Subject orthorhombic

Over the last few years the DCA and ACA compounds have been the subject of a number of interesting chemical developments including their use as templates in polymerisation 23,24), and as media for stereospecific chemical reactions 98 100). This work has been carried out on the orthorhombic systems and is therefore outside the scope of this review. 

The scale factor can be measured experimentally by a number of techniques, using either single crystal or powder samples (Stevens and Coppens 1975). Measurement for a number of crystals, including orthorhombic sulfur (S8) and x-deutero-glycylglycine, and comparison with least-squares values, indicate that scale factors from spherical-atom refinements are subject to a positive bias of... 

The degree of crystallinity may be calculated from the density of the polymer if the density is known for the amorphous and crystalline states. Some crystallizable polymers are polymorphic, i.e., they may exist in more than one crystalline form. An unstable crystalline form may change to a more stable form, and crystalline forms may change under stress. For example, hdpe changes from an orthorhombic crystalline polymer to a monoclinic form when subjected to compressive forces. 

Subjecting these vectors to the crystallographic symmetry operations generates the four possible representations of the base vectors shown in table 1. The x, y, z directions in table 1 are taken parallel to the a, b, c axes of the orthorhombic unit cell in space group Pbnm. 

The shift of the A line in the epilayers has been connected with the variation of the lattice parameters of GaN [1,11,12], The shift of this line was also measured in samples subjected to hydrostatic pressure (see Datareview A3.1). Combination of all these data permits one to obtain the whole series of excitonic deformation potentials [6,16], Two sets of data are available which are consistent with each other and are given in TABLE 1. The discrepancies between them are linked to the differences in the values of the stiflhess coefficients of GaN used by the authors. Gil and Alemu [6] in their work subsequent to the work of Shan et al [16] used data not available when Shan et al calculated their values. The notations are the same and are linked to the relationship with the quasi cubic model of Pikus and Bir [17], Deformation potentials as and a6 have been obtained by Alemu et al [8] who studied the anisotropy of the optical response in the growth plane of GaN epilayers orthorhombically distorted by growth on A-plane sapphire. For a detailed presentation of the theoretical values of deformation potentials of GaN we refer the reader to Suzuki and Uenoyama [20] who took the old values of the stiflhess coefficients of GaN [21]. 

Orthorhombic black phosphorus, the most thermodynamically stable and least reactive form, is obtained by heating white phosphorus under pressure. It has a graphitic appearance and consists of double layers as shown in Fig. 10-2(a). When subjected to pressures above 12 kbar, the orthorhombic form transforms successively to the rhombohedral and cubic forms, Fig. 10-2(b) and (c). 

Polyethylene conforms to the situation of Figure la, l.e. under normal conditions it does not display a mesophase, only the familiar orthorhombic crystal form (o). Nevertheless, there exists a virtual mesophase which can be "uncovered" the ways in which this can be achieved is the subject of what follows. 

The yellow orthorhombic modification of sulfur is the thermodynamically most stable one. The crown-like structure of a-cyclo-Sg was discovered in 1935. fi-Cyclo-Sf, is monoclinic and is formed slowly above 95 °C, the transition temperature of orthorhombic sulfur. The third crystalline modification is y-cyc/o-Sg, which melts at 106.8 C on rapid heating and can be prepared as discussed above. There is a large number of well-studied cyclic oligomers of sulfur here we refer the reader to a standard textbook. When sulfur heated to about 400°C is subjected to quenching the product is polycatenasulfur, which exhibits rubber-like properties. 

The reader may have noticed in the previous examples that some of the information given was not used in the calculations. In (a), for example, the cell was said to contain only one atom, but the shape of the cell was not specified in (b) and (c), the cells were described as orthorhombic and in (d) as cubic, but this information did not enter into the structure-factor calculations. This illustrates the important point that the structure factor is independent of the shape and size of the unit cell. For example, any body-centered cell will have missing reflections for those planes which have ft + k + 1) equal to an odd number, whether the cell is cubic, tetragonal, or orthorhombic. The rules we have derived in the above examples are therefore of wider applicability than would at first appear and demonstrate the close connection between the Bravais lattice of a substance and its diffraction pattern. They are summarized in Table 4-1. These rules are subject to... 

Ammonium Perchlorate. The thermal decomposition of AP has been the subject of several studies, and the results have been summarized in several excellent articles. [40-43]At room temperature AP has a stable phase with orthorhombic crystal symmetry, space group Pnma, with four formula units per cell. [44] The C104" and NH4 ions are essentially tetrahedral in structure and are linked by N-H -O hydrogen bonds. However, the NH4 are not rigidly fixed in the lattice starting from temperatures as low as 10 K they undergo rotational motion, which increases in complexity as the temperature increases. [44-46]... 

A new unsolvated form of pyrazine-2,3-dicarboxylic acid 100 was subjected to X-ray crystallography <04AX(E)1305>, as was an orthorhombic polymorph of A,A -bis(2-pyridyimethyl)pyrazine-2,3-dicarboxamide 101 <04AX(E)210> and two triclinic polymorphs of... 

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