Local structure of gallate proton conductors

Lanthanum barium gallate proton conductors are based on disconnected GaO₄ groups. The insertion of hydroxyls in the LaBaGaO₄ network proceeds through self-doping with Ba²⁺, consequent O^2- vacancy formation to fulfill charge neutrality. With a structural investigation on self-doped LaBaGaO₄ oxides using synchrotron XRD and EXAFS on the Ga K-edge, we find that: (a) the GaO₄ tetrahedra retain their size throughout the whole series; (b) the GaO₄ tetrahedra rotate as rigid bodies on hydration, leading to the formation of a network of shorter O-O configurations that are stabilized by hydrogen bonds; (c) contraction of the lattice occurs along the a unit cell axis, as a consequence of an overall structural rearrangement of the hydrated solid.