Thermodynamics and phase stability in the In–N system

Results of a chemical vapor deposition crystal growth method intended to produce large amounts of InN needed for thermodynamic experiments are reported. Polycrystalline films of InN were grown by reaction of InCl₃ and NH₃ in a hot-wall silica reactor under nearly atmospheric pressure. Samples were analyzed using x-ray diffraction and chemical analysis. The decomposition of InN was studied in both thin film and powder form. InN films were investigated by isothermal heating under nitrogen and subsequent microscopic inspection. The removal of the nucleation barrier of forming the first liquid phase was emphasized. InN powder decomposition experiments involved two different customized thermogravimetric methods: (i) dynamic oscillation thermogravimetric analysis (TGA), and (ii) isothermal stepping TGA for a higher resolution of the decomposition start. The decomposition start was found consistently at (773 ±5) K under 1 bar of nitrogen. Nevertheless, it is suggested that InN may be metastable even below room temperature based on Computer Coupling of Phase Diagrams and Thermo chemistry-type thermodynamic analysis of all available phase equilibrium and thermodynamic data. This included the determination of the absolute entropy of InN, 31.6 ±3 J/mol-formula K, based on a Debye and Einstein analysis of the experimental data on the heat capacity. All calculations of pressure are corrected for the fugacity of nitrogen, which becomes crucial above 1000 bar. The contradictory literature data in the In–N system are discussed based on three different internally consistent thermodynamic analyses of the system that highlight the consequences of different choices made on the decomposition temperature of InN. Widely reproduced data in the literature are shown to produce thermodynamically impossible negative absolute entropy of InN. Complete P-T-x phase diagrams are given, which strongly suggest that solid InN is metastable under ambient conditions. To find out that InN crystals could be reproducibly superheated more than 500 K before they actually decompose comes as a surprise compared to other III-V systems, especially Ga–N.