Cooperativity in a spin transition ferrous polymer: Interacting domain model, thermodynamic, optical and EPR study

A new thermodynamic model is proposed in order to account for the high spin <-> low spin conversion in metal-organic polymers. The model, based on the idea that the spin conversion occurs in interacting domains of like-spin m etal ions, allows to explain most of the important features of various type s of spin conversion. The sine qua non condition of the existence of spin t ransitions with hysteresis is obtained. In the case of very large cooperati vity, the model predicts unusual behaviour of the spin conversion system du e to a low-temperature metastable high spin state. Existence of such a stat e is interesting in the context of the light induced excited spin state tra pping recently observed in some ferrous compounds. The model is applied to interpret the spin transition in polycrystalline ferrous polymer [Fe1-yCuy (Htrz)(2)trz] (BF4) with y = 0.00, 0.01 and 0.10, detected by differential scanning calorimetry, optical reflectivity and electron paramagnetic resona nce. The domain size and the interaction energy between the domains are est imated as, respectively, n = 11 and Gamma = 2.010 kJ mol(-1) for the y = 0 compound. As the copper content is growing, n and Gamma tend to decrease, r esulting in transformations of the shape of hysteresis loop which becomes l ess steep, narrows and shifts to lower temperatures. The electron paramagne tic resonance gives further evidence of the presence of like-spin domains.