HYSTERESIS COOPERATIVITY

In general, therefore, there are three processes, prior to the kind of capillary condensation associated with the hysteresis loop of a Type IV isotherm, which may occur in a porous body containing micropores along with mesoporesia primary process taking place in very narrow micropores a secondary, cooperative process, taking place in wider micropores, succeeded by a tertiary process governed by a modified Kelvin equation. 

On the other hand, when associated with [Ni(dmit)2]-, [Fe(sal2-trien)]+ exhibits a cooperative spin transition behavior with a wide hysteresis loop (30 K) around... 

Somewhat unusual pressure dependence of the nature of the spin transition curve has been found for chain-like SCO systems containing substituted bridging triazole ligands [163, 164]. Although the transition is displaced to higher temperatures with increase in pressure, the shape of the transition curve, unusually, is effectively constant, i.e. there is no significant change in the hysteresis width and the transition remains virtually complete. This has been taken to indicate that the cooperativity associated with the transitions in these and related systems is confined within the iron(II) triazole chains. 

These structural features observed for [Fe(abpt)2(TCNQ)2] involving pronounced and extended n-n stacking interactions lead to a duality with respect to its gradual spin crossover behaviour. It has generally been accepted that extended n-n interactions may lead to the occurrence of thermal hysteresis in mononuclear Fe(II) spin crossover compounds [62-65]. Clearly, the requirements responsible for cooperative Fe(II) spin crossover behaviour are not easy to define, since obviously [Fe(abpt)2(TCNQ)2] represents an exception to this rule in spite of the pronounced TCNQ n-n stacking interactions, the Fe(II) spin crossover displays at best weak cooperativity. 

In these polymeric species, the M,AT2-1,2,4-triazole linkage is rigid, and allows an efficient transmission of cooperative effects. Consequently, abrupt ST with broad thermal hysteresis loops have been observed [26, 32-34]. The absorption spectra of these compounds show a broad band at 520 nm corresponding to the Aig Trg d-d transition in the LS state whereas no band is found in the visible region in the HS state, the 5T2g-5Eg transition being located around 850 nm [7a]. The ST is thus accompanied by a thermochromic effect, purple (LS) and white (HS). These characteristics make these compounds potential candidates for practical applications, e.g. thermal display devices [7, 8, 17]. Such behaviour has been observed, for example, in the compound [Fe(4-amino-l,2,4-triazole)3](NC>3)2 [32] whose SCO is associated with a hysteresis loop of width 35 K, centred above room temperature [8]. 

The influence of pressure has also been used to tune the ST properties of these ID chain compounds. Application of hydrostatic pressure ( 6 kbar) on [Fe(hyptrz)3] (4-chlorophenylsulfonate)2 H20 (hyptrz=4-(3 -hydroxypro-pyl)-l,2,4-triazole) provokes a parallel shift of the ST curves upwards to room temperature (Fig. 5) [41]. The steepness of the ST curves along with the hysteresis width remain practically constant. This lends support to the assertion that cooperative interactions are confined within the Fe(II) triazole chain. Thus a change in external pressure has an effect on the SCO behaviour comparable to a change in internal electrostatic pressure due to anion-cation interactions (e.g. changing the counter-anion). Both lead to considerable shifts in transition temperatures without significant influence on the hysteresis width. Several theoretical models have been developed to predict such SCO behaviour of ID chain compounds under pressure [50-52]. Figure 5 (bottom) also shows the pressure dependence of the LS fraction, yLS, of... 

The magnetic properties of the py and pz compounds are compared in Fig. 15. The 3D derivatives undergo more strongly cooperative spin transitions than the corresponding 2D counterparts as indicated by the increase in width of the hysteresis loop (range 20-40 K). The significantly higher transition temperatures observed for the pz derivatives compared with their py counterparts cannot be explained in terms of the spectrochemical series... 

The great majority of transitions observed for iron(III) are gradual and the observation of thermal hysteresis associated with them is relatively rare. In the only instances where features indicative of significant cooperativity have been reported, extensive hydrogen-bonding networks (formed in some thiosemicarbazone compounds [111, 115,118, 119]) or n-n stacking interactions (operative in several compounds of N20 Schiff base systems [135-138, 164, 165]) have been invoked as the origin of the cooperativity. 

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