TWINNING IN ZNO CERAMICS WITH SB2O3 ADDITIONS

Crystallographic twins which form in sintered ZnO varistor ceramics with Sb2O3 additions are considered in terms of established crystallographic twinning mechanisms. Possible twin origins from either phase transformations or a result of deformation are both rejected. It is concluded that the ZnO twins are growth twins which nucleate in the early stages of sintering and grain growth of the ceramic powder compact. Consideration of the oxygen anion stacking layer sequences in the hexagonal ZnO wurtzite structure and the cubic Zn7Sb2O12 spinel structure suggests that the nuclei for the twins may form as embryos consisting of a faulted region of the spinel-oxygen anion layer stacking sequence. The faulted layer sequence is created by the presence of the antimony oxide and its reaction with the ZnO. The fact that the wurtzite structure is polar and the ZnO twins are inversion twins explains why there is only a single twin per grain, as multiple twins would result in the unfavorable tail-to-tail structural configuration.