The structural phase transition and phase stability of TiO2 (P42/nmc) under high pressure from first-principles calculations

TiO2 (P42/nmc) structural phase transition and phase stability are investigated by first principles based on density functional theory. It is observed that the columbite can directly change to a tetragonal P42/nmc structure above 45.8 GPa for GGA and 50.1 GPa for GGA + U. A new TiO2 polymorph with P42/nmc structure is proposed, and its elastic constants and phonon dispersions are discussed. It is seen that P42/nmc structure is mechanically stable above 30 GPa, and satisfies the dynamical stability at 60 GPa. We also analyze the XRD patterns and the equations of state for related TiO2 polymorphs. This paper concludes with a discussion that fluorite and P42/nmc may be coexisting phases, which is in accordance with the cubic TiO2 experimental result.