  Titanium dioxide (TiO2) is a wide band gap semiconducting material which is
promising for photocatalysis. Here we present first-principles calculations to
study the pressure dependence of structural and electronic properties of two
TiO2 phases: the cotunnite-type and the Fe2P-type structure. The band gaps are
calculated using density functional theory (DFT) with the generalized gradient
approximation (GGA), as well as the many-body perturbation theory with the GW
approximation. The band gaps of both phases are found to be unexpectedly robust
across a broad range pressures. The corresponding pressure coefficients are
significantly smaller than that of diamond and silicon carbide (SiC), whose
pressure coefficient is the smallest value ever measured by experiment. The
robustness originates from the synchronous change of valence band maximum (VBM)
and conduction band minimum (CBM) with nearly identical rates of changes. A
step-like jump of band gaps around the phase transition pressure point is
expected and understood in light of the difference in crystal structures.
