Magnetic Properties of the Two Allotropic PuGa3 Phases

Abstract

PuGa3 crystallizes in either a trigonal structure type R-3m or in the hexagonal DO19 type P63 /mmc. The magnetic properties of both allotropes were investigated by magnetization, specific heat, and electrical resistivity measurements down to low temperatures and under magnetic fields and high pressures. Both phases order magnetically; the trigonal modification corresponds to a soft ferromagnet below TC=20 K with a saturated moment of 0.2 B/ Pu, whereas the hexagonal one exhibits antiferromagnetic order below TN=24 K which undergoes a metamagnetic transition at 0HM6.1 T at T=5 K. In the paramagnetic state, the effective magnetic moment inferred from a modified Curie-Weiss law amounts to eff0.78B for both phases, indicative of the occurrence of a Pu3+ charge state. The values of the electronic specific heat coefficient 110 and 220 mJ/mol K2 for the trigonal and hexagonal allotropes, respectively, indicate a moderate heavy fermion character. Comparisons with related compounds PuCoGa5, UGa3, and NpGa3 suggest a strong tendency toward 5f delocalization.