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

Abstract

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· nH 2O and M II 2[Fe II(CN) 6]· nH 2O, 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 Fm3̄ m, F4̄3 m and Pm3̄ m. 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 Co 3[Co(CN) 6] 2·12H 2O. All of the M II 3[Co III(CN) 6] 2· nH 2O compounds show negative thermal expansion behavior, which correlates with the Irving–Williams series for metal complex stability. The thermal expansion behavior for the PBAs of the M II 3[Fe III(CN) 6] 2· nH 2O 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]· nH 2O compounds show positive thermal expansion behavior.