Defect structures table

The homogeneity ranges and defect structures of the hexaborides lead to deviations from stoichiometry through the cation defects (see Table 1). 

There are large numbers of anion excess fluorite-related structures known, a small number of which are listed in Table 4.4. The defect chemistry of these phases is enormously complex, deserving of far more space than can be allocated here. The defect structures can be roughly divided into three categories random interstitials, which in... 

Table I. An extensive amount of work has been done to investigate the defect structures of many of these materials and a full review of these results is not intended here. Only generalizations which are useful for understanding these materials and their defect structures are discussed.

Table XXXVIII). Brindley (1955) has suggested that stevensite is a mixed-layer talc-saponite however, Faust et al. (1959) considered it to be a defect structure with a random distribution of vacant sites in the octahedral sheets. A small proportion of domains with few or no vacancies would then be present having characteristics of talc. The layer charge in stevensite is due to an incompletely filled octahedral sheet (Faust and Murata, 1953). This deficiency is minor (0.05—0.10) and the resulting cation exchange capacity is only about one-third that of the dioctahedral montmorillonites (100 mequiv./lOO g.).

The EH calculations agree with the CNDO results if a planar nondefect geometry is used. When the model containing defects serves as the lattice, electron-capture processes are favored at the expense of Ag+ capture at the Ag center, as shown in Table XIII. This leads to the alternative pathway shown in Fig. 24, and would explain a dependence of photochemistry on surface-defect structure. 

A number of interesting features can be seen in Figure 11.21. First, the level of water adsorption at p/p° = 0.90 by Silicalite-I is only about 10% of the capacity available for nitrogen and other small adsorptive molecules (see Table 1 l.S). This is increased to about 18% for HZSM-5, when the Si/Al ratio is reduced to 90. The presence of the hysteresis loops in the capillary condensation range indicates that a high proportion of the water adsorption has occurred within the secondary pore structure or defect structure rather than in the zeolitic channel structure. Similar findings have been reported by Llewellyn et al. (1996). 

At the ends of the polymer chains and at the ends of the short oligomer units (see for example the trimer molecule of Table 1) a bond defect structure is expected. For the acetylene structure of the polymer chain this is a carbene —C— with two free valence electrons and in the case of the butatriene structure this is a radical carbon atom —C= with one free valence electron. In both rases there is a reactive chain end, which allows reaction of the chain with the neighbouring monomer molecules. These reactive structures and a possible nonreactive structure are listed in Table 1 as examples of the trimer molecules. 

For the catalysts of different composition two polypropylene fractions can be singled out which differ greatly in their stereoregular structure (Table 6) a) a fraction insoluble in boiling n-heptane with an insignificant amount (1-2%) of steric defects of the type... 

Note this table does not distinguish between the most symmetrical form of a structure and distorted variants, superstructures, or defect structures for more details the text should be consulted. 

These equations are used to derive a defect structure for LiAl as summarized in Table 4. The Vu defects are dominant for low Li concentrations whereas with increasing x Li i defects become more important. 

Table 7. Fermi energies (in eV) and densities of states per eV and lattice site at the Fermi level for B32-type compounds AB deduced from the RAPW procedure " . (The values in brackets are gained from the ASA method - >.) The bottom of the valence states is the zero of the energy scale. For the AB compounds the Fermi energies Ep arc listed for both the ideal stoichiometric compounds (cvE is equal to the electron to atom ratio e/a = 2) and the compounds with defect structure, for which it is assumed that the fifths valence bands are non-occupied and Ep lies in the minimum of the density states curve, see Figs. 5-7. (cve = 1-98)...

In Table 7 the Fermi energies Ep and the densities of states at the Fermi level N(Ef) are listed for all binary compounds covered. In order to study the defect structure of the AB" compounds, Ep and N(Ep) are given for two valence electron concentrations Cve = 2.0 and Cve 1.98. For the latter Cve Ep lies in the minimum of the density of states curve and the 5th band is not occupied. Values of N(Ef) obtained by the two different procedures (RAPW and ASA) are listed for LiAl, LiZn and LiCd. It seems that the differences in the N(Ep) values are caused by differences in the hybridization effect in both methods (cf. Sect. C.II above). 

For the AB " compounds the values are quite different for the stoichiometric (st, CvE = 2) and the defect (de, Cve 1.98) phases. KW(AB , st) is much larger than K AB " ). This is due to the occupied states at the Fermi level of the 5th band, which have a large s character (cf. Sect. C.IV). K )(AB" , de) is much smaller than K AB , st) because the 5th band is unoccupied for the defect phases. For these phases the Fermi surface is formed by the 3rd and 4th bands which have a predominant p character at the Fermi level. The disagreement between K( )(AB st) and K"p supports the mechanism assumed for the defect structure in the AB" phases discussed in Sect. B as this mechanism result in cve values <2 (see Table 2). Assuming a net paramagnetic orbital contribution (see below) and studying as a function of cve one finds that the agreement between K and Ks " + K ) is only satisfactory for... 

There are four well-established intermetallic phases in this system, Th3Si2, ThSi, ThsSis and ThSi2. The structural data on these compounds have been assembled by [1981CHI/AKH] and are shown in Table XI-8. In addition Brown and Norreys [1960BRO/NOR] found evidence for a phase of composition TheSin, with two polymorphs with a transition temperature of ca. 1573 K. The structural data in Table XI-8 suggest that they are defect structures of ThSi2. 

Furthermore, the orientation of the alkyl chain (i.e. hexyl, octyl or decyl chain) on the imidazolium cation was pointing in a nearly perpendicular orientation away from the silica surface with a high degree of conformational order and few gauche defects. In Table 5.6-1 some of the structural parameters determined for the examined ionic liquids are summarized. 

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