Structural and spectroscopic investigation of the charge-ordered, short-range ordered, and disordered phases of the Co3O2BO3 ludwigite

Charge-ordering is prone to occur in crystalline materials with mixed-valence ions. It is presumably accompanied by a structural phase transition, with possible exceptions in compounds that already present more than one inequivalent site for the mixed-valence ions in the charge-disordered phase [1 – 3]. 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. X-ray and neutron diffraction 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. Minor structural anomalies with no space group modification are observed near 475 and 495~K, where sharp phase transitions were previously revealed by calorimetry and electrical resistivity data. An increasing structural disorder, beyond a conventional thermal effect, is noted above $\approx 370$~K, manifested by an anomalous increment of XRD Debye-Waller factors and broadened vibrational modes observed by Raman scattering. 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). Fe$_3$O$_2$BO$_3$ also shows a transition at $\approx 500$~K as revealed by differential scanning calorimetry (DSC), indicating that high-temperature phase transitions involving short-range charge-ordered states are not restricted to the Co ludwigite.

[1] C. W. Galdino et al. Physical Review B 100, 165138 (2019)
[2] N. Kazak et al. Physical Review B 103, 094445 (2021)
[3] D. C. Fretas et al. Physical Review B 94, 174409 (2016)