Interactions between alkali cations and cyanide-bridged network in A$_2$Co$_2$[Fe(CN)$_6$]$_{3.3}$ Prussian blue analogues revealed by far-infrared spectroscopy

A series of Prussian Blue Analogues (PBAs) having the chemical formula A$_2$Co$_2$[Fe(CN)$_6$]$_{3.3}$.nH$_2$O (A+ = Na+, Rb+ and Cs+) was investigated by variable temperature far-infrared spectroscopy. Despite the same chemical composition of the cyanide-bridged CoFe network, the three compounds are known to exhibit different switching properties assignable to different interactions between the alkali cations and the cyanide-bridged CoFe network. Na$_2$CoFe exhibits the thermally activated Co$^{II}$(High Spin)Fe$^{III}$ - Co$^{III}$(Low Spin)Fe$^{II}$ electron transfer upon cooling, whereas Rb$_2$CoFe and Cs$_2$CoFe are already in the Co$^{III}$(Low Spin)Fe$^{II}$ electronic state at room temperature and remain in this electronic state upon cooling. Our variable temperature far-infrared spectroscopy study shows that the bands corresponding to the vibrations of the cyanide-bridged CoFe network and to the alkali cations exhibit different thermal behaviors upon cooling in the three compounds. These differences can be related to different interactions between the alkali cations and the cyanide-bridged CoFe network. Thus, far Infrared spectroscopy turns out to be a valuable tool to study these interactions so difficult to experimentally probe but which nevertheless play a crucial role in the properties of the compounds.