Cooperative mechanisms of fast-ion conduction in gallium-based oxides with tetrahedral moieties.

The need for greater energy efficiency has garnered increasing support for the use of fuel-cell technology, a prime example being the solid-oxide fuel cell. A crucial requirement for such devices is a good ionic (O(2-) or H+) conductor as the electrolyte. Traditionally, fluorite- and perovskite-type oxides have been targeted, although there is growing interest in alternative structure types for intermediate-temperature (400-700 ( composite function)C) solid-oxide fuel cells. In particular, structures containing tetrahedral moieties, such as La(1-x)Ca(x)MO(4-x/2)(M=Ta,Nb,P) (refs 7,8), La(1-x)Ba(1+x)GaO(4-x/2) (refs 9,10) and La(9.33+x)Si(6)O(26+3x/2) (ref. 11), have been attracting considerable attention recently. However, an atomic-scale understanding of the conduction mechanisms in these systems is still lacking; such mechanistic detail is important for developing strategies for optimizing the conductivity, as well as identifying next-generation materials. In this context, we report a combined experimental and computational modelling study of the La(1-x)Ba(1+x)GaO(4-x/2) system, which exhibits both proton and oxide-ion conduction. Here we show that oxide-ion conduction proceeds via a cooperative 'cog-wheel'-type process involving the breaking and re-forming of Ga(2)O(7) units, whereas the rate-limiting step for proton conduction is intra-tetrahedron proton transfer. Both mechanisms are unusual for ceramic oxide materials, and similar cooperative processes may be important in related systems containing tetrahedral moieties.