Defect crystal chemistry lattice

In this chapter, we present some latest analysis results of lanthanides (Lns Eu and Gd)-M6ssbauer structure and powder X-ray diffraction (XRD) lattice parameter (oq) data of defect-fluorite (DF) oxides with the new defect crystal chemistry (DCC) Oq model [ 1,2] as an upgrade of the former random oxygen coordination number (CN) Oq model [3,4]. This is, thus, the first report of our ongoing efforts to further elaborate the model and extend its applicability to more various systems, especially to pyrochlore (P)-type stabilized zirconias (SZs) and stabilized hafnias (SHs). 

DEFECT CRYSTAL CHEMISTRY (DCC) LATTICE PARAMETER MODEL... 

Dislocation theory as a portion of the subject of solid-state physics is somewhat beyond the scope of this book, but it is desirable to examine the subject briefly in terms of its implications in surface chemistry. Perhaps the most elementary type of defect is that of an extra or interstitial atom—Frenkel defect [110]—or a missing atom or vacancy—Schottky defect [111]. Such point defects play an important role in the treatment of diffusion and electrical conductivities in solids and the solubility of a salt in the host lattice of another or different valence type [112]. Point defects have a thermodynamic basis for their existence in terms of the energy and entropy of their formation, the situation is similar to the formation of isolated holes and erratic atoms on a surface. Dislocations, on the other hand, may be viewed as an organized concentration of point defects they are lattice defects and play an important role in the mechanism of the plastic deformation of solids. Lattice defects or dislocations are not thermodynamic in the sense of the point defects their formation is intimately connected with the mechanism of nucleation and crystal growth (see Section IX-4), and they constitute an important source of surface imperfection. 

The crystal chemistry of scheellte-type structures Is further complicated by defect stoichiometries with extensive vacancies at cation sites. In general, random cation vacancies are examples of point defects. However, at high concentrations, an ordering of vacancies can occur at which time the vacancy can no longer be considered as a point defect and a new periodic lattice (or unit cell) will have been generated. 

Finally, there are zero-dimensional or point defects that could also be considered part of the microstructure of the material. These include vacancies or foreign atoms substituting at atomic positions in a crystal lattice as well as interstitial atoms residing in between the normal atomic sites. They are not discussed further in this chapter, partly due to limited space and partly because they are not amenable to observation by the same sorts of microscopic techniques typically used to define the other aspects of the material microstructure. More information can readily be found in the many existing treatises on point defects and crystal chemistry in the literature. ... 

When stoichiometric defects form, the crystal chemistry, that is, the ratio of cations to anions, does not change. The examples for such defects are the Schottky and Frenkel defects. These are shown in Figure 10.1. In a Schottky defect, a pair of cation and anion is missing from their sites in the lattice, whereas in a Frenkel defect, one of the ions goes from its lattice position to an interstitial position. Normally, the ion going will be the cation, as the interstice is small in size and the cations are smaller than the anions. 

A suggested third method might be based on the concept of defects in the crystal lattice. Whatever methods are used may be chosen for convenience, but must be consistent with the chemistry or assumed chemistry of the system and with the particular problem of interest. 

Co-condensed EtOH-water mixtures reveal the formation of distinct EtOH hydrate phases in different temperature domains. A hydrate 1 appears in the 130 K - 163 K range depending on the EtOH content. It is proposed to have a cubic lattice similar to that of the clathrate type I. Hydrate 2 is found to crystallize at 158 K or 188 K-193 K in correlation with the absence or the presence of ice Ic and EtOH content. Its composition seems to correspond to the monohydrate. The deposited solids undergo crystallization 10 K lower in comparison to frozen aqueous solutions. This reflects the remarkable ease with which water molecules initiate molecular rearrangement at low temperature. This seems most likely due to EtOH generating defects that facilitate the water reorientation . This may also reflect the generation of clusters (in the vapour phase before deposition) having a different nature relative to those encountered in the liquid solutions. These unusual structures may have implications in atmospheric chemistry or astrophysics. 

Several investigators have used combined approaches, particularly in the in situ precipitation of active material in the pores of sintered substrates, using cathodic polarization and caustic precipitation in simultaneous or nearly simultaneous steps. A considerable amount of the reported information on the chemistry, electrochemistry, and crystal structure of the nickel electrode has been obtained on thin films (qv) made by the anodic corrosion of nickel surfaces. However, such films do not necessarily duplicate the chemical and/or crystallographic condition of active material in practical electrodes. In particular, the high surface area, space charge region, and lattice defect structure are different. Some of the higher (3.5+) valence state electrochemical behavior seen in thin films has rarely been reproduced in practical electrodes. 

In compound crystals, balanced-defect reactions must conserve mass, charge neutrality, and the ratio of the regular lattice sites. In pure compounds, the point defects that form can be classified as either stoichiometric or nonstoichiometric. By definition, stoichiometric defects do not result in a change in chemistry of the crystal. Examples are Schottky (simultaneous formation of vacancies on the cation and anion sublattices) and Frenkel (vacancy-interstitial pair). 

Contrary to the impressions conveyed by most textbooks of structural inorganic chemistry and many of the standard works on X-ray crystallography, the architecture of solids is seldom faultless. These texts often imply that all crystals are made up of a large number of properly packed and regularly stacked unit cells, without suggestion of imperfection. Yet, if it were not for the presence of lattice defects, it would be extremely difficult to account for a wide range of phenomena, including... 

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