The Atomic Arrangement of the Sherwoodite Structural Unit: Redefinition of the Sherwoodite Vanadoaluminate Heteropoly Complex as [AlV4+V5+12O39]11−

Abstract

Abstract Sherwoodite, putatively Ca4.5(AlV5+12V4+2O40)·28H2O, was among the early naturally occurring polyoxometalates to be reported from the Uravan Mineral Belt; the ideal formula of sherwoodite is here redefined as Ca5.5[AlV4+V5+12O39]·28H2O. In an earlier preliminary structure report (R = 22%), researchers defined the composition of vanadoaluminate heteropoly complex in sherwoodite as (AlV4+,5+14O40)n−, the first natural occurrence of that polyoxometalate cluster. The cluster forms a framework around intermolecular channels that contain disordered arrays of H2O and charge-balancing Ca ions. A detailed structure study described herein [a = 27.9085(7), c = 13.8090(5), space group I41/amd, R1 < 6%] reveals new details of the vanadoaluminate heteropoly complex, indicating that it crystallizes as a structural variant of the putative (AlV4+,5+14O40)n− vanadoaluminate complex, [AlV4+V5+12O39]11−. That variant is derived through the random removal of one of the two apical vanadate octahedra, elucidating the cause of difficulties in the refinement of the preliminary structure report. The [AlV4+V5+12O39]11− complex is capped on one end by a Ca atom that bonds to four octahedral apices in a half-occupied site, and on the opposite end by the V1 and O1 atoms in half-occupied sites, similar to the compound potassium 13-vanadanomangate(IV). The capping octahedron is the locus of the V4+ ions in the mixed-valence complex. New electron microprobe analyses yielded an empirical formula, on the basis of (V4+V5+12), of (Ca4.26Mg1.09Na0.35K0.04Sr0.03)[Al0.87(V4+V5+12)O39]·28.15 H2O.