Ab initio study of Raman modes and thermodynamic properties of TinO2n−1

Abstract

Titanium suboxides with formula TinO2n−1 have attracted increasing attention owing to their widespread potential applications. Generally, the experimental samples of TinO2n−1 are a mixture of TiO2 suboxides, and the characterization of these phases is not solely possible using x-ray diffraction (XRD). In these cases, Raman modes with XRD can characterize these phases within the samples more precisely. Therefore, the investigation of Raman modes is important. On the other hand, the experimental thermodynamic study predicts that TinO2n−1 can be fabricated with a high index n, where n can take up to 100. So, the Raman modes and thermodynamic properties of these phases, nominally Ti2O3, Ti3O5, and Ti4O7, were investigated within the framework of density functional theory. Comparing the calculated and experimental Raman modes reveals that it is essential to consider the dipole–dipole interaction to reproduce the high Raman modes. Moreover, this electrostatic interaction can be responsible for stabilizing Ti3O5 polymorphs. According to the results, the bipolaronic bonds in Ti3O5 and Ti4O7 lead to a large Born effective charge, which makes these materials plausible for piezoelectric applications. Additionally, thermodynamic calculations reveal that as the n index in TinO2n−1 compounds increases, more stability is achieved at high temperatures, following the experiment. The stability in high temperatures suggests that TinO2n−1 phases can be suitable candidates for thermoelectric devices.