Anharmonic entropy and thermal expansion coefficient of SiC polytypes

Silicon carbide polytypes have been extensively studied for decades. However, their relative thermodynamic stability remains controversial due to persistent discrepancies between theoretical predictions and experimental observations. A key missing contribution in previous studies is the role of anharmonic vibrational entropy in determining the free energy at elevated temperatures. Here, we employ molecular dynamics simulations based on a machine-learned and an empirical interatomic potential to investigate the thermodynamic properties of SiC polytypes. The total entropy is evaluated using thermodynamic integration and decomposed into harmonic and anharmonic contributions. The relative stability is analyzed in terms of potential energy, harmonic free energy, and total free energy. When anharmonic effects are included, 3C-SiC is found to be the most stable polytype, while its relative stability with respect to the hexagonal polytypes decreases with increasing temperature, consistent with experimental trends. In addition, the anharmonic contribution is correlated with the thermal expansion coefficient, which is calculated along both the a- and c-axes over a wide temperature range. The calculated thermal expansion coefficients agree well with available low-temperature experimental data and provide predictions in the high-temperature regime relevant to crystal growth, where measurements are currently limited. These results establish a link between anharmonic entropy and thermal expansion, providing a unified thermodynamic framework for understanding the temperature-dependent stability of SiC polytypes.