X-Ray Spectroscopy of Cobaltites

Abstract

The ability of cobalt to take, depending on the temperature, pressure, and doping, various charge and spin states in oxide compounds periodically becomes the topic of scientific controversy and is a frequent cause of ambiguity in the interpretation of experimental results. In this review, we will discuss cobaltites, i.e., oxide compounds of cobalt, in which cobalt is in an oxidation state (charge state) of 3+. In these compounds, Co3+ ions can take the following spin configurations: high-spin, low-spin, and intermediate-spin configurations. The conditions for the formation of various spin states are given on the basis of the Tanabe–Sugano diagrams. It is shown how X-ray spectral methods, such as X-ray photoelectron spectroscopy, X-ray emission spectroscopy, X-ray absorption spectroscopy, and X-ray magnetic dichroism, can be used to study cobaltites. By using Co L2,3 X-ray absorption spectra that have a rich structure, one can determine the charge and spin states of cobalt ions. These spectra can be reproduced by atom-like multiplet calculations that involve many electrons and use the energy splitting of atomic orbitals by the ligand field as a free parameter. Absorption K spectra of oxygen are of a band nature, i.e., these spectra can be used, by mixing the 3d states of cobalt with the 2p states of oxygen, to estimate the spin state of cobalt ions and to analyze cobaltite phases after external influences. It is shown that the spin states of cobalt ions can be determined from Co Kβ X‑ray emission spectra (electronic transition $$3p \to 1s$$ ) that are sensitive to the spin state of 3d electrons because of the strong exchange interaction between the 3p hole and 3d orbitals in the final state of the emission process. A classic example of cobaltites with a low-spin configuration of cobalt ions is lithium cobaltite LiCoO2 that is a well-known material for power source cathodes. The nature of holes in defective cobaltites of the LixCoO2 (x < 1) type is considered. It is shown that holes arising from defects in the lithium sublattice are of an oxygen nature. In cobaltite LaCoO3 that also contains trivalent cobalt ions, a transition from a low-spin state to a high-spin state occurs at temperatures above 90 K. The feasibility of an intermediate-spin state in this compound is also considered. The results of X-ray spectral studies of double substitution perovskites Ln1 – xAxCo1 – yMyO3 (Ln is a lanthanide, A is Ca or Sr, and M is a transition element) are given, and changes in the charge states of cobalt ions upon doping of perovskites are shown. The results of experimental determination of spin states in octahedrons and pyramids of cobaltites LnBaCo2O6 – δ ( $$0 \leqslant \delta \leqslant 1$$ ) at temperatures below and above the temperature of the metal-insulator transition are systematized. Methods for determining spin states on the basis of X-ray spectral data are discussed. It is shown in the last section of the article how the charge state and concentration of cobalt ions in doped layered cobaltites LnBaCo4O7 can be determined using X-ray absorption spectroscopy. In these compounds, cobalt ions are in tetrahedral positions and, consequently, trivalent cobalt ions should be exclusively in the high-spin state.