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.