J. Am. Ceram. Soc., 84 [12] 3013–16 (2001)

journalPhase Relationship between 3C- and 6H-Silicon Carbide atHigh Pressure and High Temperature
Shin Sugiyama† and Motohiro Togaya
Graduate School of Engineering Science, Division of Material Physics, Osaka University,Toyonaka, Osaka 560-8531, Japan
The phase relationship between 3C- and 6H-SiC is investigatedin the pressure range 2.5– 6.5 GPa and the temperature range
400°–2500°C, by analyzing recovered samples, using X-ray
diffractometry and Raman-scattering techniques. The phasetransition from 3C- to 6H-SiC occurs at 2200°C and 2.5 GPa.
In the pressure range >4.5 GPa, 6H-SiC transforms to 3C-SiC
at 2500°C, via an intermediate state, as indicated by broadening peaks in the X-ray diffraction profile. Thermodynamically,
3C-SiC appears to be the low-temperature stable form, and thetemperature of transition to 6H-SiC, which is stable at hightemperature, appears to increase with pressure.
I.

interconversion between the 3C and 6H forms in the region of 6.0
GPa pressure and temperatures up to 2000°C. Those researchers
ascribed the discrepancy between their results and those of Sokhoret al. to impurities contained in the starting material used bySokhor et al. Their further study suggested that impurities which
are insoluble to SiC (e.g., Fe) lead to the formation of ␣-SiC. On
the other hand, Nakamura et al.7 reported that the transition
temperature of 6H- to 3C-SiC decreases from 1100° to 500°C aspressure increases from 1 to 4 GPa. However, the X-ray diffraction(XRD) peaks for their sample after treatment at 1000°C and 5 GPa
were broad, and peaks from the 6H form seemed to remain in thesample that they claimed had completely converted to the 3C form.
Transformation to the 3C form from the 6H form is difficult to
identify by XRD, because the diffraction peak positions of the 3C
form are nearly the same as those of the 6H form in the starting
sample, and the 3C form, which has a symmetry higher than thatof the 6H form, shows no intrinsic diffraction peak.
The purpose of the present study was to investigate the phaserelationship between 6H- and 3C-SiC and the stabilities of thoseforms at high pressure and high temperature, using XRD andRaman-scattering techniques.

Introduction

SILICON CARBIDE (SiC) exhibits many polytypes, which are basedon the tetrahedral coordination of carbon and silicon. Thestructures of the polytypes differ only in the stacking sequences of
the atomic planes. However, despite many studies on the structuralrelationships and transformations among the polytypes, the question of which factors (e.g., stoichiometry, impurity, pressure, ortemperature) control the formation or stability of those polytypes isstill controversial.
For many years, 3C-SiC with a zinc blende type of cubicstructure was accepted as the low-temperature stable form of SiC
up to 2100°C, at which temperature it converted to the hexagonal
form (6H-SiC).1 On the other hand, numerous reports of 6H-SiC
formation at ⬍2000°C, as well as of 3C formation at temperatures⬎2200°C and interconversions of 3C and 6H forms, in addition tothe coexistence of the two forms, have shown that these phaserelationships are not necessarily simple. If we look at the stability
diagrams of the polytypes ever proposed, Inomata et al. allocatedthe stable region to 3C-SiC at temperatures below 1700°C2 while
Knippenberg suggested that 3C-SiC is thermodynamically unstable under all conditions.1 In addition to temperature conditions, theinfluence of impurities is also crucial when we discuss the stabilityof SiC polytypes. As Jepps and Page reviewed in their paper,3
electrically p-type materials to SiC (e.g., B and Al) tend to stabilize
␣-SiC and n-type materials (e.g., N and P) stabilize ␤-SiC. In thepresence of high pressures of nitrogen, for instance, “reverse”
transformation from 6H- to 3C-SiC was observed in the temperature range 1800°–2500°C where commonly 3C is converted into6H.4
During high-pressure and high-temperature treatments of SiC,Sokhor et al.5 found that 6H-SiC transforms to the 3C form under
3.0 –7.0 GPa pressure and at temperatures in the range 1200°–1400°C. In contrast, Whitney and Shaffer6 failed to identify

II.

Experimental Procedure

In the present study, the sources of the 3C- and 6H-SiC used as
samples were ␤-SiC (Betarandom, Ibigawa Denko Co., Ltd.,Japan) and ␣-SiC (GC #16, Showa Denko Co., Ltd., Tokyo,Japan), respectively. The structural composition of the ␤-SiC wasdetermined by XRD as completely cubic 3C-SiC, and it contained
only a trace of 2H polytype. The ␣-SiC consisted of 6H-SiC with
⬃6% 14R and 4H polytypes. Recent computer simulation study,
however, proposed the possibility that those traces of other
polytypes in XRD profile were due to the presence of stacking
faults in SiC.8 Chemical analysis showed inclusion of 0.06% SiO2,0.09% Al2O3, 0.07% Fe2O3, 0.07% CaO for the ␤-SiC, and0.0005% Fe, 0.0004% Ca, 0.0002% Na, 0.0001% K, N.D. Ni, Cr,Cu, Zn, B, Al, Nb for the ␣-SiC.
The experiments were conducted in a 6 – 8-anvil-type highpressure apparatus9 in the pressure range 2.5– 6.5 GPa. Pressure
calibration of the apparatus was performed using the pressure fixed
points of bismuth (I–II, 2.55 GPa; II–III, 2.7 GPa), tellurium(II–III, 3.7 GPa), and barium (I–II, 5.5 GPa) at room temperature.
The sample was put directly into a capsule of graphite used as aheater, to avoid the influence of contamination with the wallmaterial and to achieve a C–Si system. A W–Re (3–25%)
thermocouple was used to measure the temperature of the sample
up to 2300°C; at ⬎2300°C, the temperature was obtained byextrapolating the relationship between temperature and electricalheating power. The sample was compressed in a high-pressure cellof pyrophyllite and heated to the heat-treatment temperature at arate of 100°C/min. After the sample had been held at temperaturefor 10 min, it was quenched to room temperature and thenrecovered at ambient pressure. This treated sample then wasexamined by powder XRD and Raman-scattering techniques.

N. P. Padture—contributing editor

Manuscript No. 188535. Received June 2, 2000; approved September 17, 2001.
Based in part on a thesis submitted by S. Sugiyama for the M.S. degree in material
physics, Osaka University, Osaka, Japan, 1993.
†Present address: Hokkaido University, Sapporo Hokkaido 060-0819, Japan.

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III.

Results and Discussion(1) 3C 3 6H Transformation
Figure 1 shows a series of powder XRD patterns for 3C-SiC
after treatments at 2200°C and 2.5– 6.5 GPa. Here we present theresults in the range of 29 – 41° 2␪ as no considerable change wasobserved in their diffraction patterns after the treatments in otherregions. Two diffraction peaks, other than those of the starting
material, appeared at 2.5 GPa and are identified with the diffraction lines of the (101) and (103) planes for 6H-SiC. Those peaks
indicate a phase transition from the 3C to the 6H form, and thefraction of that transition was estimated at ⬃30%, using themethod of Ruska et al.10 This result at 2.5 GPa is consistent withthose of previous works,1,11–14 which reveal a 3C-to-6H transitionat ⬎2000°C and 1 atm pressure.
On the other hand, the diffraction profiles of the samples treatedunder 4.5 and 6.5 GPa pressure and at temperatures up to 2200°C
are the same as that of the starting 3C form. This result isconsistent with the experimental data of Whitney and Shaffer,6
which indicate that the 3C form is stable in the pressure region
4.0 – 6.0 GPa and up to a temperature of 2000°C. Therefore, ourpresent results suggest that the temperature required to attain thetransformation from the 3C to the 6H form tends to increase withpressure.
Here we discuss the influence of impurities contained in our␤-SiC sample such as 0.09% of Al2O3 and 0.07% of Fe2O3. If wefollow the rule mentioned in an earlier section of this paper bothAl, which is electrically p-type material to SiC, and Fe, which isinsoluble to SiC, tend to stabilize 6H rather than 3C. Even if theresults described above are influenced by those impurities, therefore, higher stability of 3C-SiC under high pressure is still true.
Moreover, the concentration of those impurities was much lower
than those reported in the experiments which seemed to becontrolled by impurities.5,6(2) 6H 3 3C Transformation
The XRD peaks of 3C-SiC are common to all of the SiC
polytypes; hence, it is difficult to identify the 3C form in a mixed

Fig. 1. Powder X-ray diffraction profiles for 3C-SiC retrieved after
high-pressure treatment at 2200°C.

Vol. 84, No. 12

phase of 3C and 6H forms without taking into account the intensity
ratio of the diffraction peaks. Diffraction profiles of 6H samples
after high-pressure treatment under pressures of 2.5, 4.5, and 6.5
GPa at 2500°C are shown in Fig. 2. The intensities of those lines
are normalized with that of the (102) line for the 6H form, whichcorresponds to the (111) line of the 3C form. The unknown peakdesignated by an arrow in Fig. 2 is attributed to an aluminumsilicate produced by reaction of the SiC with the alumina tube used
to protect the thermocouple, because the peak was absent when noalumina tube was used. A comparison of the results with andwithout the alumina tube confirmed that the contamination had noeffect on the transformation of SiC.
For high-temperature treatment at 2500°C, the characteristic
diffraction lines of the 6H form, such as the 6H (101) and 6H (103)lines, broadened at 2.5 GPa, and the intensities of the lines at 4.5
and 6.5 GPa pressure were lower than those at 2.5 GPa. The otherdiffraction peaks appearing at higher diffraction angles, 6H (104)
and 6H (105), for instance, also broadened and weakened after thetreatment. There was no diffraction peak for SiC polytypes otherthan the 6H and 3C forms. These results suggest that the 6H form
partially transformed to the 3C form and that this transition
accelerated under higher pressure.
Figure 3 shows the results for various temperature treatments at
4.5 GPa. The diffraction peaks of the 6H form broadened withincreasing temperature, especially at 1000°–1900°C, and thensharpened at 2500°C. This broadening is attributed to “nonrandom
insertion of faults” during transformation of the 6H form to theother form, much the same as the transition of the 3C to the 6Hform pointed out by Sebastion and Krishna15 and Kabra et al.16Those researchers suggested that diffusion of the diffraction peaks
takes place during the process of transition, when stacking faults
are introduced preferentially into the crystal. Therefore, a series ofdiffraction profiles shows an intermediate state, characterized bybroad diffraction profiles, during transition from the 6H to theother more stable form, as a phenomenon before transition.
Similar broadening also was reported by Sokhor et al.5 andNakamura et al.,7 although they simply regarded it as a sign ofFig. 2. Powder X-ray diffraction profiles for 6H-SiC retrieved after
high-pressure treatment at 2500°C. Diffraction intensities are normalized
to the 6H (102), 3C (111) peak.

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Phase Relationship between 3C- and 6H-SiC

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treatment. This 793 cm⫺1 peak coincides with only one TO modeof the 3C form and occurs in the shoulder of the main peak of 788
cm⫺1 in a starting sample of the 6H form. However, the LO modeof the 3C form at 970 cm⫺1, which is the same as that of the 6Hform, is absent in the profile of the retrieved sample. This result
might be attributable to a defect in the crystal, because this Raman
peak also was absent in other sources of 3C-SiC produced atambient pressure.
As mentioned earlier, both the XRD results and the Ramanscattering measurements indicate that the 6H form tends totransform to the 3C form at high temperatures, ranging up to2500°C, under high pressure. The broadening of the XRD peaks attemperatures ⬎1000°C implies that the 6H form is a metastableform of SiC and starts transformation to the other form. This
broadening in the lower-temperature region can be interpreted as
the intermediate state of transition from 6H to 3C, 4H, 15R orother polytypes. However, it is a plausible interpretation that6H-SiC starts transformation to 3C-SiC at low temperature because no diffraction peak intrinsic to 4H, 15R, or other polytypes
was observed in the broadened XRD patterns and XRD andRaman-scattering showed the final product at 2500°C to be3C-SiC.
As for influence of impurities on the experiments started from
␣-SiC, chemical analysis showed that our ␣-SiC sample did notcontain a considerable amount of impurities and we believe theeffect of metals faintly contained in the sample is negligible.

Fig. 3. Powder X-ray diffraction profiles for 6H-SiC retrieved after
high-temperature treatment at 4.5 GPa. Diffraction intensities are normalized to the 6H (102), 3C (111) peak.

transition from the 6H to the 3C form. Because the broadening
decreased after treatment at higher temperature and resulted in thetransition to 3C-SiC in our work, those researchers observed thestarting process of the transition.
Figure 4 shows Raman-scattering data for samples retrieved
after high-pressure treatment at 2500°C, together with data for thestarting sample of the 6H form. The three TO modes of the 6Hform (768, 788, and 793 cm⫺1) are reduced to a single peak afterFig. 4. Raman spectrums for starting 6H-SiC and samples retrieved after
high-pressure treatment at 2500°C.

(3) Phase Relationship between 3C and 6H
The present experimental results for the phase relationshipbetween the 3C and 6H forms are summarized in Fig. 5. At
pressures of 2.5 GPa, the 3C form is stable at temperatures up to1900°C; above 1900°C, it transforms to the 6H form. In contrast,
the instability of the 6H form causes a broadening of the peaks in
the XRD profiles as the temperature increases above 1000°C. Attemperatures ⬎2200°C, the intermediate state characterized by thebroadening starts an inverse transformation to the 6H form, as itenters a stable p–T region of the 6H form. However, because thetransition proceeds slowly, the intermediate state remains at2500°C and 2.5 GPa. All of these experimental results suggest thatthe phase boundary of the 3C and 6H forms exists at a temperature
just below 2200° at 2.5 GPa and that the 3C form is a lowtemperature phase of SiC and the 6H form a high-temperaturephase.

Fig. 5. Summary of high-pressure and high-temperature experiments.
Black circle (square) indicates phase transition between 3C- and 6H-SiC;
open circle (square) indicates no characteristic change; shadow lined circle
indicates broadening in X-ray diffraction profile.

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Journal of the American Ceramic Society—Sugiyama and Togaya

On the other hand, at pressures of 4.5 and 6.5 GPa, the 3C form
is stable to 2500°C, and the transformation of the 6H to the 3Cforms, through the intermediate state, is accomplished at 2500°C.
These results indicate that the phase boundary has a positive slope
and that the stable region of the 3C form tends to extend to 2500°C
with increasing pressure.
IV.

Conclusions

The phase relationship between the 3C- and 6H-SiC in the p–T
field was examined by the phase analysis of samples recovered
after high-temperature and high-pressure treatment, using XRD
and Raman-scattering techniques.
3C-SiC transformed to 6H-SiC after treatment at 2.5 GPa and2200°C. At higher pressures, 4.5 and 6.5 GPa, this transformationwas not observed, and the 3C form was stable up to 2200°C. For
6H-SiC, the intensity of the 6H characteristic XRD peaks decreased after treatment at 2500°C under 4.5 and 6.5 GPa pressure,a result ascribed to transformation from the 6H to the 3C form.
This transformation also was confirmed by Raman-scatteringresults. In the temperature range 1000°–1900°C, the XRD peaks of
the 6H-SiC broadened, implying that the 6H form started a slow
transformation to the 3C form, by the introduction of stacking
faults.
The present experimental results led to the following conclusions:(1) 6H- and 3C-SiC have high- and low-temperature stableregions, respectively.
(2) The phase boundary between the 6H and the 3C forms
apparently occurs at just below 2200°C under 2.5 GPa pressureand has a positive slope.
Acknowledgments
We are grateful to Dr. M. Kobayashi for carrying out Raman scattering
measurement and also to Dr. M. Hirota for valuable discussion. We would like tothank anonymous reviewers who encouraged and helped improve this paper.

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