Spin-state ordering and intermediate states in the mixed-valence cobalt oxyborate Co3O2BO3 with spin crossover

Spin-state ordering - a periodic pattern of ions with different spin-state configurations along a crystal lattice - is a rare phenomenon, and its possible interrelation with other electronic degrees of freedom remains little explored. Here we perform a structural investigation of the mixed-valence Co homometallic ludwigite Co₂²⁺Co³⁺O₂BO₃. A superstructure consistent with a long-range Co³⁺ spin-state ordering is observed between T₄=580 K and T₃=510 K. Intermediate states with mesoscopic correlations are detected below T₃ down to T₁=480 K with a change of dimensionality at T₂=495 K. The spin-state correlations are connected to the charge sector as revealed by the abrupt changes in the electrical resistance at T₁ and T₂. The evolution of the structural parameters below T₁ indicate that the spin crossover is ignited by a moderate degree of thermally-induced Co²⁺/Co³⁺ charge disorder. Charge and spin-state degrees of freedom can be interrelated in mixed-valence spin-crossover materials, leading to sharp transitions involving intermediate spin-state/charge correlated states at the mesoscale.