Cation mobility in a series of zeolite-like coordination polymers

The cation mobility for the series of coordination polymers Zn ₃A ₂[Fe(CN) ₆] ₂·xH ₂O with A = Na, K, Rb, Cs was studied from impedance spectroscopy data. Spectra for anhydrous and hydrated samples were recorded and interpreted. Two types of jumps for the cation within the cavity were detected. The regions close to the N ends of the ZnN ₄ tetrahedron, where the framework negative charge is concentrated, are the available structural positions for the cation. According to the materials formula unit, Zn ₃A ₂[Fe(CN) ₆] ₂·xH ₂O, the framework has six available sites per cation. Such fractional occupancy explains the occurrence of cation jumps between equivalent sites. Because in the structure there are two different N-N distances, the existence of two types of thermally activated jumps was predicted and really observed. The two types of cation jumps show strong dependence on both, the cation nature and the amount of water molecules found in its coordination environment. A potential contribution of proton conduction to the observed effects was discarded. An increase in the local relative humidity leads to a higher resistance for the cation mobility.