Behavior of actinide dioxides under pressure:: UO2 and ThO2 -: art. no. 014113

The structural high-pressure properties of ThO2 and UO2 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: ThO2: B-0=198(2) GPa and B-0(')=4.6(3); UO2: B-0=207(2) GPa and B-0(')=4.5(4). In the case of UO2 the ambient pressure cubic phase was still found to be present at 69 GPa, which is in contradiction with earlier measurements. With regards to these results and re-calculations performed on other actinide dioxides PuO2 and AmO2, we obtain a different evolution of the bulk moduli through the actinide dioxide series.