Characterisation of secondary phases [resent in ZnO-based varistor ceramics
A study of compound formation, crystal structures, phase transformations and phase equilibrium in the system ZnO-Sb2Oy-Bi2O3 has been carried out. Rietveld refinement of combined X-ray Diffraction (XRD) and Neutron Diffraction (ND) data and phase diagram analysis have shown that instead of the previously reported single pyrochlore, Zn2Sb3Bi3O14, there are two closely related but separate pyrochlore phases. One, a Bi-rich phase, labelled P2, is a fully oxygenated line phase of fixed composition, Zn2-Sb2.92Bi3.08O14, in which some of the Bi3+ may have oxidised to Bi5+, to give a mixed valence state, Biv0.08BiIII3.0. The other, an Sb-rich phase, labelled P1, forms a solid solution of general formula Zn2+2xSb3.04Bi2.96-2xO14-2, 0.0 < x < 0.08, with the replacement mechanism, 2Bi3+ + O2- « 2Zn2+. It is thought that antimony may be present in the mixed valence state, Sbv3.0SbIII0.04. Rietveld refinement has confirmed (i), variable oxygen content present in P1 when x > 0, (ii), the preferential site occupancies of Bi, Sb and Zn and (iii), the displacement of Zn2+ off the centre of the A site, 16d, to a site of higher multiplicity, 96g, referred to as A’; this site gives rise to a 5-fold co-ordination environment and reasonable Zn-O bond lengths.