Phase transitions in several metal dioxides as studied by angle dispersive X-ray diffraction up to 50 GPa

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Abstract
The following dioxides TiO2, ZrO2, HfO2 and RuO2 have been studied up to 50 GPa in a diamond anvil cell by angle‐dispersive x‐ray powder diffraction at room temperature. Phase transitions were observed in all four compounds. Both zirconium and hafnium dioxides have baddeleyite type structures under ambient conditions and transform to several orthorhombic or tetragonal structures. No evidence for an increase in the coordination number beyond seven was found. Anatase‐TiO2 was found to undergo two phase transitions: the first to an α‐PbO2 type structure, and the second to a monoclinic structure which could be of the baddeleyite type. Rutile‐RuO2 transformed to an orthorhombic structure and then to a cubic, fluorite‐type structure. Several reflections were found to be common to all three phases of RuO2 indicating that the rutile to fluorite transformation mechanism can occur via diffusionless processes involving oxygen displacement in the xy plane of rutile. The highest pressure phases of these compounds are extremely incompressible with bulk moduli ranging from 399 to 553 GPa.

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