The dielectric and magnetic properties and their correlations were investigated in polycrystalline perovskite ${\text{LaCoO}}_{3-\delta }$. 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 ${\text{Co}}^{3+}$ low-spin (LS) $\left(S=0;t_{2g}^6{e}_g^0\right)$ to a higher spin state occurs at $T_{s1}\approx 80\text{K}$, which is controversial in nature and has been suggested to be an intermediate-spin (IS) state $\left(S=1;t_{2g}^5{e}_g^1\right)$ or a high-spin (HS) state $\left(S=2;t_{2g}^4{e}_g^2\right)$ 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 $T>80\text{K}$ was interpreted as a reflection of the coexistence of magnetic LS and IS/HS states. At lower temperatures $\left(T\le 40\text{K}\right)$ perceptible variation in bulk dielectric permittivity with temperature appeared to be correlated with the magnetic susceptibility associated with a magnetic defect structure. At $40\text{K}<T<T_{s1}$, 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.

• Received 11 November 2008

DOI:https://doi.org/10.1103/PhysRevB.79.125105