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TOTAL-ENERGY CALCULATIONS; AB-INITIO; STABILITYChemistry; Science & Technology - Other Topics; Materials Science;
Abstract:
Transformation of the metastable lambda-phase to the stable beta-phase of Ti3O5 is triggered by external pressure, irradiation, or electric current. Recent investigations of the photoinduced phase transition revealed a selection rule according to which the photoinduced phase transformation only proceeds when the pump pulse is applied to the ab plane. In the present study, possible reasons for this phenomenon are investigated theoretically. We calculate the relative free energy of the beta-phase, transition state, and lambda-phase under external pressure at the density functional theory (DFT) level. In light of the experimentally proposed selection rule, we investigate the phase transition process in more detail by considering the pressure-dependent formation and propagation of a beta-phase front in lambda-Ti3O5 in a large supercell. This mixed-phase model inherently features an interface region, which connects both phases. We investigate several lattice planes as possible interfaces and their energetic contribution to the phase transition process. Compared to traditional solid-state nudged elastic band calculations, this approach offers a more realistic model of the phase transition by incorporating the gradual conversion of the phases with differing ratios of beta:lambda-phase fractions and volume change. These simulations featuring extended supercells necessitate the use of machine learned potentials, which, in our case, employ r(2)SCAN-D3 as a reference method. Our approach reveals significant anisotropy in the energetic pathways and confirms that phase transitions in Ti3O5 involving the ab interface are energetically favored, which offers a rationalization of the experimental findings. We analyze exemplary acoustic-like phonon modes and find that the pressure effect on the phase transition is rooted in the softening of these modes, which distort the lambda structure toward the transition state.
