Far‐Infrared Spectroscopic Investigation of CoFe Prussian Blue Analogues at Thermally Activated Electron Transfer

Abstract A series of Prussian Blue Analogues (PBAs) having the chemical formula K x Cs y Co 4 [Fe(CN) 6 ] (8+x+y)/3 ⋅nH 2 O was investigated by variable temperature far‐infrared spectroscopy. The compounds with the lowest amounts of interstitial alkali cations exhibit the thermally activated Co II (High Spin)Fe III ⋅ Co III (Low Spin)Fe II electron transfer, whereas above a threshold of two alkali cations per unit cell the compounds are in the Co III (Low Spin)Fe II electronic state at room temperature and remain in this electronic state over the 300–15 K temperature range. Our variable temperature study shows that FIR spectroscopy turns out to be particularly well‐suited to probe local interactions between the alkali cations in interstitial sites of PBAs and the surrounding cyanide‐bridged bimetallic network as well as to disentangle electronic and structural phenomena involved in the switching properties.