Thermal hysteresis

The transformation is beHeved to occur by a diffusionless shear process (83). It is often referred to as martensitic transformation, having a thermal hysteresis between the cooling and heating cycles. The transformation is dependent on particle size finer particles transforming at a lower temperature than... 

The small effects and the progressive evolutions observed seem to be in agreement with a second order phase transition. The lack of coexistence of the low- and high-temperature phases in the temperature range close to the transition temperature and the apparent lack of thermal hysteresis confirm this conclusion. 

CaCl2 formed wide but relatively short strands without forming apparent continuous network structures in most cases (Fig. 6.13D). For i-carra-geenan, the addition of or Ca formed localized networks through side-by-side aggregation between helices, which was consistent with the thermal hysteresis between sol-to-gel and gel-to-sol transitions. However, this interhelical aggregation was not necessarily a prerequisite for gelation. 

FIGU RE 8.5 The thermal hysteresis that occurs when the rate of the phase change on cooling is lower than that on heating at the same temperature. 

Fig. 9 Magnetic susceptibility measurements for [Fe(sal2-trien)][Ni(dmit)2] showing the thermal hysteresis loop...

Several other light-induced phenomena associated with spin transition systems have recently been reported. These include light induced thermal hysteresis (LITH), which is another example of light induced bistability, discovered for the SCO compound [Fe(PMBiA)2(NCS)2] which undergoes a very abrupt thermal ST around 170 K with hysteresis [174]. Irradiation of the sample at 10 Kwith green light resulted in the population of the LIESST state. When the temperature was raised to 100 K and lowered back to 10 K under continuous irradiation a wide thermal hysteresis loop resulted. The... 

Among all Fe(II) spin crossover compounds known to date, the extensively studied polymeric [Fe(4-R-l,2,4-triazole)3](anion)2 systems (R=amino, alkyl, hydroxyalkyl) appear to have the greatest potential for technological applications, for example in molecular electronics [1, 24, 25] or as temperature sensors [24, 26]. This arises because of their near-ideal spin crossover characteristics pronounced thermochromism, transition temperatures near room temperature, and large thermal hysteresis [1, 24, 27]. 

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. 

Light-induced excited spin state trapping Light-induced thermal hysteresis 4- (p-Methoxyphenyl)-1,2,4-triazole 4- (p-Tolyl) -1,2,4-triazole Pyridine... 

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]. 

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