Effect of Zn doping on the structure and electrical conductivity of Mn_1.5Co_1.5O₄ spinel

Electrical conductivity is one of the most important concerns in spinel-structured antioxidant films coated on a solid oxide fuel cell separator, and the mechanism of electrical conduction involves the small polaron hopping through corner-shared octahedral sites. Arranging favorable ions in the octahedral sites rather than in the tetrahedral sites is better for achieving higher electrical conduction. In this study, Zn, which prefers the tetrahedral site, was substituted for the Mn_1.5Co_1.5O₄ spinel to arrange site distribution. The crystal structure, electrical conductivity, and thermal expansion behavior of Zn-substituted ZMCOs [Zn_xMn_1.5−0.5xCo_1.5−0.5xO₄ (0.0 ≤ x ≤ 0.6)] were investigated. At Zn = 0.2, the highest electrical conductivity of 42.1 S/cm was observed, while at Zn > 0.2, the electrical conductivity decreased due to decreased Co²⁺/Co^III and Mn³⁺/Mn⁴⁺ pair concentration because Zn ions started to occupy the octahedral site. The higher the Zn concentration of ZMCO, the lower the distortion of the MnO₆ octahedron via the Jahn–Teller effect, resulting in a higher symmetry of the crystal structure. Therefore, with the increase in the Zn concentration, a mixture of tetragonal-cubic phase changed to cubic phase. In addition, the phase transition temperature of the tetragonal-cubic phase decreased.