Abstract:
An oxide represented by Formula 1: A2M1−xCxD2O7+δ Formula 1 wherein, in Formula 1, x is in the range of 0.4≦x≦1.0; δ is selected such that the oxide electrically neutral; A is at least one metal selected from an alkaline earth metal; M is an alkaline earth metal that differs from A; C is a transition metal; and D is at least one selected from germanium (Ge) and silicon (Si).