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 ~60 GPa and ~2500 K. It undergoes a transition from the cubic fluorite-type structure (thorianite) to the orthorhombic α-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” (<xref>Boulfelfel et al. 2006</xref>). The thermal equation of state parameters for both thorianite [<italic>V</italic>₀ = 26.379(7), <italic>K</italic>₀ = 204(2), α<italic>K</italic>_T = 0.0035(3)] and the high-pressure cotunnite-type phase [<italic>V</italic>₀ = 24.75(6), <italic>K</italic>₀ = 190(3), α<italic>K</italic>_T = 0.0037(4)] are reported, holding K 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. [ABSTRACT FROM AUTHOR]