| Abstract
| - The two-dimensional (2D) polymeric spin crossover (SCO) compound Fe(py)2[Ag(CN)2]2 has been synthesized.The compound shows a two-step spin transition detected by magnetic, heat capacity, and X-ray diffractionmeasurements. The magnetic moment shows a high-temperature step (step 1) occurring at 146.3 K withouthysteresis, while the low-temperature step (step 2) happens at 84 K on cooling and 98.2 K on heating. Thesemeasurements reveal a large amount of residual high spin (HS) species (23%) and that HS state trappingoccurs at cooling rates of around 1 K min-1 or higher. The two-step behavior has been confirmed by heatcapacity, which gives, for steps 1 and 2, respectively, ΔH1 = 3.33 kJ mol-1, ΔS1 = 22.6 J mol-1 K-1, andΔH2 = 1.51 kJ mol-1, ΔS2 = 15.7 J mol-1 K-1. For step 2 a hysteresis of 10 K has been determined withdynamic measurements. Powder X-ray diffraction at room temperature shows that the compound is isostructuralto Cd(py)2[Ag(CN)2]2 previously reported. Powder X-ray diffraction indicates that there is only onecrystallographic site for iron(II) in the whole temperature range, confirmed by Mössbauer spectroscopy. TheX-ray diffraction study at different temperatures do not show any superstructure in the region between thetransitions, discarding a crystallographic phase transition as the origin of the two-step behavior. However, anunexpected increase of the thermal factor is detected on lowering the temperature and considered as amanifestation of a disordered state between the two steps, consisting of a mixing of HS and LS specieswithout long-range order.
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