Phase relations and equation of state of ZrO2 to 100 GPa.

The phase relations and equation of state of ZrO2 were investigated up to 100 GPa by means of in situ observation using laser heating in a diamond anvil cell and synchrotron radiation. A cotunnite (PbCl2)-type phase, which appears above 12.5 GPa, is stable to a pressure of 100 GPa and a temperature of 2500 K. No post-cotunnite phase was observed under the present experimental conditions. The unit-cell parameters and the volumes of the cotunnite-type ZrO2 were determined as a function of pressure at room temperature using a laser-annealing technique. The cotunnite-type ZrO2 shows rather isotropic compression. The bulk modulus calculated using the Birch-Murnaghan equations of state is 278 GPa, which suggests that high-density ZrO2 is a candidate for potentially very hard materials. In situ high-temperature experiments performed below 12.5 GPa revealed that a tetragonal fluorite (CaF2)-type phase is stable up to 3000 K, although a cubic fluorite-type phase has been assumed to exist in this high-temperature regime. The result suggests the possibility that stoichiometric ZrO2 does not show the cubic structure up to the melting temperature