Novel high/ultrahigh pressure structures of TiO2 with low band gaps

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The high photoactivity of titanium dioxide at UV region has attracted ample attention over recent. The present work aims to predict highly photoactive TiO2 crystal structures under high/ultrahigh pressure (up 300 GPa). Global minimizations of rutile TiO2 band gap were performed using first-principles calculations combined with the evolutionary algorithm USPEX (Universal Structure Prediction: Evolutionary Xtallography). Among several phases predicted, three semiconductors with low band gaps were selected to study their structural, thermodynamic, dynamical, and electronic properties. These phases show new symmetry (P4/nmm), coordination number up to nine, and the lowest enthalpy of formation values (until −9.27 eV) ever reported to TiO2 compounds. In order to confirm the USPEX efficiency, a comparison between band gaps obtained by GGA, GGA + U, and HSE06 calculations was performed. Relative stability and bulk modulus of dynamically stable phases are also investigated. Finally, our calculations reveal that R-semiconductor-300 exhibits the lowest direct band gap predicted for TiO2 to date