Soft x-ray absorption spectroscopy study of the electronic structures of the MnFe Prussian blue analogs (Rb_xBa_y)Mn_([3-(x+2y)]/2)[Fe(CN)_6]H_2O

摘要

The electronic structures of Prussian blue analog (Rb_xBa_y)Mn_([3-(x+2y)]/2)[Fe(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 Fe~(2+)-Fe~(3+) mixed valent, with an average valency of v(Fe) ~ 2.8 and nearly divalent (Mn~(2+)), respectively. Our Mn/Fe 2p XMCD study supports that Mn~(2+) ions are in the high-spin states while Fe~(2+)-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 ≤ T ≤ 300 K, suggesting that a simple charge transfer upon cooling from Fe~(3+)-CN-Mn~(2+) to Fe~(2+)-CN-Mn~(3+) does not occur in (Rb_xBa_y)Mn_([3-(x+2y)]/2)[Fe(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 Fe~(3+) ions have a strong covalent bonding with the C ≡ N ligands and are under a large crystal-field energy of 10Dq ~ 3 eV, in contrast to the weak covalency effect and a small 10Dq ~ 0.6 eV for the divalent Mn~(2+) ions.