Partial melting of the charge order in the mixed-valent homometallic ludwigite Co₂²⁺Co³⁺BO₅

Co₂²⁺Co³⁺BO₅ is investigated from 6 to 800 K by single crystal X-ray diffraction (XRD), Raman spectroscopy, neutron diffraction (NPD) and Co K-edge X-ray absorption spectroscopy (XANES and EXAFS), where the latter is analyzed by reverse Monte Carlo methods with a genetic algorithm [1]. XANES data demonstrate the coexistence of Co²⁺ and Co³⁺ ions over the investigated temperature interval. XRD, NPD and EXAFS data show that all four Co1-Co4 sites present nearly rigid CoO₆ octahedra up to ~400 K, with the smallest Co4 site hosting the Co³⁺ ions. Above 400 K, a substantial lattice disorder is perceived, accompanied by an increment (reduction) of the mean Co4-O (Co2-O) distances up to ~500 K that indicates a redistribution of the Co charges inside the Co4-Co2-Co4 ladders in this temperature interval [2]. The phonon Raman spectra are sensitive to the partial melting of charge order at ~400-500 K, also showing anomalies related to short-range magnetic correlations below ~70 K and the long-range magnetic ordering transition at T_C = 43 K. Our results bring insight into the nature of the electronic states and lattice configurations in this intriguing mixed-valent Co oxide.
[1] J. Timoshenko et al., J. Phys.: Condens. Matter 26, 055401 (2014).
[2] C. Galdino et al., PRB 100, 165138 (2019).