Compositional dependence of negative thermal expansion in the Prussian blue analogues M^IIPt^IV(CN)₆ (M = Mn, Fe, Co, Ni, Cu, Zn, Cd)

The effect of M^II substitution on the magnitude of the negative thermal expansion (NTE) behavior within a series of Prussian Blue analogues, M^IIPt^IV(CN)₆ for M^II = Mn, Fe, Co, Ni, Cu, Zn, Cd, has been investigated using variable-temperature powder X-ray diffraction (100-400 K). The NTE behavior varies widely with M^II substitution, from near zero thermal expansion in NiPt(CN)₆ (α = dl/l dT = -1.02(11) × 10^-6 K^-1) up to a maximum in CdPt(CN)₆ (α = -10.02(11) × 10^-6 K ^-1). The trend in the magnitude of the NTE behavior, with increasing atomic number (Z) of the M^II ion, follows the order Mn^II > Fe^II > Co^II > Ni^II < Cu ^II < Zn^II < Cd^II, which correlates with the trends for M^II cation size, the lattice parameter, and structural flexibility as indicated by the temperature-dependent structural refinements and Raman spectroscopy, Analysis of the temperature dependence of the average structures suggests that the differences in the thermal expansion are due principally to the different strengths of the metal-cyanide binding interaction and, accordingly, the different energies of transverse vibration of the cyanide bridge, with enhanced NTE behavior for more flexible lattices.