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: $\mathrm{Th}{\mathrm{O}}_2$: $B_0=198\left(2\right)\mathrm{GPa}$ and $B_0^{\prime }=4.6\left(3\right)$; $\mathrm{U}{\mathrm{O}}_2$: $B_0=207\left(2\right)\mathrm{GPa}$ and $B_0^{\prime }=4.5\left(4\right)$. In the case of $\mathrm{U}{\mathrm{O}}_2$ the ambient pressure cubic phase was still found to be present at $69\mathrm{GPa}$, 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.

• Received 17 February 2004

DOI:https://doi.org/10.1103/PhysRevB.70.014113