Phase relations, thermal conductivity and elastic properties of ZrO 2 and HfO 2 polymorphs at high pressures and temperatures.

In the present study, P - T phase diagrams of ZrO 2 and HfO 2 for a wide pressure range of 0-150 GPa at 0-2500 K were calculated for the first time using density functional theory with the method of lattice dynamics within the quasi-harmonic approximation. We calculated P - T conditions for a full sequence of high-pressure transformations, P 2 1 / c → Pbca → Pnma → P 6̄2 m , for both compounds. At low temperatures, these transformations for ZrO 2 are obtained at 7.6 GPa ( P 2 1 / c → Pbca ), 13.4 GPa ( Pbca → Pnma ), and 143 GPa ( Pnma → P 6̄2 m ), while for HfO 2 similar polymorphic transitions are obtained at 9 GPa ( P 2 1 / c → Pbca ), 16 GPa ( Pbca → Pnma ), and 126 GPa ( Pnma → P 6̄2 m ), correspondingly. At high temperatures, for both ZrO 2 and HfO 2 the P 2 1 / c and Pbca structures transform into the P 4 2 / nmc modification. In addition, the thermal conductivity and elastic properties of the ZrO 2 and HfO 2 polymorphs were calculated and compared with the available experimental and theoretical data.