RAMAN SPECTROSCOPY OF NANOCRYSTALLINE Li-Ti-O SPINELS AND COMPARATIVE DFT CALCULATIONS ON Ti_yO_z AND Li_xTi_yO_z CLUSTERS

摘要

Raman spectra of cubic nanocrystalline insertion hosts in Li-Ti-O system were measured for Li_(1.1)Ti_(1.9)O_(4-delta) spinels and the corresponding delithiated titanium dioxide material. To interpret the measured data optimized geometries of cubic Ti_yO_z and Li_xTi_yO_z were calculated using the DFT cluster approach for clusters containing up to 23 atoms. The calculated geometries were used to predict the density-of-states (DOS) diagrams and vibrational spectra of the corresponding structures upon harmonic approximation. While the DFT approach reproduces satisfactorily the main structural characteristics of Ti_yO_z clusters, the agreement in the case of Li_xTi_yO_z clusters between calculated and real structures is less satisfactory due to the small cluster size. The band gap energies obtained from DOS diagrams represent overestimates of the actual experimental band gap energies measured on cubic Li-Ti-O oxides. The agreement between calculated and measured band gap energy improves with increasing cluster size. Raman spectra calculated under harmonic approximation from the optimized geometries reproduce all important features of the experimental Raman spectra. The agreement of DFT-calculated and experimental Raman spectra improves with increasing size of the cluster.