Crystallographic sites

The crystal stmcture of the calcium fluoroapatite has two different crystallographic sites for the Ca " ion. The Ca(I) site has a threefold axis of symmetry and is coordinated to six oxygen ions at the vertices of a distorted trigonal prism. The Ca(Il) ions are located at the corners of equilateral... 

KMn8016). The Ba2+ and K+ cations partially occupy crystallographic sites at the center of the (2 x 2) tunnels. The presence of these large cations led, for many... 

Cul) is not due to point defects but to partial occupation of crystallographic sites. The defective structure is sometimes called structural disorder to distinguish it from point defects. There are a large number of vacant sites for the cations to move into. Thus, ionic conductivity is enabled without use of aliovalent dopants. A common feature of both compounds is that they are composed of extremely polarizable ions. This means that the electron cloud surrounding the ions is easily distorted. This makes the passage of a cation past an anion easier. Due to their high ionic conductivity, silver and copper ion conductors can be used as solid electrolytes in solid-state batteries. 

M[pzTp](H20) (M = Na, K) have been determined by x-ray diffraction (30). In contrast to the discrete monomeric structures of the tris (pyrazolyl)hydroborato complexes Tl[TpRR ], K[TpBut2], and Cs[TpBut2] described earlier, the hydrated tetrakis(pyrazolyl)borato complexes M[PzTp](H20) (M = Na, K) exhibit an interesting polymeric-type structure, a portion of which is illustrated in Fig. 14. In each case, the cations occupy two different crystallographic sites, and the cation in one of the sites is coordinated to two pyrazolyl groups in a rr-type fashion. 

Finally, the tris(pyrazolyl)hydroborato ligand system has also provided an interesting example in which a crystallographic site is disordered between a vacancy and a chain of three atoms. Thus, the x-ray structure of the cobalt complex [TpAnt]CoNCS (Ant = 9-anthryl) revealed the presence of the cocrystallized thallium derivative... 

TpAnt]Tl, and that the composition of the crystal studied was approximately [TpAnt](CoNCS)0.94TI0.06. As a consequence of the cocrystallization, the crystallographic site occupied by the CoNCS moiety was disordered with a thallium atom (Fig. 57). The ability of [TpAnt]CoNCS and [TpAnt]Tl, which differ substantially in the number of atoms at the disordered site, to cocrystallize is presumably due to the fact that the disordered site is enclosed within the pocket that is provided by the tris(pyrazolyl)hydroborato ligand. 

Figure 7.13 The evolution along the series of lanthanides of the sum of Ln-0 bond lengths in homometallic [Ln2] complexes for each crystallographic site. The solid lines...

Fig. 13 87Rb NMR spectra of polycrystalline RbN03 (a) sheared z-filter 2D triple-quantum MQMAS spectrum, (b) MAS spectrum, (c) MQMAS isotropic projection, and (d-f) cross-sections of the 2D spectmm showing the three different crystallographic sites...

For the case of muonium, nonresonant spin precession in a magnetic field provides a copious source of information about its crystallographic sites and the associated unpaired electron distribution around them (see Chapter 15). Here, the concentration of muons is always too low for molecule formation, and migration to impurities and implantation defects can be kept small by the short muon lifetime and use of pure material and low temperature. 

The formation of defects can be considered as the reaction of a nominally perfect crystal with dopant. The rules for writing equations including defects are similar to those of elementary chemistry, but as the matrix is a crystal structure, quantities must be specified with respect to crystallographic sites rather than molecules or moles. 

Almost all the crystalline materials discussed earlier involve only one molecular species. The ramifications for chemical reactions are thereby limited to intramolecular and homomolecular intermolecular reactions. Clearly the scope of solid-state chemistry would be vastly increased if it were possible to incorporate any desired foreign molecule into the crystal of a given substance. Unfortunately, the mutual solubilities of most pairs of molecules in the solid are severely limited (6), and few well-defined solid solutions or mixed crystals have been studied. Such one-phase systems are characterized by a variable composition and by a more or less random occupation of the crystallographic sites by the two components, and are generally based on the crystal structure of one component (or of both, if they are isomorphous). 

Finally, there are the inclusion complexes, which in some senses behave like solid solutions, in others like the above complexes. Thus, for example, many inclusion complexes are of variable composition and their structures are usually determined by the host structure on the other hand, guest and host must occupy different crystallographic sites (217a). 

As we have seen, the interlayer galleries in LDHs can be considered to be made up of hexagonal close-packed sites parallel to the close-packed layers of hydroxyl groups and metal cations, with one crystallographic site per metal [17,238]. The maximum amount of water which can be accommo-... 

A guide to the manner in which structural theory may be applied to a detailed consideration of the mechanism of a surface-catalyzed reaction is found in papers by Cossee (113), Arlman (114), and Arlman and Cossee (115) concerning the mechanism of the stereoregular heterogeneous catalyzed polymerization of propylene. Particular crystallographic sites are shown to be the active centers at which the reactants combine and ligand field theory is used to demonstrate a plausible relationship between the activation energy for the conversion of adsorbed reactants to the product and the properties of the transition metal complex which constitutes the reaction center. 

The various TPR peaks may correspond to different active sites. One hypothesis assumed cyclization over metallic and complex (Section II,B,4) platinum sites (62e) the participation of various crystallographic sites (Section V,A) cannot be excluded either. Alternatively, the peaks may represent three different rate determining steps of stepwise aromatization such as cyclization, dehydrogenation, and trans-cis isomerization. If the corresponding peak also appears in the thermodesorption spectrum of benzene, it may be assumed that the slow step is the addition of hydrogen to one or more type of deeply dissociated surface species which may equally be formed from adsorbed benzene itself (62f) or during aromatization of various -Cg hydrocarbons. Figure 11 in Section V,A shows the character of such a species of hydrocarbon. 

---