Effect of germanium substitution on the structure and ionic conductivity of the hexagonal perovskite derivative compound Ba3WVO8.5

Abstract

A series of hexagonal perovskite derivative compounds Ba₃WV_1−xGe_xO_8.5−x/2 (x = 0, 0.1, 0.2, and 0.3) were synthesized using a solid-state reaction route and characterized by powder XRD, SEM-EDX and dielectric spectroscopy. All the samples were found to have a rhombohedral lattice (space group: Rm) and showed a systematic increasing trend in unit cell parameters with increasing Ge⁴⁺ content. High-temperature XRD studies on the samples confirmed their stability up to 1023 K. Temperature- and frequency-dependent dielectric studies indicated high ionic conduction in all these samples and systematic enhancement in ionic conductivity with increasing Ge⁴⁺ incorporation. Among the studied samples, Ba₃WV_0.7Ge_0.3O_8.5 showed the highest dc conductivity (∼1.80 × 10⁻⁵ S cm⁻¹ at 973 K), while Ba₃WVO_8.5 exhibited a value of 1.03 × 10⁻⁷ S cm⁻¹ under the same conditions. This enhancement in conductivity upon incorporation of Ge⁴⁺ ions was attributed to the easy diffusion of oxide ions in the lattice due to increasing unit cell volume, oxygen ion vacancies and anion disorder in the BaO₃ layer owing to preferential formation of tetrahedral GeO₄. The temperature evolution of ionic conductivity of the samples showed Arrhenius behaviour, and activation energies were in the range of 0.9–1.6 eV. In addition, a changeover in conduction mechanism was observed at higher temperatures. The shape parameters in the electrical modulus spectra were determined using Havriliak–Negami (H–N) function fitting, and it was found that the distribution of relaxation times increased with the Ge⁴⁺ content due to a widely distributed spatial arrangement of carriers and energetics for carrier migration.