Phase relations and equations of state of ${\mathrm{ZrO}}_2$ under high temperature and high pressure

The phase relations and pressure volume dependences of ${\mathrm{ZrO}}_2$ 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 ${\mathrm{ZrO}}_2)$ transforms to two distorted fluorite $\left({\mathrm{CaF}}_2\right)$-type phases depending on temperature: an orthorhombic phase, orthoI, below $600°\mathrm{C}$ and a tetragonal phase above $600°\mathrm{C}.$ Both orthoI and tetragonal phases then transform into another orthorhombic phase, orthoII, with a cotunnite $\left({\mathrm{PbCl}}_2\right)-\mathrm{t}\mathrm{y}\mathrm{p}\mathrm{e}$ structure above 12.5 GPa and the phase boundary is almost independent of temperature. OrthoII is stable up to $1800°\mathrm{C}$ 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 $2.052\pm 0.003\times {10}^{-5}{\mathrm{K}}^{\mathrm{-}1}$ over $25–1400°\mathrm{C}.$ The bulk modulus calculated using Birch-Murnaghan’s equations of state is 296 GPa for orthoII, which suggests that the high-density ${\mathrm{ZrO}}_2$ is a candidate for potentially very hard materials. The phase relation of stabilized cubic ${\mathrm{ZrO}}_2,$ ${\mathrm{C}\mathrm{a}\mathrm{O}-\mathrm{Z}\mathrm{r}\mathrm{O}}_2,$ under pressure at elevated temperature has also been examined. Distorted fluorite-type phases do not appear in ${\mathrm{C}\mathrm{a}\mathrm{O}-\mathrm{Z}\mathrm{r}\mathrm{O}}_2$ but the direct transition from cubic phase to orthoII is observed on the same P-T conditions as in pure ${\mathrm{ZrO}}_2.$