Two new Np-Ga phases: α-NpGa2 and metastable m-NpGa2

Following an earlier study of metastable Np-rich Np-Ga alloys, rapidly quenched Np-Ga alloys with 63-80 at.% Ga were prepared and studied. Two new NpGa2 phases, both with an AlB2 type structure, were found: α-NpGa2, with a=4.246A, c=4.060A, c/a=0.956 and m-NpGa2, with a=4.412A, c=3.642A, c/a=0.825. While m-NpGa2 was observed only in very fast quenched (splat cooled) samples and appears to be metastable, α-NpGa2 is probably an equilibrium phase. In a splat cooled alloy with 75 at.% Ga, another, unidentified, metastable phase was observed. Crystal chemical discussions of atomic volumes, interatomic distances and axial ratios are given; the volume difference between the two forms of NpGa2 is correlated with a valence change of Np. (Auth.)