Inert structural transition in 4H and 6H SiC at high pressure and temperature: A Raman spectroscopy study

We conducted Raman spectroscopy measurements of 4H-SiC and 6H-SiC up to 69 GPa and 1023 K to assess the stability and bonding of SiC at high pressure and temperature. Both optic and acoustic modes were observed at wide pressure and temperature ranges for the first time. The temperature shifts of the Raman frequencies were fitted by the equation with the Bose-Einstein distribution function, and we found that the shifts were almost insensitive to the pressure. The mode Grüneisen coefficients weakly depend on the pressure and temperature, suggesting the sluggish transition of the crystal structure, unlike the previous experiments showing the transition or decomposition of SiC at high pressure and temperature conditions. Inert transitions are confirmed by Raman measurements and annealing experiments using multiple high-pressure apparatuses. The crystallinity may be a hidden critical parameter in the experiments to determine the stable polytypes of SiC under high pressure and temperature.