Delocalisation of 5f electrons in curium metal under high pressure

Abstract
Curium metal was studied by X-ray diffraction up to 52 GPa. Starting with the double-hexagonal close-packed structure (Cm I), a cubic close-packed structure (Cm II) was formed at 23 GPa and another high-pressure phase (Cm III) at 43 GPa. The latter phase can be indexed as an orthorhombic cell of the alpha -uranium-type structure. The volume difference between the cell of Cm III and that of Cm II, together with the formation of a low-symmetry (orthorhombic) crystal structure, suggests that itineracy of the 5f electrons in curium metal starts at 43 GPa. The fact that the delocalisation pressure is higher in curium than in the neighbouring americium, berkelium and californium metals is explained qualitatively on the basis of a comparison of localisation energies.

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