Enhanced oxygen ion conductivity in Ba3W1+xV1−xO8.5+x/2 (− 0.2 ≤ × ≤ 0.2) hexagonal perovskite derivative compounds

Abstract

A series of hexagonal perovskite derivative compounds Ba₃W_1+xV_1−xO_8.5+x/2 (x =  − 0.2, − 0.1, 0, 0.1, 0.2) with varying oxygen content was synthesized by high-temperature solid-state reaction route and characterized using X-ray diffraction, SEM–EDX, Raman spectroscopy, XPS, and dielectric spectroscopy. All samples were isostructural, having features of both palmierite and 9R hexagonal perovskite. The unit cell volume showed a continuously decreasing trend with increasing oxygen content. The XPS studies showed no deviation of oxidation states of W⁶⁺ and V⁵⁺ and hence confirmed that the oxygen stoichiometry is solely controlled by the W to V ratio in the samples. The presence of both octahedral MO₆ and tetrahedral MO₄ units in all samples was inferred from temperature-dependent Raman spectroscopic studies. The translational and rotational motion of MO₄ tetrahedra are appreciably affected by temperature. The dc conductivity was obtained directly from the complex ac conductivity derived from temperature-dependent dielectric measurements. It was found that the dc conductivity increases when the composition deviates from x = 0.0, i.e., W:V = 1:1. An estimate of the ion mobility and mobile ion concentration was obtained using the Almond-West formalism. The conductivity was found to be significantly higher in W-rich compounds (x > 0), and the ion mobility was also correspondingly higher. It could be inferred that the compositional dependence of unit cell parameters, particularly a- or b-axis, and the oxygen stoichiometry, play crucial roles in governing the ionic conductivity of these hexagonal perovskite derivatives.