A Raman study of the temperature-induced low-to-intermediate-spin state transition in LaCoO3

The effect of the thermally induced spin-state transition on the phonon mode behavior of LaCoO3 has been investigated by using Raman spectroscopy. The number of observed Raman modes is larger than the number predicted by group theory for a rhombohedrally distorted perovskite structure with the space group R 3 ̄ c. There is a strong similarity between the Raman spectrum of LaCoO3 and LaMnO3 at ambient conditions due to the Jahn–Teller (J–T) distortion associated with the intermediate-spin (IS) state of Co3+. The J–T distortion splits the external mode associated with La–O vibration into a doublet above the spin-state transition temperature. There is an increase in the anharmonic coupling between the one-phonon states with increase in temperature above the transition temperature, due to the gradual increase in the population of Co3+ in the IS state.