2 C3N4 formula units) 

and has P63/m or closely related P3 symmetry. In the former symmetry, all NC3

polyhedra are considered to be planar in geometry, thus implying formal sp2-

hybridization of N-atoms similar to that observed for β-Si3N4.

108,112 Other 

authors considered only the half NC3 units to be planar while the other half to 

have pyramidal geometry (sp3hybridized nitrogen), which resulted in the 

change of the symmetry to P3.111,113
36

Fig. 2.8 Theoretically predicted dense carbon(IV) nitride polymorphs that have been 

proposed in literature and which were examined theoretically: a) β-C3N4 (P63/m), b) αC3N4 (P31c); c) “pseudo-cubic” or “defect zinc blende” C3N4 (P-43m); d) “cubic” or 

“willemite-II” C3N4 (I-43d); e) spinel-type γ-C3N4 (Fd-3m). The carbon and nitrogen 

atoms are represented as gray and blue spheres, respectively.

Following the prediction of the β-phase, the α-C3N4 structure was similarly 

deduced from α-Si3N4. It can be described as an ABAB stacking sequence of 

layers of β-C3N4 (a) and its mirror image (b) (Fig. 2.8b). The hexagonal unit cell 

has P31c symmetry and contains 4 C3N4 formula units. According to theoretical 

calculation, NC3 polyhedra are exclusively pyramidal in geometry, thus 

indicating sp3-hybridization of nitrogen.111,113 The issues of nitrogen 

hybridization and structural uncertainties in the predicted α- and β-C3N4 are

critically reviewed by Malkow.120

Pseudocubic C3N4 polymorph represents a defect zinc blende structure 

(isostructural to α-CdIn2Se4). It can be constructed from a c-BN structure by 

substituting 4 B-atoms of the unit cell by 3 C-atoms and one vacancy. The 

structure exhibits either P-43m or P-42m symmetry with 7 atoms (one formula 

unit) in the unit cell. The network consists of corners-linked CN4 tetrahedra 

(Fig. 2.8c) in such a way that the C-N-C angle is close to 109°, which suggests 

sp3hybridization for nitrogen. 

(a) (b)

(c)

(d)

(e)
37

The cubic “wmII”- type structure is derived from a high-pressure phase of a zinc 

silicate (Zn2SiO4) willemite-II, via replacing the cations (Zn and Si) by C and 

the anions (O) by N (Fig. 2.8d). The initially tetragonal structure was found to 

relax toward the more symmetrical cubic one having I-43d symmetry. For this 

structure the extremely high bulk moduli (~450–500 GPa), exceeding the 

experimental one of diamond (442 GPa), and high density 3.8–3.9 g/cm3 were 

reported. However, both pseudocubic and cubic C3N4 phases were found to be 

less thermodynamically stable than the previously described α and β 

polymorphs. 

Finally, the discovery of the high-pressure cubic spinel silicon nitride (γSi3N4)

121 suggested the existence of isostructural γ-C3N4.114,119 The structure 

(space group Fd-3m) comprises C-atoms in 2 different environments: 8 

tetrahedrally and 16 octahedrally coordinated carbons per unit cell (Fig. 2.8e). 

Despite the evident existence of 6 coordinated carbons in transition metal 

complex cations, octahedral carbon sites in γ-C3N4 are considered to be very 

unlikely in terms of hybridization. Nevertheless, this structural form is expected 

to be attainable under ultra-high-pressure conditions.119

By simulating the compression of polymeric carbon nitride precursors—in 

particular, carbon dicarbodiimide C(NCN)2—Kroll and Hoffmann have found 

transformation to a completely novel and unusual solid structure having C3N4

stoichiometry.115 The hypothetical λ-C3N4 consists of tetrahedrally coordinated 

carbon with nitrogen polyhedra both in planar (sp2) and pyramidal (sp3) 

geometry. However, the λ-phase also contains C-C and N-N bonds, which, in 

the sense of oxidation state formalism, does not allow to refer λ-C3N4 to 

carbon(IV) nitrides.

Despite most predicted carbon nitrides that exhibit the C3N4-stoichiometry, first 

synthesis attempts, mainly devoted to deposition of thin films, have indicated 

the difficulty to obtain a C:N ratio of 3:4. With this in view, Cote and Cohen 

have examined 8 new structural models for carbon nitrides with 1:1 

stoichiometry by means of ab initio calculations.122 They have considered cubic 

rock salt and zinc-blende types, rhombohedral (buckled carbon nitride sheets 

with interplanar covalent bonds), orthorhombic β-InS type, tetragonal GePtype, and 3 hexagonal structures being 3-D networks of pure sp2

-bonded solids 

“bct-4”, “H-6”, and GaSe-type. The authors reported that for this stoichiometry,

sp2bonding of nitrogen is energetically preferable to sp3. The cubic phases were 

found to be mechanically unstable and relaxed to a rhombohedral phase with a 

bulk modulus of 253 GPa. The bulk moduli of 345 and 375 GPa were calculated 

for hexagonal H-6 and bct-4 phases, respectively. Finally, the GaSe structure, 

made of layers where the C-C bond is perpendicular to the plane of the layers, 

was found to be the most energetically favorable of all phases considered, and 

it has a bulk modulus of 199 GPa. Kim et al.123 investigated cubic CN phases 

(rocksalt, BCC, and zinc-blende) using DFT and found that they relax to their 
38

tetragonal counterparts (e.g., zinc-blende to β-tin type). A body-centered 

tetragonal CN phase with 1:1 stoichiometry is predicted to be highly stable; it

has a bulk modulus of 243 GPa and an interesting atomic structure, with 

complicated C-C and N-N dimerization along the c axis. Another wave in the 

theoretical exploration C:N = 1:1 was initiated by the discovery of “polymeric” 

nitrogen having a cubic gauche-type structure, cg-N (Fig. 2.9a).124 Replacing 

half of the nitrogen atoms in the cg-N by carbon, Wang et al.125 suggested that 

a novel carbon nitride phase consisted of sp3hybridized bonds and possessed a 

cubic P213 symmetry with 8 atoms/cell, cg-CN (Fig. 2.9b). The bulk modulus 

and hardness of the cubic gauche CN were calculated to be 335–348 GPa and 

54 GPa, respectively. A further search of hypothetical CN phases by means of 

ab initio total-energy calculations has indicated the potential existence of 7 other 

low-energy polymorphic structures of sp3-hybridized CN.126 Among them, the

Pnnm structure with 8 atoms per unit cell was found to be energetically more 

favorable than all previously reported crystalline structures with 1:1 

stoichiometry. Furthermore, it has the theoretical hardness of 62 GPa.126

Fig. 2.9 Unit cells of the simple cubic gauche cg-N (a) and cg-CN (b)125

Aiming to explain nitrogen evolution during the CVD of carbon nitride thin 

films, Betranhandy et al.127 have suggested 2 hypothetical models based on a 

pyrite-type structure with different C:N stoichiometries: CN2 and C2N. In the 

considered AX2 compounds with the pyrite-type structure, X2 diatomic units are 

arranged in the octahedral holes of the A-atoms fcc-sublattice. The authors 

indicated structural instability of both carbon nitrides and a tendency to release 

N2 and C2N2 for CN2 and C2N, respectively. As many reports explained the 

nitrogen substoichiometry of carbon nitrides by an extraordinary stability of N2

molecule, Sandre et al.128 have proposed many structural models for the 

composition C3N, making the substitution of one-quarter of carbon atom by 

nitrogen in the carbon graphite structure. Ab initio calculations have led to 2 

graphitic 2-D structures and 2 3-D ones (poly-C3N-a and -b) with orthorhombic 

symmetry. Authors mainly insist that the dynamic structural transformations 

from one polymorph to another are favored, which makes very difficult 

structural characterizations.
39

In addition to such dense, saturated or sp3-hybridized carbon(IV) nitride 

structures, many soft unsaturated structural variants have been suggested in the 

literature. Different graphitic C3N4 structures112,113,129,130; numerous polymeric 

triazine-, heptazine-, carbodiimide-, and cyanamide-based networks115,131; and

amorphous C3N4-phases132 are considered as suitable precursors for the 

synthesis of a hard C3N4 phase and will be referred in the following subsection.

2.2.2 CNx Thin Films via Vapor Deposition

Up-to-date descriptions of the proposed C3N4 phases and reviews of the 

experimental results can be found elsewhere120,133,134. Numerous attempts to 

obtain hypothetical phases have been undertaken since the Liu and Cohen 

publication. Two main preparative approaches can be clearly distinguished: thin 

films deposition and bulk synthesis. First attempts to obtain CNx coatings were 

undertaken already in the 1970s. But after the above-described prediction of 

metastable β-C3N4 phase possessing diamond-like properties, an exponential 

increase of attempts to synthesize the postulated material via gas phase 

deposition techniques was documented. Currently, scientific databases consist 

of more than 2,000 papers related to carbon nitride films. The numerous papers 

have been summarized and critically analyzed in several review articles120,133–

135 allowing to conclude that the existence of crystalline stoichiometric C3N4 has 

not been clearly evidenced yet.

Practically all known variants of chemical vapor deposition techniques were 

applied to produce CNx-coatings. These include thermal low-pressure CVD, 

microwave- and RF-plasma-enhanced CVD, and laser- or hot filament CVD. 

The applied physical vapor deposition techniques include ion implantation or 

ion beam deposition; magnetron, electron cyclotron resonance, and other 

sputtering methods; laser ablation; vacuum arc deposition; and laser-induced 

gas phase reaction. The CVD-approaches are dominated by plasma-enhanced 

techniques, while in the physical vapor deposition (PVD) area, predominantly 

various kinds of sputtering methods were used. Important deposition parameters 

like temperature, pressure, precursors, target materials, and substrates were 

varied in wide range. Some of the obtained films showed an extremely high 

hardness (e.g., ~65 GPa for coatings with C:N-ratio of 0.6–0.7 obtained by ion 

beam PVD136), which, however, is still significantly below the corresponding 

values for diamond (~100 GPa). Independent of applied methods and apparatus, 

several reports on the successful synthesis of partially or fully crystalline 

coatings were published. Many authors claim unambiguous formation of βC3N4, α-C3N4, cubic, graphitic, unknown crystalline C3N4 phases, or their 

mixtures in the obtained coatings. 

However, in most cases there are only very small crystallites formed, sometimes 

with various compositions or mixed with other (amorphous) phases. Their 

diffraction intensities from XRD and TEM studies and derived lattice constants 
40

match usually relatively poorly with the calculated values for the theoretically 

proposed modifications. Moreover, a detailed systematic analysis of the 

published diffraction data have shown that the reported X-ray and/or electron 

diffraction patterns may also be assigned to different crystalline carbon 

phases.

134 The elemental composition of the coatings is usually insufficient: 

such techniques as EDX, XPS, Rutherford backscattering spectrometry, or 

EELS are, in general, not suitable to provide an exact and reliable quantification 

of the light elements C, H, N, and O, and even the C:N ratio is usually very 

difficult to determine for a small single crystal. In many cases, the obvious 

contamination of the obtained material with hydrogen is ignored. Interpretation 

of such results on the basis of the C:N ratio is unacceptable, since molecular or 

polymeric crystalline C/N/H-phases may also be formed. So, it is not unlikely

that many of the crystalline phases described in experimental reports contain 

significant amounts of oxygen, hydrogen, or other elements.

Critical analysis of the existent literature devoted to the preparation of carbon 

nitride films leads to the conclusion that it is relatively unlikely that the vapor 

deposition techniques can provide dense crystalline stoichiometric C3N4—in 

particular, its existence has not been clearly evidenced yet. To date, the soobtained deposits are generally mainly amorphous and can eventually contain 

nanometric CNx crystallites. Nevertheless, amorphous nonstoichiometric CNxmaterials are interesting from a fundamental, as well as from a practical, point 

of view because of their interesting mechanical, tribological, and optical 

properties. Much of the recent work in the field of carbon nitride coatings is 

focused on the synthesis and characterization of amorphous, homogeneous 

materials.137

2.2.3 Ambient- and Low-Pressure Bulk Synthesis Approaches

Another route to C-N compounds suggested by many authors devotes bulk 

synthesis approaches, which will provide hard ceramic carbon nitride materials 

in macroscopic amount. The present section briefly overviews the most 

significant results on carbon nitrides synthesis under ambient- or “low-” 

pressure conditions, implying the pressures below 1 GPa. It should be 

mentioned that the application of such moderate conditions in most cases does 

not provide dense C-N compounds. Thermal decomposition of organic 

precursors, moderate temperature synthesis, or solvothermal synthesis in most 

cases was resulting in amorphous/turbostratic and low-density crystalline 

(graphitic) carbon nitrides. Nevertheless, the obtained CNx materials are 

nitrogen rich and predominantly single phase, so they (in particular, graphitic 

C3N4 networks A and B on Fig. 2.10) can be considered as perfect precursors 

for the production of dense C3N4.
41

Fig. 2.10 Examples for graphitic C3N4 networks with relative high symmetry based on 

the s-triazin unit (A) and the s-heptazine (tri-s-triazine) unit (B). Structure A was initially 

predicted to be the thermodynamically most stable C3N4 modification. However, recent 

DFT calculations have shown that B is significantly more stable than A.

138

2.2.3.1 Solid Phase Pyrolysis and Synthesis

Pyrolysis of N,N-diethyl-1,4-phenylen-diammonium-sulfate (C10H18N2O4S) in 

a nitrogen atmosphere at 800 °C and in the presence of SeO2 as catalyst results 

in a heterogeneous product containing crystalline carbon nitride with a zinc 

blende structure, according to TEM examinations.139 The authors propose the 

following reaction: 

C10H18N2O4S + 8N2 → 3“C3N4”+ CO2 + 6NH3 + SO2

The composition of the crystalline product could only be estimated with EELS 

because of the small crystallite size of 5–50 nm. The crystallites are surrounded 

by an amorphous matrix with varying nitrogen content.

Thermal treatment of melamine (C3H3N6), cyanoguanidine (C2H4N4), 1,2,4-

triazol (C2H3N3), diaminomaleonitrile (C4H4N4), polymerised HCN, 

dicyanoimidazol (C5H2N4), TCNE (C6N4), TCNE with ammonia, and

hexaazatriphenylene hexacarbonitrile (C18N12) at 700 °C / 225 MPa results in 

the products with composition deviating from the ideal C3N4. The observed 

ammonia release during thermal treatment allowed the authors to conclude that 

an ideal precursor should contain no or at least the lowest possible hydrogen 

content.140

An alternative method to synthesize the graphitic C3N4 precursor A (Fig. 2.10) 

is based on reactions of cyanuric chloride (2,4,6-trichloro-1,3,5-triazine, 

C3N3Cl3) with metal nitrides and/or amides. The major challenge of this 

approach is the separation of the side product, i.e., the metal chloride, from the 

polymer. Kawaguchi et al.141 used lithium nitride and removed LiCl by 

dissolving in water:

C3N3Cl3+Li3N→“C3N4”+3LiCl
42

Two different products were obtained, with the composition C3N4.5O1.2H4.1 and 

C3N3.6O1.1H4.2. The oxygen content indicates a hydrolysis reaction during the 

treatment with water. Amorphous carbon nitrides as well as graphitic C3N4

hollow spheres have been reported to be formed upon reacting cyanuric chloride 

or fluoride with lithium nitride in a steel or Monel reactor at temperatures 

between 300 and 500 °C.142,143 Interestingly, the addition of materials such as 

glass wool to the educts seems to have a catalytic effect resulting in the 

formation of hollow spheres with diameters from 30 nm to 1 μm (Fig. 

2.11).142,143 SEM, TEM, FTIR, XRD, and solid-state NMR examinations 

indicated the presence of spherical CNx particles with graphitic structure. The 

authors supported their interpretation with force-field calculations, which 

indicate that triazine-based C3N4 structures cross-linked with nitrogen atoms 

(structure A in Fig. 2.10, Fig. 2.11) tend to form curved layers. Guo et al.144

have mixed cyanuric chloride with either potassium or nitriding agents, such as 

NaN3 and NaNH2, and submitted to temperatures of 300, 380 and 220 °C, 

respectively, in a solvent-free autoclave. While at higher temperatures nitrogenpoor materials were obtained, the lower-temperature treatment (when the 

organic reagent was mixed with NaNH2) resulted in graphitic carbon nitride 

with a nitrogen-to-carbon (N/C) ratio of 1.20 and interplanar distance of 3.28 

A. The authors proposed that a single layer of the graphitic carbon nitride is 

composed of triazine rings interconnected by nitrogen bridges (structure A in 

Fig. 2.10).

Fig. 2.11 g-C3N4: s-triazine-based chemical structure and TEM image of nanosized

hollow spheres142,143

Komatsu et al.145,146 reported preparation of graphitic C3N4 materials based on 

the tri-s-triazine nucleus (C6N7) (e.g., structure B in Fig. 2.10) by ambient 

pressure chemical synthesis. Pyrolysis of tricyanomelamine or of mixtures of 

melamine with metal halides like ZnCl2 at temperatures up to 800 °C results in 

the C/N/H-polymer melon, as supported by IR and mass spectrometry (MS) and 

N

N

N N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N N

N

N

N N

N

N

N

N

N

N

N

N

N N N N
43

C/H/N elemental analyses.146 Much more promising appeared to be the 

reactions of potassium melonate K3[(C6N7)(NCN)3], with heptazinetrichloride, 

C6N7Cl3.

145 Taking into account the IR and MS examinations, as well as XRD 

and the C:N-ratio determined by combustion chemical analysis, the author 

suggested for the synthesized product a network structure like that shown in Fig. 

2.10B. Zhao et al.147 have focused on the de-ammonation polycondensation of 

melamine at moderate temperatures (300 °C then 600 °C in atmosphere). The 

resulting carbon nitride nanowires adopted a turbostratic stacking with an 

interplanar distance of 3.21 A. High nitrogen content (C3N4.63H1.61) and high 

thermal stability (up to 700 °C) suggested this material as a good precursor for 

the high-temperature and high-pressure synthesis of dense carbon nitride 

crystals.

Careful thermal treatment of C–N–H compounds, such as melamine 

C3N3(NH2)3, dicyandiamide H4C2N4, ammonium dicyanamide NH4[N(CN)2],

or cyanamide H2CN2 to about 450 °C was found to lead to the formation of a 

nitrogen-rich condensed molecular structure so-called “melem” (2,5,8-

triamino-tri-s-triazine).148 Melem consists of nearly planar C6N7(NH2)3

molecules that are organized into parallel layers with an interplanar distance of 

3.27 A. Being also highly nitrogen rich (the C:N ratio 1:1.67), melem is of 

significant interest as an intermediate phase on the way to the graphitic C3N4. 

Indeed, when heated up to 580 °C, it transforms into a graphitic carbon nitride 

with an interplanar distance of 3.40 A. Same behavior was observed for 

guanylurea dicyanamide [(NH2)C(=O)NHC(NH2)2][N(C≡N)2], which forms 

melamine at about 130 °C and leads to a melem by heating at about 380 °C and, 

upon further heating to 490 °C, transforms into graphitic carbon nitride.149 An

investigation of the thermal behavior of nonmetal tricyanomelaminates in the 

temperature range of 380–500 °C revealed the formation of a graphitic 

hydrogenated carbon nitride through the intermediate melem phase.150 The

authors relate the structure of the obtained g-C3N4 to the “A” structure shown 

in Fig. 2.10.

Amorphous bulk nitrogen-rich carbon nitrides are produced via slow thermal 

decomposition of 2,4,6-triazido-1,3,5-triazine [(C3N3)(N3)3] at 185 °C in a highpressure reactor.151 Under autogenous nitrogen pressure (~1 atm), the product 

with C3N3.9H1.4O0.2 composition (~C3N4) is obtained, while the application of 

initial nitrogen pressure of 6 atm yields a solid with one of the highest reported 

nitrogen-to-carbon ratios corresponding to C3N5, C3N4.7H1.3O0.4. Both products 

were found to have significant sp2carbon bonding in a conjugated doubly 

bonded network. Electron microscopy reveals that these powders have a glassy 

microstructure with large irregular pores and voids. Similar amorphous 

nitrogen-rich carbon nitride material, C3N4+x where 0.5 < x < 0.8, is obtained 

through the rapid decomposition of the trichloromelamine [(C3N3)(NHCl)3] 
44

upon heating above 185 °C.152 The ideal decomposition should occur according 

to the following equation:

[(C3N3)(NHCl)3] → C3N4 + 3HCl + (2-x)/2*N2

IR, NMR, optical, and X-ray photoelectron spectroscopy revealed in the product 

primarily sp2hybridized carbon centers and triazine (C3N3) rings bridged by 

nitrogen species. The authors have shown that pressure is not essential for high 

nitrogen content in resulting layered carbon nitrides.

Spherical carbon nitride nanoparticles with a graphitic structure were also 

synthesized using a template process.153 A nanoporous silica matrix was soaked 

with liquid cyanamide (CH2N2) and dried. The carbon nitride material was

obtained via heating the monolith under N2 flow at 550 °C followed by 

treatment with hydrofluoric acid to remove the matrix. The authors have 

reported high nitrogen content (N/C = 1.56) and suggested the s-heptazinebased structure (B-type in Fig. 2.10) for the single layer of g-C3N4. The oxygen 

and hydrogen content of the synthesized material was not, however, specified. 

Semencha et al.154 reported simple methods for preparing crystalline carbon 

nitrides—namely, g-C3N4 and β-C3N4. The submicrometer-sized graphitic 

crystallites were obtained via thermal decomposition of a mixture of sodium 

and potassium thiocyanates at 450 °C. Reaction of carbon tetrachloride 

CCl4 with ammonia NH3 at an elevated temperature was claimed to lead to βC3N4. The CNx phases were investigated using TEM, XRD, IR spectroscopy,

and MS. Because of a poor agreement of experimental XRD patterns with that 

of the theoretically predicted ones, the presence of nonindexed XRD peaks, and 

the absence of any data on elemental composition, further characterization of 

the synthesized materials is indispensable.

Very recently Goglio et al.155 suggested another simple method to synthesize 

carbon nitrides, including a low-compressibility CNx-polymorph. The carbon 

nitride material was obtained through the decomposition of commercial 

thiosemicarbazide (H2NC(S)N2H3) powder at ambient pressure and 600 °C 

under nitrogen flow. Besides the presence of a graphitic CNx phase, a 

nanocrystalline material is obtained and is identified as a cubic carbon nitride 

with a cell parameter of 3.163 Å. The authors report a high bulk modulus of 355 

GPa for the cubic phase, which is still below those from numerous theoretical 

predictions. Finally, a bulk elemental combustion analysis of C, H, N, O, and S 

indicated no sulfur and high nitrogen content, but also showed the presence of 

significant H and O amounts. The composition of the cubic phase was not 

determined.

Another ambient-pressure approach toward C3N4 is presented by Schmidt and

Jansen.156 Using C2N2O, a hydrogen-free inorganic polyisocyanate obtained by 

the polymerization of molecular cyanogen isocyanate (NC–NCO), they 
45

prepared a phase-pure 3-D structured C–N network by a carefully controlled 

polycondensation reaction. Only gaseous CO2 was released, providing 

amorphous and porous C3N4 at 400 °C. Further densification of this inorganic 

cross-linked polymer up to 2.3 g/cm3 was achieved by annealing in the presence 

of elemental mercury as a catalyst. The densified C3N4 network was analyzed 

using elemental analysis; IR, Raman, and electron paramagnetic resonance

spectroscopy; thermal analysis, and MS studies. The authors have concluded 

that the obtained material with the composition C3N4 is best described as an 

inorganic resin-like thermosetting network (Fig. 2.12), structurally related to the 

well-known melamine-based resins (polytriazines).

Fig. 2.12 The 3-D resin-like structural model for C3N4

156

2.2.3.2 Solvothermal Route

Of all the synthesis methods for the carbon nitrides, the solvothermal process is 

considered to be of significant interest because the moderate temperature 

conditions are able to prevent the loss of nitrogen, and the chemical reactivity 

of the precursors at the interface between solid and liquid phases or between 

solvated species can be strongly improved. Montigaud et al.157 tried to prepare 

the triazine-based structure A (Fig. 2.10) via solvothermal techniques at 250 °C 

/ 130 MPa starting from cyanuric chloride and melamine in the presence of 

ethyl-diisopropylamine. The product was characterized by XRD, reflection 

electron microscopy, EDX, Raman, and XPS. The resulting poorly crystallized

graphitic phase had an N/C ratio of 1.28 and contained hydrogen. It is very 

likely that the C/N/H-polymer melon was formed. Melon, [C6N9H3]n, contains 

only 1.5 wt% hydrogen; therefore, its composition is very close to the 

carbon(IV) nitride stoichiometry. Treatment of the same reagents in the 

presence of nickel with supercritical benzene as a solvent at 400 °C and 

autogenously pressure (or even without solvent) results in the formation of 

nitrogen-rich graphitic carbon nitride hollow spheres (10–40 nm in size) with 

an N/C equal to 1.6.158 In this case, nickel is claimed to be necessary to induce 

a polycondensation reaction. FTIR, UV–vis, XPS, and photoluminescence 
46

spectroscopic studies of the carbon nitride materials show that the products 

reserve the s-triazine-based structure (A, Fig. 2.10). The solvothermal 

condensation of melamine without a secondary reagent with pure hydrazine 

(NH2NH2) as a dehydrogenating agent was also studied at higher pressure and 

temperature conditions (P = 3 GPa, T = 800–850 °C) in a belt apparatus.159,160

The product was characterized with XRD, SEM, XPS, EMPA, elemental 

analysis, and IR spectroscopy. A hydrogen content of 2 wt% was reported,

indicating the presence of a C/N/H phase. The resulting material was a carbon 

nitride with a new unpredicted graphitic structure having an orthorhombic cell 

and a composition close to C3N3.36O0.14H1.24. Similar solvothermal conditions 

were applied to test 2 further triazine derivatives for their suitability to prepare 

a C3N4-material: 2-amino-4,6-dichloro-1,3,5-triazine and 5-azacytosin.161 The 

products also showed relatively high hydrogen contents and/or significant 

oxygen impurities.

Solvothermal reaction of CCl4 with NH4Cl at 400 °C in a stainless steel 

autoclave was reported to provide graphite-like C3N4 nanocrystals without any 

catalyst.162 The obtained material exhibited very good crystallinity and its XRD 

pattern is consistent with graphite-like C3N4. IR spectroscopy indicated the 

presence of C=N, C–N, and N-H bonds. The overall elemental composition was 

not reported.

Further attempts to synthesize carbon nitride material via a solvothermal route 

involved cyanuric chloride C3N3Cl3 as a source of both carbon and nitrogen. 

The reactant was usually mixed with different additives able to trap chloride 

anions or/and to supply additional nitrogen (e.g., alkali metals or their nitrides, 

NaNH2 or NaN3) in a nonpolar solvent (C6H6 or C6H12). In these experiments, 

moderate pressure and temperature conditions have been developed in a 

solvothermal environment. Lu et al.163 tried to reproduce the solvothermal 

carbon nitride synthesis using cyanuric chloride with lithium nitride as reactants 

in benzene solution at 300–400 °C / 5–7 MPa. They claimed the formation of 

α- and β-C3N4. Nevertheless, the reported N/C ratio was very low compared

with the theoretical value (0.66–0.76 vs. 1.33), and a number of nonindexed 

XRD peaks implied the presence of unknown crystalline phase(s). Here further 

characterization is necessary to unambiguously identify the structure and 

chemical nature of the phases observed. In order to investigate the nitridation 

mechanism, Guo et al.164 have performed nitridation of C3N3Cl3 using NaN3 as 

the nitriding agent according to the following chemical equation:

C3N3Cl3 + 3NaN3 → C3N4 + 3NaCl + 4N2

The reaction has been carried out in the presence of benzene as the solvent at 

220 °C for 15 h. The obtained material is composed of high-quality carbon 

nitride nanotubes having a graphite-like sp2bonded structure confirmed by 

XPS. Elemental analysis has shown the atomic N/C ratio of 1.25, which is very 
47

close to the theoretical value for C3N4 (1.33). However, the presence of 

hydrogen bonded to nitrogen was detected. An equivalent benzene-thermal 

synthesis with NaNH2 instead of NaN3 at moderate temperatures (180–

220 °C)165 has led to the material mainly consisting of graphitic carbon nitride 

nanosized crystallites with N/C ratio 1.39, in agreement with chemical reaction:

C3N3Cl3 + 3NaNH2 → C3N4 + 3NaCl + 2NH3

The N–H bonds, however, were still present in the product. The reaction of 

cyanuric chloride with sodium metal at 250 °C without a catalyst in the presence 

of cyclohexane as solvent, in contrast to the above 2 experiments, resulted in 

carbon nitride with an atomic N/C ratio of 1.166 Analysis of the XRD, FTIR, and 

XPS data suggested a graphite-like sp2-bonded structure composed of blocks of 

s-triazine rings bridged by carbon-carbon atoms in the bulk carbon nitride. The 

TEM revealed spherical particles with an average diameter of 50 nm. Carbon 

nitrides having CN stoichiometry were prepared from the same reagents by Li 

et al.167 between 230 and 290 °C and 1.8 and 4.5 MPa in a stainless steel 

autoclave. CN nanotube bundles were formed at 230 °C and 1.8 MPa using 

NiCl2 as a catalyst. Without any catalyst, aligned nanoribbons were formed at 

290 °C and 3 MPa, while microspheres consisting of hundreds of nanoribbons 

were formed at 260 °C and 3.5–4.5 MPa.

Recently, well-crystallized graphitic carbon nitride was prepared from C3N3Cl3

and NaN3 in CCl4 at 180 °C without any catalyst.168 The elemental analysis of 

the prepared sample indicated that the average composition was 

C3N4.88H3.87O1.58, with the N/C ratio to be 1.63, which is higher than the 

expected one (1.33) of the C3N4 empirical stoichiometry. The presence of 

hydrogen and oxygen in the sample was explained by hydrolysis during the 

washing procedure and/or by the presence of –NHx end groups. XPS and FTIR 

studies confirmed a graphite-like sp2bonded structure of the A-type (Fig. 2.10). 

TEM investigation revealed the presence of nanometer-size spherical particles 

and crinkly lamellas.

When comparing the results obtained by solvothermal methods, one can notice 

that the obtained morphologies of carbon nitrides are very different 

(microspheres of nanoribbons, nanotube bundles, nanospheres, lamellas, etc.) 

even when the only difference deals with the concentration of reagents and a 

slight modification in their ratios. Then the mechanisms responsible for all these 

morphologies are very sensitive to all experimental parameters (concentration, 

temperature, reducing agent amount, catalyst presence, and pressure), which 

underlines the great complexity of the phenomena involved in the solvothermal 

route.
48

2.2.4 HP-HT Synthesis of Dense C-N Phases

Despite many claims of making new CNx films and C3N4 crystallites, the 

scientific community still doubts the existence of dense C3N4 phases other than 

the graphite-like ones.

110,169,170,171 After years of unsuccessful attempts to 

synthesize the new C3N4 phases, Badzian et al. argued that based on chemical 

considerations, the “extensive network with C-N single bonding has never been 

documented and only very local C-N bonds are known as in amino acids”.170

The difficulties faced in the synthesis of carbon nitrides with a C3N4

stoichiometry are most probably related to their low thermodynamic stability 

with respect to carbon and nitrogen, as indicated by the positive values of their 

enthalpies of formation.172 Taking this issue into account, together with 

theoretical studies mentioned in the previous section, as well as keeping in view 

the reported successes in the high-pressure synthesis of other nitrides, it 

becomes clear that HP-HT techniques similar to the synthesis of diamond and 

c-BN should be applied to prepare hard C3N4 materials (Fig. 2.13). Using 

suitable precursors with high nitrogen content may allow obtaining carbon 

nitride materials of good crystallinity in macroscopic amounts, which will also 

facilitate their characterization.

Fig. 2.13 The diagram presents an overview of the reported experimental routes to 

produce C3N4 phases. The bold route indicates the most promising synthesis pathways to 

dense C3N4.

An overview of the reported attempts to synthesize dense carbon nitride 

polymorphs under pressures exceeding ~1 GPa is listed in Table 2.3 and 

includes the utilized precursors, synthesis conditions, characterization methods,

and main results. Even though most of the products obtained from the highpressure experiments are amorphous materials with nitrogen contents much 

lower than the expected value of 57 at% for C3N4 (x < 1.33 for CNx), several 

groups have reported the synthesis of polycrystalline materials consisting of 

small α-, β-, and c-C3N4 crystallites embedded in an amorphous carbon nitride 

matrix. Moreover, a recently conducted synthesis of several new C-N-based 

crystalline phases demonstrates how little is known about the behavior of the CN compounds under high pressure.

CVD, PVD

CVD, PVD

Laser Ablation

Laser Ablation

Precursor Synthesis

Precursor Synthesis

Bulk Materials

Bulk Materials

Films

Films

Dynamic

Dynamic

Static

Static

Shock Wave

Synthesis

Shock Wave

Synthesis

Solvotherm.

Synthesis

Solvotherm.

Synthesis

P > 1 GPa

P > 1 GPa

P < 1 GPa

P < 1 GPa

Pyrolysis

Pyrolysis

HP-HT

HP-HT

C+N2

C+N2

Mechanochem. 

Synthesis

Mechanochem. 

Synthesis
49

Table 2.3 Experimental details and main results related to the HP-HT synthesis of 

carbon nitrides

Starting 

Materials

Synthesis 

Conditions Analyses Methods x in CNx

Claimed Phases, Contaminations, 

Comments Ref.

Static High-Pressure Synthesis

[C3N3(NH2)3 +

NH2NH2]

2.5–3 GPa

800-850 °C

FTIR, XPS, XRD,

REM, TEM, ESCA

1.14–1.34 Cryst. graphitic C3N3.36O0.14H1.24 [129, 160]

[C3N3(NH2)3

+ C3N3Cl3]

1.0–1.5 GPa

500–550 °C

XRD, TEM, EELS,

FTIR, NMR

1.50 Crystalline C6N9H3·HCl with 

hexagonal graphitic structure 

[173]

C3N3(NH2)Cl2

C3N3(NH2)(OH)2

3.0 GPa

450–1100 °C

XRD, FTIR, EA 0.80–1.46 cryst. C3N2.4O0.7H2.23 , C3N4.4H4.4; 

unknown structure

[161]

C3N3(NH2)3 5.0 GPa 

400–900 °C

EA, FTIR, XPS, 

XRD

0.17–1.91 T = 650 °C: partially cryst. graphitic 

CNx + melamine + unknown; H

T > 900 °C: amorphous carbon, low N 

content

[174]

a-CxNyHz 6–7 GPa 

300–400 °C

FTIR, XRD, Raman 0.25 Cryst. β-C4N + graphite + disordered 

carbon; O

[175]

[C6Cl6 + 3NaN3] 7.7 GPa 

400–500 °C

HR-TEM, EELS, XRD, 

FTIR, Raman

0.70–1.94 Nanosized hexagonal graphitic 

crystallites in amorphous matrix; O, H 

[176]

C3N3(NH2)H2

catal.: NiMnCo, 

Co

7 GPa 

1400 °C

EDX, XRD, REM, 1.09–1.44 Partially cryst. α-C3N4 + β-C3N4 + 

catal. + unknown phase

[177]

C6N4 18–42 GPa 

2000 °C

TEM, EELS, EDX, 

FTIR, Raman

0.56-0.88 Amorphous sp2-bonded CNx + carbon [172, 178]

[C6N4 + 2C3N12] 115 GPa 

2500 °C

XRD n.d. Cryst. CNx, nitrogen content? [179]

[C60 + N2] 30 GPa, 

2000–2500 °C

XRD n.d. Cryst. cubic phase; nitrogen presence? [180]

C3N4H2.5 4.8 GPa 

950 °C

EELS, SIMS, XRD 1.1 Turbostratic-CN [181]

a-C3N4 8.5 GPa

500 °C

XRD, TEM n.d. Cryst. cubic CNx, nitrogen content? [143]

g-C3N4 23–38 GPa

<2700 °C

XRD, SEM, EDX 1.29–1.47 Unsolved cubic C3N4 + diamond [182]

C2N4H4 27–41 GPa

1700–2300 °C

TEM, EELS, EDX, 

SIMS, Raman

1.61 Well crystalline (~1 μm) orthorhombic 

“defect-wurtzite” C2N2(NH)

[171]

a-C3N4 + cr-CNx

film

3–7.7 GPa, 

300–1200 °C

XRD, XPS, TEM n.d. Cryst. α-C3N4 + β-C3N4 + unknown 

phase

nitrogen content?

[183]

Shock-Wave Compression Synthesis

C3N3Cl3, C3N3H3,

C3N3(OH)3

15–50 GPa

1000–3000 °C 

EA, XRD 1.0 Nanocrystalline cubic CNx (H, O, Cl) [184]

NaN(CN)2+CI4+

NaN3

20–25 GPa TEM, EELS, IR 0.09–0.15 Cubic sp3-bonded CNx

nanocrystallites embedded in 

amorphous matrix (low N content)

[185]

NaN(CN)2+CI4/C

Br4

13–45 GPa XRD, EA 0.38–1.3 Amorphous or poorly crystallized 

graphitic CNx

[186]

Graphitic C3N2 30 GPa 

2700 °C

TEM, EELS, EDX, 

XRD,EA

0.55 Cubic heterodiamond C2N [187]

a-C3N3.9O0.9 (1) 

C2N4H4 (2)

(1): 42–44 GPa 

2200–2400 °C 

(2): 28 GPa 

280 °C

XRD n.d. Cryst. β-C3N4 + monoclinic phase. 

nitrogen/oxygen content?

[188]
50

Table 2.3 Experimental details and main results related to the HP-HT synthesis of carbon 

nitrides (continued)

Starting 

Materials

Synthesis 

Conditions Analyses Methods x in CNx

Claimed Phases, Contaminations, 

Comments Ref.

Mechano-Chemical Synthesis

Graphite + NH3 High energy ball 

milling

XRD, EELS, FTIR, 

XPS, TEM, EA

n.d. Nanocryst. β-C3N4 embedded in 

amorphous carbon. Nitrogen content?

[189]

Graphite + N2 High energy ball 

milling

XPS, IR <0.12 Amorphization of graphite, weak 

nitridation

[190]

Nanosized graphite 

+ NH3

High energy ball 

milling

XRD, FTIR, EELS, 

TEM, Raman, EDX, 

XPS

~1.33 Nanosized β-C3N4 embedded in 

amorphous carbon

[191]

Ball milled 

graphite + NH3

High energy ball 

milling

350 -450 °C + 

NH3

XRD, TEM, XPS, 

EELS, FTIR

~1.33 β-C3N4 nanorods [192]

Ball milled 

graphite + NH3 

flow

1100 °C + 

NH3 flow

XRD, TEM, Raman,

EELS, FTIR, EDX

~1.33 Graphitic C3N4, excellent crystallinity [193]

Ball milled 

graphite + NH3 

flow

(1): 1050 °C + 

NH3 flow;

(2): 1350 °C + 

NH3 flow

of g-C3N4 from 

(1)

XRD, TEM, EELS,

EDX

~1.33 (1) Graphitic g-C3N4, excellent 

crystallinity

(2) sp3-bonded pseudocubic C3N4

[194]

C3N3Cl3 + Li3N High energy ball 

milling

XRD, SEM, XPS, 

TEM, FTIR

1.23–1.3 Graphitic carbon nitride [195]

One of the first noteworthy HP-HT syntheses of carbon nitrides has been 

attempted in an LH-DAC, where the molecular precursor TCNE (C6N4) was 

subjected to up to 2500 °C and 42 GPa.172,178 TEM, SAED, EELS, and EDX 

analysis revealed the formation of amorphous sp2-bonded carbon nitrides. The 

amount of nitrogen incorporated into the network increases with pressure, 

ranging from 24% (~C3N) at 18 GPa to 38% at 42 GPa (~C3N2), which was the 

most nitrogen-rich carbon nitride prepared under pressure by that time. The 

authors argued that, in agreement with their thermodynamic analysis, pressures 

of 40 GPa are insufficient for the synthesis of sp3-bonded carbon nitrides. 

Interestingly, that heating of a mixture of buckminster-fullerene C60 with 

nitrogen in LH-DAC to 2,500 °C at pressures up to 30 GPa resulted in a 

3-D carbon nitride in which the XRD pattern was consistent with a dense 

crystalline cubic system.180 The proposed crystal structures were not compatible 

with any theoretical C3N4, thus suggesting the formation of an unpredicted 

carbon nitride. However, further characterization of the high-pressure product 

in order to prove the presence of nitrogen and to determine the structure was not 

performed.

Dymont et al.175 have heat-treated the amorphous C-N-H precursor at 7 GPa by 

means of a toroid-type apparatus. Among various carbon phases, the authors 

also detected a crystalline carbon nitride with a hexagonal structure 

(a = 6.65 Å, c = 4.82 Å). After comprehensive XRD, IR, and Auger electron 

spectroscopy analysis, the model of the hexagonal β-C4N structure was 
51

proposed, which is derived from the theoretical C3N4 structure by replacing

some of the nitrogen atoms with carbon and forming C–C bonded pairs. 

Crystalline carbon nitride has been also synthesized from 3-amino-1,2,4-

triazine, C3N4H4, having (C3N3) a molecular skeleton in the presence of a 

nickel-based alloy or cobalt as a catalyst at similar pressures of 7 GPa and 

temperature of about 1,400 °C.177 SEM showed rod-like C-N crystals of several 

micrometers in size having 47–62 at% of nitrogen. Based on XRD, the authors 

claimed the presence of α-C3N4 and β-C3N4 crystals with lattice constants of a0 

= 6.425, c0 = 4.715, and a0 = 6.419, c0 = 2.425, respectively, in reasonable 

agreement with earlier ab initio calculations. Recently, Kim and Zorov183

reported preparation of a mixture of crystalline α-, β-C3N4, and an unknown 

phase at 7 GPa and 550 °C from amorphous carbon nitride (a-C3N4.2) in a toroidtype apparatus. To promote crystallization of the dense phase, the author used 

crystalline carbon nitride films, obtained by laser-electric discharge method, as 

crystallization seeds. The exact composition of the HP products was not 

determined.

In 2007 the LH-DAC synthesis of a new well-crystallized compound with an

N/C ratio of 3/2—namely, carbon nitride imide, C2N2(NH)—in which all of the 

carbon atoms are tetrahedrally coordinated was reported.171 The crystals of 

C2N2(NH) were formed at HP-HT conditions from a single source precursor 

1-cyanoguanidine (dicyandiamide, C2N4H4):

After the laser heating, the products were examined in-situ in a DAC by Raman 

spectroscopy. The recovered samples were investigated by means of TEM, 

EELS, and nano-SIMS (secondary ion mass spectrometry). Treatments of 

dicyandiamide at P < 27 GPa and T < 2,000 K yielded black amorphous CNx

products. At higher P-T conditions the formation of a new optically transparent 

phase having a characteristic Raman spectrum was observed. The TEM 

investigation of the recovered products revealed the presence of a wellcrystallized material (Fig. 2.14a). The N/C ratio of the crystals was determined 

by EELS to be 1.61 ± 0.06. The electron-energy loss near-edge fine structure of 

the EELS spectra proofed the formation of tetrahedrally coordinated C and N 

atoms with sp3-hybridization in the synthesized material. Additionally, 

quantitative nano-SIMS measurements revealed the presence of hydrogen in the 

recovered crystals with an H/C ratio of 0.5 ± 0.15. Thus, the resulting 

composition of the synthesized material could be expressed as C2N3H, 

corresponding to C2N2(NH).171 The crystal structure of the new compound was 

determined by combining SAED data, experimentally measured composition,

and first-principles calculations. Only a defect-wurtzite (dwur) structure type 

gave good agreement with the electron diffraction and EELS data, especially 
52

regarding the sp3-hybridized N sites. The SAED patterns for different zone 

orientations suggested an orthorhombic cell with the space group Cmc21 and 

lattice parameters a = 7.546(2) Å, b = 4.434(8) Å, and c = 4.029(8) Å for the 

new dwur-C2N2(NH) compound (Fig. 2.14b). The density of the carbon nitride 

imide (3.21 g/cm3) was found to be close to that of diamond (3.52 g/cm3), while 

the calculated bulk modulus of C2N2(NH) of 277 GPa is significantly lower than 

those for diamond or predicted dense C3N4 polymorphs. 

Fig. 2.14 a) Bright field TEM image of the C2N2(NH) crystal synthesized at P 

= 41 GPa and T > 1700 °C and b) crystal structure of the defect wurtzite-type carbon 

nitride imide, dwur-C2N2(NH)

171

The above finding was confirmed by Salamat et al.

196 The authors have obtained 

XRD patterns in the DAC at high pressure and during decompression,

confirming that no phase transitions occur upon release of pressure and recovery 

to ambient conditions. Analysis of the XRD patterns from different zones 

resulted in an orthorhombic C-centered cell with a0 = 7.618 Å, b0 = 4.483 Å,

and c0 = 4.038 Å, which supported the defect-wurtzite structure type (Fig. 

1.14b) for the synthesized C2N2(NH) (space group Cmc21) analogous to those 

of Si2N2(NH) and Si2N2O (sinoite). The C2N2(NH) is suggested to be a 

promising precursor for dense C3N4, which could form above 40 GPa via further 

polycondensation reaction accompanied by complete elimination of NH3:

3 C2N2(NH) → 2 C3N4 + NH3

The 2 hardest materials studied (diamond and cubic BN (c-BN)) were 

successfully synthesized under HP-HT conditions by direct conversion of a 

graphite-like phase with sp2bonding to a cubic phase with sp3bonding.

197

Naturally, one can assume that the experiment with direct conversion of the 

graphite-like C3N4 to diamond-like phases should provide an answer to the 

question of the existence of dense C3N4 phases. Thermodynamic analysis 

conducted by Badding et al.172,198 showed that direct phase transition should 

occur at pressures above 50 GPa at 1300 K. Figure 2.15 shows the computed 

phase boundary between the cubic phase of C3N4 and diamond + nitrogen from 

Odintsov and Pepekin199 and Horvath-Bordon et al.200 If temperatures of 2,000–

(a) (b)
53

2,500 K (typical for the direct synthesis of diamond from graphite) are applied, 

pressure of at least 50 GPa is required for g-C3N4  dense C3N4 phase 

transition. If temperatures of 1,200–1,500 K are required to overcome the 

activation barrier for the g-C3N4  dense C3N4 phase, then the optimal pressure 

for synthesis of the dense C3N4 phase should be between 20 and 40 GPa (Fig. 

2.15).

Fig. 2.15 Computed phase boundary between carbon + nitrogen and the dense phases 

of C3N4: willemite-type phase w-C3N4 (blue line200) and -C3N4 (red line199). For 

comparison, the phase diagram of carbon is shown by dashed lines.

Indeed, Khabashesku et al.143 synthesized nanocrystals of a dense C3N4 phase 

treating nanosized spherical powder of C3N4 at 8.5 GPa and 500 °C. TEM 

analysis of the product indicated the presence of well-faceted crystallites 

embedded in a graphic carbon nitride phase. The diffraction patterns obtained 

from these crystallites were indexed by a cubic cell with the lattice parameter 

of 5.3–5.4 Å, which is in good agreement with the theoretical value of 5.397 

Å.113 The authors also concluded that the temperature of the “nanosizespherical-C3N4  dense-C3N4” transition should be lower than that of the 

crystalline g-C3N4  dense-C3N4 transformation. Using a well-characterized 

graphitic phase165,201 with the LH-DAC technique has led to the discovery of a 

new cubic C3N4 (c-C3N4) and monoclinic m-C3N4 phases.

182 A cubic phase was 

recovered at ambient conditions from the graphite-like C3N4 (g-C3N4) phase 

subjected to pressures between 16 and 30 GPa and temperatures between 1600 

and 3,000 K.

182 The density of the cubic phase (2.62 g/cm3

) is less than that 

predicted for the high-pressure phases but is 12% denser than the low-pressure 

graphitic phase ( = 2.336 g/cm3). The same research group took another 

approach to synthesize a dense C3N4 phase at HP-HT conditions: they used the 

above-mentioned c-C3N4 phase as a starting material. The c-C3N4 was converted 

into a new monoclinic (m-C3N4) phase at a pressure of 50 GPa and high 
54

temperature. The XRD pattern of the recovered sample contained more than 30 

well-defined peaks, which were indexed with a monoclinic unit cell having the 

following parameters: a = 5.607Å; b = 7.628 Å; c = 4.226 Å, and β = 126.915.

The authors, however, noted that the observed XRD patterns are strongly 

dependent on the P-T synthesis conditions when starting with g-C3N4 (g-C3N4

nanospheres143 or turbostratic g-C3N4 phases) materials.165 They explain this 

finding by the possible existence of several structures with similar energies but 

different equilibrium volumes, for which the formation will strongly depend on 

the preparation conditions.202

With respect to dynamic high-pressure techniques, several shock-wave 

syntheses of dense (nano)crystalline C3N4 phases were reported. In particular,

the cubic heterodiamond C2N phase was obtained by shock compression of 

graphitic C3N2 around 30 GPa and 3,000 K.187 The recovered product was 

analyzed using authentic methods such as TEM, SAED, EELS EDX, powder 

XRD, IR spectroscopy, and CHN/S/O combustion elemental analysis. The C2N 

heterodiamond was found to consist of sp3-bonded carbon and nitrogen atoms 

settled within a cubic diamond-like cell with a lattice constant of 3.51 Å, 1.6%–

1.9% smaller than that of diamond. A series of shock recovery experiments up 

to 50 GPa on 3 nitrogen-rich C-N-based materials188 have shown that an 

amorphous C–N–O material and dicyandiamide C2N4H4 transform to a new 

carbon nitride phase (“phase-X”) as well as to β-C3N4 at peak shock pressures 

of 28–44 GPa. The powder XRD of recovered samples allowed to index the 

“phase-X” with a monoclinic cell having parameters of a = 0.981 nm, b = 0.723 

nm, c = 0.561 nm, β = 95.28°, and Vcell = 0.3966 nm3. The composition of the 

sample—in particular, N/C ratio and O-content—was not determined.

High-energy ball mills may also be used to generate high pressures exceeding 

5 GPa as well as high temperatures. The high P-T conditions occur at localized 

spots during impacts between the balls in the reaction mixture. Fahmy et al.189

were the first to try the reaction of pure graphite with liquid ammonia in a highenergy ball mill. Examination of the products with XRD, EELS, FTIR, XPS, 

high-resolution TEM, and elemental analyses revealed a nanocrystalline phase 

that could be attributed to β-C3N4. 

Another research group published several reports on the mechanochemical

synthesis of β-C3N4.

191,192 Graphite nanopowder was reacted with ammonia in 

a high-energy ball mill to produce nanosized β-C3N4 particles. Subsequent 

thermal annealing of the ball-milled powder at 300–450 °C under flow of NH3

gas resulted in the formation of nanorods (Fig. 2.16). Examination of the 

products by XRD, FTIR, high-resolution TEM, XPS, and EELS supported the 

formation β-C3N4 phase. Surprisingly, the ammonolysis of the high-energy ballmilled amorphous nanostructured graphite powders at 1,050 and 1,350 °C was 

reported to result in crystallization of graphitic- and pseudocubic-C3N4, 

respectively.

194 The XRD, TEM, EELS, and EDX were used to analyze the 
55

structure, bonding, and composition (N/C ~1.33) of the synthesized materials. 

The true elemental composition (namely, the hydrogen and oxygen content) of 

the products remains unclear.

Fig. 2.16 Low-magnification TEM images of -C3N4 single-crystal nanorods obtained 

via reaction of graphite nanopowder with ammonia in a high-energy ball mill followed 

by the thermal treatment under NH3 flow192

2.3 B-C Compounds

Within B-C-N system binary compounds, which should be certainly mentioned,

are boron carbides. The well-known “B4C” itself combines a number of 

important properties like high hardness, high thermal and ionizing radiation 

stability, chemical inertness, and neutron absorption. (Strictly speaking, boron 

carbide, commonly referred to as B4C (B12C3), may have compositions varying 

between B13C2 and B12C3.

203,204) Discovered more than a century ago, this 

compound was extensively studied and now has a number of useful 

applications.203,205 For a long time boron carbide was considered the third 

hardest material after diamond and c-BN and still the hardest one above 1,300 

°C.203,205 On the other hand, the interest in the high-pressure studies of the 

boron-carbon system originated from the fact that doping of diamond with other 

elements has a drastic effect on its properties—in particular, doping with boron 

turns it from an insulator into a semiconductor or even into a quasi-metallic 

electrical conductor with high chemical resistance.206 Moreover, boron-doped 

diamond was reported to be a type-II superconductor with a transition 

temperature of Tc = 5 K,207 while heavily boron-doped diamond (20 at% of B) 

is predicted to be a superconductor with Tc up to 55 K.208 However, employment 

of the common CVD and HP-HT synthesis methods allows one to incorporate

a limited amount of boron (~2–3 at%) into the diamond lattice. The diamondlike B-C phases with high boron content (which can be increased by using 

extreme P-T conditions) are expected to combine the best properties of diamond 

and boron carbide, which could be further extended to advanced optoelectronic 

applications.
56

2.3.1 HP-HT Synthesis of Diamond-like B-C Phases

Most of the earlier HP-HT experiments in the B-C system were related to the 

synthesis of the boron-doped diamond. The main starting material for such 

experiments was a mixture of boron or boron carbide B4C with graphite 

powders. Thus, Ekimov et al.207,209 investigated interaction in the B4C(B)–C 

system in the toroid-type apparatus at pressures of 8–9 GPa and temperatures 

of 2,500–2,800 K. They reported the formation of a superhard polycrystalline 

diamond material containing less than 5 wt% B4C. The increase in the lattice 

parameter of the synthesized diamond, observed via Raman spectroscopy and 

NMR studies, confirmed the formation of heavily boron-doped diamond with 

boron content estimated to reach ~2 at% (the overall boron content being 2.8 

at%). The authors also tried HP-HT synthesis from homogeneous mixtures of 

naphthalene (C10H8) with o-carborane (C2B10H12) or with amorphous boron, so 

that the B-content was varied from 0 to 10 at%. They observed that under 

employed P-T conditions, the level of boron doping of diamond reaches the 

limit already for 5 at% of boron in the starting material and is still lower than 

the B-doping level observed for the B4C(B)–C system.210 Using the mixture of 

B13C2 and graphite with a C/B ratio of 13 (∼7 at% B) as a starting material, 

Dubrovinskaia et al.211 performed experiments in a MAP at pressures from 9 to 

20 GPa and temperatures 2,500–2,700 K. The chemical composition and texture 

of the samples were studied using SEM, TEM/parallel EELS, and EPMA. The 

overall B content was determined to be 2.6 at% for the sample synthesized at 

20 GPa and 2700 K and 1.8 at% for the sample synthesized at 9 GPa and 2600 

K. XRD data revealed the presence in boron-doped diamond of a very small 

amount of a boron-rich phase (B50C2) together with traces of the starting B13C2. 

The Raman spectra and increased lattice constants of the synthesized diamond 

crystals are in agreement with earlier reported heavily B-doped diamond phases.

For a long time it has been known that boron atoms can substitute carbon atoms 

in the structure of graphite, thus forming a graphite-like solid solution. 

Lowell212 synthesized boron-doped graphite by thermal treatment of a mixture 

of graphite flakes and B4C to 2,350 °C and reported the maximum boron 

solubility of 2.35 at%. Since that time a number of graphite-like B-C phases 

(boron-substituted graphites) with boron content up to 50 at% were successfully 

prepared by thermal chemical vapor deposition methods (e.g., Kouvetakis et 

al.62,213). Since the graphitic structures are the usual precursors for HP-HT 

synthesis of superhard diamond-like phases (graphite → diamond, h-BN → cBN), the boron-substituted graphites are considered to be perfect precursors for 

diamond-like B-C phases. However, HP-HT treatment of turbostratic graphitelike g-BC3 at 20 GPa and 2,300 K using a MAP resulted in a bulk composite 

material consisting of intergrown boron carbide B4C and B-doped diamond with 

1.8 at% B. The material was found to exhibit semiconducting behavior and 

extreme hardness of approximately 88 GPa comparable with that of single-
57

crystal diamond.214 Subjecting the same g-BC3 to shock pressures ranging from 

10 to 30 GPa resulted only in local segregation of g-BC3 and its partial 

transformation into a mixture of amorphous boron carbide and carbon. No 

expected diamond-like phases were observed in the shock products, probably 

due to the low shock temperatures (~600 K) reached in the samples.215

In contrast to above experiments, a direct transformation of the g-BC1.6 phase 

to a diamond-like c-BC1.6 phase in an LH-DAC at 2,230 K and 45 GPa was 

reported.216 The recovered samples were examined using both synchrotronbased X-ray diffraction and confocal micro-Raman spectroscopy at ambient 

conditions. XRD examination revealed the presence of another orthorhombic or 

hexagonal phase in the less heated areas. The lattice parameter of the c-BC1.6

phase was found to be slightly higher than that of diamond, thus supporting the 

theoretical prediction that incorporation of the B atoms into a diamond structure 

should lead to a negligible distortion of the cubic diamond cell.217 The same 

research group continued LH-DAC experiments using other g-BCx starting 

materials (x = 4 and 3) at pressures above 40 GPa and temperatures above 2,000 

K and claimed the formation of corresponding diamond-like c-BC4 and c-BC3

phases.218 Their cubic lattice parameters were found to be 3.587 and 3.589 Å as 

well as the C/B ratio (determined by EPMA/EELS) to be 3.9 ± 0.3 and 2.8 ± 0.7, 

respectively.

Solozhenko et al.219 have performed a systematic investigation of phase

transformations of turbostratic g-BCx phases at pressures up to 25 GPa and 

temperatures up to 2,500 K using both an LH-DAC and MAP technique. In situ 

HP-HT experiments with g-BC and g-BC3 in the 3–7 GPa pressure range using 

MAP and energy-dispersive XRD revealed no phase transformation up to 2000 

K in both compounds. At pressures above 20 GPa in the 2,000–2,500 K range, 

t-BCx phases (1 ≤ x ≤ 4) decompose into boron-doped diamond (1–2 at% B) 

and boron carbides (B4C and B50C2) that are accompanied by the formation of 

cubic B-C phases (c-BCz) with lattice parameters up to 2% higher than that of 

diamond. From these findings the authors concluded that the synthesis of a pure 

cubic B-C phase may be expected only for g-BCx with a lower B-content (i.e.,

x ≥ 5). Indeed, g-BC5 compressed to 24.0 GPa and heated above 2200 K 

transformed to a (pseudo)cubic BCz phase. Since no amorphous phase has been 

observed in the recovered products by TEM and Raman spectroscopy, the 

authors suggested the formation of a c-BC5 phase, the composition of which 

was further confirmed by EELS and EPMA analysis. At higher temperatures 

the cubic phase was found to decompose to boron-doped diamond and B4C. In 

contrast to the other reported c-BCx phases, the lattice parameter of c-BC5

(3.635 Å) was reported to be higher than that of both diamond and c-BN. 

Significantly, the authors were able to produce the sintered polycrystalline 

bodies (~1 mm3) at similar P-T conditions using MAP. The Vickers hardness of 

these samples was found to be ~71 GPa, which is close to the hardness of 
58

nanostructured bulk c-BC2N. The authors also reported for c-BC5 a remarkable 

indentation fracture toughness of 9.5 MPa∙m1/2 and thermal stability up to 1,890 

K in a nonoxidizing environment. These findings make diamond-like c-BC5 an 

exceptional superabrasive superior to diamond.

2.3.2 Potential Superhard B-C Phases

In contrast to carbon nitrides and boron-carbon nitrides, theoretical exploration 

of superhard diamond-like B-C phases and the calculation of their properties 

has begun quite recently. The interest in diamond-like BCx structures has 

originated from the knowledge that superhard diamond-like structures are 

usually derived from their graphitic counterparts. In turn, graphitic structures of 

boron–carbon phases, g-BCx, are also well known to exist in various 

stoichiometries with x greater than or equal to 1. Potential diamond-like phases 

can be formed from a unit cell or a supercell of diamond by replacing C atoms 

with boron at the various lattice positions according to the nominal lattice 

stoichiometry. Thus, various stoichiometries like BC, BC2, BC3, BC5, and BC7

were investigated, and structures of different symmetries (cubic, tetragonal, 

orthorhombic, hexagonal, or trigonal) were derived depending on the 

composition and positions of B-atoms (Table 2.4).

Among all considered BCx compositions, diamond-like c-BC5 attracted the

interest of experimentalists and theoreticians. The recent synthesis of c-BC5 in 

macroscopic amounts using MAP219 provided the community with the initial 

experimental data on its structure (XRD and Raman spectrum) and properties 

(extreme hardness and stiffness). This was followed by theoretical work using 

DFT calculations, where Calandra et al.220 have shown that superhard c-BC5 is 

metallic, making it the hardest conductor studied to date, and superconducting 

with a Tc = 45 K, the largest Tc ever for a phonon-mediated superconductor. 

Because of the similar atomic numbers of boron and carbon, the position of B 

in the cell could not be determined using XRD. Consequently, the structure of 

the synthesized c-BC5 was assigned to a pseudocubic cell with a0 = 3.635 Å. 

However, Calandra et al. have shown that c-BC5 should adopt a hexagonal 

structure, which is compatible with a cubic symmetry for c/a = √6 ≈ 2.45 (Fig. 

2.17). Performing volume and force optimization of a 6-atom hexagonal 

diamond-like supercell with 2 C atoms replaced by boron, the authors have 

found that the obtained c/a ratio was only slightly larger than the ideal one for 

cubic symmetry. Thus, the BC5 cell can be obtained via a small elongation of 

the cubic cell along the cubic (111) axis (Fig. 2.17).
59

Table 2.4 Structural parameters, bulk moduli, shear moduli, and estimated hardness 

(if available) of the predicted superhard diamond-like BCx phases calculated employing 

GGA (LDA). The experimental values for diamond, c-BN, and recently discovered c-BC5

are given for comparison.

Compd/Structure V/atom

(Å3)

a

(Å)

b

(Å)

c

(Å)

B

(GPa)

G

GPa)

H

(GPa) Ref.

Diamond1 ... 0.3567 0.3567 0.3567 442-433 534-544 60–150 [1,50]

c-BN1 ... 0.3616 0.3616 0.3616 369-401 409 46–80 [1,50]

c-BC5

1 ... 3.635 3.635 3.635 335 ... 71 [219]

BC cubic ...

3.790 

(3.745)

3.790 

(3.745)

3.790 

(3.745) ... ... ... [217]

BC2 t-I41/amd ... 2.520 2.520 11.919 349 319 56 [221]

BC3 tetragonal ...

3.553 

(3.509)

3.553 

(3.509)

3.913 

(3.876) 352 (379) (344) ... [217,222]

tetr-P-42m 5.975 3.513 3.513 3.871 361 337 41 [223]

tetr-P-42m ... 2.510 2.510 3.915 359 ... 66 [224

o-Pmma-a ... 2.513 2.520 7.788 354 ... 62 [224]

o-Pmma-b ... 2.484 2.531 7.891 352 ... 65 [224]

trigonal ...

3.685 

(3.642)

3.685 

(3.642)

3.685 

(3.611) 337 (378) ... ... [217]

BC5 tetragonal 5.993 3.6332 ... ... 385 405 ... [225]

hex-P3m1 5.871 3.6082 ... ... 376 394 ... [226]

hex-P3m1 6.02 2.55 2.55 6.39 337 ... 83 [227]

hex-P3m1

6.006 

(5.805)

2.552 

(2.522)

2.552 

(2.522)

6.392 

(6.324) 379 (407) 386 (410) 62 [228]

hex-P3 (5.867) (2.539) (2.539) (6.303) (410) ... ... [229]

hex-P3m1 ...

2.210 

(2.161) ...

6.694 

(6.326) (405) (372) ... [223]

o-Pmma-2 6.007 ... ... ... 379 ... 70 [230]

o-Pmma-I ... 2.488 2.498 11.214 382 ... ... [231]

o-Pmma-II ... 2.472 2.495 11.346 388 ... ... [231]

tetr-I-4m2 ... 2.525 2.525 11.323 376 379 80 [232]

trig-P-1 ... 4.455 4.459 4.462 370 374 79 [232]

h-P3m1-II 6.02 2.532 2.532 13.017 401 ... ... [233]

BC7 c-P-43m

... 3.621 

(3.581)

3.621 

(3.581)

3.621 

(3.581) 387 (418) (412) ... [217,223]

o-Amm2 ... ... ... ... 388 430 49.5 [234]

hex 5.933 

(5.733)

... ... ... 387 (417) 399 (421) 63 [228]

P-4m2 ... 2.516 2.516 7.450 377 421 75 [235]

P-4m2 ... 2.514 2.514 7.466 388 419 78 [236]

B4C3 α-P31c 7.86 7.085 7.085 5.061 231 ... 60 [237]

β-P63/m 7.61 6.750 6.750 2.701 203 ... 56 [237]

c- I-43d 7.19 5.860 5.860 5.860 260 ... 65 [237]

pc-P-42m 7.89 3.809 3.809 3.809 220 ... [237]

cs-Fd-3m 6.11 6.996 6.996 6.996 333 ... 64 [237]

1Experimental values

2Effective cubic lattice parameter derived from the volume of the unit cell
60

Fig. 2.17 A representative atomic arrangement in the hexagonal supercell of c-BC5

220

with carbon and boron atoms represented by gray and red spheres, respectively. The 

hexagonal supercell is compatible with a cubic symmetry (dashed blue line) for c/a = √6 

≈ 2.45.

Based on Calandra and Mauri’s structural model220, Jiang et al.225 considered a 

12-atom hexagonal supercell and, via replacing 2 of the carbon atoms with 

boron, constructed 9 nonequivalent structures. The authors found only a small 

total-energy difference between the considered ordered configurations and the 

fully disordered state of BC5 (with boron randomly distributed in the diamond 

lattice). Moreover, the calculated volume per atom, bulk modulus, and XRD 

pattern of the disordered BC5 structure were in good agreements with 

experimental data.219 The authors concluded that the c-BC5 phase synthesized 

at HP and HT may be disordered in nature. 

Yao et al.232 have shown that recently synthesized diamond-like BC5 is 

metastable and suggested the existence of a thermodynamically stable 

tetragonal (I-4m2) polymorph of BC5. The predicted phase is expected to be 

harder than c-BC5, metallic, and superconducting with an estimated 

superconducting critical temperature Tc of 47 K. Another candidate structure 

with trigonal P-1 symmetry was found to fit very well with the experimental 

XRD pattern of c-BC5, being, however, energetically less favorable. Li et al.230

suggested another 2 energetically more preferable orthorhombic (Pmma) 

structures for c-BC5. Their simulated XRD patterns, Raman modes, and Vickers 

hardness revealed remarkable agreement with the experimental data. Both

structures were also found to be hole-conducting and superconducting with a 

critical temperature of 11–23 K. The main result of the theoretical studies of 

potential c-BC5 structures and other diamond-like B-C phases are summarized 

in Table 2.4. All of the considered c-BCx are expected to be ultraincompressible and superhard. Moreover, they may exhibit conductivity and 

superconductivity with a remarkably high Tc of 11–48 K.

221–224,226–228,233–235,238

The combination of such properties suggests that they are an extremely

superabrasive and promising functional material for electronics extreme 
61

environments. Also, notice the trend in the mechanical properties: the larger the

concentration of boron in the considered B-C phase, the lower the elastic moduli 

of c-BCx and the less hard the material would be. Last but not least, the

incorporation of large amounts of boron also affects the conducting character of 

c-BCx.

2.4 Summary

During last 2 decades, many studies were dedicated to the exploration of the BC-N phase system. Undoubtedly, the synthesis of diamond-like boron carbon 

nitrides, c-BC2N and c-BC4N, and boron carbide, c-BC5, is the most remarkable 

achievement in the search of novel superhard materials. These dense phases 

were obtained under high static pressures above 20 GPa and temperatures about 

2,000 K, employing various precursors, either single-source graphitic phase or 

an amorphous mechanical mixture of the constituting compounds. Furthermore, 

c-BC2.5N heterodiamond was successfully obtained as a nanocrystalline powder 

via shock compression of its graphitic counterpart above 30 GPa and 2,700 K. 

As the authors emphasized, one of the most important factors in the successful 

synthesis of dense diamond-like B-C-N phases lies in the nature and quality of 

the starting materials employed. A serious achievement was that the preparation 

of macroscopic single-phase samples made possible a detailed examination of 

properties of the high-pressure products. Both c-BCxN and c-BC5 were 

confirmed to be superhard, with a hardness of ~70–80 GPa, approaching that of 

diamond, and possess exceptional fracture toughness, high thermal stability, and 

oxidation resistance. Moreover, while c-BCxN reveals semiconducting 

behavior, c-BC5 is a conductor and potential high-Tc superconductor. Thus,

these novel compounds not only represent exceptional superabrasives

surpassing diamond, but also, thanks to their intriguing functional properties, 

can expand the boundaries of high-temperature electronics and electrochemistry 

at extreme conditions. 

However, the solid-state structure of the synthesized diamond-like B-C-N 

phases and their structure-properties relationship still remain a matter of 

controversy. So far, there is no unambiguous proof of the existence of the 

experimentally obtained crystalline phases. Thus, there is more detailed 

research to be done to clarify the composition and structure of the published 

phases in the B-C-N system. Nevertheless, despite the successful realization of 

superhard diamond-like B-C-N and B-C phases in macroscopic amounts, 

commercialization of their production in the form of powders or sintered bodies 

is quite unlikely at the moment. The reason is that extremely high thermobaric 

conditions are required for the direct transformation of the suitable precursors—

namely, pressures and temperatures above 20 GPa and 2,000 K, respectively. 

However, it would be of interest to investigate the feasibility of such a 

transformation using different solvent catalysts (similar to those employed for 
62

the industrial synthesis of diamond and c-BN) to reduce the P-T conditions to 

commercially utilizable values. A different way to synthesize large amounts of 

high-pressure B-C-N phases is by the shock-wave technique (e.g., by utilizing 

explosives). The disadvantage here is that the desired P-T conditions can only 

be maintained for very short times, which usually result in the formation of 

ultradispersed powders with reduced crystallinity. Therefore, once the synthesis 

and purification of the powder product is accomplished, an appropriate sintering 

technique (including conditions and suitable sintering additives) to obtain dense 

superhard B-C-N bodies should be developed.

Concerning dense C3N4, the direct synthesis under extreme static or dynamic 

pressures and temperatures has unambiguously led to dense C-N phases, 

although the results are far from numerous. However, in most cases the 

identification of their structures and compositions (with respect to the amount 

of nitrogen and contaminations) remains problematic. Moreover, because of the

severe pressure and temperature conditions required, such studies are mainly of 

fundamental character. Another significant point is that the properties predicted 

for dense C3N4, such as extreme incompressibility and superhardness, have 

never been experimentally confirmed. It is worth noting that despite the fact that

extreme pressures and temperatures were claimed to be necessary for the C3N4

synthesis, dense carbon nitrides were also obtained under quite mild 

conditions—namely, via solvothermal reactions and a high-energy ball-milling 

process. These 2 routes appear to be very promising in the future, either for 

crystal growth or large-scale synthesis of dense C3N4. Significant efforts are 

being undertaken to investigate and understand the chemistry of dense carbon 

nitrides and the mechanisms involved during their synthesis. These mechanisms

are strongly dependent on the nature of the precursors, catalysts, pressure, 

temperature, and duration.
63

3. Experimental: Synthesis of Transparent Spinel-Type 

γ-Si3N4

Dmytro Dzivenko and Ralf Riedel

3.1 High-Pressure, High-Temperature (HP-HT) Synthesis

Commercial silicon nitride powder (Goodfellow, α-Si3N4 > 85%, particle size 

< 10 μm, oxygen ~1 wt%, nitrogen > 37.5 wt%) was used as a starting material 

for high-pressure synthesis. In spite of the declared chemical composition, 

elemental combustion analysis of the powder employing LECO TC-436 

analyser revealed the presence of 2.5 ± 0.1 wt% of oxygen. Nitrogen was found 

to amount 38.5 ± 0.9 wt%, which is in reasonable agreement with the theoretical 

value of 39.9 wt%. 

The starting Si3N4 powder was packed into a 3.8-mm-long platinum capsule 

with an outer diameter of 2 mm, which was preliminary welded, closed at one 

end, and mechanically sealed after sample loading. The sample loading and 

capsule-sealing procedures were performed in a glove box under a controlled 

argon atmosphere to prevent further contamination of the sample powder with 

oxygen and/or moisture.

For the sample pressurization, a hydraulically driven 6/8 type multianvil (MA) 

apparatus was employed. The setup consisted of an 18-mm-edge-length Crdoped MgO octahedron containing the sample in a set of coaxially arranged 

parts, including a resistance-heated LaCrO3 furnace and a ZrO2 thermal 

insulation tube that accommodated the cylindrical capsule (Fig. 3.1). The 

octahedron was placed inside the octahedral volume formed by 8 tungsten 

carbide cubes with truncated corners. The sample assembly was brought to a 

pressure of 17 ± 0.5 GPa at room temperature and then heated to 1,825 °C in 

about 30 min. After holding the maximum temperature (±5 °C) for 1 h, the 

sample was quenched to room temperature in about 15 s by turning off the

heating power. The temperature during experiment was monitored using a 

W3%Re-W25%Re thermocouple coaxially inserted into the furnace assembly 

(Fig. 3.1). A detailed description of the MA-press assembly, pressure 

calibration, and temperature measurement can be found elsewhere.239
64

Fig. 3.1 Cross section of an octahedral pressure assembly used for HP-HT synthesis of 

γ-Si3N4

3.2 Sample Characterization

After decompression, the sample capsule (optically intact) was recovered from 

the pressure medium and embedded in epoxy resin. The capsule cap was 

removed for further examination by polishing, and 2 slices about 0.3 and 0.25 

mm thick were cut from the sample by a diamond wire saw. The sample slices 

were investigated by optical microscopy, Raman spectroscopy, and powder 

X-ray diffraction (XRD). Preliminary elemental analysis of the high-pressure

product was performed using a scanning electron microscope equipped with an 

energy dispersive X-ray (EDX) detector. The residual HP product weighing 

5.5 mg was completely recovered by cutting and unwrapping the capsule 

cylinder. 

3.2.1 Optical Observation

Figures 2.2 and 2.3 show optical micrographs of the 2 slices of the HP product 

made in reflected and transmitted light. On slice 1, the areas of a light-yellowish 

“milky” material were observed under reflected light (Fig. 3.2a). These areas 

appeared to be translucent under transmitted illumination (Fig. 3.2b). Likewise, 

the translucent phase was observed in slice 2 (Fig. 3.3). The translucent phase 

is mainly concentrated on the sample surface in contact with the capsule. The 

rest of the sample (internal volume) is, in contrast, dark gray and opaque. 
65

Fig. 3.2 The microscopic photographs of “slice 1” of the HP product in a platinum

capsule ring taken under combined transmitted and reflected light (a) and in 

transmitted light only (b)

Fig. 3.3 The microscopic photographs of “slice 2” of the HP product in a platinum

capsule ring taken under combined transmitted and reflected light (a) and in 

transmitted light only (b)

3.2.2 Raman Spectroscopy

Raman spectra were excited by a HeNe laser with the wavelength λ = 632.8 nm 

and measured using a LabRam HR800 spectrometer (Horiba Jobin Yvon). All 

Raman shifts detected from the translucent areas of slices 1 and 2 (Fig. 3.4a) 

can be attributed to the cubic spinel γ-Si3N4 and are in excellent agreement with 

the previously reported data.117,240,241 The dark areas also revealed the main 

features of γ-Si3N4 (bands at 523 and 846 cm-1) as well as broad peaks at 123 

and 360 cm-1(Fig. 3.4b), which have not been (yet) assigned to any of the 

known Si-N-O phases and can be probably related to an impurity or defects in 

γ-Si3N4.

240

 

(a) (b)

 

(a) (b)
66

Fig. 3.4 Typical Raman spectra measured from the translucent (a) and dark (b) areas 

of the HP product. The characteristic Raman shifts of the γ-Si3N4 with corresponding 

Raman-active phonon modes (according to Fang et al.242) are indicated.

3.2.3 Powder X-ray Diffraction (XRD)

A powder XRD pattern (Fig. 3.5) was collected from slice 2 in the transmission 

geometry using a STOE Stadi P diffractometer (Stoe & Cie GmbH) equipped 

with a Molybdenum X-ray source, a flat Ge (111) monochromator (λ(Mo-Kα1) 

= 0.70932 Å) and a linear position-sensitive detector covering the 2θ region of 

6°. The XRD pattern shows only reflexes of a cubic phase, the structure of 

which could be attributed to that of spinel type γ-Si3N4. The refined lattice 

parameter a0= 7.7467(6) Å is slightly higher than those reported earlier.240,241,243

The anion parameter (nitrogen coordinate) is calculated to be 0.2589(2).
67

Fig. 3.5 Powder XRD pattern (red circles) of the HP product (slice 2) measured in 

transmission “STOE” geometry. The results of the full profile Rietveld refinement for 

the cubic spinel γ-Si3N4 and the corresponding difference curve are presented by black 

and blue lines, respectively. The calculated peak positions of the cubic spinel-type 

structure (space group Fd-3m) are denoted by green tick marks. The refinement 

agreement factors are RBragg = 7.2%, RF = 6.1%, and χ2 = 1.2.

3.2.4 Energy-Dispersive X-ray Spectroscopy (EDX)

Preliminary elemental analysis of the HP product (slice 1, unpolished) was 

performed by means of EDX spectroscopy using a scanning electron 

microscope (HR-SEM Philips XL30 FEG) equipped with an EDX detector 

(EDAX Genesis). Semi-quantitative EDX analysis revealed the presence of Si, 

N, and a small amount of O (as well as C from adhesive carbon tape) throughout 

the sample (e.g., Fig. 3.6).

Fig. 3.6 Typical EDX spectrum measured from translucent part of the HP product. 

C-K peak is due to the adherent carbon tape.
68

However, because of the unprepared sample surface and low resolution of the 

EDX detector at low energies, the scattering of measured values did not allow 

an unambiguous conclusion about the compositional difference between 

translucent and dark sample areas. The element mapping also showed no 

significant composition variation (Fig. 3.7).

Fig. 3.7 a) Optical and b) SEM micrographs of the sample surface together with EDX 

elemental maps for c) nitrogen, d) oxygen, e) silicon, and f) platinum

(a) (b)

(c) (d)

(e) (f)
69

3.3 Conclusion

The investigation of the HP product allows us to conclude that the major constituent 

(and the only crystalline) phase of both the translucent and opaque sample parts is 

γ-Si3N4. The nature of the sample translucency is not, however, clarified. The 

resulting translucency and opaque sections of the final product, instead of the 

transparency that was desired, probably resulted from microstructure or the 

elemental composition variation (mainly oxygen content) of γ-Si3N4. The variation 

of γ-Si3N4 can result from the temperature/pressure gradients in the sample and 

oxygen diffusion from the oxidic environment through the platinum capsule into 

the sample during heating. Moreover, the presence of an insignificant amount of an 

amorphous oxidic phase241,243 cannot be excluded. Therefore, further detailed 

investigation of the HP product with respect to its (micro)structure and elemental 

and phase composition by means of transmission electron microscopy combined 

with electron energy loss spectroscopy and electron-probe microanalysis is 

required. These studies will provide more insight into the relationship between the 

experimental environment/conditions and the optical properties of the HP product. 

Thus, optimization of the synthetic approach for obtaining fully translucent γ-Si3N4

bulks can be possible.
70

4. References for Sections 2 and 3

1. Brazhkin VV, Lyapin AG, Hemley RJ. Philos Mag A. 2002;82(2):231.

2. McMillan PF. Nat Mater. 2002;1(1):19; Solozhenko VL, Gregoryanz E. Mater 

Today. 2005;8(11):44.

3. Rafaja D, Klemm V, Motylenko M, Schwarz MR, Barsukova T, Kroke E, Frost 

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List of Symbols, Abbreviations, and Acronyms

2-D 2-dimensional

3-D 3-dimensional

Ar argon

B4C boron carbide

BBr3 boron tribromide

BCN boron-carbon-nitrogen

c-BCN cubic boron-carbon-nitride

c-BN cubic boron nitride

CCl4 carbon tetrachloride

CHN/S/O carbon, hydrogen, nitrogen, sulfur, oxygen

CVD chemical vapor deposition

DAC diamond anvil cell

DEPT distortionless enhancement by polarization transfer

DFT density functional theory

dwur defect-wurtzite

EDX energy-dispersive X-ray

EELS electron energy loss spectroscopy

EPMA electron probe microanalysis

EPR electron paramagnetic resonance

FTIR Fourier-transform infrared

g-BC2N graphitic BC2N

GGA generalized gradient approximation

h-BCN hexagonal boron–carbonitride

He helium

HP high pressure
86

HT high temperature

LDA local density approximation

LH-DAC laser-heated diamond anvil cell

Li3N lithium nitride

MA multianvil

MAP multianvil press

MS mass spectrometry

N2 nitrogen gas

N/C nitrogen carbon ratios

NMR nuclear magnetic resonance

Pt platinum

PVD physical vapor deposition

REM reflection electron microscopy

SAED select area electron diffraction

SEM scanning electron microscope

SIMS secondary ion mass spectrometry

TCNE tetracyanoethylene

TEM transmission electron microscopy

THF tetrahydrofuran

w-BCN wurtzite boron-carbonitride

XPS X-ray photoelectron spectroscopy

XRD X-ray diffraction
87

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