Crystal structure 16 INDEX

Figure 1. shows the measured phase differenee derived using equation (6). A close match between the three sets of data points can be seen. Small jumps in the phase delay at 5tt, 3tt and most noticeably at tt are the result of the mathematical analysis used. As the cell is rotated such that tlie optical axis of the crystal structure runs parallel to the angle of polarisation, the cell acts as a phase-only modulator, and the voltage induced refractive index change no longer provides rotation of polarisation. This is desirable as ultimately the device is to be introduced to an interferometer, and any differing polarisations induced in the beams of such a device results in lower intensity modulation. 

In most cases crystal densities differ from the densities of amorphous polymers. This leads to differences in refractive index, which in turn cause scatter of light at boundaries between amorphous and crystalline zones. Such materials are opaque except in certain instances where the crystal structure can be carefully oriented to prevent such scatter of light. 

Fig. 20.6 (a) XRD of as-synthesized AgSCi2H25 precursor with indexes of reflections, (b) Cross-sectional representation of the layered crystal structure of AgSCi2H25. (c) TEM image of... 

BIBLIOGRAPHY OF CRYSTAL STRUCTURES OF CARBOHYDRATES, NUCLEOSIDES, AND NUCLEOTIDES FOR 1979 AND 1980 ADDENDA AND ERRATA FOR 1970-1978 AND INDEX FOR 1935-1980... 

The physical properties of substances do not involve chemical changes. Color (see Textbox 17) and crystal structure (see Textbox 21), for example, are physical properties that are characteristic of a substance that serve to identify most substances. Other physical properties, such as density, hardness (see Table 3), refractive index (see Table 19), and heat capacity (see Table 101), are also useful for characterizing and identifying substances as well as distinguishing between different substances. 

Recent developments and prospects of these methods have been discussed in a chapter by Schneider et al. (2001). It was underlined that these methods are widely applied for the characterization of crystalline materials (phase identification, quantitative analysis, determination of structure imperfections, crystal structure determination and analysis of 3D microstructural properties). Phase identification was traditionally based on a comparison of observed data with interplanar spacings and relative intensities (d and T) listed for crystalline materials. More recent search-match procedures, based on digitized patterns, and Powder Diffraction File (International Centre for Diffraction Data, USA.) containing powder data for hundreds of thousands substances may result in a fast efficient qualitative analysis. The determination of the amounts of different phases present in a multi-component sample (quantitative analysis) is based on the so-called Rietveld method. Procedures for pattern indexing, structure solution and refinement of structure model are based on the same method. 

Figure 6. Successive changes of the phase value of a Fourier wave with index h = 2 moves the region with high potential (black areas) from the origin at X = 0 in the top map towards X = 1/4 in the map at the bottom. This shows that the value of the phase (f) determines the positions with high potential within the unit cell, whereas the amplitude A just affects the intensity. Note, that the maps with a phase shift of (j) = 0° and (f) = 180° have a centre of symmetry at the origin of the unit cell, whereas the other maps have no symmetry centre. From this we can draw another important conclusion if we put the origin of the unit cell on a centre of symmetry we have only two choices for the phase value, = 0° or (j)= 180°. As we will see later, this feature is of great importance for solving centrosymmetric crystal structures.

This surface is therefore the (111) surface. This surface is an important one because it has the highest possible density of atoms in the surface layer of any possible Miller index surface of an fee material. Surfaces with the highest surface atom densities for a particular crystal structure are typically the most stable, and thus they play an important role in real crystals at equilibrium. This qualitative argument indicates that on a real polycrystal of Cu, the Cu(l 11) surface should represent a significant fraction of the crystal s surface total area. 

Figure 4.7 shows top-down views of the fee (001), (111), and (110) surfaces. These views highlight the different symmetry of each surface. The (001) surface has fourfold symmetry, the (111) surface has threefold symmetry, and the (110) has twofold symmetry. These three fee surfaces are all atomically flat in the sense that on each surface every atom on the surface has the same coordination and the same coordinate relative to the surface normal. Collectively, they are referred to as the low-index surfaces of fee materials. Other crystal structures also have low-index surfaces, but they can have different Miller indices than for the fee structure. For bcc materials, for example, the surface with the highest density of surface atoms is the (110) surface. 

All three basic carbonates, artinite, hydromagnestite and dypingite, are white crystalline substances of monoclinic crystal structures refractive index 1.488, 1.523 and 1.508, respectively the index of refraction for the basic carbonate octahydrate is 1.515 the densities are 2.02 and 2.16 g/cm for artinite and hydromagensite the basic carbonates are all practically insoluble in water. 

Zinc sulfide is white to gray-white or pale yellow powder. It exists in two crystalline forms, an alpha (wurtzite) and a beta (sphalerite). The wurtzite form has hexagonal crystal structure refractive index 2.356 density 3.98 g/cm3 melts at 1,700°C practically insoluble in water, about 6.9 mg/L insoluble in alkalis soluble in mineral acids. The sphalerite form arranges in cubic crystalline state refractive index 2.368 density 4.102 g/cm changes to alpha form at 1,020°C practically insoluble in water, 6.5 mg/L soluble in mineral... 

White, heavy, amorphous powder or monoclinic crystals refractive index 2.13 density 5.68 g/cm Mohs hardness 6.5 transforms to tetragonal structure above 1,100°C and cubic form above 1,900°C melts at 2,710°C and vaporizes at about 4,300°C insoluble in water soluble in hydrofluoric acid and hot sulfuric, nitric and hydrochloric acids. 

The Physical Properties are listed next. Under this loose term a wide range of properties, including mechanical, electrical and magnetic properties of elements are presented. Such properties include color, odor, taste, refractive index, crystal structure, allotropic forms (if any), hardness, density, melting point, boiling point, vapor pressure, critical constants (temperature, pressure and vol-ume/density), electrical resistivity, viscosity, surface tension. Young s modulus, shear modulus, Poisson s ratio, magnetic susceptibility and the thermal neutron cross section data for many elements. Also, solubilities in water, acids, alkalies, and salt solutions (in certain cases) are presented in this section. 

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