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 L_2,3 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 Li_xCoO₂ (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 Ln_{1 – x}A_xCo_{1 – y}M_yO₃ (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 LnBaCo₂O_{6 – 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.

中文翻译：

---

钴的X射线光谱

摘要

钴根据温度，压力和掺杂而周期性地吸收氧化物化合物中各种电荷和自旋态的能力成为科学争议的话题，并且经常在解释实验结果时产生歧义。在这篇综述中，我们将讨论钴，即钴的氧化物，其中钴处于3+的氧化态（电荷态）。在这些化合物中，Co ³⁺离子可以采取以下自旋配置：高自旋，低自旋和中自旋配置。根据田边一states野图给出了形成各种自旋态的条件。它显示了如何使用X射线光谱方法（例如X射线光电子能谱，X射线发射光谱，X射线吸收光谱和X射线磁二色性）来研究钴。通过使用具有丰富结构的Co L _2,3 X射线吸收光谱，可以确定钴离子的电荷和自旋态。这些光谱可以通过涉及多个电子的类原子多重峰计算来重现，并利用配体场对原子轨道的能量分裂作为自由参数。吸收钾 氧的光谱具有带性质，即可以通过将3d钴状态与2p氧状态混合，估计钴离子的自旋态并在外部影响后分析钴相来使用这些光谱。结果表明，钴离子的自旋态可以通过Co KβX射线发射光谱（电子跃迁\（3p \ to 1s \））确定，该光谱对3 d电子的自旋态很敏感，因为它们之间的交换很强在发射过程的最终状态下3 p孔和3 d轨道之间的相互作用。钴离子具有低自旋构型的钴酸锂的经典示例是锂钴酸锂LiCoO_图2是用于电源阴极的公知材料。考虑了Li _x CoO ₂（x <1）型缺陷钴矿中空穴的性质。结果表明，由锂亚晶格缺陷引起的空穴具有氧性质。在还包含三价钴离子的钴LaCoO _3中，在高于90 K的温度下会发生从低自旋态到高自旋态的转变。还考虑了该化合物中自旋态的可行性。钙钛矿双取代Ln _{1 – x} A _x Co _{1 – y} M _y O ₃的X射线光谱研究结果给出了（Ln是镧系元素，A是Ca或Sr，M是过渡元素），并且示出了在钙钛矿掺杂时钴离子的电荷态的变化。系统地确定了钴金属LnBaCo ₂ O _6-δ（\（0 \ leqslant \ delta \ leqslant 1 \））的八面体和棱锥中自旋态的实验结果。讨论了基于X射线光谱数据确定自旋状态的方法。本文的最后一部分显示了掺杂层状钴LnBaCo ₄ O ₇中钴离子的电荷状态和浓度可以使用X射线吸收光谱法测定。在这些化合物中，钴离子处于四面体位置，因此，三价钴离子应仅处于高自旋状态。