Effect of chromium substitution on the lattice vibration of spinel lithium manganate: A new interpretation of the Raman spectrum of LiMn₂O ₄

The variation of the lattice vibration of spinel lithium manganate upon chromium substitution has been investigated by performing a combinative micro-Raman and X-ray absorption spectroscopic (XAS) analysis. As the manganese ion in the spinel LiMn_2-xCr_xCr_xO₄ is replaced by a chromium ion, the relative intensities of two main Raman peaks at ∼580 and ∼620 cm^-1 are remarkably changed. This change cannot be understood by the previous peak assignment on the basis of the average crystal structure of the cubic spinel lattice. According to the Mn K-edge XAS analysis, it is certain that there are two kinds of local structures around the manganese in the spinel LiMn_2-xCr_xO ₄ and the substitution of Mn with Cr gives rise to a depression of the local tetragonal distortion around the manganese ion. On the basis of this finding, we are able to re-interpret two main Raman features of the cubic spinel lithium manganate as totally symmetric A_1g modes without and with tetragonal distortion, respectively. This new interpretation is further supported not only by the comparison with the reference spectra of λ-MnO₂ and Li₂Mn₂O₄ but also by the good correlation between the ratio of Mn^-IV/Mn^+III concentration and the relative intensities of two Raman peaks.