Missing Piece in the Crystal Chemistry of Zn-Sb Secondary Phases in ZnO-Sb2O3-Bi2O3 Varistor Ceramics: Orthorhombic β-Zn7Sb2O12. An Experimental and Theoretical Study of the Crystal Structure and Its Thermal and Vibrational Spectroscopic Characterization

Abstract

The ternary Zn-Sb oxide β-Zn₇Sb₂O₁₂ was prepared by solid state synthesis from ZnO and Sb₂O₃ at 1250 °C and a reaction time of 168 h. The crystal structure of β-Zn₇Sb₂O₁₂ was solved from powder diffraction data by direct-space methods and was refined by the Rietveld technique. The title compound β-Zn₇Sb₂O₁₂ crystallizes in the orthorhombic space group Cmme with a = 1210.6(1) pm, b = 1856.7(1) pm, c = 852.2(1) pm, V = 1.9154(1) nm³, and eight formula units per unit cell. The structure can be described as a distorted cubic closest packing of oxide ions with Zn²⁺ and Sb⁵⁺ in the tetrahedral and octahedral interstitial sites. According to group-subgroup relations, the anion packing is directly derived from the spinel structure of α-Zn₇Sb₂O₁₂; however, the occupation pattern of the interstitials is completely different than in the spinel structure type. The most prominent structural feature in β-Zn₇Sb₂O₁₂ is the clustering of all [ZnO₄] polyhedra into a [Zn₇O₂₀] moiety representing an excerpt from the sphalerite structure. The structural model is corroborated by vibrational spectroscopy as well as density functional theory calculations. Thermal analysis reveals an irreversible phase transition of β-Zn₇Sb₂O₁₂ into the α-polymorph at 1330 °C.