Thermal expansion in Prussian Blue analogs M3[Cr(CN)6]2.nH2O (M = Mn, Fe, Co, Ni)

Thermal expansion in Prussian Blue Analogs (PBAs) M₃[Cr(CN)₆]₂.nH₂O (M = Mn, Fe, Co, Ni; n = 10–16) was studied using powder X-ray diffraction (XRD) as a function of temperature in the range 123–298 K. Standard chemical precipitation was used to prepare the materials and they were characterized using standard characterization techniques XRD, X-ray fluorescence (XRF), thermogravimetric analysis (TGA) and Fourier transform infrared (FTIR) spectroscopy. All materials were found to crystallize in the cubic structure with space group $Fm\bar{3}m$. Strong compositional dependence of thermal expansion is found in this series of PBAs. While Mn₃[Cr(CN)₆]₂.12H₂O and Ni₃[Cr(CN)₆]₂.16H₂O show positive thermal expansion (PTE) behavior the other two PBAs, Fe₃[Cr(CN)₆]₂.10H₂O and Co₃[Cr(CN)₆]₂.14H₂O, show strong negative thermal expansion (NTE) behavior with as large coefficient of thermal expansion (CTE) as −19.7 x 10⁻⁶ K⁻¹ (for M = Fe) in the temperature range 123–223 K. For the PBAs showing NTE, the magnitude of NTE coefficients can be correlated with the trends for M cation size and cell (or lattice) parameter.