Structural Consequences of Different Metal Compositions in the Doped Spin-Crossover Crystals [FezM1-z(bpp)2][BF4]2 (M=Ni, Zn; bpp=2,6-Bis{Pyrazol-1-yl}Pyridine)

Variable temperature crystallographic characterization of [FezZn1-z(bpp)(2)][BF4](2) (bpp=2,6-bis{pyrazol-1-yl}pyridine; z=0.88, 0.72 and 0.27) and [FezNi1-z(bpp)(2)][BF4](2) (z=0.83, 0.72 and 0.32) is presented. Comparison with previously published data confirms the isothermal unit cell volume change during spin-crossover (Delta V-SCO) behaves differently in the Zn- and Ni-doped crystals. For the FeZn crystals, the relationship between Delta V-SCO and z is continuous for z >= 0.3 but is steeper than expected, so Delta V-SCO approximate to 0 for z=0.27. In contrast Delta V-SCO in the FeNi materials shows only a small variation between 0.83 >= z >= 0.46, before decreasing more strongly at higher dilution. Delta V-SCO in each FeZn crystal is smaller than for its FeNi analogue with a similar composition. As well as the dopant ion ionic radius, the smaller Delta V-SCO for the Zn-doped materials reflects their T-1/2 values, which are lower than for their FeNi counterparts. The contribution of T-1/2 to this behavior is especially evident at high metal dilution.