Effect of Surface Free Energies and Particle Diameter on the Ti₃O₅ β → λ Phase Transition Temperature: A Theoretical Study

The solid–solid heat-storage material Ti3O5 is experimentally known to store thermal energy for long time periods and to release the accumulated energy on demand. During the heat-storage process the lower-energy β-phase is transformed into the higher-energy λ-phase, which is retained at ambient conditions, unless external pressure is applied. Therefore, the metastability of the λ-phase is a crucial aspect of the heat-storage system. In previous theoretical studies it was shown that the hysteresis cannot be explained by conventional bulk models. Here, we present a comprehensive theoretical study of the surfaces of both Ti3O5 phases in order to demonstrate the effect of the surface energy on the relative phase stability. We apply the r2SCAN-D3 method to the calculation of surface free energies and show that most λ-phase facets are more stable than the corresponding β-phase surfaces. On the basis of calculated surface free energies we predict the temperature dependency of the crystallite morphology and estimate the size-dependent relative free energy of β- and λ-particles. We show that for particles in the experimentally synthesized diameter range the inclusion of surface effects has a substantial effect on the theoretical prediction of phase transition temperatures.