MicroRaman spectroscopy on LiMn2O4: warnings on laser-induced thermal decomposition

Raman spectroscopy, together with X-ray diffraction, is a well-established technique to characterize samples among which the manganospinels that find applications in lithium rechargeable batteries. However, for this class of samples the relative intensities of the peaks and the position of the less intense features of the Raman spectra of LiMn2O4 as they are reported in literature are quite different from sample to sample. In this paper it will be shown that the number of Raman-active modes of LiMn2O4 and their energies depend drastically on the laser intensity used for the measurement. This effect was attributed to heating of the sample under the intense laser beam. In our experimental set-up the sample can reach temperatures of about 1500 K locally, that is values well above the known decomposition temperature. The present experimental results show that the intense peak at 625 cm(-1), which is commonly considered as the Raman phonon of LiMn2O4, is the spectral signature of the phases which are produced by its decomposition. We propose that the correct Raman spectrum of LiMn2O4 consists of a broad peak at 580 cm(-1), with two additional poorly defined features between 300 and 400 cm(-1).