Mixed conductor, electrochemical device,and method of preparing mixed conductor

A method of preparing a mixed conductor, the method comprising:

• providing a mixture comprising an element A precursor, a titanium precursor, and an element G precursor,
• wherein the element A precursor provides a monovalent cation, and the element G precursor provides at least one of a monovalent cation, a divalent cation, a trivalent cation, a tetravalent cation, a pentavalent cation, or a hexavalent cation, with the proviso that G is not Ti or Cr;
• and
• heat-treating the mixture to prepare the mixed conductor.