Phase relations and equations of state of ZrO2 under high temperature and high pressure - art. no. 174108

The phase relations and pressure volume dependences of ZrO2 under high pres sure and high temperature have been investigated by means of in situ observ ation using multianvil-type: high-pressure devices and synchrotron radiatio n. By compression of 3-4 GPa, baddeleyite (monoclinic ZrO2) transforms to t wo distorted fluorite (CaF2)-type phases depending on temperature: an ortho rhombic phase, orthoI. below 600 degreesC and a tetragonal phase above 600 degreesC. Both orthoI and tetragonal phases then transform into another ort horhombic 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 degreesC 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 acco mpanies about 9% volume decrease. The thermal expansion coefficient of orth oII at 20 GPa is 2.052+/-0.003 x 10(-5) K-1 over 25-1400 degreesC. The bulk modulus calculated using Birch-Murnaghan's equations of state is 296 Cpa f or orthoII, which suggests that the high-density ZrO2 is a candidate for po tentially very hard materials. The phase relation of stabilized cubic ZrO2, CaO-ZrO2, under pressure at elevated temperature has also been examined. D istorted fluorite-type phases do not appear in CaO-ZrO2, but the direct tra nsition from cubic phase to orthorII is observed on the same P-T conditions as in pure ZrO2.