Silicon nitride:: Enthalpy of formation of the α- and β-polymorphs and the effect of C and O impurities

High-temperature oxidative drop solution calorimetry was used to measure the enthalpy of formation of alpha- and beta-Si3N4. Two different solvents, molten alkali berate (48 wt% LiBO2 52 wt% NaBO2) at 1043 and 1073 K and potassium vanadate (K2O 3V(2)O(5)) at 973 K, were used, giving the same results. Pure a- and beta-Si3N4 polymorphs have the same molar enthalpy of formation at 298 K of -850.9 +/- 22.4 and -852.0 +/- 8.7 kJ/mol, respectively. The unit cell dimensions of impure alpha-Si3N4 samples depend linearly on the O and C impurity contents, and so does the molar enthalpy of formation. The energetic stability of the alpha-Si3N4 phase decreases when the sample contains O and C impurities, The experimental evidence strongly suggests that the impurities dissolve into the alpha-Si3N4 structure to form a (limited) isostructural solid solution series but that this solid solution series is energetically less stable than a mechanical mixture of pure (alpha or beta) Si3N4, SiO2, and SiC. Thus, the alpha-phase is not stabilized by impurities and is probably always metastable.