Doping Effect on the Electronic Structure and Heat-Storage Properties of Ti3O5

It is known that substitution of a small fraction of Ti atoms in Ti3O5 by aliovalent metal atoms alters its heat-storage properties, i.e., β → λ phase-transition enthalpy and temperature, and extends its range of application to, for example, the capture and utilization of waste heat. Heat-storage properties vary depending on the substituting element and its concentration. The exploration of the vast space of possible combinations of these quantities is challenging due to the small energy differences under consideration. Thus, computational approaches which reliably predict heat-storage properties of such defective systems can aid in the screening of potential material candidates for subsequent synthesis. Substituted compounds MxTi3–xO5 with trivalent M = Sc, Al, and Mg have been reported in the literature. Here we present the first thorough study of the doping effect on the electronic structure and the heat-storage properties of Ti3O5 from first principles. Electronic ground states were calculated for all Ti–M substitution positions using the M06 hybrid functional. Doping leads to increased metallic character in both phases, which primarily results from a change in atomic positions and not from the substituting element itself. We applied the r2SCAN-D3 method to the study of heat-storage properties of those materials and found good agreement in phase-transition enthalpies with experimentally recorded data. Calculated phase-transition entropies show larger deviations from experiment. The phase-transition mechanism is studied as a function of the defect concentration by calculating the minimum-energy path. Doping primarily changes the relative energy of both phases and leaves the activation barrier virtually unchanged. Our results suggest that heat-storage systems of this kind are efficient only for M concentrations below 4 atom %.