Evidence for a Low-Spin to Intermediate-Spin State Transition in LaCoO3

TL;DR

This study provides evidence for a temperature-induced transition in LaCoO3 from a low-spin to an intermediate-spin state of Co3+ ions, linked to Jahn-Teller distortions and an insulator-metal transition.

Contribution

It offers experimental evidence for a spin-state transition in LaCoO3 and links it to structural distortions and electronic phase change.

Findings

Spin-state transition observed between 100K and 500K.

Orbital degeneracy appears above 500K.

Jahn-Teller distortions diminish near the insulator-metal transition.

Abstract

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 to an intermediate-spin state without (100K - 500K) and with (>500K) orbital degeneracy. We attribute the lack of orbital degeneracy up to 500K 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.