Thermodynamic modeling of YO1.5-TaO2.5 system and the effects of elastic strain energy and diffusion on phase transformation of YTaO4

Thermodynamic parameters of the YO1.5-TaO2.5 system were obtained, and the effects of elastic strain energy and diffusion on phase transformation of YTaO4 were analyzed in this work. The YO1.5-TaO2.5 system was critically modeled using the CALPHAD technique based on our calculated formation energies by DFT and available experimental data. According to DFT calculations, M'-YTaO4 was suggested to be the thermodynamically stable phase at low temperature. For a displacive transformation of T -> M between the equilibrium tetragonal and monoclinic YTaO4, our calculations suggested it cannot be hindered by elastic strain energy. For a diffusive transformation of T -> M', it can be divided into T -> M and M -> M'. Diffusive transition of M -> M' was likely to be impeded due to large diffusion energy barrier, which was calculated to be 3.260 eV. However, the driving force Delta G (M -> M') is about -0.121 kJ/mol. The large diffusion energy barrier and small driving force may be the main reason that T cannot transform to M' after cooling.