Evidence for a low-spin to intermediate-spin state transition in ${\mathrm{LaCoO}}_3$

We present measurements of the magnetic susceptibility and of the thermal expansion of a ${\mathrm{LaCoO}}_3$ single crystal. Both quantities show a strongly anomalous temperature dependence. Our data are consistently described in terms of a spin-state transition of the ${\mathrm{Co}}^{3+}$ ions with increasing temperature from a low-spin ground state ${(t}_{2g}^6{e}_g^0)$ to an intermediate-spin state ${(t}_{2g}^5{e}_g^1)$ without (100–500 K) and with $(>\mathrm{}500\mathrm{}\hspace{0.5em}\mathrm{K})$ orbital degeneracy. We attribute the lack of orbital degeneracy up to 500 K to (probably local) Jahn-Teller distortions of the ${\mathrm{CoO}}_6$ octahedra. A strong reduction or disappearance of the Jahn-Teller distortions seems to arise from the insulator-to-metal transition around 500 K.