Phase relations and equations of state of ZrO2 under high temperature and high pressure

The phase relations and pressure volume dependences of ZrO2 under high pressure and high temperature have been investigated by means of in situ observation using multianvil-type high-pressure devices and synchrotron radiation. By compression of 3-4 GPa, baddeleyite (monoclinic ZrO2) transforms to two distorted fluorite (CaF2)-type phases depending on temperature: an orthorhombic phase, orthoI, below 600 deg. C and a tetragonal phase above 600 deg. C. Both orthoI and tetragonal phases then transform into another orthorhombic phase, orthoII, with a cotunnite (PbCl2)-type structure above 12.5 GPa and the phase boundary is almost independent of temperature. OrthoII is stable up to 1800 deg. C and 24 GPa. The unit-cell parameters and the volumes of these high-pressure phases have been determined as functions of pressure and temperature. The orthoI/tetragonal-to-orthoII transition accompanies about 9% volume decrease. The thermal expansion coefficient of orthoII at 20 GPa is 2.052±0.003x10-5 K-1 over 25-1400 deg. C. The bulk modulus calculated using Birch-Murnaghan's equations of state is 296 GPa for orthoII, which suggests that the high-density ZrO2 is a candidate for potentially very hard materials. The phase relation of stabilized cubic ZrO2, CaO-ZrO2, under pressure at elevated temperature has also been examined. Distorted fluorite-type phases do not appear in CaO-ZrO2 but the direct transition from cubic phase to orthoII is observed on the same P-T conditions as in pure ZrO2