Effect of chromium substitution on the lattice vibration of spinel lithium manganate:: A new interpretation of the Raman spectrum of LiMn2O4

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 LiMn2-xCrxO4 is replaced by a chromium ion, the relative intensities of two main Raman peaks at similar to580 and similar to620 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 LiMn2-xCrxO4 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 lambda-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.