Influence of vacancies on phonons and pressure phase transitions in TiO: a DFT study

Abstract: Since recently, some interests appeared in TiO as a thin film coating material due to its promising physical properties. Moreover, because of the existence of intrinsic vacancies, TiO demonstrates memristive properties. To use these compounds in active region for memristive switching, the investigation of the dynamical stability of these compounds is of importance. Therefore, the structural, electronic, and dynamical properties of TiO in two different phases, namely cubic and monoclinic structures along with vacancies, are debated from the framework of density functional theory. The structural calculations show that the vacancies have quite an impact on the bulk modulus of these compounds. The electronic band structure remarks that these structures are metallic. In addition, there is an energy gap between O-2p and Ti-3d states in these compounds below the Fermi level that can be altered via vacancy concentration. The phonon calculations reveal that monoclinic structure is dynamically stable whereas the cubic phase is unstable at room temperature. Considering ~12.5% of both Ti and O vacancy concentration, eliminate the imaginary modes for the cubic structure. Moreover, the vacancy-free cubic structure can be stable above ~ 10 GPa applied pressure, which is consistent with the experiment. Additionally, phonon dispersion curves of TiOx compounds suggest that these materials are not favorable for thermal conductivity. The pressure phase transition results indicate the transition from monoclinic to vacant cubic TiO at ~8 GPa, whereas the monoclinic to vacancy-free cubic TiO transition occurs at ~ 44 GPa, which are in a good agreement with the recent reports. Therefore, it can be concluded that engineering of the vacancy sites and their concentration in TiO phases can play an important role in applying them in technological applications.