Thermally-Induced Spin-Crossover in Fe_1-xT_x(pyrazine)[Fe(CN)₅NO] with T=Co, Ni - Effects of Iron Atom Dilution

2D ferrous nitroprusside with the pyrazine molecule as a pillar between adjacent layers shows a thermally induced spin-crossover behavior. This supposes that the energy gap between the high and low spin states is the order of kT in the temperature region where such a spin transition is observed. If a fraction of the iron atoms involved in that transition is progressively replaced by a second metal, to form a solid solution (alloy), the thermal effect could be modified due to the iron atom dilution, but such behavior also depends on the bonding properties of the ligands for the iron atom and of the NO and CN interaction in the interlayer region. This hypothesis was the idea that motivated the preparation of the titled systems of solid solutions and their study from magnetic and Mossbauer data recorded at different temperatures in the range of 5-300 K, complemented with XRD, Raman, and IR measurements. Co and Ni were considered to form the solid solutions under study because they form isostructural solids in the parent 3D coordination polymers. For both metals, the formed solids show a spin-crossover effect for all the compositions range but from a certain degree of dilution for the iron atom, above 75 % substitution by the second metal, the thermal hysteresis almost disappears. This suggests that from that dilution degree, the spin transition involves structural changes at the local coordination environment for the iron atom but not for the entire solid framework. This hypothesis is properly supported by the recorded Raman and Mossbauer spectra. The presence of Co and Ni atoms in the solid solutions results in higher stability for the entropy-driven high spin state. This is appreciated as lower values for T down arrow(HS),(1/2), and T up arrow(LS),(1/2) corresponding to the temperatures where the subscript 1/2 indicates the temperature where 50 % of the metal centers have changed their spin state.