Low temperature mixed spin state of Co3+ in LaCoO3 evidenced from Jahn-Teller lattice distortions

TL;DR

This study uses Raman spectroscopy to reveal Jahn-Teller lattice distortions in LaCoO3, indicating a low-temperature mixed spin state of Co3+ ions, with evidence of orbital order and phonon anomalies.

Contribution

It provides new evidence of Jahn-Teller distortions linked to the intermediate-spin state of Co3+ in LaCoO3 through detailed phonon analysis.

Findings

Additional phonon modes indicate lattice distortions.

Anomalous behavior of the 608 cm^-1 mode suggests Jahn-Teller activity.

Second-order phonon spectra support Jahn-Teller orbital order.

Abstract

One- and multi-phonon excitations of the single crystalline LaCoO3 were studied using Raman spectroscopy in the temperature region of 5 K - 300 K. First-order Raman spectra show a larger number of phonon modes than allowed for the rhombohedral structure. Additional phonon modes are interpreted in terms of activated modes due to lattice distortions, arising from the Jahn-Teller (JT) activity of the intermediate-spin (IS) state of Co3+ ions. In particular, the 608-cm-1 stretching-type mode shows anomalous behavior in peak energy and scattering intensity as a function of temperature. The anomalous temperature dependence of the second-order phonon excitations spectra is in accordance with the Franck-Condon mechanism that is characteristic for a JT orbital order.