Dielectric response to the low-temperature magnetic defect structure and spin state transition in polycrystalline LaCoO3

The dielectric and magnetic properties and their correlations were investigated in polycrystalline perovskite LaCoO3-δ. The intrinsic bulk and grain-boundary (GB) dielectric relaxation processes were deconvoluted using impedance spectroscopy between 20 and 120 K, and resistivity and capacitance were analyzed separately. A thermally induced magnetic transition from a Co3+ low-spin (LS) (S=0; t 2g 6 eg0) to a higher spin state occurs at Ts1 80 K, which is controversial in nature and has been suggested to be an intermediate-spin (IS) state (S=1; t 2g 5 eg1) or a high-spin (HS) state (S=2; t 2g 4 eg2) transition. This spin state transition was confirmed by magnetic-susceptibility measurements and was reflected in the impedance by a split of the single GB relaxation process into two coexisting contributions. This apparent electronic phase coexistence at T>80 K was interpreted as a reflection of the coexistence of magnetic LS and IS/HS states. At lower temperatures (T≤40 K) perceptible variation in bulk dielectric permittivity with temperature appeared to be correlated with the magnetic susceptibility associated with a magnetic defect structure. At 40 K<T< Ts1, separated GB and bulk resistivity vs T curves were consistent with localized polaron Mott variable-range hopping (VRH) based on impurity conduction. Below 40 K, a crossover from impurity Mott's VRH to another type of thermally activated charge transport was detected, which was correlated with the appearance of the defect-related magnetism.