Disorder geometric

The geometric data reported for 3,4-bis(p-chlorophenyl)furoxan (71TL2907) seem to be in error. A reinvestigation has shown that there are at least two polymorphic forms of this compound (monoclinic and orthorombic), which are both disordered T. Traulsen, C. Nather, and W. Friedrichsen, unpublished results. 

From a mechanical point of view the polymer may be regarded as a composite consisting of an alternative stiff (crystalline) and soft-compliant (disordered) elements. Given the geometrical arrangement of these two alternating phases and the hardness value of both of them, the arising question is to predict the hardness value of the material. On the other hand, it is known that density is a crystallinity parameter... 

The structural picture that was envisaged to represent the temperature-dependent fluctuations of the EFG tensor [15] is based on the X-ray structure of MbOa that exhibits a geometric disorder of Fe02 with two different positions of the terminal O-atom [28]. Within this stmcture, the projection of the 0-0 bond on the heme plane is rotated by about 40° in position 2 compared to 1 (Fig. 9.10). Conventional Mossbauer studies of single crystals of Mb02 have shown that the principal component of the FFG tensor lies in the heme plane and is oriented along the projection of the 0-0 bond onto this plane [29]. If the terminal O-atom is located in position 2, the EFG should be of the same magnitude as in position 1, but its orientation is different. The EFG fluctuates between positions 1 and 2 with a rate that depends on temperature. 

Table 1 Thermal and geometrical data of selected semiflexible polymers giving rise to thermotropic hexagonal mesophases or main-chain disordered crystalline phases (adapted from [11])... 

Two types of geometric effects have been found in this context. One of these effects is exemplified by isoleucine (16), in which the sec-butyl group adopts a gauche conformation and disorder occurs through interchange of ethyl and methyl groups (57). In this situation the sec-butyl group has pseudo mirror symmetry. 

When solid NaCl is heated to 801° C, it melts, and the ions are no longer fixed in a simple geometric pattern. They will move relative to each other, subject only to the constraint of electrostatic attraction and repulsion. Each Na, therefore, will be adjacent to as many Cl anions as possible, and each Cl will tend to be surrounded by Na cations. No longer would each ion be surrounded by precisely 6 ions of the opposite charge. This looser arrangement of ions in molten sodium chloride shows that the liquid state displays an increased disorder than the solid state, so it is of higher entropy. 

We have demonstrated in this contribution that the structural order-disorder transitions in the KDP family are by far more complex than a pseudo-spin model, for example, could describe. Geometrical constraints and couphng to the soft mode, as well as cluster formation of nanometric size, indicate that here the conventional picture of cooperative phenomena has to be revised. 

However, in the 1980s some doubts arose about the validity of this model. Results of several experiments indicated that a model based on order-disorder motion of PO4 groups could be more appropriate to describe the phase transition [3,4]. Within such a type of a model, the isotope effect on Tc could be explained in terms of a geometric isotope effect. More precisely. 

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