hP8-to-cP4 structural transition in Ni3In compounds at high temperature and pressure: Theoretical assessment of compression and phase stability information

According to Webb et al. (1986) by heating the Ni 3 In (hP8) compound at 800–1200 ° C under a pressure ( P ) of 6.5 GPa, a cP4 phase was formed, which reverted to hP8 when annealed at low pressure. A striking X-ray result is that the atomic volume ( V ) of cP4 was higher than that of hP8. Webb et al. determined the V versus P relations in samples quenched to room temperature and reported that the compressibility of the cP4 phase was significantly larger than that of the hP8 phase. These various findings have been theoretically analyzed in the current work by using ab initio density-functional-theory (DFT) calculations, including an account of the vibrational and electronic contributions to the thermodynamic properties. Calculations of the V versus P relations are used to reassess the compressibility relation between the cP4 and hP8 phases, and Gibbs energy calculations to characterize the relative stability of these structures. In particular, detailed comparisons are reported with the only information available on the thermal properties of the hP8 phase, viz., the Gibbs energy estimates obtained in CALPHAD-type phenomenological modeling of the Ni–In equilibrium diagram. The key qualitative result of the current work is that cP4 should be considered as a high-temperature phase, which might be stabilized by heating to the temperature chosen to anneal the hP8 material under pressure. On this basis it is suggested that the seemingly anomalous relative stability relations discussed by Webb et al. might be a consequence of considering only the effect of pressure on the cP4/hP8 relative phase stability.