Behaviour of Actinide Dioxides under Pressure: UO2 and ThO2

The structural high-pressure properties of $\mathrm{Th}{\mathrm{O}}_{2}$ and $\mathrm{U}{\mathrm{O}}_{2}$ have been studied in diamond-anvil cells up to maximum pressures of 80 and 69 GPa, respectively. These results show that both thorium and uranium dioxides exhibit an identical sequence of structural transitions under pressure; both transform rather sluggishly to the cotunnite-type (orthorhombic $Pnma$) high-pressure structure. This study which was performed under better hydrostatic conditions than previous experiments has enabled us to determine reliable compressibility parameters for the two dioxides: $\mathrm{Th}{\mathrm{O}}_{2}$: ${B}_{0}=198(2)\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and ${B}_{0}^{\ensuremath{'}}=4.6(3)$; $\mathrm{U}{\mathrm{O}}_{2}$: ${B}_{0}=207(2)\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and ${B}_{0}^{\ensuremath{'}}=4.5(4)$. In the case of $\mathrm{U}{\mathrm{O}}_{2}$ the ambient pressure cubic phase was still found to be present at $69\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, which is in contradiction with earlier measurements. With regards to these results and re-calculations performed on other actinide dioxides $\mathrm{Pu}{\mathrm{O}}_{2}$ and $\mathrm{Am}{\mathrm{O}}_{2}$, we obtain a different evolution of the bulk moduli through the actinide dioxide series.