Atomic occupancy precision

Quantitative analysis of ESP is important for several reasons. The precise knowledge of the ESP is necessary to allow comparison of atomic potentials in different structures, analysis of composition (partial occupancy) and chemical bonding (crystal formation, structure property relations). 

Cation Site Distribution, Thin-film EDS analysis can also be used to quantitatively determine the site occupancy of atoms in a known crystal structure. Atom Location by Channeling Enhanced Microanalysis (ALCHEMI) is a technique which utilises electronchanneling enhanced X-ray emission for specific atoms in a crystal when appropriately oriented relative to the incident beam [43]. The method involves no adjustable parameters, can be used on relatively small areas of sample and provides fractional occupancies of atom positions [44] Unlike X-ray diffraction which has had limited success with adjacent elements in the periodic table [e.g. 45], ALCHEMI can provide site occupancies for adjacent elements and is relatively insensitive to sample thickness or the precise electron beam orientation [44] ... 

The oxygen atom was found to be disordered at the crystallographic inversion center and therefore refined with half occupation factors on each side of the B-B bond (model A, Space Group C2/c, No. 15). This disorder is connected with the centrosymmetry in the crystal lattice, which is also strictly valid for the C(SiMe3)3 substituents. In spite of the disorder of the oxygen atom in the crystal, the bond length of the B-B bond could be determined with satisfactory precision [1.601 (7)... 

In the spin-coupled description of a molecule such as SF6, the sulfur atom contributes six equivalent, nonorthogonal sp -like hybrids which delocalize onto the fluorine atoms. Each of these two-centre orbitals overlaps with a distorted F(2p) function and the perfect-pairing spin function dominates. Of course, using only 3s, 3px, 3p and 3pz atomic orbitals, we can at most form four linearly independent hybrid orbitals localized on sulfur, with a maximum occupancy of 8 electrons, as in the octet rule. However, the six sulfur+fluorine hybrids which emerge in the spin-coupled description are not linearly dependent, precisely because each of them contains a significant amount of F(2p) character. It is thus clear that the polar nature of the bonding is crucial. 

Type A and type C centers can be readily distinguished. Type B centers are usually readily identified, but it may be difficult to determine the respective occupancies of the two shells precisely, because the contributions are typically not well resolved in the Fourier transform. Also, it is unfortunate that EXAFS cannot distinguish between car-boxylate oxygens and water as ligands of zinc. However, coordination by imidazole is usually clearly evident because of the multiple-scattering pathways involving all atoms of the five-membered ring (23, 26). 

Refinement of the crystal structure is, therefore, a powerful chemical analysis technique. Unlike conventional chemical analysis, which only yields the bulk composition of the sample, powder diffraction analysis facilitates accurate determination of the occupancies of different crystallographic sites by various chemical elements, or in other words, establishes precise chemical composition of the crystal at the atomic resolution. It should be noted that the results may be considered reliable only when the difference in the scattering ability of atoms in question is significant, in addition to a very high quality of experimental data. This is indeed the case here because scattering factors of Sn and Ni are related as-1.8 1. 

Although x-ray diffraction techniques can yield data on atomic positions and occupation frequency which appear highly precise, these data... 

The comparative analysis of the positional parameters of barrerite framework atoms between the samples studied by [75G3] and [99S1] displayed only minor differences. Differences in occupancy factors of almost all extra-framework sites were found between the two samples studied by [99S1]. The occupancy factors of Cl and W1 and the positional parameters of WIP and W4P rendered the strongest deviations with respect to those of the type locality sample. The cations in five C sites could not be located precisely. 

The definitive method for Pb quantification in biological media, and notably whole blood, is IDMS. IDMS accuracy traces to the fact all analytical manipulations are on a weight basis and entail simple procedures (NAS/NRC, 1993 U.S. EPA, 1986). There are essentially two reference methods, both validated with IDMS and in widespread use for routine measurements in environmental and occupational epidemiology and clinical applications (NAS/NRC, 1993 U.S. EPA, 2006). These are a spectroscopic method, graphite-furnace atomic absorption spectrometry (GE-AAS), and an electrochemical approach, ASV. Both ASV and GE-AAS demonstrate the requisite track record in terms of accuracy, precision, time requirements, and cost-effectiveness for routine but reliable methodology (Flegal and Smith, 1995 NAS/NRC, 1993). ICP-MS can also be viewed as a reference method for those analytical settings where costs for operator expertise, instrumentation, and procedures are not critical, such as academic research laboratories. 

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