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

In the present study, - phase diagrams of ZrO and HfO 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 - conditions for a full sequence of high-pressure transformations, 2/ → → → 6̄2, for both compounds. At low temperatures, these transformations for ZrO are obtained at 7.6 GPa (2/ → ), 13.4 GPa ( → ), and 143 GPa ( → 6̄2), while for HfO similar polymorphic transitions are obtained at 9 GPa (2/ → ), 16 GPa ( → ), and 126 GPa ( → 6̄2), correspondingly. At high temperatures, for both ZrO and HfO the 2/ and structures transform into the 4/ modification. In addition, the thermal conductivity and elastic properties of the ZrO and HfO polymorphs were calculated and compared with the available experimental and theoretical data.