Preparation of cubic spinel-type oxides, Mn(1.75−1.25X)Co2.5XNi1.25(1−X)O4 (0 ≤X≤ 0.6), and their electrical properties were investigated. The starting oxides, containing metals with a prescribed molar ratio, were heated to 1400 oC and held for 3 h in argon. The sintered bodies were cooled to 800 oC and then oxidized for 48 h in air to convert them into a cubic spinel structure. The electrical conductivities of the sintered bodies were confirmed to increase exponentially with an increase in the temperature, indicating that they have intrinsic NTC thermistor characteristics. In the region of 0 ≤ X≤ 0.4, the electrical conductivity increased with an increase in X and decreased with an increase in X in the region of 0.4 < X ≤ 0.6. The concentration of Mn4+ in the octahedral sites of the spinel structure was considered to be larger than that of Mn3+ because the sintered bodies were n-type semiconducting. The electrical conduction of the oxides prepared in this study was concluded to be controlled by a small polaron hopping mechanism.

Keywords: Mn-Co-Ni monophase cubic spinel-type oxide, Electrical conductivity, Seebeck coefficient, Cation distribution, Small polaron hopping, Thermistor.