Charge order inFe2OBO3: AnLSDA+Ustudy

Charge ordering in the low-temperature monoclinic structure of iron oxoborate $\left({\mathrm{Fe}}_2{\mathrm{OBO}}_3\right)$ is investigated using the local spin density approximation $\left(\mathrm{LSDA}\right)+U$ method. While the difference between $t_{2g}$ minority occupancies of ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ cations is large and gives direct evidence for charge ordering, the static “screening” is so effective that the total $3d$ charge separation is rather small. The occupied ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ cations are ordered alternately within the chain which is infinite along the $a$ direction. The charge order obtained by $\mathrm{LSDA}+U$ is consistent with observed enlargement of the $\beta$ angle. An analysis of the exchange interaction parameters demonstrates the predominance of the interribbon exchange interactions which determine the whole $L$-type ferrimagnetic spin structure.