The proposal addresses the average structures of new lead-free ferroelectrics, low-temperature ionic conductors and mixed conductors which are candidate solid oxide fuel cell cathodes. The objectives are to identify phase transitions upon doping to set up candidate morphotropic phase boundaries, to locate catalytic dopant sites in both site-ordered simple perovskite and complex superstructure mixed conductors and to idnetify interstitial oxide stabilisation mechanisms in melilite-related systems. The proposal focusses on using the high resolution of HRPD to add to parallel studies on POLARIS in a separate proposal.