We study in detail the potential energy surface of Ga and Sr substituted Ba2In2O5 by considering the changes in the relative energies of the local structures of Ba2In2O5 when replacing 1/8th of the indium atoms with gallium or 1/8th of the barium atoms with strontium. The calculations are subsequently used to interpret the increase in ionic conductivity of cubic Ba2In2O5 when strontium is substituted for barium and the decrease in ionic conductivity when gallium is substituted for indium. The effects of replacing 1/8th of the indium atoms with gallium or 1/8th of the barium atoms with strontium are significant and affect considerably the relative stability of the different low energy local structures present for Ba2In2O5. While a higher density of low energy structures is observed for Ba1.75Sr0.25In2O5 than for Ba2In2O5, the opposite occurs for Ba2In1.75Ga0.25O5. This observation supports our main hypothesis: a high density of low energy local structures is a prerequisite for high ionic conductivity. (c) 2005 Elsevier B.V. All rights reserved.