Thermal expansion in 3d-metal Prussian Blue Analogs - A survey study

We present a comprehensive study of the structural properties and the thermal expansion behavior of 17 different Prussian Blue Analogs (PBAs) with compositions M^II ₃[(M′)^III(CN) ₆]₂·nH₂O and M^II ₂[Fe^II(CN)₆]·nH₂O, where M^II=Mn, Fe, Co, Ni, Cu and Zn, (M′)^III=Co, Fe and n is the number of water molecules, which range from 5 to 18 for these compounds. The PBAs were synthesized via standard chemical precipitation methods, and temperature-dependent X-ray diffraction studies were performed in the temperature range between -150 °C (123 K) and room-temperature. The vast majority of the studied PBAs were found to crystallize in cubic structures of space groups Fm3m, F43m and Pm3m. The temperature dependence of the lattice parameters was taken to compute an average coefficient of linear thermal expansion in the studied temperature range. Of the 17 compounds, 9 display negative values for the average coefficient of linear thermal expansion, which can be as large as 39.7×¹0^-6 K^-1 for Co₃[Co(CN)₆]₂·12H₂O. All of the M^II ₃[Co^III(CN)₆] ₂·nH₂O compounds show negative thermal expansion behavior, which correlates with the IrvingWilliams series for metal complex stability. The thermal expansion behavior for the PBAs of the M ^II ₃[Fe^III(CN)₆]₂· nH₂O family are found to switch between positive (for M=Mn, Co, Ni) and negative (M=Cu, Zn) behavior, depending on the choice of the metal cation (M). On the other hand, all of the M^II ₂[Fe ^II(CN)₆]·nH₂O compounds show positive thermal expansion behavior.