Spin-state ordering and intermediate states in the mixed-valence cobalt oxyborate Co${}_3$O${}_2$BO${}_3$ with spin crossover

Abstract

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 Co${}_2^{2+}$Co${}^{3+}$O${}_2$BO${}_3$. A superstructure consistent with a long-range Co${}^{3+}$ spin-state ordering is observed between $T_4=580$ K and $T_3=510$ K. Intermediate states with mesoscopic correlations are detected below $T_3$ down to $T_1=480$ K with a change of dimensionality at $T_2=495$ K. The spin-state correlations are connected to the charge sector as revealed by the abrupt changes in the electrical resistance at $T_1$ and $T_2$. The evolution of the structural parameters below $T_1$ indicate that the spin crossover is ignited by a moderate degree of thermally-induced Co${}^{2+}$/Co${}^{3+}$ 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.

arXiv comment28 pages, 8 figures

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