Valence missing atoms

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. 

Approximately 3,500 structures were removed because of missing atoms (usually H) in the coordinate sets, atoms with incorrect valences (usually H with 0 or more than one bond) and bad bond angles. Finally, duplicate structures were removed to leave just the lowest R-factor entry. The space groups included in the analysis are given in Table 2. 

The third weighting scheme is related to vertex valence and takes into account the number of bonds incident to a vertex and its pairs of unshared electrons. Each electron pair present counts one, each missing electron pair contributes —1 to the total valence of atoms, and in the case of free radicals the unique electron present in the outer valence shell contributes half a bond to the total valence. The vertex weight Wj is then defined as... 

In structure analysis, bond valence sums can be used to test different models, assign oxidation states, examine the distribution of atoms among different sites, locate missing atoms and examine diffusion paths, as the following examples illustrate. 

Defects in crystalline solids occur for structural reasons, because the atoms (or ions) are not arranged ideally in the crystal when all the lattice sites are occupied, and for chemical reasons, because inorganic compounds may deviate from the fixed composition determined by the valence of the atoms. There are different types of structural defects in a crystalline solid which are normally classified into three groups (1) point defects, (2) line defects, and (3) planar defects. Point defects are associated with one lattice point and its immediate vicinity. They include missing atoms or vacancies, interstitial atoms occupying the interstices between atoms, and substitutional atoms sitting on sites that would normally be occupied by another type of atom. These point defects are illustrated in Fig. 7.2 for an elemental solid (e.g., a pure metal). The point defects that are formed in pure crystals (i.e., vacancies and interstitials) are sometimes referred to as intrinsic or native defects. 

Lattice vacancies (missing atoms) may considerably alter the valence bonds and cause electrons to be exited by a much smaller amount of energy (such as produced by a photon of red light) that would normally be required in a perfect lattice. A diamond containing such lattice vacancies appears blue since the red components of light (the one with less photon energy) are absorbed. A minimum of one vacancy per 10 atoms is necessary for the blue color to be noticeable.l l... 

The 3B premise enables the STM and STS to probe individual atomic valence, which assists formulation of the reaction kinetics. Electron clouds of metal dipoles, induced by either the 0 or the non-bonding lone pairs of the 0 , dominate the STM protmsions ions of guest or oxygen (M", 0 or 0 ) or vacancies of missing atoms produce STM depressions. Even though it locates... 

Similarly, trivalent atoms may be added to group IV semiconductors to form acceptor states close to the valence band. Atoms from the valence band may be easily thermally ionized to replace the missing electrons in the covalent bond, leaving holes in the valence band. Materials in which the majority carriers are holes are referred to as p-type materials. Adding acceptor states moves the Fermi level toward the valence band. 

When the entry of a structure has been completed the operator depresses the stylus on the OUTPUT option. The software checks for missing bonds (unfilled valences on atoms for which attached hydrogens are not Implied), unbalanced atomic charges, and mis-... 

CoNiMn408, Ni2Mn40s and NisMnsOj. In the spectra of the noncubic Mn40 and the noncubic CoMn204, the extended fine structure is missing. It is apparent that the extended fine structure does pertain solely to the crystal lattice and is not dependent on the valence, coordination, or nature of the atom giving rise to the absorption spectrum. 

An ordered cation-deficient derivative of the NaCl structure was found in the orthorhombic SC2S3 type (168). The unit cell contains twelve rocksalt units (a = 2 ao, b = ]/2 ao, c = 3J/2 o) The cation vacancies are three-dimensionally distributed without pronounced directional preference. In each sulfur-centered octahedron two scandium atoms are missing. SC2S3, ScaSe3, Y2S3 and Y2Se3 (213) Eire normal valence compounds with non-metallic properties. 

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