Electrical properties of La 2− xSr xCoO 4—II: Models and analysis of the relationship between cobalt 3 d electron state and structural, electrical and magnetic properties

Abstract

Statistical dynamic calculations were made for the localized electronic state of cobalt 3 d electrons in the K 2NiF 4 −type La 2− x Sr x CoO 4 ( x = 0.5, 1.0 and 1.5) in order to clarify the relation between the conductivity, Seebeck coefficient, magnetic properties, and lattice parameters. First, the chemical potential of electrons, Seebeck coefficient, number of cobalt ions in each electronic state, and the conductivity, were formulated based on the following assumptions: due to tetragonal distortion of the Co-60 octahedra, the Co-3 d levels split into levels d xz − d yz (lowest), d xy , d z 2 and d x 2− y 2 (highest) by crystal field interaction; by the exchange interaction, each level splits into two levels; cobalt ions are in the high spin state for x = 0.5 and x = 1.0, and intermediate spin state for x = 1.5; the highest occupied level and the lowest unoccupied level contribute to the conduction. Then, by fitting the equations to the data reported in Part I, the energy gap between the highest occupied and lowest unoccupied levels, the electron interaction energy of the d xz − d yz level, the number of cobalt ions in each electronic state, and the hopping frequency were determined. The energy gap and interaction energy were of the order of 0.01–0.4 eV and decrease with increasing strontium content and temperature. The activation energy of the hopping frequency is 0.6–0.15 eV and decreases with increasing x. Finally, the average Co-O bond length and the effective magnetic moment were calculated. They agreed with those measured by earlier investigators.