The temperature dependence of thermal conductivity, $\kappa \mathrm{}\left(T\right),$ and magnetic susceptibility, $\chi \mathrm{}\left(T\right),$ have been measured on single crystals of $L{\mathrm{CoO}}_3$ $(L=\mathrm{La},$ Pr, Nd) grown by the floating-zone method. The susceptibility measurement shows a progressive stabilization of the low-spin (LS) state of Co(III) with decreasing size of the ${\mathrm{L}}^{3+}$ ion, and the population of excited intermediate-spin (IS) or high-spin (HS) state Co(III) ions begins to increase at 200 K and 300 K for ${\mathrm{PrCoO}}_3$ and ${\mathrm{NdCoO}}_3$ compared with 35 K in ${\mathrm{LaCoO}}_3.$ The low-temperature Curie-Weiss paramagnetic susceptibility of $L{\mathrm{CoO}}_3$ is an intrinsic property arising from surface cobalt and, possibly, a LS ground state bearing some IS character caused by the virtual excitation to the IS state. The transition from a LS to a IS/HS state introduces bond-length fluctuations that suppress the phonon contribution to $\kappa \mathrm{}\left(T\right)\mathrm{}$ below 300 K. The suppressed $\kappa \mathrm{}\left(T\right)\mathrm{}$ could be further reduced by dynamic Jahn-Teller distortions associated with the IS/HS species. A smooth transition in $\rho \mathrm{}\left(T\right)\mathrm{}$ and $\alpha \mathrm{}\left(T\right)\mathrm{}$ and a nearly temperature independent $\alpha \mathrm{}\left(T\right)\mathrm{}\approx 20\mu \mathrm{V}/\mathrm{K}$ above 600 K do not support a thermally induced, homogeneous Mott-Hubbard transition model for the high-temperature transition of ${\mathrm{LaCoO}}_3$ from an insulating to a conductive state. A two-phase process is proposed for the interval $300\mathrm{K}<T<\mathrm{}700\mathrm{}\mathrm{K}$ with a conductive phase growing in a matrix of IS states with localized $e$ electrons that are stabilized by Jahn-Teller distortions that may be dynamic.

• Received 1 October 2003

DOI:https://doi.org/10.1103/PhysRevB.69.134409