Crystal chemistry of transuranium pnictides

The synthesis and crystal chemistry of M transuranium element pnictides (M = Np, Pu, Am and Cm) are described. Msub(a) Xsub(b) binary compounds have been prepared (X = P, As, Sb, and Bi) with the following (a:b) stoechiometric ratios: (1:2), (3:4), (1:1), (4:3). MXY and M₂XY₂ ternary compounds (Y = O, S, Se and Te) have also been prepared. The crystal structure encountered in these compounds of Pu, Am and Cm is generally the same as in the corresponding rare-earth compounds. Thorium and uranium compounds were grouped in a distinct set where the actinide ion was considered as tetravalent. Neptunium exhibits an intermediate behaviour and was found to give either uranium-like compounds such as NpAs₂, or rare-earth like compounds such as NpSn₂. From the plots of the lattice constants versus the actinide atomic number, we could infer that the 5f electrons are likely to be delocalised in the compounds; no 5f - shell contraction seems to occur. Furthermore these results lead to assume that in the isostructural diantimonides of the light rare-earths, 4f wave-fonctions must be involved in the bonds just; like in the LnSbTe ternary compounds. As the f electrons delocalization is dependent on crystal structure, the various structures found in our system have been studied.