Low temperature heat capacity measurements of β-Si3N4 and γ-Si3N4: Determination of the equilibrium phase boundary between β-Si3N4 and γ-Si3N4

Isobaric heat capacities of beta-Si3N4 and gamma-Si3N4 were measured at temperatures between 1.8 and 309.9 K with a thermal relaxation method. The measured heat capacities of gamma-Si3N4 are smaller than those of beta-Si3N4 in this temperature range. Using these data, we determined the standard entropies of beta-Si3N4 and gamma-Si3N4 to be 62.30 Jmol(-1) K-1 and 51.79 Jmol(-1) K-1, respectively. The equilibrium phase boundary between beta-Si3N4 and gamma-Si3N4 was calculated using these values and thermodynamic parameters reported in previous studies. The obtained equilibrium phase transition pressure at 2000 K is 11.4 GPa. It is lower than the experimental pressures at which gamma-Si3N4 was synthesized in previous studies. The calculated Clapeyron slope at this temperature is 0.6 MPa K-1, which is consistent with those of theoretical studies.