Order-disorder transformations detection

A modification of the simple grazing-angle experiment is the multiple reflection -absorption spectroscopy, which was first described by Gaines (Figure 6) (24). It has a unique value for the detection of order-disorder transformations in the structure of a... 

Reports on order-disorder transformations by heating are uncommon in zeolite literature, probably because of a lack of careful investigation on this subject and objective detection difficulties. As far as the framework (Si, Al) distribution order... 

Sharpataya et al. [37] measured the heat capacity of caesium hexafluoroarsenate in the temperature range 300-850 K and detected a solid-solid transformation from the rhombohedral to cubic phase, occurring in ftie range 235-360 K. The transition has been interpreted as an order-disorder type, and its enthalpy and entropy have been determined. The magnetocaloric effect has been discussed by Kalva and Sestak [38], who proposed ftie use of a quasi-particle formalism to model it. 

One way to see that a transition is discontinuous is to detect a coexistence of two phases, in this case the orientationally ordered and disordered phases, in a temperature interval. This is revealed by time variation of the potential energy of the cluster. In the temperamre region of phase coexistence, each cluster dynamically transforms between the phases, and its potential energy fluctuates around two different mean values (Fig. 4). In an ensemble of clusters, the coexistence of different phases is observable insofar as a fraction of the clusters (e.g., in a beam [17]) can exhibit the structure of one phase, while another fraction takes on the stmcture of another phase. 

Fredrickson and Binder [9] further improved this theory to describe the kinetics of the ordering process. Their concentration dependent free energy potential shows two side minima, which have the same depth as the middle one at the microphase separation transition temperature Tmst- Therefore, they presume a coexistence of the disordered and the lamellar phase at Tmot- As the temperature is further lowered, these side minima become dominant and the transition comes to completion. For a supercooled material, they expect after a completion time an Avrami-type ordering transformation with an exponent of 4 equivalent to spherically growing droplets of ordered material. This characteristic time corresponds to the time to form stable droplets of ordered material plus the time needed for the structures to grow to a size that they can be detected by the used technique. 

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