Competition between Backbonding and Electron Correlation in Fe Cyanides

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We calculate the Fe 2p X-ray absorption spectrum (XAS) of an Fe(CN)6 model cluster in order to understand the electron correlation effect on the electronic states of RbMn[Fe(CN)6], K3Fe(CN)6, and K4Fe(CN)6. The Hamiltonian includes the Fe on-site Coulomb interaction and the interatomic electron hopping terms, and it has some adjustable parameters, such as the Fe 3d–3d Coulomb interaction strength ($U$) and the Fe 3d–C 2p and C 2p–N 2p hopping strengths. We diagonalize the Hamiltonian using the basis wavefunctions that are constructed according to configuration interaction and determine the numerical values of the relevant physical parameters so as to reproduce the observed Fe 2p XAS. As shown in the studies performed so far, backbonding, in other words, the charge transfer from Fe 3d to ligand $ p\pi$ orbitals, is also important in the present study. On the other hand, we point out that $U$ plays a role of suppressing excess charge transfer. Therefore, the observed Fe 2p XAS of the Fe cyanides is realized by the competition between backbonding and electron correlation. We also note the importance of considering the realistic electronic structure of the cyano group in reproducing the Fe 2p XAS as well as the Fe 1s XAS.
2010-11-15