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

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Abstract
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−Si3N4phase 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.