High-pressure phase behavior and equations of state of ThO2 polymorphs

ThO2 is 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 ThO2 to similar to 60 GPa and similar to 2500 K. It undergoes a transition from the cubic fluorite-type structure (thorianite) to the orthorhombic alpha-PbCl2 cotunnite-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 [V-0 = 26.379(7), K-0 = 204(2), alpha K-T = 0.0035(3)] and the high-pressure cotunnite-type phase [V-0 = 24.75(6), K-0 = 190(3), alpha K-T = 0.0037(4)] are reported, holding K-0(') 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.