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

It is known that substitution of a small fraction ofTi atoms inTi(3)O(5) by aliovalent metal atoms alters its heat-storageproperties, i.e., beta -> lambda phase-transition enthalpyand temperature, and extends its range of application to, for example,the capture and utilization of waste heat. Heat-storage propertiesvary depending on the substituting element and its concentration.The exploration of the vast space of possible combinations of thesequantities is challenging due to the small energy differences underconsideration. Thus, computational approaches which reliably predictheat-storage properties of such defective systems can aid in the screeningof potential material candidates for subsequent synthesis. Substitutedcompounds M x Ti3-x O5 with trivalent M = Sc, Al, and Mg have been reportedin the literature. Here we present the first thorough study of thedoping effect on the electronic structure and the heat-storage propertiesof Ti3O5 from first principles. Electronic groundstates were calculated for all Ti-M substitution positionsusing the M06 hybrid functional. Doping leads to increased metalliccharacter in both phases, which primarily results from a change inatomic positions and not from the substituting element itself. Weapplied the r(2)SCAN-D3 method to the study of heat-storageproperties of those materials and found good agreement in phase-transitionenthalpies with experimentally recorded data. Calculated phase-transitionentropies show larger deviations from experiment. The phase-transitionmechanism is studied as a function of the defect concentration bycalculating the minimum-energy path. Doping primarily changes therelative energy of both phases and leaves the activation barrier virtuallyunchanged. Our results suggest that heat-storage systems of this kindare efficient only for M concentrations below 4 atom %.