Thermodynamics of Ternary Nitride Formation by Ammonolysis: Application to LiMoN₂, Na₃WN₃, and Na₃WO₃N

An approximate method for predicting the feasibility of synthesis of ternary nitrides from appropriate oxides and ammonia gas is outlined. The known thermodynamic data for binary oxides and nitrides may be a helpful guide in predicting the formation of ternary nitrides by ammonolysis of ternary oxides. When the difference between the free energy of formation of the ternaries from the binary oxides (ΔG_f^b(O)) and binary nitrides (ΔG_f^b(N)) is small, the predictions are expected to be reliable. Such considerations suggest that ternary oxides containing the most electropositive metals (alkaline, alkaline earth, rare earth) will not form ternary nitrides by ammonolysis but perhaps will form oxynitrides or decompose to the electropositive metal oxide/hydroxide and binary transition-metal nitride. When the metals in the ternary oxide are from group V or greater, ternary nitride formation by reaction with ammonia is likely. We have developed a new high-temperature calorimetric procedure for determining the standard enthalpies of formation of ternary nitrides and applied it to: LiMoN₂, Na₃WN₃, and Na₃WO₃N. The standard enthalpies of formation: ΔH_f^o(LiMoN₂) = −386.0 ± 6.4 kJ/mol, ΔH_f^o(Na₃WN₃) = −358.7 ± 53.3 kJ/mol and ΔH_f^o(Na₃WO₃N) = −1358.8 ± 18.2 kJ/mol were obtained. An approximate method for predicting the feasibility of synthesis of ternary nitrides from appropriate oxides and ammonia gas is outlined. The known thermodynamic data for binary oxides and nitrides may be a helpful guide in predicting the formation of ternary nitrides by ammonolysis of ternary oxides. When the difference between the free energy of formation of the ternaries from the binary oxides (ΔG_f^b(O)) and binary nitrides (ΔG_f^b(N)) is small, the predictions are expected to be reliable. Such considerations suggest that ternary oxides containing the most electropositive metals (alkaline, alkaline earth, rare earth) will not form ternary nitrides by ammonolysis but perhaps will form oxynitrides or decompose to the electropositive metal oxide/hydroxide and binary transition-metal nitride. When the metals in the ternary oxide are from group V or greater, ternary nitride formation by reaction with ammonia is likely. We have developed a new high-temperature calorimetric procedure for determining the standard enthalpies of formation of ternary nitrides and applied it to: LiMoN₂, Na₃WN₃, and Na₃WO₃N. The standard enthalpies of formation: ΔH_f^o(LiMoN₂) = −386.0 ± 6.4 kJ/mol, ΔH_f^o(Na₃WN₃) = −358.7 ± 53.3 kJ/mol and ΔH_f^o(Na₃WO₃N) = −1358.8 ± 18.2 kJ/mol were obtained.