Method for producing pyrochlore-type oxide

A method for producing a pyrochlore-type oxide that contains a plurality of cations including an alkali metal cation and a lanthanoid cation, in its composition, the method comprising:

• a mixing process of mixing a composite oxide that contains at least cations other than the alkali metal cation with an alkali metal compound that contains the alkali metal cation;
• and
• a firing process of heating a mixture containing the composite oxide and the alkali metal compound at a predetermined temperature to produce the pyrochlore-type oxide, wherein
• the composite oxide contains the lanthanoid cation,
• in a case where the composite oxide contains the alkali metal cation, a composition ratio of the alkali metal cation in the composite oxide is less than a composition ratio of the alkali metal cation in the alkali metal compound,
• the mixture contains the alkali metal compound in an amount exceeding a stoichiometric ratio relative to the pyrochlore-type oxide, and
• in the firing process, the alkali metal compound is liquefied by heating at the predetermined temperature.