Alkali-metal vanadium sulfate β alums, MV(SO4)2·12H2O (M = K, Rb or Cs): structural anomalies related to unsymmetrical occupancy of the t2g(Oh) orbitals

Abstract

Crystal structure determinations of the alums MV(SO₄)₂·12H₂O (M = K, Rb or Cs) showed that they all belong to the β structural class. This observation, while predicted earlier from the crystal morphology, is not expected from trends based on the sizes of the monovalent cations and results in formal 12-co-ordination for K⁺ and Rb⁺. The structures were determined at 120 (K) and 295 (Rb and Cs) K and refined to R values of 0.025, 0.040 and 0.022 for 877, 348 and 2008 independent ‘observed’ reflections, respectively. The crystals are cubic, space group Pa with a= 12.253(5), 12.367(2) and 12.434(1)Å, respectively. Despite the relatively imprecise determination of the hydrogen-atom positions by X-ray diffraction, analysis of the available neutron structures indicates that the water co-ordination geometry may be deduced with good precision from the heavy-atom coordinates. On this basis the stereochemistry of water co-ordination to M^III is conserved in the three structures. The occurrence of the β structure for these salts is likely to be due to a preferred stereochemistry for water co-ordination to vanadium(III) which arises from lifting of the degeneracy of the ³T_2g(O_h) ground term.