Bond-length fluctuations and the spin-state transition inLCoO3(L=La,Pr, and Nd)

Abstract

The temperature dependence of thermal conductivity, $\ensuremath{\kappa}(T),$ and magnetic susceptibility, $\ensuremath{\chi}(T),$ 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 $\ensuremath{\kappa}(T)$ below 300 K. The suppressed $\ensuremath{\kappa}(T)$ could be further reduced by dynamic Jahn-Teller distortions associated with the IS/HS species. A smooth transition in $\ensuremath{\rho}(T)$ and $\ensuremath{\alpha}(T)$ and a nearly temperature independent $\ensuremath{\alpha}(T)\ensuremath{\approx}20\ensuremath{\mu}\mathrm{V}/\mathrm{K}$ 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 $300\mathrm{K}lTl700\mathrm{K}$ 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.