Crystal second kind

A second kind of electronic defect involves the electron. Let us suppose that the second plane of the cubic lattice has a vacancy instead of a substitutional impurity of differing valency. This makes it possible for the lattice to capture and localize an extraneous electron at the vacancy site. This is shown in the following diagram. The captured electron then endows the solid structure with special optical properties since it ean absorb photon energy. The strueture thus becomes optically active. That is, it absorbs light within a well-defined band and is called a "color-center" since it imparts a specific color to the crystal. 

If one of two equilibrating diastereomers crystallizes out of solution, shifting the equilibrium in one direction, the process is referred to as an asymmetric transformation of the second kind lUPAC Compendium of Chemical Technology, 2nd Edition, Mc-Naught, A. D., Wilkinson, A., Eds. Blackwell Science, 1997. 

The fact that the water molecules forming the hydration sheath have limited mobility, i.e. that the solution is to certain degree ordered, results in lower values of the ionic entropies. In special cases, the ionic entropy can be measured (e.g. from the dependence of the standard potential on the temperature for electrodes of the second kind). Otherwise, the heat of solution is the measurable quantity. Knowledge of the lattice energy then permits calculation of the heat of hydration. For a saturated solution, the heat of solution is equal to the product of the temperature and the entropy of solution, from which the entropy of the salt in the solution can be found. However, the absolute value of the entropy of the crystal must be obtained from the dependence of its thermal capacity on the temperature down to very low temperatures. The value of the entropy of the salt can then yield the overall hydration number. It is, however, difficult to separate the contributions of the cation and of the anion. 

The differential technique described under (a) has an advantage in removal of the liquid-junction potential and of mechanical faults often encountered in work with reference electrodes of the second kind. The procedure described under (b) suppresses the potential fluctuations, but difficulties can arise from the very high resistance of a cell containing two ISEs. A differential amplifier was designed for this prupose [15]. The two ISEs used can also exhibit different slopes electrode membranes were therefore prepared by cutting a single crystal into two halves, where each half contains a chaimel for passage of the solution and functions as an ISE [163]. 

The major difference between block and graft copolymers is the position of the second kind of unit. Thus, information that applies to block copolymers can often be applied to graft copolymers. So, domains where physical cross-linking occurs via crystallization can occur in either block components or within graft copolymers where the necessary symmetry occurs. 

A brief description of the type of "racemic" compounds is necessary for the reader to better understand the principles behind the application of crystallization methods to the separation of enantiomers. Three fundamental types of crystalline racemates exist. In the first, the crystalline racemate is a conglomerate, which exists as a mechanical mixture of crystals of two pure enantiomers. The second, which is the most common, consists of the two enantiomers in equal proportions in a well-defined arrangement within the crystal lattice this is termed racemic compound. The third possibility occurs with the formation of a solid solution between the two enantiomers that coexist in an unordered manner in the crystal. This kind of racemate is called a pseu-... 

Asymmetric transformation The conversion of a mixture (usually 1 1) of stereoisomers into a single stereoisomer or a mixture in which one isomer predominates. An asymmetric transformation of the first kind involves such a conversion without separation of the stereoisomers. An asymmetric transformation of the second kind also involves separation, such as an equilibration accompanied by selective crystallization of one stereoisomer [76]. The terms first- and second-order asymmetric transformations to describe these processes are inappropriate. See also stereoconvergent. 

For a perfect crystal, all lie on a perfect lattice, and therefore the correlation function Tz(r) is nonzero only when r coincides with the lattice. However, for crystals with imperfections, rjk are subject to statistical fluctuations, and the correlation function rz(r) described by (3.35) is no longer strictly on a lattice but is smeared out, as illustrated in Figure 3.13. Note that in the imperfection of the first kind the autocorrelation function is smeared out equally at every lattice point except at the origin, but in the imperfection of the second kind the degree of smearing of the autocorrelation function becomes more severe as the distance from the origin is increased. 

Figure 3.13 Imperfections of (a) the first and (b) the second kind in two-dimensional crystals, (c) and (d) are the corresponding autocorrelation functions.

It is demonstrated in [8] that the transport coefficients (thermal diffusivity, diffusion coefficient, fluidity, etc.) considered in the Fourier approximation are proportional to the stability coefficients. This makes it possible to determine whether we are dealing with a critical transition or a limited phase transition of the second kind and, in the latter case, which of the parameters are characteristic. In critical transitions, the transport coefficients decrease strongly, whereas in limited transitions of the second kind they tend to infinite values. This criterion shows that phase transitions of the second kind which occur in binary alloys, polymers, ferromagnets, ferroelectrics, liquid crystals, etc., are essentially transcritical transitions, which are sometimes close to the critical conditions because the values of the transport coefficients decrease strongly at the transition point. The occurrence of superfluidity in He H demonstrates that, even in the absence of a coordinate or a derivative which tends to zero, this substance is a superphase in the kinetic sense. 

So, at first, during melting, the film of nonstructural second liquid is formed on the surface of crystal substance. When the film thickness gets equal he, the phase transition of second kind happens, and second liquid transforms into ordinary structural liquid. 

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