Crystal chemistry of Co-doped Zn7Sb2O12

Zn7Sb2O12 forms a full range of Co-containing α solid solutions, Zn7−xCoxSb2O12, with an inverse-spinel structure at high temperature. At low temperatures for x<2, the solid solutions transform into the low temperature β-polymorph. For x=0, the β→α transition occurs at 1225±25 °C; the transition temperature decreases with increasing x. At high x and low temperatures, α solid solutions are formed but are non-stoichiometric; the (Zn+Co):Sb ratio is >7:2 and the compensation for the deficiency in Sb is attributed to the partial oxidation of Co2+ to Co3+. From Rietveld refinements using ND data, Co occupies both octahedral and tetrahedral sites at intermediate values of x, but an octahedral preference attributed to crystal field stabilisation, causes the lattice parameter plot to deviate negatively from the Vegardʹs law. Sub-solidus compatibility relations in the ternary system ZnO–Sb2O5–CoO have been determined at 1100 °C for the compositions containing ⩽50% Sb2O5.