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ThO 2  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 ThO 2  to ~60 GPa and ~2500 K. It undergoes a transition from the cubic fluorite-type structure (thorianite) to the orthorhombic α-PbCl 2  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), α K  T  = 0.0035(3)] and the high-pressure cotunnite-type phase [ V  0  = 24.75(6),  K  0  = 190(3), α K  T  = 0.0037(4)] are reported, holdingK  0′$\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.

High-pressure phase behavior and equations of state of ThO 2 polymorphs