Order-disorder structure

Structures with one- or two-dimensional disorder are also called order-disorder structures (OD structures). 

Theoretical treatment of order-disorder structural phase transitions in hydrogen-bonded ferroelectrics implies, in principle, the determination of the transition critical temperature Tc and accompanying crystal structure transformations. 

Backhaus, K. O., Grell, K. O., Grell, H., and Fichtner, K. Database of OD (order-disorder) structures. In Crystallographic Databases. Information Content. Software Applications. Scientific Applications. Section 3.4, pp. 178-181. Published by the Data Commission of the International Union of Crystallography Bonn, Cambridge, Chester (1987). 

Physical processes Order-disorder structures, ordered-phase transitions, symmetry breaking, spontaneous magnetization, non-equilibrium crystallization phenomena, percolation, electrodeposition, formation of dissipative structures, turbulence and instabilities in fluid dynamics, and diffusion-limited aggregation process. Biological processes Excitation in muscles, pulsation of heart, calcium waves, natural fold-up of protein molecules, deposition of lipid bilayers, auto-regulation of homeostasis morphogenesis, hyper-cycles and autocatalytic networks, etc. 

This method has been applied to experiments of multiple internal reflection infrared spectroscopy on thin films of commercial polyethylene [120]. It is known that optical microtomy can be achieved by changing the incidence angle of the incoming beam. Structural depth profiling has then be performed thus allowing to probe the ordered-disordered structure of such films (Figure 3-32). 

Other examples of order-disorder second-order transitions are found in the alloys CuPd and Fe Al. Flowever, not all ordered alloys pass tlirough second-order transitions frequently the partially ordered structure changes to a disordered structure at a first-order transition. 

A large number of ordered surface structures can be produced experimentally on single-crystal surfaces, especially with adsorbates [H]. There are also many disordered surfaces. Ordering is driven by the interactions between atoms, ions or molecules in the surface region. These forces can be of various types covalent, ionic, van der Waals, etc and there can be a mix of such types of interaction, not only within a given bond, but also from bond to bond in the same surface. A surface could, for instance, consist of a bulk material with one type of internal bonding (say, ionic). It may be covered with an overlayer of molecules with a different type of intramolecular bonding (typically covalent) and the molecules may be held to the substrate by yet another fomi of bond (e.g., van der Waals). 

The balance between these different types of bonds has a strong bearing on the resulting ordering or disordering of the surface. For adsorbates, the relative strength of adsorbate-substrate and adsorbate-adsorbate interactions is particularly important. Wlien adsorbate-substrate interactions dominate, well ordered overlayer structures are induced that are arranged in a superlattice, i.e. a periodicity which is closely related to that of the substrate lattice one then speaks of commensurate overlayers. This results from the tendency for each adsorbate to seek out the same type of adsorption site on the surface, which means that all adsorbates attempt to bond in the same maimer to substrate atoms. 

Some materials undergo transitions from one crystal structure to another as a function of temperature and pressure. Sets of Raman spectra, collected at various temperatures or pressures through the transition often provide useftil information on the mechanism of the phase change first or second order, order/disorder, soft mode, etc. 

The order-disorder transformation is not unique to two-layer fluids, which is readily concluded from the second maximum of n in the vicinity of s 3.55 where the fluid consists of three strata. However, it turns out that only the innermost, middle stratum undergoes the same kind of structural reorganization just explained for the two-layer fluid the two contact strata (i.e., the strata closest to the substrate) do not participate in the transformation. The intensity of the second maximum in n is therefore reduced by roughly 2/3 compared with the first one, as one would expect. 

The ternary Ge halides, MGeX3 (M = Rb, Cs X = Cl, Br, I) are polymorphic with various distorted perovskite-like (p. 963) structures which reflect the influence of the nonbonding pair of electrons on the Ge" centre. Thus, at room temperature, rhombohedral CsGel3 has three Ge-I at 275 pm and three at 327 pm whereas in the high-temperature cubic form (above 277°C) there are six Ge-I distances at 320 pm as a result of position changes of the Ge atoms (reversible order-disorder transition). Again, RbGel3 has a lemon-yellow, orthorhombic form below —92° an intermediate, bordeaux-red orthorhombic perovskite form (—92° to —52°) a black rhombohedral form (—52° to —29°) and... 

In the second section a classification of the different kinds of polymorphism in polymers is made on the basis of idealized structural models and upon consideration of limiting models of the order-disorder phenomena which may occur at the molecular level. The determination of structural models and degree of order can be made appropriately through diffraction experiments. Polymorphism in polymers is, here, discussed only with reference to cases and models, for which long-range positional order is preserved at least in one dimension. 

Let us consider a structural limiting model, in which the polymer molecules, presenting a periodic conformation, are packed in a crystal lattice with a perfect three-dimensional order. Besides this limiting ordered model, it is possible to consider models of disordered structures having a substantially identical lattice geometry. 

This is, for instance, the case of PTFE, which at atmospheric pressure presents two reversible first-order transitions at 19 °C and 30 °C [67], In the transition at 19 °C the molecular conformation changes slightly, from a 13/6 to a 15/7 helix and the molecular packing changes from an ordered structure with a triclinic unit cell (corresponding to a positioning of the chain axes nearly hexagonal) toward a partially disordered structure (partial intermolecular rotational disorder) with a... 

For the case of polymers which present reversible solid-solid transitions, producing more disordered forms, with increasing the temperature, (e.g. PTFE, ETFE, 1,4-fran.r-poly butadiene (see Sects. 2.5 and 3.2)) the introduction in the chains of comonomeric units, as well as of other constitutional defects, tends to stabilize the more disordered structure with respect to the more ordered one, and hence to lower the transition temperatures. 

The /3-alloys are different in nature from the 7-alloys and the a-manganese and /3-manganese structures discussed above, in that they are not complex structures, but are simple, being based upon the body-centered arrangement. /3-Brass, for example, has either a disordered structure, above 480°K, the copper and zinc atoms in essentially equal number being distributed largely at random over the points of a body-centered cubic lattice, or an ordered structure, below 300°K, with copper and zinc at the positions 000 and, respectively, of the cubic unit. Moreover, the physical properties of /3-brass are not those that indicate a filled zone structure. 

---