Experimental and theoretical identification of a new high-pressure TiO2 polymorph.

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 ( 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 (TiO2, ZrO2, HfO2) on compression at ambient temperature all follow the common path: rutile -->alpha-PbO2-type --> baddeleyite-type (MI) --> orthorhombic OI (Pbca) structure --> cotunnite-type (OII).