Subías Gloria, Blasco Javier, Lafuerza Sara, Cuartero Vera, Sánchez M Concepción, Boada Roberto, Díaz-Moreno Sofía, Fauth François, García Joaquín
Instituto de Nanociencia y Materiales de Aragón (INMA), CSIC-Universidad de Zaragoza, Zaragoza 50009, Spain.
Departamento de Física de la Materia Condensada, Universidad de Zaragoza, Zaragoza 50009, Spain.
Inorg Chem. 2020 Nov 2;59(21):15757-15771. doi: 10.1021/acs.inorgchem.0c02174. Epub 2020 Oct 19.
We have investigated the role of oxygen stoichiometry and structural properties in the modulation of Co valence and spin state in single-layer LaACoO (A = Sr, Ca; 0 ≤ ≤ 1) perovskites as well as the interplay between their local structural properties and the magnetic and charge-ordering phenomena. We show the results of high angular resolution powder X-ray diffraction and Co -edge X-ray absorption and emission spectroscopy experiments on polycrystalline and single-crystal samples. The different doping-induced changes in the Co valence and spin state by Ca (or Sr) substitution can be understood in terms of the evolving oxygen stoichiometry. For Ca doping, the interstitial oxygen excess around the La/Ca atoms in underdoped samples is rapidly lost upon increasing the Ca content. The creation of oxygen vacancies leads to the stabilization of a mixed-valence Co independently of the Ca content. In contrast, Sr substitution leads to almost stoichiometric samples and a lower oxygen vacancy concentration, which allows higher mixed-valence states for Co up to Co. The Co mixed-valence state along the two series is fluctuating between two valence states, Co as in LaCoO and Co as in LaSrCoO, that become periodically ordered for the charge-ordered phases around the half-doping. The X-ray emission derived spin states agree well with the Co fluctuating mixed-valence state derived from X-ray absorption spectroscopy on consideration of a distribution of high-spin Co and low-spin Co. Furthermore, there is no quenching of the orbital contribution for the high-spin Co, as concluded from a comparison with macroscopic magnetization measurements. Doping holes are mainly located in the plane and have a strong oxygen 2p character. The major lattice distortions, which are different for Sr and Ca doping, occur along the axis, where changes in the oxygen stoichiometry take place. Moreover, charge-order transitions are clearly shown from the anomalous increase of the lattice parameter with an increase in the temperature above 500 K but there is no signature for a temperature-dependent spin-state transition.
我们研究了氧化学计量和结构性质在单层LaACoO（A = Sr、Ca；0 ≤ ≤ 1）钙钛矿中对Co价态和自旋态的调制作用，以及它们的局部结构性质与磁有序和电荷有序现象之间的相互作用。我们展示了对多晶和单晶样品进行的高角分辨率粉末X射线衍射以及Co K边X射线吸收和发射光谱实验的结果。通过Ca（或Sr）取代引起的Co价态和自旋态的不同掺杂诱导变化，可以根据不断变化的氧化学计量来理解。对于Ca掺杂，欠掺杂样品中La/Ca原子周围的间隙氧过量在增加Ca含量时会迅速损失。氧空位的产生导致了混合价Co的稳定，而与Ca含量无关。相比之下，Sr取代导致几乎化学计量的样品和较低的氧空位浓度，这使得Co能够达到更高的混合价态直至Co³⁺。沿着这两个系列的Co混合价态在两个价态之间波动，即LaCoO中的Co³⁺和La₀.₅Sr₀.₅CoO中的Co²⁺，在半掺杂附近的电荷有序相中它们会周期性地有序排列。考虑到高自旋Co和低自旋Co的分布，X射线发射得出的自旋态与X射线吸收光谱得出的Co波动混合价态非常吻合。此外，与宏观磁化测量结果比较得出，高自旋Co的轨道贡献没有淬灭。掺杂空穴主要位于ab平面，并且具有很强的氧2p特征。Sr和Ca掺杂不同的主要晶格畸变沿c轴发生，氧化学计量在该轴上发生变化。此外，在温度高于500 K时，随着温度升高，c晶格参数异常增加，清晰地显示出电荷有序转变，但没有温度依赖性自旋态转变的迹象。
