High-temperature proton conductors with structure-disordered oxygen sublattice

Abstract

Data on the thermogravimetry, spectroscopy, and electrical charge transfer as functions of T, aH₂O, and aO₂ for niobates and tantalates of alkali-earth metals with structure disordering of the oxygen sublattice, which can show high-temperature proton conduction, are summarized. It is shown that in the solid solution series with decreasing x (that is, with the increasing of the oxygen vacancies concentration) the proton conductivity increase, which is caused by the increasing of both the concentration of proton defects formed in the structure (in compliance with the formula Sr_{6 − 2x} M ⁺⁵_{2 + 2x} O₁₀(OH)_2−6x and their mobility. The proton transfer dominates for the compositions with x < 0.15 at temperatures below 550°C. In the solid solutions (Ba_1−y Ca _y )₆Nb₂O₁₁ (0.23 ≤ y ≤ 0.47) characterized by equal concentration of oxygen vacancies, with the increasing of barium content (correspondingly, with the increasing of the lattice parameter) the oxygen-ion conductivity (at aH₂O = 3 × 10⁻⁵) grows monotonically, which is caused by the decreasing of the oxygen atom migration energy and increasing of their mobility. In this series, the proton conductivity (at aH₂O = 2 × 10⁻²) increased. It was shown, by using IR-spectroscopy and the ¹H NMR method, that the protons exist in the complex oxide structure mainly as energy-wise nonequivalent OH⁻ groups: isolated, closely set, and paired, whose quantitative ratios are determined by the coordination preference of the B-sublattice elements.