Phase equilibria, crystal chemistry and polymorphism of Zn7Sb2O12 doped with Cr and Ni

The binary phase diagram for Cr-doped Zn7Sb2O12 was determined and compared with that for Ni-doped Zn7Sb2O12. The β → α transition temperature, 1225 °C for undoped Zn7Sb2O12, decreases extremely rapidly with increasing Cr content; the solubility limit of Cr in β-Zn7Sb2O12 is <1% but 37.5% in α-Zn7Sb2O12. Bond valence sum calculations for the tetrahedral site, which contains exclusively Zn, show it to be significantly underbonded in undoped α-Zn7Sb2O12, but less underbonded with increased Ni2+ and especially Cr3+ content, thus providing an explanation for the stabilisation of α-Zn7Sb2O12 to lower temperatures on doping with Ni and especially Cr. Sub-solidus compatibility relations in the ternary system ZnO–Sb2O5–Cr2O3 were determined at 1100 °C for compositions containing ≤50% Sb2O5.