Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Optical properties crystal-like structures

Of the morphological phenomena mentioned in the last few paragraphs, that of twinning is likely to be of most frequent value in identification problems, but all the phenomena are significant from the point of view of crystal structure and the relation between internal structure and growth characteristics. The subject of crystal morphology in relation to internal structure will not, however, be pursued further at present it will be taken up again in Chapters VII and VIII. For the present, we shall continue our consideration of the problem of the identification of microscopic crystals we pass oij to discuss crystal optics, the relation between optical properties and crystal shape and symmetry, and the determination of refractive indices and other optical characteristics under the microscope. [Pg.63]

Crystals of stoichiometric 1 1 mixtures of compounds that can complex with each other have been shown to form preferentially to pure crystals of the individual components. In some cases these crystals may have potential non-linear optical properties. An interesting example is the 1 1 mixture of p-aminobenzoic acid and 3,5-dinitrobenzoic acid. (15) A view of the crystal structure is shown in figure 3. Examination of this figure leads one to the hypothesis that the preference for the mixed crystal may be due to a) a more stable H-bonding interaction between the different benzoic acids in the hetero-dimer than in the homo-dimer b) the ability of the mixed crystal (hetero- dimers) to H-bond between their amino and nitro groups. It is likely that both of these factors play a role in the stability of the crystal structure. Calculational modelling can aid in determining the importance of these factors. [Pg.466]

Crystallization of 2-(2-thienyl)pyrroles 125 produced gold-like and bronze-like metallic crystals <02BCJ2359, 02T10233>, while structurally related derivatives produced red-violet metallic crystals <02T10225>. A crystal structure analysis of 2,5-bis(4-biphenylyl)thiophene and the effect of structure on optical properties has been reported <02AM498>. [Pg.131]

The structure of the memory switching materials in the ON state have been examined and found to be crystalline. The crystallized portions exhibit metallic conductivity or behave like small band gap semiconductors. Correspondingly, their optical properties like... [Pg.349]

The crystal structure of canine MPO has recently been determined to 3-A resolution (7, 14). This mammalian enzyme is a covalently linked dimer of molecular weight 140 kDa that can be cleaved into two identical halves by reduction of a single disulfide bond. Each half of the dimer, termed hemi-MPO, has the same optical properties and catalytic activity as the parent. Hemi-MPO consists of two polypeptides of 466 and 108 amino acid residues, and a heme-type prosthetic group is covalently bound to the larger polypeptide in a crevice 15 A below the protein surface. Like CCP and LIP, the secondary structure of MPO is dominated by a-helices with relatively little /3-sheet structure (Fig. [Pg.88]

In crystals, impurities can take simple configurations. But depending on their concentration, diffusion coefficient, or chemical properties and also on the presence of different kind of impurities or of lattice defects, more complex situations can be found. Apart from indirect information like electrical measurements or X-ray diffraction, methods such as optical spectroscopy under uniaxial stress, electron spin resonance, channelling, positron annihilation or Extended X-ray Absorption Fine Structure (EXAFS) can provide more detailed results on the location and atomic structure of impurities and defects in crystals. Here, we describe the simplest atomic structures more complicated structures are discussed in other chapters. To explain the locations of the impurities and defects whose optical properties are discussed in this book, an account of the most common crystal structures mentioned is given in Appendix B. [Pg.31]

Chapter 1 of the present volume provides the basic concepts related to the properties and characterization of the centres known as shallow dopants, the paradigm of the H-like centres. This is followed by a short history of semiconductors, which is intimately connected with these centres, and by a section outlining their electrical and spectroscopic activities. Because of the diversity in the notations, I have included in this chapter a short section on the different notations used to denote the centres and their optical transitions. An overview of the origin of the presence of H-related centres in crystals and guidelines on their structural properties is given in Chap. 2. To define the conditions under which the spectroscopic properties of impurities can be studied, Chap. 3 presents a summary of the bulk optical properties of semiconductors crystals. Chapter 4 describes the spectroscopic techniques and methods used to study the optical absorption of impurity and defect centres and the methods used to produce controlled perturbations of this absorption, which provide information on the structure of the impurity centres, and eventually on some properties of the host crystal. Chapter 5 is a presentation of the effective-mass theory of impurity centres, which is the basis for a quantitative interpretation... [Pg.479]


See other pages where Optical properties crystal-like structures is mentioned: [Pg.10]    [Pg.10]    [Pg.534]    [Pg.262]    [Pg.329]    [Pg.500]    [Pg.1263]    [Pg.261]    [Pg.782]    [Pg.615]    [Pg.322]    [Pg.54]    [Pg.370]    [Pg.218]    [Pg.63]    [Pg.320]    [Pg.917]    [Pg.382]    [Pg.391]    [Pg.394]    [Pg.579]    [Pg.332]    [Pg.437]    [Pg.162]    [Pg.157]    [Pg.156]    [Pg.44]    [Pg.228]    [Pg.274]    [Pg.437]    [Pg.225]    [Pg.189]    [Pg.219]    [Pg.326]    [Pg.203]    [Pg.339]    [Pg.247]    [Pg.144]    [Pg.419]    [Pg.131]    [Pg.248]    [Pg.62]    [Pg.302]    [Pg.361]   
See also in sourсe #XX -- [ Pg.456 , Pg.456 , Pg.466 ]




SEARCH



Crystal optical properties

Crystal properties

Optical crystal

Optical structuring

Properties crystallization

© 2024 chempedia.info