Experimental and Theoretical Identification of a New High-PressureTiO2Polymorph

Our combined theoretical and experimental investigations have led to the discovery of a new polymorph of titanium dioxide, where titanium is seven-coordinated to oxygen in the orthorhombic OI ( $\mathrm{Pbca}$) structure. The zero-pressure bulk modulus of the new phase measured in the pressure range 19 to 36 GPa is 318(3) GPa. We demonstrate that the group IVa dioxides ( ${\mathrm{TiO}}_2$, ${\mathrm{ZrO}}_2$, ${\mathrm{HfO}}_2$) on compression at ambient temperature all follow the common path: rutile $\to \alpha -{\mathrm{PbO}}_2$-type $\to$ baddeleyite-type (MI) $\to$ orthorhombic OI ( $\mathrm{Pbca}$) structure $\to$ cotunnite-type (OII).