Ternary neptunium and plutonium compounds MXY (X = P, As, Sb, Bi; Y = S, Se, Te)

We describe the preparation and X-ray characterization of neptunium and plutonium ternary compounds MXY (X = P,As,Sb,Bi; Y = S,Se,Te). From their powder patterns, all the compounds prepared were found to crystallize in the tetragonal system of anti-Fe2As type. The variations of the molar volumes of these compounds as a function of the actinide atomic number, are similar to those of the monochalcogenides and monopnictides. This analogy suggests that the variations of the molar volumes have to be interpretated by changes in the valency state of the actinides in these compounds (4+ for uranium and 3+ for neptunium and plutonium) and a progressive localization of the 5f electrons when going from uranium to plutonium. (author)