Observation of an Ultrahard Phase of Graphite Quenched 

from High-pressure 

by Jennifer A. Ciezak-Jenkins 

ARL-TR-5466 February 2011 

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Army Research Laboratory 

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ARL-TR-5466 February 2011 

Observation of an Ultrahard Phase of Graphite Quenched 

from High-pressure 

Jennifer A. Ciezak-Jenkins 

Weapons and Materials Research Directorate, ARL 

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Observation of an Ultrahard Phase of Graphite Quenched from High-pressure 

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14. ABSTRACT 

In-situ isothermal high-pressure synchrotron x-ray diffraction, and optical Raman spectroscopy were used to examine the 

structural properties, equation of state, and vibrational dynamics of highly ordered pyrolytic graphite. The x-ray measurements 

show that the pressure-volume relations remain smooth to near 5.9 GPa, where there is an abrupt discontinuity. This 

discontinuity is correlated to a hexagonal-to-monoclinic phase transition, which was previously predicted via ab initio crystal

structure algorithms. However, discontinuities in the Raman vibrational modes do not appear until near 19 GPa. The 

discrepancy in the onset pressure of the phase transition is correlated to edge effects. The high-pressure monoclinic phase was 

recovered to ambient pressure and remains stable for long periods. 

15. SUBJECT TERMS

High-pressure, diamond anvil cell, pyrolytic graphite, ultrahard materials 

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Jennifer A. Ciezak-Jenkins 

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iii

Contents 

List of Figures iv

List of Tables iv

Acknowledgments v

1. Introduction 1

2. Experimental Methodology 2

3. Results 3

3.1 X-ray Diffraction .............................................................................................................3

3.2 Raman Spectroscopy .......................................................................................................6

4. Discussion and Conclusions 8

5. References 10

Distribution List 13
iv 

List of Figures 

Figure 1. Graphite model, showing the planar orientation. ............................................................2

Figure 2. Ab initio predicted crystal structure of monoclinic carbon. Reproduced directly 

from (7). .....................................................................................................................................2

Figure 3. XRD of HOPG; the patterns are offset for clarity. ..........................................................4

Figure 4. The P-V equation of state of cold compressed HOPG. The obvious discontinuity 

near 6 GPa is indicative of a phase transition. Both data sets were fit using a third-order 

Birch-Murnaghan equation of state (22). ...................................................................................5

Figure 5. Anisotropic ratio of HOPG as a function of pressure. .....................................................6

Figure 6. Raman frequency shifts of HOPG as a function of pressure. Fits were obtained 

using least squared analysis. ......................................................................................................7

Figure 7. Cracked diamond anvil after monoclinic phase was decompressed from highpressures. ....................................................................................................................................8

Figure 8. Approximately 15 x 15 micron HOPG sample, quenched from high-pressure, on a 

copper slide. ...............................................................................................................................8

List of Tables 

Table 1. Lattice parameters of HOPG as a function of pressure. ...................................................4
v 

Acknowledgments 

The Geophysical Laboratory of the Carnegie Institution of Washington is thanked for access to 

their high-pressure gas loading facility. The Advanced Light Source is supported by the 

Director, Office of Science, Office of Basic Energy Sciences of the U.S. Department of Energy 

under contract no. DE-AC02-05CH11231. 

 
vi 

INTENTIONALLY LEFT BLANK. 
1 

1. Introduction 

The search for ultrahard materials, which may have mechanical properties that rival or exceed 

that of diamond, has stimulated research under extreme conditions for over 50 years (1). Recent 

studies of simple materials under thermomechanical extremes of pressure and temperature have 

revealed extraordinary physical and chemical behavior accompanied by unexpectedly complex 

structures that were not envisioned by conventional chemical models (see, for example, 

references 2–7). The general theories that describe these transformations and permit 

extrapolation to rationally design novel materials with exotic properties are unknown. It is only 

through the development of new fundamental physics-based principles that the capability to 

manipulate the structures of elements and compounds into novel, yet practical, materials will be 

achieved. Innovative experiments coupled with state-of-the-art computational simulations are 

needed to not only describe this unique behavior, but to provide a framework for making useful 

and accurate predictions. By studying the formation of exotic phases of materials under extreme 

conditions, an understanding of how to predict and ultimately influence metastability will be 

developed; such capability is an important first step in understanding how material properties can 

be manipulated and exploited to design new, novel, and improved materials that cannot be 

obtained through conventional synthesis approaches. Such materials, particularly those made of 

first and second row elements, represent an entirely new class of materials that may exhibit many 

interesting properties of relevance to Department of Defense (DoD) interests, such as superhardness (4 ,8), optical nonlinearity (2) and high-energy-density (2, 3, 5). 

This work examines the synthesis, physical and chemical properties, metastability, and recovery 

mechanism of an ultrahard phase of carbon, formed from isothermal compression of pyrolytic 

graphite. Materials with bulk moduli and a hardness rivaling that of diamond have been 

theoretically predicted for many years (see, for example, references 9–11) but the existence of 

ultrahard phases of carbon has only recently been experimentally documented under extreme 

conditions in graphite (4, 7), nanotubes (12), and fullerenes (13). It is well known that 

polycrystalline graphite, shown in figure 1, undergoes a structural phase transition slightly above 

16 GPa at room temperature; this transition is accompanied by striking changes in several 

physical and chemical properties, including electrical and optical reflectivity (14), x-ray 

diffraction (XRD) patterns, and hardness (4, 7). It has been speculated that the hardness of the 

high-pressure phase of polycrystalline graphite is comparable to or slightly exceeds that of bulk 

diamond, due to the indentations left by the transformed material on the planar surface of the 

diamond anvil upon decompression (4). Attempts to quench the ultrahard high-pressure phase of 

graphite to ambient conditions were unsuccessful (4). Although several potential 

crystallographic structures were proposed, such as hexagonal or amorphous diamond (15–18), 

experiments gave largely conflicting results as to the actual crystallographic structure. Recent ab 

initio crystal structure predictions (7) have shown a close correlation between the simulated 
2 

properties of a monoclinic carbon phase, shown in figure 2, and those derived from experimental 

measurements (4), including the optical transparency and sharp drop in reflectivity after the 

phase transition. The simulations have also suggested the hardness (83.1 GPa) and bulk modulus 

(431.2 GPa) of the monoclinic material are comparable to diamond; however, this report gave no 

insights into possible mechanisms to introduce metastability into the material (7). In this work, 

the synthesis of a quenchable high-pressure monoclinic carbon phase formed by cold 

compression of highly ordered pyrolytic graphite (HOPG) is detailed. XRD and Raman 

spectroscopic data clearly confirm a significant change in the physical properties of the pyrolytic 

graphite material after the phase transition, which remain after a quench to ambient conditions. 

Figure 1. Graphite model, showing the planar orientation. 

Figure 2. Ab initio predicted crystal structure of 

monoclinic carbon. Reproduced directly from 

reference 7. 

2. Experimental Methodology 

Grade ZYA HOPG was purchased from SPI Supplies and used without further purification. A 

flake of approximately 50 microns in diameter and a thickness of 20 microns was removed from 
3 

the larger SPI sample and positioned in a piston-cylinder-type diamond anvil cell equipped with 

300-micron diamond culets and a stainless steel gasket. For both the Raman spectroscopic and 

XRD measurements a neon pressure medium was loaded into the diamond anvil cell using a 

specialized high-pressure gas loading system (19). The in-situ pressure within the diamond anvil 

cell was determined from the frequency shift of the ruby R1 luminescence of several micrometer 

sized rubies placed around the sample (20). In-situ Raman scattering measurements were 

conducted using a diode-pumped solid-state laser operating at 532 nm and a laser spot diameter 

of ~4 μm. Prior to the experimental measurements, the spectrograph was calibrated using a neon 

lamp. This method of calibration has an accuracy of ±1 cm–1. The spectral resolution for all 

Raman measurements was ±4 cm–1. 

Room temperature isothermal angle dispersive x-ray powder microdiffraction experiments were 

conducted on beamline 12.2.2 of the Advanced Light Source by using focused monochromatic 

x-rays (λ = 0.4959 Å) and a high-resolution image plate detector. The recorded two-dimensional 

diffraction images were then integrated to produce high-quality angle dispersive XRD patterns 

using FIT2D. 

3. Results 

3.1 X-ray Diffraction 

HOPG is a unique polycrystalline polymorph of graphite with a hexagonal structure of planar 

layers and a high degree of preferred orientation. The lattice parameters for the hexagonal cell of 

HOPG at ambient conditions are a = c = 3.42 Å. Good agreement of the XRD patterns with 

previously reported results (21) was noted, with five characteristic diffraction peaks below 20° at 

0.5 GPa: (111), (201), (112), (202), and (203). To understand the high-pressure behavior of 

HOPG, XRD upon isothermal compression was measured at room temperature and several 

representative XRD patterns are shown in figure 3. Upon compression to 5.9 GPa, a 

discontinuous change in the XRD patterns was observed, but the Bragg peaks observed at 

0.5 GPa remained traceable to the original pattern, indicating that the high-pressure form 

remained in a graphite-like atomic rearrangement and was not amorphous. The appearance of 

several new peaks at 5.9 GPa suggests at least a partial phase transition to a crystallographic 

symmetry lower than hexagonal. These new peaks were indexed on the basis of the monoclinic 

symmetry proposed by Li et al. (7) as (201), (202), (–203), (220), (420), and (620). In contrast to 

Mao et al. (4), the onset of this transition began near 5.9 GPa. However, between 5.9 and 

12.2 GPa, the a and c lattice parameters do not change significantly, suggesting the phase 

transition from hexagonal to monoclinic is very sluggish at room temperature and, that between 

5.9 and 12.2 GPa, the material can be approximately considered as a distorted hexagonal 

structure, as previously proposed (4). The lattice parameters calculated from the diffraction 

patterns are listed in table 1. 
4 

Figure 3. XRD of HOPG; the patterns are offset for clarity. 

Table 1. Lattice parameters of HOPG as a function of pressure. 

Pressure (GPa) a b c beta 

0.0 3.42(4) 3.42(4) 

0.2 3.32(6) 3.34(5) 

2.1 3.32(1) 3.33(5) 

3.4 3.31(5) 3.31(4) 

5.9 3.09(8) 3.76(2) 102.7(2) 

7.3 3.06(4) 3.74(1) 101.0(2) 

12.2 3.04(1) 2.17(4) 3.71(9) 99.1(5) 

15.9 3.00(0) 2.15(1) 3.67(2) 98.4(7) 

18.1 2.97(3) 2.14(5) 3.62(6) 97.2(5) 

22.9 2.92(5) 2.13(3) 3.60(3) 96.0(2) 

29.6 2.87(7) 2.11(1) 3.57(5) 95.6(6) 

33.2 2.87(1) 2.09(9) 3.55(5) 94.9(5) 

42.1 2.83(4) 2.08(4) 3.48(4) 94.1(5) 

50.9 2.79(3) 2.08(3) 3.43(3) 93.5(9) 

The relationship between pressure and volume (P-V) of HOPG to near 51 GPa is represented in 

figure 4. The P-V isotherm was fit to the third-order Birch-Murnaghan equation of state (22): 

ܲሺܩܲܽሻ ൌ   3

2  ൗ Bo [݊ି଻

ଷൗ – ݊ିହ

′ܤ –1) 3 – 1] – [ ଷൗ

4ൗ  ) (݊ିଶ

ଷൗ – 1)], (1) 
5 

where η = ܸ

ܸ݋ ൗ and B0 and B ́ are the bulk modulus and its pressure derivative, respectively. 

The best data fits were obtained when B0 = 432 GPa, which exceeds that of cubic boron nitride 

(401 GPa) and rivals that of diamond (468 GPa). Above 5.9 GPa, the smoothly varying curve 

shows no evidence of discontinuities that could indicate additional phase transitions. 

Figure 4. The P-V equation of state of cold compressed HOPG. The 

obvious discontinuity near 6 GPa is indicative of a phase 

transition. Both data sets were fit using a third-order 

Birch-Murnaghan equation of state (22). 

The a and c lattice parameters of the monoclinic cell determined from the diffraction patterns 

give the pressure dependence of the axial ratio c/a for the unit cell, also called the anisotropic 

ratio. The anisotropic ratio is shown in figure 5 as a function of pressure. Above the phase 

transition at 5.9 GPa, the a and c values decrease smoothly with pressure, as is shown in table 1, 

with the c axis being slightly more compressible. Beginning near the phase transition at 5.9 GPa, 

the anisotropic ratio increases and reaches its maximum by 30 GPa. The broad pressure range 

associated with the increase in the anisotropic ratio suggests a sluggish phase transition, which 

does not near completion until near 30 GPa. The increase in the anisotropic ratio over this 

pressure range may be attributed to a decrease in the spacing between the planes (figure 1); at 

relatively low pressures, the van der Waals forces between these layers are relatively weak, 

permitting larger compression, whereas after the phase transition, interplanar bonds are formed 

between the graphene sheets. Only when the sheets buckle does the anisotropic ratio decrease. 

This trend has also been observed in other layered materials including tungsten disulfide (23), 

molybdenum disulfide (24), and uranium (25). 
6 

Figure 5. Anisotropic ratio of HOPG as a function of pressure.

3.2 Raman Spectroscopy 

The evolution of the Raman frequencies of HOPG on isothermal compression at room 

temperature to near 25 GPa are shown in figure 6. The physical origin of the Raman bands in 

graphite is a subject of great debate; however, several general discussions regarding the 

symmetry selection rules in graphite are available (26, 27). The main spectral features of the 

Raman spectra of HOPG are the so-called G and D peaks, which lie around 1590 cm–1 and 

1795 cm–1, respectively, for visible excitation. Except for when using ultraviolet (UV) 

excitation, Raman spectra of all graphitic carbons are dominated by the sp2 sites, because visible 

excitation always resonates with the strong π bonding states of the graphene sheets (figure 1). 
7 

Figure 6. Raman frequency shifts of HOPG as a 

function of pressure. Fits were obtained 

using least squared analysis. 

The assignment of the D and G peaks is straightforward in the molecular picture of HOPG, as 

these bands are present in all poly-aromatic hydrocarbons (28, 29). The G peak is due to the 

bond stretching of all pairs of sp2 atoms in the graphite rings. The D peak is due to the breathing 

modes of sp2 atoms in rings (28, 30, 31). While second-order scattering is, in general, allowed in 

HOPG, the intensities of the spectral features are weak compared with the vibrational intensities 

of the first-order Raman modes; therefore, the second-order features are not discussed. 

The following trends in figure 6 are immediately noticeable: 

1. The Raman G mode centered near 1590 cm–1 shifts towards slightly higher frequencies 

until around 10 GPa and then softens between 10 and 18 GPa. The vibrational softening of 

this band suggests a general weakening of the ring structures of the graphene layers, 

perhaps due to buckling of the layers. 

2. The Raman D mode softens over the pressure range from ambient to 18 GPa, implying 

weakening of the graphene ring structures. 

3. Slightly above 18 GPa, there is an abrupt discontinuity in the vibrational frequencies of 

both modes, which corresponds to a phase transition. 

4. Above 19 GPa, the G mode increases in frequency, suggesting a general strengthening of a 

new ring structure, whereas the D mode, in general, softens suggesting a collapse of the 

well-defined layers and a structure that is closer packed than the original material. 

 
8 

Upon decompression from the Raman experiments catastrophic diamond failure occurred (figure 

7). It is unclear whether the diamond failed due to issues relating to the sample or an inherent 

condition of the diamond anvil. Despite failure of the diamond anvil, an approximate 15 x 15 

micron chip of the ultrahard monoclinic phase was able to be recovered (figure 8) to ambient 

conditions. Raman characterization of the semi-transparent recovered sample indicates 

similarities with the high pressure phase. To date, the recovered sample has remained stable 

under ambient conditions for several months. 

Figure 7. Cracked diamond anvil after monoclinic phase 

was decompressed from high-pressures. 

Figure 8. Approximately 15 x 15 micron HOPG sample, 

quenched from high-pressure, on a copper slide. 

4. Discussion and Conclusions 

Revolutionary ab initio crystal structure predictions by Li et al. (7) suggest that the well-known 

high-pressure phase transition in graphite is an ultrahard monoclinic polymorph of carbon. 

Several simulated properties, including the XRD and near K-edge inelastic x-ray scattering were 

compared to the experimental data of Mao et al. (4) and favorable agreement was noted, with the 

onset of the phase transition near 14 GPa. The XRD collected on HOPG in this work suggests 

the phase transition may near 5 GPa, due to the presence of small crystallites near the sample 
9 

edge. However, as pressure increases, additional HOPG is transformed into the new phase and 

can be imaged using Raman spectroscopy. In future Raman experiments, the sample will be 

rastered to test this hypothesis of edge effects. A similar type phenomenon is thought to occur in 

boron carbide, where amorphous regions grow in response to defects and pressure (32). 

Using the monoclinic crystal structure proposed by Li et al. (7), the XRD peaks of HOPG were 

indexed to the calculated monoclinic d-spacings with an average root mean square deviation of 

0.013 Å over the pressure range between 5.9 and 50.9 GPa. However, a significantly smaller a

lattice parameter of the monoclinic phase was observed during these experiments relative to the 

calculated structure (7). This discrepancy between the computational and experimental results 

may be attributed to a higher degree of buckling in the graphene planes than is theoretically 

predicted, but additional experimental data are needed before this can be definitively concluded. 

The high-pressure Raman spectroscopic results suggest that the transition is accompanied by 

significant destabilization of the ring structure of the graphene sheets. The G band of HOPG 

involves the in-plane displacements of the planar graphene sheets, while the D band involves the 

ring-breathing motions. We speculate that as a result of compression and the strong anisotropic 

properties of HOPG, the interplanar spacing decreases and there is a gradual buckling of the 

graphene sheets. But even more interesting is that the continued softening of the phonons above 

the transition suggests that the transition is not isosymmetric; it has the appearance of a zoneboundary soft mode that derives intensity from other disorder or strain. This would imply the 

formation of a superlattice, but additional experiments needed to be conducted before any 

definitive conclusions can be drawn. 

The present study has provided new information on the high-pressure behavior of HOPG. Under 

pressure, evidence of structural changes was documented starting near 5 GPa and continuing to 

19 GPa by XRD and Raman spectroscopy. On compression, both the D and G Raman modes 

show softening and a significant discontinuity near 19 GPa, which implies a structural 

transformation. Concordant with the Raman results, several new peaks appear in the XRD 

pattern near 5.9 GPa, which may be attributed to a phase transition. The discrepancy between 

the Raman and XRD has been justified as a beamsize problem; the x-ray beam, being larger, is 

likely sampling the edge of the sample where defects and possible seed crystals form. The 

pressure dependence of the c/a ratio is anomalous between 18 and 30 GPa, suggesting the phase 

transition is not complete until 30 GPa. The phase can be quenched to ambient conditions for 

extended periods of time. 
10 

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 ATTN RDRL CIM G T LANDFRIED 

 BLDG 4600 

 ABERDEEN PROVING GROUND MD 

 21005-5066 

 3 HCS US ARMY RSRCH LAB 

 ATTN IMNE ALC HRR 

 MAIL & RECORDS MGMT 

 ATTN RDRL CIM L TECHL LIB 

 ATTN RDRL CIM P TECHL PUB 

 ADELPHI MD 20783-1197 

 1 ELEC US ARMY RSRCH LAB 

 ATTN RDRL WML B J CIEZAK-JENKINS 

 BLDG. 390 

 ABERDEEN PROVING GROUND MD 

 21005-5069 

TOTAL: 6 (2 ELEC, 4 HCS) 

 
14 

INTENTIONALLY LEFT BLANK.