Bond-length fluctuations and the spin-state transition in LCoO₃ (L=La, Pr, and Nd)

The temperature dependence of thermal conductivity, κ(T), and magnetic susceptibility, χ(T), have been measured on single crystals of LCoO₃ (L = 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 L³⁺ 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 PrCoO₃ and NdCoO ₃ compared with 35 K in LaCoO₃. The low-temperature Curie-Weiss paramagnetic susceptibility of LCoO₃ 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 κ(T) below 300 K. The suppressed κ(T) could be further reduced by dynamic Jahn-Teller distortions associated with the IS/HS species. A smooth transition in ρ(T) and α(T) and a nearly temperature independent α(T)≈20 μV/K above 600 K do not support a thermally induced, homogeneous Mott-Hubbard transition model for the high-temperature transition of LaCoO₃ from an insulating to a conductive state. A two-phase process is proposed for the interval 300 K<T<700 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.