Phase transition character and thermodynamic modeling of the [formula omitted] and [formula omitted] hexagonal Si–N system supplemented by first-principles calculations

Abstract

The phase boundaries of the β→P6¯′→δ transitions are investigated using first-principles calculations within the quasi-harmonic approximation. The agreement between the calculated properties of β-Si3N4 (benchmark system) and other results has proven the validity of our calculation. The lattice parameters, elastic constants and bulk moduli of P6¯′-, P6¯- and δ-Si3N4 are also analysed. At 300K, the transition pressures of the β→P6¯′→δ transitions are 40.0 and 53.1GPa, respectively. The predicted β→P6¯′→δ phase boundaries have positive slopes, hence, at higher temperatures it require higher pressures to synthesize P6¯′- and δ-Si3N4. It turns out that the bulk modulus and thermal expansion coefficient of the P6¯′ phase are greater than those of the P6¯ phase. The bulk moduli are high and only weakly temperature dependent. Moreover, the heat capacity, entropy, thermal expansion coefficient and Debye temperature are also analysed. The different temperature dependences of entropy between P6¯′-Si3N4 and P6¯-Si3N4 can be ascribed to the fact that large channels in P6¯-Si3N4 which allow for more vigorous thermal motion than in P6¯′-Si3N4.