Structure and dehydration mechanism of the proton conducting oxide Ba2In2O5(H2O)x

Abstract

The structure and dehydration mechanism of the proton conducting oxide Ba₂In₂O₅(H₂O)_x are investigated by means of variable temperature (20–600 °C) Raman spectroscopy together with thermal gravimetric analysis and inelastic neutron scattering. At room temperature, Ba₂In₂O₅(H₂O)_x is found to be fully hydrated (x = 1) and to have a perovskite-like structure, which dehydrates gradually with increasing temperature and at around 600 °C the material is essentially dehydrated (x ≈ 0.2). The dehydrated material exhibits a brownmillerite structure, which is featured by alternating layers of InO₆ octahedra and InO₄ tetrahedra. The transition from a perovskite-like to a brownmillerite-like structure upon increasing temperature occurs through the formation of an intermediate phase at ca. 370 °C, corresponding to a hydration degree of approximately 50%. The structure of the intermediate phase is similar to the structure of the dehydrated material, but with the difference that it exhibits a non-centrosymmetric distortion of the InO₆ octahedra that is not present in the dehydrated material. The dehydration process upon heating is a two-stage mechanism; for temperatures below the hydrated-to-intermediate phase transition, dehydration is characterized by a homogenous release of protons over the entire oxide lattice, whereas above the transition a preferential desorption of protons originating in the nominally tetrahedral layers is observed. Furthermore, our spectroscopic results point towards the co-existence of two structural phases, which relate to the two lowest-energy proton configurations in the material. The relative contributions of the two proton configurations depend on how the sample is hydrated.