Structural Effects in the Protonic/Electronic Conductivity of Dion-Jacobson Phase Niobate and Tantalate Layered Perovskites

The layered Dion-Jacobson phase perovskites HLaTa2O7·xH2O, HLaNb2O7·0.5H2O and structural variants of HLaNb2O7·0.5H2O were prepared and their AC and DC conductivities were measured as a function of temperature and atmosphere. The layered structure in HLaNb2O7·0.5H2O was modified by pillaring, exfoliation-restacking, and further annealing. Exfoliation followed by restacking markedly increased the ionic conductivity, either by creation of mesopores for hydration or by enhanced contact between layers, but pillaring had little effect. HLaNb2O7·0.5H2O was prone to reduction under hydrogen, yielding a mixed ionic-electronic conductor. In-plane conductivity measurements of an oriented film of HLaNb2O7·0.5H2O showed enhanced ionic conductivity attributed to the anisotropic conductivity of the sheets, but reduction under a hydrogen atmosphere was apparent. The reduction could be suppressed by adding water vapor to the ambient atmosphere or by replacing the B-site niobium with tantalum, which is less easily reduced.