Evidence for a low-spin to intermediate-spin state transition in LaCoO 3

We present measurements of the magnetic susceptibility and of the thermal expansion of a LaCoO 3 single crystal. Both quantities show a strongly anomalous temperature dependence. Our data are consistently described in terms of a spin-state transition of the Co 3 + ions with increasing temperature from a low-spin ground state (t 6 2 g e 0 g ) to an intermediate-spin state (t 5 2 g e 1 g ) without (100-500 K) and with (>500 K) orbital degeneracy. We attribute the lack of orbital degeneracy up to 500 K to (probably local) Jahn-Teller distortions of the CoO 6 octahedra. A strong reduction or disappearance of the Jahn-Teller distortions seems to arise from the insulator-to-metal transition around 500 K.