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

Abstract

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^1,2. A crucial requirement for such devices is a good ionic (O²⁻ or H⁺) conductor as the electrolyte^3,4. Traditionally, fluorite- and perovskite-type oxides have been targeted^3,4,5,6, although there is growing interest in alternative structure types for intermediate-temperature (400–700 ^∘C) solid-oxide fuel cells. In particular, structures containing tetrahedral moieties, such as La_1−xCa_xMO_4−x/2(M=Ta,Nb,P) (refs 7,8), La_1−xBa_1+xGaO_4−x/2 (refs 9,10) and La_9.33+xSi₆O_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−xBa_1+xGaO_4−x/2 system, which exhibits both proton and oxide-ion conduction^9,10. Here we show that oxide-ion conduction proceeds via a cooperative ‘cog-wheel’-type process involving the breaking and re-forming of Ga₂O₇ 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.