Localization of 5f electrons in delta-plutonium: Evidence for the Kondo effect

Among the actinides, plutonium is at the critical position between light actinides (up to plutonium) with itinerant 5f electrons and the heavy actinides (from americium on) with localized 5f electrons. The change of behaviour has been described by Johansson (Philos. Mag., 30 (19741 469) as a Mott-like transition of the 5f electrons between Pu and Am. However, in plutonium itself there is a huge volume expansion of 26% between the low temperature low symmetry alpha-phase and the high temperature high symmetry delta-phase. From the 20% expansion on going from 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 temperature susceptibility measurements and high temperature resistivity measurements are analysed within a Kondo-like model of localized 5f electrons. High temperature susceptibility measurements of stabilized delta-Pu alloys have been made for the first time and it is found, in agreement with previous data and model, that a Curie-Weiss behaviour is recovered. This adds strong support to the hypothesis of 5f localization in delta-plutonium: the delocalization-localization transition of the 5f electrons along the actinide series starts in fact within the plutonium phase diagram, as also proposed by Antropov et al. (Sov. Phys. Solid State, 32 (1990) 1612) from band structure calculations.