COMPOSITE OXIDE HAVING N-TYPE THERMOELECTRIC PROPERTY

PROBLEM TO BE SOLVED : To provide a novel material having excellent properties as an n-type thermoelectric conversion material. SOLUTION : A composite oxide comprises a composition expressed by a general formula Ln1-xMxNiO2.7-3.3 (where Ln is a lanthanide element, and M is at least an element of one kind, i.e. a rare earth element other than Ln, selected from Na, K, Li, Zn, Pb, Ba, Ca, Al and Bi, and 0≤x≤0.8) or a composition expressed by a general formula (Ln1-xMx)2NiO3.6-4.4 (where Ln is a lanthanide element, and M is at least an element of one kind, i.e. a rare earth element not identical with Ln, selected from Na, K, Li, Zn, Pb, Ba, Ca, Al and Bi, and 0≤x≤0.8). The composite oxide has a negative Seebeck coefficient at a temperature of 100°C or more.