The Fe-Cu intersite charge transfer and Fe charge disproportionation are interesting phenomena observed in some $Ln$Cu${}_3$Fe${}_4$O${}_{12}$ ($Ln$ = lanthanides) compounds containing light and heavy $Ln$ atoms, respectively. We show that a change in the spin state is responsible for the intersite charge transfer in the light $Ln$ compounds. At the high-spin state, such systems prefer an unusual Cu $d^8$ configuration, whereas at the low-spin state they retreat to the normal Cu $d^9$ configuration through a charge transfer from Fe to the Cu $3d_{xy}$ orbital. We find that the strength of the crystal-field splitting and the relative energy ordering between Cu $3d_{xy}$ and Fe $3d$ states are the key parameters determining the intersite charge transfer (charge disproportionation) in light (heavy) $Ln$ compounds. It is further proposed that the size of $Ln$ affects the on-site interaction strength of Cu 3$d$ states, leading to a strong modification of the Cu $L_3$-edge spectrum, as observed by the x-ray-absorption spectroscopy.

• Received 15 December 2013
• Revised 12 March 2014

DOI:https://doi.org/10.1103/PhysRevB.89.125125