Lattice dynamics and vibrational spectra of lithium manganese oxides: A computer simulation and spectroscopic study

The lattice vibrational modes of spinel-structured lithium manganese oxides have been calculated using atomistic modeling methods. The simulations all ow the Raman and infrared spectra of lithiated, fully delithiated, and part ially delithiated phases to be assigned for the first time. Calculations fo r the spinels LiMn2O4, lambda-MnO2, and Li0.5Mn2O4 are compared with experi mental Raman data measured for thin films of the oxides coated on a platinu m electrode. The appearance of a number of new bands in the Raman spectrum of LiMn2O4 following partial extraction of lithium is shown to result from local lowering of the symmetry and Raman activation of modes which are opti cally inactive or only infrared active in LiMn2O4. The results support a mo del for the Li0.5Mn2O4 lattice in which the lithium ions are ordered. The d eformation vibrations of lattice hydroxyl "defects" in lambda-MnO2 have als o been calculated; comparison of the calculated and experimental vibrationa l data supports a model in which hydroxyl species are localized at octahedr al Mn vacancies.