Mixed Orbital Ground States of Fe2+ in Prussian Blues

Abstract

We report on the mixed-orbital ground state compounds analogous to the mixed-valence ones. Orbital doublet and singlet ground states of the Fe 2+ ion displayed in Mössbauer spectra of the Prussian blue analogues AyFe3−y[Co(CN)6]2 •xH2O (y 1) are interconverting to each other as temperature changes for A = K, Na. Relative weight of orbital singlet ground states increases with lowering temperature. In the alcali-free cobalticyanides, the Mössbauer spectra are dominated by quadrupole-splt doublets of the same origin with a large splitting, characteristic of the Fe 2+ species coordinated by two (at least) oxygen atoms of water molecules in cis-configuration. Single-type octahedral Fe 2+ coordination is isolated in A-filled cobalticyanides for A=Rb, Cs to be characterized by the narrower quadrupole spectrum associated with the orbital doublet ground states. Ionic exchange of K for Cs in KFe[Co(CN)6] •H2O results in predominance of the orbital singlet, i.e., in enhanced anisotropy of the Fe 2+ valence electrons. We found hence that the Fe 2+ charge distribution can be modified together with crystal size at the step of synthesis owing to distortion isomerism.