Soft x-ray absorption spectroscopy study of the electronic structures of the MnFe Prussian blue analogs (RbxBay)Mn[3−(x+2y)]/2[Fe(CN)6]H2O

Abstract

The electronic structures of Prussian blue analog $({\mathrm{Rb}}_{x}{\mathrm{Ba}}_{y}){\mathrm{Mn}}_{[3\ensuremath{-}(x+2y)]/2}[\mathrm{Fe}{(\mathrm{CN})}_{6}]$ cyanides have been investigated by employing soft x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) at the Fe and Mn $L$ $(2p)$ edges. The measured XAS spectra have been analyzed with the configuration-interaction (CI) cluster model calculations. The valence states of the Fe and Mn ions are found to be ${\mathrm{Fe}}^{2+}\text{\ensuremath{-}}{\mathrm{Fe}}^{3+}$ mixed valent, with an average valency of $v(\mathrm{Fe})\ensuremath{\sim}2.8$ and nearly divalent $({\mathrm{Mn}}^{2+})$, respectively. Our Mn/Fe $2p$ XMCD study supports that ${\mathrm{Mn}}^{2+}$ ions are in the high-spin states while ${\mathrm{Fe}}^{2+}\text{\ensuremath{-}}{\mathrm{Fe}}^{3+}$ ions are in the low-spin states. The Fe and Mn $2p$ XAS spectra are found to be essentially the same for $80\ensuremath{\le}T\ensuremath{\le}300$ K, suggesting that a simple charge transfer upon cooling from ${\mathrm{Fe}}^{3+}\text{\ensuremath{-}}\mathrm{CN}\text{\ensuremath{-}}{\mathrm{Mn}}^{2+}$ to ${\mathrm{Fe}}^{2+}\text{\ensuremath{-}}\mathrm{CN}\text{\ensuremath{-}}{\mathrm{Mn}}^{3+}$ does not occur in $({\mathrm{Rb}}_{x}{\mathrm{Ba}}_{y}){\mathrm{Mn}}_{[3\ensuremath{-}(x+2y)]/2}[\mathrm{Fe}{(\mathrm{CN})}_{6}]$. According to the CI cluster model analysis, it is necessary to take into account both the ligand-to-metal charge transfer and the metal-to-ligand charge transfer in describing Fe $2p$ XAS, while the effect of charge transfer is negligible in describing Mn $2p$ XAS. The CI cluster model analysis also shows that the trivalent ${\mathrm{Fe}}^{3+}$ ions have a strong covalent bonding with the $\mathrm{C}\ensuremath{\equiv}\mathrm{N}$ ligands and are under a large crystal-field energy of $10Dq\ensuremath{\sim}3$ eV, in contrast to the weak covalency effect and a small $10Dq\ensuremath{\sim}0.6$ eV for the divalent ${\mathrm{Mn}}^{2+}$ ions.