Ionic interactions in actinide tetrahalides

We determine a model of the ionic interactions in AX(4) compounds (where A is an atom in the actinide series from Th to Am and X = f, Cl, Br or I) by an analysis of data on the static and dynamic structure of their molecular monomers. The potential energy function that we adopt is taken from earlier work on rare-earth trihalides [Z. Akdeniz, Z. Cicek and M. P. Tosi, Z. Nat urforsch. 55a, 861 (2000)] and in particular allows for the electronic pola rizability of the actinide ion. This polarizability quantitatively determin es the antisymmetric-bending vibrational mode, but its magnitude remains co mpatible with a symmetric tetrahedral shape of the molecule at equilibrium. The fluorides have an especially high degree of ionic character, and the i nterionic-force parameters for each halide of the U, Np, Pu and Am series s how regular trends, suggesting that extrapolations to the other transuranic -element halides may usefully be made. The Th compounds show some deviation s from these trends, and the interionic-force model that we determine for T hCl4 differs somewhat from that obtained in a previous study. We therefore return on the evaluation of the relative stability of charged oligomers of ThCl4 and ZrCl4 and find confirmation of our earlier results on this proble m.