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 Ba3W1+xV1−xO8.5+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.