Crystallization sites

On metal electrodes having coarse-crystalline structure, the number of special crystal sites (vertices, edges) is small relative to the total number of all surface atoms, so that their (positive or negative) contribution to the overall rate must be quite small. At highly disperse, fine-crystalhne deposits, however, the situation is different. Here special effects are really observed they are considered in more detail in Section 28.5.4. 

Fig. 23 The bonds that constitute crystalline domains must lie nearly parallel to the jy-axis with an angle 6 of less than 20°. Furthermore, the bonds must have at least three neighbors that satisfy 0.7a < Jr + r < 1.3a and ry < r0/2. Note that the crystalline stems deep inside the crystal (black spheres) have six neighbors, while those on the free sin-faces (hatched spheres) have four neighbors. The stems at the half-crystal site, or at the kink site, (white sphere) have three neighbors. Stems attached on the free surface, and those floating in the melt phase have less than three neighbors...

The subsequent Figs. 3.29 (a)-(d) correspond to an analysis of the structure carried out in order to show the different local atomic arrangements (coordinations around the atoms in the two crystal sites). 

The same structure is formed in a number of binary (or ternary) phases, for which a random distribution of the two (or three) atomic species in the two equivalent sites is possible. Typical examples are the (3-Cu-Zn phase (in which the equivalent 0,0,0 A, A, A positions are occupied by Cu and Zn with a 50% probability) and the (3-Cu-Al phase having a composition around Cu3A1 (in which the two crystal sites are similarly occupied, on average by Cu, with a 75% occupation probability, and by Al, with a 25% occupation probability). A number of these phases can be included within the group of the Hume-Rothery phases (see 4.4.5). 

While a non-phosphated topcoat/adhesive interface provided an excellent, moisture resistant, occlusive seal even under the most severe cycle testing, phosphated ZM adherends did not prove to be as durable in comparison (Figure 11). The reason for this lies in the fact that phosphate coverage on Zincrometal is incomplete. Partially crystalline phosphates are non-uniformly interspersed on randomly exposed zinc dust spheres at the surface. Consequently, the moisture resistance normally provided at the adhesive/topcoat interface was reduced due to the incomplete sealing between the topcoat/ adhesive surfaces. This became apparent as most of the failures examined after aging in these environments were concentrated at the adhesive/phosphate/paint interface. Results obtained on these samples were similar to those obtained for phosphated CRS joints, indicating that the locus of failure occurred at phosphate crystal sites. Note, however, that the durability of these joints was still considered to be very good in comparison to other metallic oxide/ adhesive interfaces. 

It is easy to determine the environment of Pb in PbZrOs accurately, since PbZrOs is a well ordered compound. But many useful ferroelectric oxides, for instance well-known ferroelectric oxides, Pb(Zr,Ti)03 (PZT), are mixed ion systems in which the crystal sites are occupied by two or more different ions. In such systems it is more difficult to assess the state of Pb off-centering with accuracy using conventional crystallographic analysis, such as the Rietveld refinement. Local Pb displacements are often non-collinear, displaced in different directions from site to site. These local displacements will be observed indirectly only as artificially large thermal factors, and the average displace-... 

Elements present in only minor or trace concentrations are partitioned in quite different ways. These elements are not abundant enough to form their own minerals, so they must be incorporated into minerals composed of the major elements. Because they must fit into crystalline sites whose sizes and coordination are already fixed, the ease with which they can be included depends on ionic size and charge. Trace ions that fit nicely into sites otherwise occupied by major ions having similar sizes and charges are said to be compatible. Conversely, large ions with high charge, referred to as incompatible elements, tend to be excluded from most crystal sites. 

Atom jumping in a crystal can occur by several basic mechanisms. The dominant mechanism depends on a number of factors, including the crystal structure, the nature of the bonding in the host crystal, relative differences of size and electrical charge between the host and the diffusing species, and the type of crystal site preferred by the diffusing species (e.g., anion or cation, substitutional or interstitial). 

In the absence of atomic order in the crystal sites we have... 

In many crystals, the lineshape is affected by self-diffusion of the atoms from one crystal site to another. The resulting lineshape can be expressed in the time domain as10... 

J. A. Weil, Use of symmetry-related crystal sites for measuring tensor properties in magnetic resonance.. Magn. Reson., 1973, 10, 391-393, Erratum Note that tiie last 4 rows of Table I are tainted. Please see the correct version in Table V of Ref. 12. 

However, experimental studies soon showed that in the original case and apparently in all similar ones, as well as more complicated ones,14 no such bond stretch isomers exist. The isomer(s) with the longer bond are simply the normal (short- bonded) molecule co-crystallized with an impurity so that at some crystal sites a M—Cl bond occurs where there would be an M=0 bond in the pure oxo compound. The refinement of X-ray data from such a mixed crystal leads to the appearance of a longer M=0 bond. Also, the contamination of a blue compound by a yellow impurity gives the appearance that there is a green compound. From the theoretical side the concept seems counter-intuitive (reminiscent of polywater ) and, indeed, a high-quality molecular quantum study15 failed to confirm the earlier theoretical support. 

Yasunaga et al. (1979) studied photoconductivity of single crystals of PbPc doped with O. The photocurrent increased sharply with increasing O. The results were explained by the ionization of a charge-transfer state created by the dissociation of a triplet exciton. The exciton dissociation was suggested to occur at crystal sites occupied by O. 

Fig. 2.7-8 shows the correlation diagram of the motional degrees of freedom of a free molecule, a molecule at its lattice site, and a unit cell. The vibrations of the species a and hi as well as of 02 and / 2 combine to give the symmetry species a and a" of the molecule at its crystal site. Since there are four molecules in the unit cell, all vibrational degrees of freedom split into four components. This is called factor group... 

Eu + ion was often used as a probe to detect the crystal environment in which the ion is located. In LaP04, Eu can be excited at 260 run because of a charge transfer band caused by the electron transfer in the Eu +-0 bond. In the emission spectrum of Eu " ", the ratio of the different peaks of the Fj (/ = 1, 2) transitions gives information about the symmetry of the crystal site in which Em is located, which is Ci in the monazite type LaP04, the same as in the bulk materials (Stouwdam et al., 2003). 

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