Structural and spectroscopic investigation of the charge-ordered, short-range ordered, and disordered phases of the Co\(_3\)O\(_2\)BO\(_3\) ludwigite

• Published in: arXiv.org 2021-08
• Main Authors: Galdino, C W, Freitas, D C, Medrano, C P C, Sanchez, D R, Tartaglia, R, Rabello, L P, Mendonça, A A, Ghivelder, L, Continentino, M A, Zapata, M J M, Pinheiro, C B, Azevedo, G M, Rodríguez-Velamazán, J A, Garbarino, G, Núñez-Regueiro, M, Granado, E
• Format: Article
• Language: English
• Subjects: Crystal defects; Crystal structure; Crystallography; Evolutionary algorithms; Fine structure; Iron; Ladders; Low temperature; Ludwigite; Neutron diffraction; Oxidation; Single crystals; Thermal energy; Valence; X ray absorption; X-ray diffraction
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2107.14757

In this work, we investigate the representative case of the homometallic Co ludwigite Co\(^{2+}_2\)Co\(^{3+}\)O\(_2\)BO\(_3\) (\(Pbam\) space group) with four distinct Co crystallographic sites [\(M1\)-\(M4\)] surrounded by oxygen octahedra. The mixed-valent character of the Co ions up to at least \(T=873\) K is verified through x-ray absorption near-edge structure (XANES) experiments. Single crystal x-ray diffraction (XRD) and neutron powder diffraction (NPD) confirm that the Co ions at the \(M4\) site are much smaller than the others at low temperatures, consistent with a Co\(^{3+}\) oxidation state at \(M4\) and Co\(^{2+}\) at the remaining sites. The size difference between the Co ions in the \(M4\) and \(M2\) sites is continuously reduced upon warming above \(\approx 370\) K, indicating a gradual charge redistribution within the \(M4\)-\(M2\)-\(M4\) (424) ladder in the average structure. An increasing structural disorder, is noted above \(\approx 370\) K, The local Co-O distance distribution, revealed by Co \(K\)-edge Extended X-Ray Absorption Fine Structure (EXAFS) data and analyzed with an evolutionary algorithm method, is similar to that inferred from the XRD crystal structure below \(\approx 370\) K. At higher temperatures, the local Co-O distance distribution remains similar to that found at low temperatures, at variance with the average crystal structure obtained with XRD. We conclude that the oxidation states Co\(^{2+}\) and Co\(^{3+}\) are instantaneously well defined in a local atomic level at all temperatures, however the thermal energy promotes local defects in the charge-ordered configuration of the 424 ladders upon warming. These defects coalesce into a phase-segregated state within a narrow temperature interval (\(475< T < 495\) K). Finally, a transition at \(\approx 500\) K revealed by differential scanning calorimetry (DSC) in the iron ludwigite Fe\(_3\)O\(_2\)BO\(_3\) is discussed.