Atomic environment type

AET atomic environment type BW Bragg-Williams (theory of ordoing)... 

ESF extrinsic stacking fault lAET irregular atomic environment type... 

Intrinsic defects (or native or simply defects ) are imperfections in tire crystal itself, such as a vacancy (a missing host atom), a self-interstitial (an extra host atom in an otherwise perfect crystalline environment), an anti-site defect (in an AB compound, tliis means an atom of type A at a B site or vice versa) or any combination of such defects. Extrinsic defects (or impurities) are atoms different from host atoms, trapped in tire crystal. Some impurities are intentionally introduced because tliey provide charge carriers, reduce tlieir lifetime, prevent tire propagation of dislocations or are otlierwise needed or useful, but most impurities and defects are not desired and must be eliminated or at least controlled. 

EXAFS is a nondestructive, element-specific spectroscopic technique with application to all elements from lithium to uranium. It is employed as a direct probe of the atomic environment of an X-ray absorbing element and provides chemical bonding information. Although EXAFS is primarily used to determine the local structure of bulk solids (e.g., crystalline and amorphous materials), solid surfaces, and interfaces, its use is not limited to the solid state. As a structural tool, EXAFS complements the familiar X-ray diffraction technique, which is applicable only to crystalline solids. EXAFS provides an atomic-scale perspective about the X-ray absorbing element in terms of the numbers, types, and interatomic distances of neighboring atoms. 

Temperature-programmed reduction combined with x-ray absorption fine-structure (XAFS) spectroscopy provided clear evidence that the doping of Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) promotes the carburization rate relative to the undoped catalyst. Since XAFS provides information about the local atomic environment, it can be a powerful tool to aid in catalyst characterization. While XAFS should probably not be used exclusively to characterize the types of iron carbide present in catalysts, it may be, as this example shows, a useful complement to verify results from Mossbauer spectroscopy and other temperature-programmed methods. The EXAFS results suggest that either the Hagg or s-carbides were formed during the reduction process over the cementite form. There appears to be a correlation between the a-value of the product distribution and the carburization rate. 

So the number of signals in the NMR spectrum tell the number of different sets of equivalent protons in a molecule. Each signal corresponds to a set of equivalent protons. Therefore, protons with identical electronic environments are identical and have the same chemical shift. Protons with different electronic environment (different adjacent atoms, different types of bond) are non equivalent and have different chemical shifts. 

In most cases a clear maximum gap is revealed (here the gap between the second and the third bar). The atomic environment is then constructed with the atoms to the left of this gap (8 + 6 in the example of CsCl). To avoid in particular cases bad or ambiguous descriptions, however, a few additional rules have been considered. When for instance two (or more) nearly equal maximum gaps were observed, a selection was made in order to keep, in a given structure type, the number of different AET as small as possible. A convexity criterion for the environment polyhedron was also considered. The coordination polyhedron has to be defined as the maximum convex volume around only one central atom enclosed by convex faces with all coordinating faces lying at the intersections of at least three faces. This rule was especially used where no clear maximum gap was detectable. 

Daams et al. (1992) have analysed all the cubic structure types reported in Villars and Calvert (1985), after excluding all oxides and a few types with improbable interatomic distances, thus leaving 128 structure types representing 5521 compounds. Their analysis showed that these cubic structure types have 13 917 atomic environments (point sets). Of those environments, 92% belong to one of the 21 most frequently occurring AET, which are those reported in the following list (see also Fig. 3.18) ... 

Of the 5521 compounds crystallizing in the mentioned 128 structure types, 46% belong to a single-environment group (structures in which all atoms have the same type of environment), 37% have two environment types, 9% three and the rest four or more environments (=98% of the cubic compounds crystallize in structure types with 1, 2, 3 or 4 AET). 

The results of a similar analysis of the intermetallic hexagonal structure types have been reported by Daams and Villars (1994). Of 442 structure types, 315 (clearly intermetallic and correctly refined) were considered. In this case too it was observed that a small group of atomic environments is greatly preferred. The 23 AET most frequently occurring in the 315 hexagonal structure types are reported in the following list (to be compared with those previously reported for cubic and rhombohedral structure types) ... 

Concluding this section, we may mention a paper by Daams and Villars (1993) concerning an atomic environment classification of the chemical elements. Critically evaluated crystallographic data for all element modifications (and recommended atomic volumes) have been reported. Special structural stability diagrams were used to separate AET stability domains and to predict the structure (in terms of environment types) of hitherto unknown high-pressure and high-temperature modifications. Reference to the use of AET in thermodynamic (CALPHAD) modelling and calculation has been made by Ferro and Cacciamani (2002). 

Daams, J.L.C. (1995) Atomic environments in some related intermetallic structure types. In Intermetallic Compounds, eds. Westbrook, J.H. and Fleisher, R.L. (John Wiley and Sons Ltd, Chichester), Vol. 1, p. 363. 

See also some comments on this point as a result of the atomic environment analysis of the structure types summarized in 3.7.5.)... 

Notice that, in this structural type, space group and atomic positions are the same as in the tI2-In type. The different c/a axial ratio, however, results in different atomic environment. 

When copper is bound to one sulfur atom of a cysteine and two nitrogens of two histidines in an essentially tetrahedrally distorted - trigonal ligand environment (type I copper proteins), the excited levels are low in energy, and the values are reduced to about 5 x 10 ° s (29). Examples are blue copper proteins, like ceruloplasmin and azurin, and copper(II) substituted liver alcohol dehydrogenase (30-32). 

The Bravais or space lattice does not distinguish between different types of local atomic environments. For example, neighbouring aluminium and silicon both take the same face-centred cubic Bravais lattice, designated cF, even though one is a close-packed twelve-fold coordinated metal, the other... 

Method of Broto, Moreau, and Vandycke The method of Broto et al. [46] is an atom contribution method including one extra contribution for conjugated double bonds. The complete set of atom constants is given in Appendix F to illustrate the relative hydrophobicity of the different types of atom contributions. Atom types are differentiated by their environment depending on whether they are C atoms or heteroatoms. The C-atom environment is limited to the adjacent bonds and to the attached H atoms. For heteroatoms, the environment additionally includes nonhydrogen neighbors. The latter are divided into two classes (1) C atoms, for which the bond environment is considered and (2) heteroatoms, Z, irrespective of... 

The activity of a catalyst often depends on the type of sites that are present on the catalytically active particles. Hence, ideally one wants to know how atoms are distributed over terraces, edges, corners, etc. - a question that has long occupied the thoughts of many investigators of catalysis. Van Hardeveld and Hartog [17] addressed this problem by analyzing the different atom environments on a number of different crystal geometries, such as cubes and truncated octahedrons, as a function of their size, and their report from 1969 has subsequently become a true citation classic. 

An indirect means of dealing with the correlation problem when the molecule is too large to be treated by advanced methods such as the G2 is to include it in a reaction (often hypothetical) in which the reactants and products are similar in terms of one or more of several electronic and structural factors, e.g. number of electron pairs, types of bonds, atom environments, etc. [15,20,21], It is hoped that in computing AH(298 K) for such a process, the errors for the reactants and products will largely cancel. The desired AHf can then be obtained if the AHf of all of the other species are known. There are several categories of such reactions isogyric ones conserve the number of electron pairs, isodesmic maintain unchanged... 

The carbon atoms in Ceo are equivalent, and as expected only a single line is observed 5142.7 ppm (CeDe). C70 has Dih symmetry and as there are five distinct carbon atom environments, a five-line spectrum is observed 5150.1, 147.5, 146.8, 144.8, and 130.3 ppm (CeDe) of intensity ratio 10 20 10 20 10 (Figure 15). The initial assignments based on chemical intuition have been subsequently confirmed by 2D C NMR. The upheld line (5130.9 ppm) corresponds to the 10 graphitic atoms around the waist (type e), that he at the intersection of three six-membered rings. 

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