Speculations on the existence of hydride ions in proton conducting oxides

The chemical and physical nature of the hydride ion is briefly treated. Several reactions of the hydride ion in oxides or oxygen atmosphere are given. A number of perovskites and inverse perovskites are listed, which contain the H− ion on the oxygen or B-anion sites in the archetype ABO3 system. H− is stable with respect to oxide and halide anions but, among cations only with respect to oxides and halides of strongly electropositive metals such as alkaline, alkaline-earth and main group III metals. H− is only stable in combination with transition metal ions of certain elements in their lowest positive oxidation state. Mixed oxide/hydride containing perovskites may thus exist. Steinsvik et al. have recently suggested a defect model for a perovskite including substitutional hydride ions on the oxygen site, HO⋅, and protons associated with a lattice oxygen, OHO⋅. The defect equations for this acceptor doped A(II)B(IV)O3 model compound are solved without using the conventional Brouwer approximations. One case is presented where hydride formation is suppressed, and another case where it is promoted. Plots of concentration versus water and oxygen partial pressures show new interesting features; these are discussed.