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 (TiO 2 , ZrO 2 , HfO 2 ) on compression at ambient temperature all follow the common path: rutile → α-PbO 2 -type → baddeleyite-type (MI) → orthorhombic OI (Pbca) structure → cotunnite-type (OII).