Magnetic, Electronic, Structural, And Thermal Properties Of The Co3o2bo3 Ludwigite In The Paramagnetic State

The mixed-valent homometallic ludwigite (Co-2(2+) Co3+)O2BO3 is investigated above the ferrimagnetic ordering temperature T-c = 43 K through structural, thermal, magnetic, electric, and spectroscopic probes. X-ray absorption at the Co L-2,L-3 edges is consistent with the coexistence of Co2+ and Co (3+ )ions, as expected by the sample stoichiometry. Magnetic susceptibility shows a relatively large net paramagnetic moment per Co3+ ion above room temperature, p = 4.87, indicating that the Co3+ ions are not in a pure low-spin configuration at high temperatures, also showing a non-Curie-Weiss behavior below 300 K. Electrical conductivity and differential scanning calorimetry measurements on single crystals indicate two phase transitions at similar to 475 and similar to 495 K. X-ray powder diffraction shows substantial lattice parameter anomalies below 500 K. These results indicate phase transitions associated with changes in the Co oxidation state in each of its four crystallographic sites. Such transitions are possibly dictated by a competition between (i) an ordered ground state with all Co3+ ions occupying the same crystallographic site and (ii) either partially or totally charge-disordered states that are favored at high temperatures due to their higher entropy.