Enhancing the oxide-ionic conductivity of Ba3Mo1+xNb1−2xGexO8.5 at intermediate temperatures: the effect of site-selective Ge4+-substitution

Abstract

Significant oxide-ionic conductivity has been recently reported for a family of cation-deficient hexagonal perovskite derivatives Ba₃M₂O_8.5 (M = Mo/W⁶⁺ and Nb⁵⁺/V⁵⁺). Herein, strong 4-fold coordination geometry preferring Ge⁴⁺ ions are doped into Ba₃Mo_1+xNb_1−2xGe_xO_8.5 to manipulate the oxygen distribution within palmierite-like layers for the enhancement of oxide-ionic conductivity. Rietveld refinement of the neutron diffraction data of Ba₃Mo_1.2Nb_0.6Ge_0.2O_8.5 reveals that Ge⁴⁺-ions are selectively incorporated into the palmierite-like layers, owing to their strong 4-fold coordination environment preference. Such a site-selective doping behavior leads to an increase in the occupation proportion of the O3 site and a concomitant decrease in the occupancy factor for O2. Ionic conduction measurements show that the bulk conductivity of Ba₃Mo_1.2Nb_0.6Ge_0.2O_8.5 is about twice higher than that of the parent compound at intermediate temperatures (300–500 °C). Furthermore, bond-valence site energy (BVSE) landscape analysis reveals that the oxygen ionic conduction of Ba₃Mo_1+xNb_1−2xGe_xO_8.5 is dominated by the two-dimensional pathways along the palmierite-like layers, despite the three-dimensional (3D) oxygen diffusion pathways being present in the hybrid structure, which strongly confirms that the enhancement in ionic conductivity at intermediate temperatures is attributed to the site-selective Ge⁴⁺-substitution-induced redistribution of oxygen ions within the palmierite-like layers.