Single deviations from ideal crystal structure

A piece of ordinary solid metal is not a single crystal of metal. One always finds the grain boundaries where the forces, even inside the metal, are not spherically symmetrical. In addition, one always finds various kinds of imperfections inside a crystal where the forces are, again, assymetric. These faults are transferred from the bulk to the surface making the surface far from ideally smooth. Moreover, the surface of a solid can never be accurately represented by a continuous plane of atoms, the way it is often done. All these deviations from ideality give rise to various effects which do not represent the major phenomenon, but must be considered in the theoretical interpretation of experiments. These side effects are often so large as to mask the major phenomenon itself. A liquid metal is, of course, free from those side effects it is smooth and not as structured as a solid one. Therefore it is more suitable for the study of the interphase itself without the side effects. Since kinetics of electrode reactions depend also on the state of the surface, liquid electrodes are also very suitable for kinetic studies. Thus one would use the liquid electrode (mercury) as often as possible in electrodics. Indeed, mercury occupies a unique position in electrodics because of the above reasons, as well as others which will become apparent later. 

Wustite, Fei j O has a positive deviation from ideality in the partial molar free energy of Fe below 1100°C. As previously mentioned, clustering of cation vacancies is observed and the clustered vacancy defects are in turn ordered. The X-ray diffraction pattern of Fej at room temperature shows spots due to a superstructure in addition to the spots from the NaCl structure. Quantitative analysis of the intensities of the superstructure and the main peaks in a single crystal of X = 0.098 quenched to room temperature led to the structure shown in Figure 23. The cluster of 13 octahedral vacancies and four associated tetrahedral cations shown gave substantial agreement with the observed intensities. In order to maintain local charge balance,... 

Deviations of this type may clearly be brought about by deviations of the structure of the zeolites imder study from the ideal structure as implied for the deviation of Eq. 17. It would be most remarkable, however, if for two completely different chabazite species, viz. the big single crystals and poly-crystalUne samples, identical deviations from the ideal structure would occur, as suggested by the experimental data. 

Even with such (chemically) rather well-defined compounds, this description of the structure is an idealized one. The real structure as determined from single crystals (table 1) exhibits characteristic deviations from the idealized structure (i) the single R atom frequently shows unusually elongated thermal ellipsoids , and (ii) excess electron density is found in all octahedral voids surrounded by X and R atoms. These deviations are elaborated in the following... 

Finally we come to deal with structures, ideal and real, assoeiated with variety of defects, impurities, vacancies, interscialities, interfaces, etc. - these terms imply the usual hierarehy, the ideal being the prototype from which we can know about real things and their inevitable deviations. But henceforth it is the defect that is interesting, for the specific properties it gives the single crystal... 

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