Charge states of cobalt ions in nanostructured lithium cobaltite: X-ray absorption and photoelectron spectra

The charge states of ions in nanostructured lithium cobaltite prepared by severe plastic deformation under pressure have been determined using X-ray absorption spectroscopy and photoelectron spectroscopy, as well as calculations of the atomic multiplets with allowance for the charge transfer. It has been found that small deformations (pressures up to 5 GPa and angles of anvil rotation up to 30$^\circ$) lead to the generation of lithium vacancies in the bulk of the nanostructured material and the formation of the Li$_2$O phase on the surface. The charge compensation occurs at the expense of holes in oxygen 2$p$ states; the electronic configuration of cobalt ions is 3$d^6\underline L$, where $\underline L$ is a hole in oxygen 2$p$ states. It has been shown that nanostructured lithium cobaltite belongs to the class of insulators with a negative charge transfer energy. An increase in the degree of deformation of lithium cobaltite (at a pressure up to 8 GPa) leads to the formation of Co$^{2+}$ ions (with the electronic configuration 3$d^7$).