Delocalisation of 5f electrons in berkelium-californium alloys under pressure

The high-pressure structural behaviour of two berkelium-californium alloys was studied up to about 47 GPa. The alloys had been formed through the ingrowth of 249Cf into 249Bk by beta - decay. The berkelium-rich alloy (about 35% Cf) exhibited the same phases as pure berkelium metal, the californium-rich alloy (about 60% Cf) the same phases as pure californium. An approximately linear increase of transition pressures with californium content was observed. The results are compared with data obtained previously for the high-pressure behaviour of the pure americium, curium, berkelium and californium metals. As in the pure metals, the transformation of the alloys to the alpha-uranium-type structure, accompanied by a sudden volume decrease, is interpreted as the onset of 5f electron itinerancy.