Preparation and electrical properties of sintered bodies composed of Mn_(2.25-X)Fe_XNi_0.75O₄ (0≦X≦2.25) with cubic spinel structure

Preparation and electrical properties of sintered bodies consisting of monophase cubic spinel oxides, Mn_(2.25-X)Fe_XNi_0.75O₄ (0≦X≦2.25), were investigated. Sintered bodies with monophase cubic spinel structure were prepared by heat-treatment for 48 h in air at 1000 °C and/or 1200 °C to convert them into a cubic spinel structure after sintering at 1400 °C in Ar. The relationship between ln (σ) and T ⁻¹ was expressed as straight lines for all compositions, indicating that these oxides have intrinsic NTC thermistor characteristics. Here, σ is electrical conduction. The relationship of the specimens in the region of 0≦X≦2.125 was seen to consist of two straight lines with slightly different slopes. Turning points were observed near 300 °C. It was thought that the carrier concentration changed based on the disproportionation reaction of Mn³⁺ in the octahedral site (B site) of the spinel structure. For specimens with X below 1.5, the electrical conduction was considered to be mainly due to an electron jump between Mn³⁺ and Mn⁴⁺ in the B site. For specimens with X above 1.875, the conduction was considered to be mainly due to an electron jump between Fe²⁺ and Fe³⁺ in the B site. It was found that the activation energy of the mobility between Fe ions is lower than that between Mn ions. The electrical conduction of the sintered bodies was concluded to be controlled by a small polaron hopping mechanism.