Electronic structure of LiCoO₂and related materials; photoemission studies

The layered oxides LiCoO 2 , LiNiO 2 , NaCoO 2 and NaNiO 2 have been studied by XPS, UPS and EELS. The interpretation of data is somewhat hampered by surface segregation, especially for the Na compounds. However, all these materials are shown by photoemission to possess a very narrow d band, in contrast to the perovskite oxides LaMO 3 , and EELS reveals the presence of local, ligand-field-type excitations for LiCoO 2 . The authors argue that this is a consequence of the layered nature of these oxides. PES also shows satellites in both core and valence regions; the valence satellite intensity appears to correlate more with ligand ionizations, rather than metal ionizations. They interpret these results in terms of semi-empirical band calculations, and an impurity model.