Order-disorder mechanism

Precipitation from solutions at about room/ambient temperature  

Further increase in temperature, up to 800°C, leads to a slight decrease in the signal, which remains unchanged following the cooling of the sample and repeated heating up to at least 600-650°C. 

A relatively strong SHG signal observed at low temperatures, can be explained only based on the interaction between fields that contain ordered 

The appearance of spontaneous polarization in the case of Li4Ta04F is related to volumetric irregularities and ordering of the Li+ - Ta5+ dipoles, as is in the case of the similar niobium-containing compound Li4Nb04F. It can be assumed that the main difference between the two compounds is that the irregularities and the Li+ - Ta5+ dipoles are thermally more stable compared to the niobium-containing system. This increased stability of the dipoles leads to the reversible phase transition at 660°C. 

The function of (T) in the vicinity of the phase transition to centrosymmetric conditions usually has a linear character. Such behavior corresponds to ferroelectrics that undergo type II phase transitions and for which the SHG signal, hm, in the vicinity of the Curie temperature is described by the Curie - Weiss Equation  

---

To determine static properties of the SeO radical in KDP and DKDP, the temperature dependence of the hyperfine interaction between unpaired electron and Se (I = 1/2) nucleus was measured [53]. The hyperfine tensor component A, where the direction is along the c-axis, shows an isotope effect, because its value is higher in DKDP than in KDP. Furthermore, its value shows a jump at Tc for DKDP and a considerable temperature dependence in the PE phase of both crystals, approximated by the relation A (T) = A (0) - B coth(ro/T), where To 570 K for both crystals. It is interesting to note that A, similarly to the As NQR frequency and P isotropic chemical shift, should be constant in the PE phase if the two-state order-disorder mechanism of the corresponding tetrahedron holds. However, while the temperature dependencies of the As NQR frequency and P isotropic chemical shift in the PE phase were explained as originating from a six-state order-disorder mechanism [42] and additional displacive mechanism [46], respectively, here it was assumed that excitation of some extra lattice vibration mode with frequency Tq affects the hyperfine tensor components and causes the temperature dependence of A. ... 

In the discussion, synthetic zeolites like zeolite A, X and D have also been considered every time they give information useful for the comprehension of the order/disorder mechanism. 

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. 

A reversible succession of order-order and order-disorder transition was observed for a poly(ethylene-a/f-propylene)- -poly(ethylene-co-butylene)-b-polystyrene terpolymer, which shows at room temperature non-hexagonally packed PS cylinders. Upon heating, this system reorganizes to a hexagonally packed one, and at higher temperatures dynamic-mechanical analysis indicates the transition to the disordered state [73],... 

In other cases more complex disordering mechanisms are observed. Non-stoichiometric oxides in which the oxygen vacancies are ordered at low temperatures illustrate convergent disordering. The oxygen atoms and oxygen vacancies are here distributed at the same sub-lattice at high temperatures. An example is... 

Indeed, in the world of tomorrow we can expect new aspects of polymer solids to extend the conventional and successful structure ideas of this century. These, of course, were the recognition as molecular identities of the chains of repeating chemical monomers. The circumstances of those entities have resulted in interesting concepts of solubilities, viscosity, and other mechanics, and especially thermodynamic limitations m mutual solubility or comparability of polymer mixtures. But we have known for decades that even homogeneous regular chain polymers such as Carothers polyesters and polyamides formed solids with manifold imperfections and irregularities, such as order-disorder crystal configurations.(22,23)... 

Because of the prediction of coexistence of order/disorder in [26], Dalai et al. [27] reinvestigated the phase transition mechanism with an ultra-high resolution NMR method, which is based on magic angle spinning (MAS) and is described in detail by Dalai et al. in Chap 2 of this volume. The advantages of this method as compared to conventional NMR techniques are that it is ... 

Abstract This chapter describes the experimentai compiement of theoretical models of the microscopic mechanism of ferroelectric transitions. We use the hydrogen-bonded compounds as examples, and attempt to show that the new experimental data obtained via recently developed high resolution nuclear magnetic resonance techniques for solids clearly support the hypothesis that the transition mechanism must involve lattice polarizability (i.e. a displacive component), in addition to the order/disorder behaviour of the lattices. 

Basis of Using j so to Distinguish Between the Order/Disorder and Displacive Mechanisms... 

Fig. 9 Temperature dependence of 5iso for the NMR peaks in squaric acid in the close vicinity of the phase transition temperature left panel). Note that the change starts as a smooth curve, followed by a jump caused by the first-order character of the phase transition. This is considered evidence for the coexistence of an order/disorder and displacive character in the phase transition mechanism [20]. The right panel gives a comparison between theoretical and experimental data. For details, see text...

---