A high-pressure high-temperature X ray study of phase relations and polymorphism of HfO2

Abstract

X ray diffraction experiments have been carried out to study the two pressure-induced phase transitions in HfO 2 . Angle-dispersive experiments were performed at room temperature in a diamond anvil cell using synchrotron radiation and imaging plates. Experiments at high temperature were performed with a multianvil device using the energy dispersive technique and a rotating-anode generator with a solid state detector. It was found that the monoclinic-orthorhombic I (ortho I) transition occurred around 5 GPa with a volume reduction of 3.4% and the ortho I - ortho II transition occurred at 26 GPa with a volume reduction of 9.4%. The phase boundary between the ortho I and ortho II phases above 400°C is expressed approximately by a linear equation, T(°C)=4720-300P(GPa). The kinetic limit curves gradually towards the higher pressure side at lower temperatures because of the sluggishness of the transition. The ortho II phase was identified as the cotunnite-type structure for both at high pressure and quenched samples.