Crystallography of Ni-doped Zn7Sb2O12 and phase equilibria in the system ZnOSb2O5NiO

Zn7Sb2O12 forms an extensive range of Ni-containing solid solutions of formula Zn7−xNixSb2O12: 0 ≤ x ≤ 4. At low x, the solid solutions are polymorphic. For x = 0, the low temperature β structure transforms to the high temperature α form with the spinel structure at 1225 ± 25 °C. Transition temperature decreases with increasing x and for x ≳ 2, only the α structure forms. The lattice parameter, a, of the cubic, α solid solution decreases linearly from 8.603 Å for x = 0 to 8.514 Å for x = 4. Rietveld refinement of powder neutron diffraction data for composition x = 4, confirmed the structural formula Zn3tet[Ni4Sb2]octO12. The grain size of α depends on both composition and synthesis temperature for samples synthesised by solid state reaction at 1000–1100 °C: it increases either with decreasing x for a given reaction temperature or with increasing temperature for a given x. Sub-solidus compatibility relations in the ternary system ZnOSb2O5NiO have been determined at 1100 °C for compositions containing ≤50% Sb2O5.