Delocalisation of 5f electrons in berkelium and californium metals under pressure

Berkelium and californium were studied by X-ray diffraction up to pressures of 57 GPa (Bk) and 48 GPa (Cf). Participation of the 5f electrons in the metallic bonding, as evidenced by the appearance of the low-symmetry alpha -uranium structure and by a sudden volume decrease, starts at about 25 GPa in berkelium and at 41 GPa in californium.