LOCALIZATION OF 5F ELECTRONS IN DELTA-PLUTONIUM - EVIDENCE FOR THE KONDO EFFECT

Among the actinides, plutonium is at the critical position between lig ht actinides (up to plutonium) with itinerant 5f electrons and the hea vy actinides (from americium on) with localized 5f electrons. The chan ge of behaviour has been described by Johansson (Philos. Mag., 30 (197 41 469) as a Mott-like transition of the 5f electrons between Pu and A m. However, in plutonium itself there is a huge volume expansion of 26 % between the low temperature low symmetry alpha-phase and the high te mperature high symmetry delta-phase. From the 20% expansion on going f rom alpha-Ce to gamma-Ce, we infer a more important localization of 5f electrons in delta-Pu compared with the situation in alpha-Pu. Low te mperature susceptibility measurements and high temperature resistivity measurements are analysed within a Kondo-like model of localized 5f e lectrons. High temperature susceptibility measurements of stabilized d elta-Pu alloys have been made for the first time and it is found, in a greement with previous data and model, that a Curie-Weiss behaviour is recovered. This adds strong support to the hypothesis of 5f localizat ion in delta-plutonium: the delocalization-localization transition of the 5f electrons along the actinide series starts in fact within the p lutonium phase diagram, as also proposed by Antropov et al. (Sov. Phys . Solid State, 32 (1990) 1612) from band structure calculations.