Site occupancy vacancies

The alnicos are polycrystalline solids, in which very complex structural disorder generally occurs, due to the wide variety of deviations from periodicity that may be present. Physical disorder corresponds to the displacement of atoms, and/or the presence of amorphous regions and non-uniform crystallite sizes, while chemical disorder corresponds to site occupation by impurity atoms, and/or the presence of vacancies accompanied by variable valence states of... 

According to this notation, the main symbol in a formula shows the species (usually elements) present in a certain site. The -> vacancies are normally denoted by the italicized symbol V. Sometimes the vacant sites may also be indicated by other symbols, such as square box ( ), especially when the symbol ofvanadium (V) is present. The right lower indexes may indicate the species occupying this crystallographic site in an ideal lattice, or may show that this position is interstitial (subscript i). The second right lower index, if any, indicates the occupancy and should be separated by a comma. 

Important information on site occupation and magnetic moment distribution may also be obtained from Mossbauer measurements if a suitable nucleus is involved. The Mossbauer spectra can be least-squares fitted on the assumption that postulated environments exist. A lattice simulation model may be used to verify the existence of these and to identify their spectral components. The correlation between neutron and Mossbauer measurements in this area was shown for a series of alloys Co(Ga1 Jt >yjtFe>,) with 0 electron concentrations and which had identical magnetic properties within experimental error (Whittle 1983). The neutron and Mossbauer measurements were in agreement in suggesting that the structure of the alloys was one in which (referring to fig. 1) discrete sublattices exist of Co atoms and 2% of vacancies of A and C sublattices and of Ga, V and Fe atoms on B and D... 

D. H. Olson (Mobil Research Development Corp., Princeton, N. J.) The results reported here are the best structural data to date pertaining to the structural transformations occurring during the preparation of ultrastable faujasite. However, I feel the warning presented earlier by J. V. Smith should be kept in mind. The large standard deviations of the population parameters which give site occupancies and vacancies should be considered while deriving a mechanistic picture from the structural data. 

The aluminum vacancy leads to phonon scattering because its scattering cross section, T, for phonons is large F is proportional to the relative difference in mass between the original site occupant and the new site occupant. 

There are many factors that determine the rate of a reaction sequence that lead to a particular product. Within the same catalytic system, reaction sequences leading to different products may compete. The two key parameters, which are important to the selectivity of a catalytic reaction, are the difference of the rate constants of elementary reaction steps controlled by electronic, geometric or steric parameters and the overlayer composition of the reactive catalytic surface or occupancy of complex or cavity. This affects the relative probability for product molecule formation from the recombination or dissociation of reaction intermediates generated during the catalytic cycle. The relative stability of the fragment molecules determines their concentration and, hence the probability that they are present at high enough concentration to result in a finite quantity for recombination. Site occupancy controls also the probability of surface vacancies necessary for dissociation. The last, for instance, is an important parameter that discriminates between associative... 

Figure 4 illustrates some of the defect sites that must exist in the polymeric superstructures of the 5NPE c [CBPQ T ] and 5NP c [CBPQT ] crystals. Since 5NPE is not a HMW polymer, a mixture of discrete pseudorotaxanes sum to a lattice with 1/6 DNP site vacancy. Although the vacancy can occur either inside (Fig. 4a) or alongside (Fig. 4b) CBPQ F, the structure refines to site occupancy factors of 0.900 and 0.767 for encircled and unencircled DNP units, respectively, indicating that almigside DNP units are preferentially vacated over encircled ones.

The density of lattice vacancies can be determined, usually accurately, by the site occupation parameter. Thus if the local atomic potential is bifurcated and atoms are locally displaced, it may appear that partial occupation of the displaced site describes the system quite well. However, in such cases there is usually strong coupling between the occupation parameter and the DW factor during the process of refinement. Also, local displacement of an atom is usually accompanied by a lattice relaxation around the displaced atom and a much increased DW factor of the displaced site. Thus, in practice, it is very difficult to describe anharmonic local displacements through partial occupation of two closely spaced sites. 

The strong dependence between site occupancy and thermal displacement parameters led us to improve the stmctural model by adding titanium vacancies and fixing B factors to acceptable values. Finally both these parameters were refined simultaneously. The best fit... 

Taking B(16)-B(20) as interstitial sites, in total there are 3 X 1.52 -I- 3 X 3.38 = 14.70 electrons per unit cell available compared with the valence electron deficiency of 5. Assigning an error of only 2% to the site occupation densities determined by X-ray fine structure investigations, the completely occupied valence band leaves 10 electrons per unit cell to be distributed on localized gap states originating from the B(13) vacancies or from interstitial B(16)-B(20) states. If one assumes that the B(13) vacancy generates a gap state, which can be occupied by two paired electrons, similar to the vacancy state in silicon (213,214), the number of interstitial B(16)-B(20) atoms exactly corresponds to the number of unoccupied sites in this orbital. If single ionization of the interstitial B atoms is assumed, the valence band and the B(13) vacancy orbitals are exactly filled up. 

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