A crystal is made up of metal ‘M1’ and ‘M2’ and oxide ions. Oxide ions form a ccp lattice structure. The cation ‘M1’ occupies 50% of octahedral voids and the cation ‘M2’ occupies 12.5% of tetrahedral voids of oxide lattice. The oxidation numbers of ‘M1’ and ‘M2’ are, respectively.