Pressure-induced phase transition in ThO2and PuO2

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Abstract
Thorium and plutonium dioxides were studied under pressure by the energy dispersive X-ray diffraction method. A double conical slit assembly was used to collect simultaneously the diffracted radiation at five and seven degrees. ThO2 undergoes a phase transformation at 40 GPa. The high-pressure phase remains stable up to 55 GPa, the highest pressure reached in the experiment. For PuO2, a structural transformation occurs near 39 GPa. The observed high-pressure phases of ThO2 and PuO2 exhibit similar diffraction spectra. Like for some other fluorite type compounds, the ThO2 and PuO2 high-pressure phase has been indexed in the PbCl2-type structure. The bulk modulus has been calculated as B0= 262 GPa with a pressure derivative of B0' = 6.7 for ThO2 and as B0 = 379 GPa with B0' = 2.4 for PuO2. The volume decrease at the transition is 12% for PuO2 and 8% for ThO2.