Host crystal lattice

Reactivity within (DL)-PheNCA crystals provides a number of simple ways to de-symmetrize the racemic mixtures of the homochiral oligopeptides. For example, L-2-(thienyl)-alanineNCA (ThieNCA) molecules have been shown to enantioselectively occupy the L-sites in the DL-PheNCA host crystals. Lattice-controlled polymerization of such D-Phe/(L-Phe L-Thie)-NCA mixed crystals yields libraries of non-racemic oligopeptides of ho-... 

Complex inorganic colored pigments are solid solutions or compounds consisting of two or more metal oxides. One oxide serves as a host and the other oxide(s) inter-diffuse into the host crystal lattice. This interdiffusing is accomplished at temperatures generally in excess of 1000°C. Of the 14 possible crystal structures, the two most important for plastics applications are rutile and spinel. A third structure, zircon, will be briefly mentioned. 

Alcock and coworkers studied the polymerization of butadiene (as well as of monoolefins, acetylene and aromatic olefins) trapped within the tunnel clathrate system of tris((9-phenylenedioxy)cyclotriphosphazene, induced by Co-y-radiation. The host was used in order to find if the concatenation and orientation of the monomer molecules under the steric forces generated within the host crystal lattice will lead to stereospecific polymerization. The clathrate was prepared by addition of liquid butadiene to the pure host at low temperature. The irradiation was conducted at low temperatures. Irradiation of pure butadiene (unclathrated bulk monomer) leads to formation of a mixture of three addition products f,2-adduct, cis- and trons-f,4-adducts. In contrast, the radiation-induced polymerization within the tunnel system of the host yielded almost pure trans-1,4-polybutadiene. A small percentage of f, 2-addition product was observed, but no evidence for the formation of c/s-f,4-adduct was found, confirming the earlier observation by Fin ter and Wegner. The average molecular weight was about 5000,... 

Crystals of tris(o-phenylenedioxyde)cyclotriphosphazene (97) can act as hosts for the inclusion of a number of organic polymers, e.g. cis-1,4-poly butadiene, 1,4-polyisoprene, polyethylene (PE), poly(ethylene oxide) (PEO) and polytetrahydrofuran. X-ray studies of the PE and PEO inclusion compounds show that the polymer chains are extended along the tunnel-like voids of the host lattice. The formation of clathrates appears to be limited by the tunnel dimension of the host crystal lattice. The melting points of the inclusion adducts appear to be higher than those of either the pure host or the pure... 

Depending on the similarity in size and structure to the host molecules, guest additive (impurity) molecules may be taken up into the host crystal lattice to form solid solutions (see Section 1.1.4). In cases of additive molecules that do not satisfy the general incorporation criteria for solid solution formation (e.g., for large polymers of distinctly different chemical structures), forced coprecipitation of the additives with the host molecules will lead to the formation of solid dispersions. Such dispersion systems are highly unstable thermodynamically and will revert back to the stable state, resulting in phase separation. 

In the 20 century, many kind of conducting ion species from monovalent to tetravalent ions have been discovered as well as a large number of host crystal lattices. Although some binary rare earth oxides for solid electrolytes have also been investigated, their ion conducting properties were, unfortunately, not enough except the ceria-based materials. 

The crystal-field Hamiltonian that describes the interaction of a lanthanide ion with the host crystal lattice has been chosen in a number of different forms. Two of these have already been described one, eq. (33), in terms of the irreducible spherical tensors Ctm and the second, eq. (36), in terms of the V polynomials. A third notation, less common than the first two, is (Morrison et al., 1970)... 

Cavity A depression in the surface of a host which has length, width and depth of comparable dimensions. Cavity can also refer to a space within a host molecule or host crystal lattice sufficient to accommodate a guest. 

The choice of the material is critical in terms of the color and intensity of the emission. For example, upconverting nanoparticles (UCNPs) doped with Er + emit mainly green light (510-570 run) and red light (630-680 nm), but the peak maxima and relative intensities of the bands depend oti the host lattice. The green emission is dominant in fluoride-based lattices, while in oxide-based lattices it is the red emission. Dopants such as Tm + result in mainly blue upconversion fluorescence (450-500 nm) that is accompanied by a weak red fluorescence in certain host crystals. Lattices doped with Ho also show a green and red luminescence that is comparable to that of Er. ... 

Most of the compounds that have been considered as electroactive materials for secondary lithium battery exhibits relatively slow kinetics of lithium insertion (as already mentioned in the previous paragraphs, D < 10 cm. s" ). This is principally due to the low intrinsic ionic conduction (few vacant sites that can ensure the mobility of the lithium ions in the host crystal lattice). To overcome this difficulty, many authors have already suggested reducing the crystallites size. Actually, for such electrochemical systems, the lithium diffusion pathway is less detrimental in a nanograin than in a micrometer-sized particle (Fig. 6.9). 

The point defects, in turn, are classified as native (intrinsic) and substitution defects. The intrinsic point defects appear as a vacancy (the absence of an atom in a crystal lattice position) or as an interstitial defect (the presence of the host crystal atom in an interstitial position). The host crystal atoms can be substituted for another atom of a different chemical species at a regular lattice site or at the interstitial position (impurity center or substitution defect). The point defects can also be classified as neutral and charged relative to the host crystal lattice. The perturbation of a solid by... 

In many cases, the spectral width of the gain profile is comparable with that of spectral lines from incoherent spectral lamps, as, for example, is the case for gas lasers with gain profiles determined by the Doppler width of the amplifying transition between two discrete states of excited atoms or molecules in the active medium. Also, in many solid-state lasers, where the active medium is composed of excited impurity atoms or ions, diluted at low concentrations in a host crystal lattice, the linewidth of the amplifying transition is often small compared with the commonly found broad absorption bands of solids. In such cases the laser wavelength is restricted... 

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