Local lattice distortion during the spin-state transition inLaCoO3

Infrared spectroscopy of a ${\mathrm{LaCoO}}_3$ crystal has revealed anomalous splitting of the phonon modes as well as their intensity variation with temperature during the spin-state transition. The observed results can be interpreted in terms of local lattice distortion arising from the Jahn-Teller effect of the thermally excited intermediate-spin (S=1) state of ${\mathrm{Co}}^{3+}$ ions. A possible role of the ${\mathrm{e}}_{\mathrm{g}}$ orbital ordering is discussed with the analysis of the intensity variation of the split Co-O stretching modes.