High-pressure phase behavior and equations of state of ThO2polymorphs

ThO2is an important material for understanding the heat budget of Earth’s mantle, as well as the stability of nuclear fuels at extreme conditions. We measured the in situ high-pressure, high-temperature phase behavior of ThO2to ~60 GPa and ~2500 K. It undergoes a transition from the cubic fluorite-type structure (thorianite) to the orthorhombic α-PbCl2cotunnite-type structure between 20 and 30 GPa at room temperature. Prior to the transition at room temperature, an increase in unit-cell volume is observed, which we interpret as anion sub-lattice disorder or pre-transformation “melting” (Boulfelfel et al. 2006). The thermal equation of state parameters for both thorianite [V0= 26.379(7), K0= 204(2), αKT= 0.0035(3)] and the high-pressure cotunnite-type phase [V0= 24.75(6), K0= 190(3), αKT= 0.0037(4)] are reported, holding K0′$\begin{array}{} K_0^{'} \end{array} $fixed at 4. The similarity of these parameters suggests that the two phases behave similarly within the deep Earth. The lattice parameter ratios for the cotunnite-type phase change significantly with pressure, suggesting a different structure is stable at higher pressure.