Spin-state transition, magnetic, electrical and thermal transport properties of the perovskite cobalt oxide Gd0.7Sr0.3CoO3

The magnetization, resistivity ρ, thermoelectric power (TEP) S, and thermal conductivity κ in perovskite cobalt oxide Gd0.7Sr0.3CoO3 have been investigated systematically. Based on the temperature dependence of susceptibility χg(T) and Seebeck coefficient S(T), a combination of the intermediate-spin (IS) state for Co3+ and the low-spin (LS) state for Co4+ can be suggested. A metal–insulator transition (MIT) caused by the hopping of σ* electrons (localized or delocalized eg electrons) from the IS Co3+ to the LS Co4+ is observed. Meanwhile, S(T) curve also displays an obvious phonon drag effect. In addition, based on the analysis of the temperature dependence of S(T) and ρ(T), the high-temperature small polaron conduction and the low-temperature variable-range-hopping conduction are suggested, respectively. As to thermal conduction κ(T), rather low κ values in the whole measured temperature range is attributed to unusually large local Jahn–Teller (JT) distortion of Co3+O6 octahedra with IS state.