A single-crystalline LnBM.sub.2O.sub.5+.delta. or LnBM.sub.2O.sub.5.5+.delta. compound is provided, which includes an ordered oxygen vacancy structure; wherein Ln is a lanthanide, B is an alkali earth metal, M is a transition metal, O is oxygen, and 0.ltoreq..delta..ltoreq.1. Methods of making and using the compound, and devices and compositions including same are also provided.
STATEMENT OF FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT
 This invention was made with government support under Grant No. DE-FE0003780 awarded by the Department of Energy. The government has certain rights in the invention.