Influence of the formal electric charge on the size of the transition metal atom cluster in YRh4B4, YRu4B4 and PbMo6S8 related compounds

A single-crystal x-ray study of the superconducting compounds MRh4B4 (M = Y, Th), Y (Ru/sub 1-x/Rh/sub x/)4B4 and M Mo6S8 (M = Ag, Cu, Pb, Sn, In, Rare Earths) shows that the size of the T atom cluster (T = transition metal) changes as a function of its formal charge. An increase of the formal charge by one electron leads to an expansion of the T4 tetrahedron by about 1% in the borides and to a contraction of the T6 octahedron by about 2% in the chalcogenides. The relative displacements of the T atoms due to this electronic effect are of the same order of magnitude as their r.m.s. amplitudes of thermal vibrations at low temperatures