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 α- and β-Si₃N₄. Two different solvents, molten alkali borate (48 wt% LiBO₂ · 52 wt% NaBO₂) at 1043 and 1073 K and potassium vanadate (K₂O · 3V₂O₅) at 973 K, were used, giving the same results. Pure α- and β-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 α-Si₃N₄ samples depend linearly on the O and C impurity contents, and so does the molar enthalpy of formation. The energetic stability of the α-Si₃N₄ phase decreases when the sample contains O and C impurities. The experimental evidence strongly suggests that the impurities dissolve into the α-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 (α or β) Si₃N₄, SiO₂, and SiC. Thus, the α-phase is not stabilized by impurities and is probably always metastable.