Phases and crystal structure

At atmospheric pressure and 298 K, zirconium has a hexagonal close packed structure which is called a-zirconium. The crystal structure of this phase has been determined by a number of workers at, or around, 298 K and values for a and c from these studies are summarised in Table V-1. The selected values are the average of the data listed in the table and are in good agreement with those selected by Fernandez Guillermet [87FER] (3.2316 and 5.1475 x 10m for a and c, respectively) in a review of the thermodynamic properties of zirconium (the value for a in Table 4 of [87FER] is written incorrectly as 0.3216 nm and should read 0.32316 nm). 

These equations are different Ifom those obtained by Fernandez Guillermet 

At high pressures, zirconium forms another phase, called ro-zirconium, as was first reported by Bridgman [52BR1]. Jamieson [63JAM] found that the phase formed under pressure persisted after the pressure had been released and used X-ray diffraction to demonstrate that the phase was hexagonal with three atoms in the unit cell. The lattice positions of the unit cell lead to a greater density for this phase than is found for a-Zr [87FER]. 

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In this chapter, we have looked at some of the intrinsic features of hydroxylamine, oxime and hydroxamic acid molecules. The insights obtained, particularly concerning the electrostatic potentials on their molecular surfaces, should provide a useful basis for proceeding to their gas phase and crystal structures and properties. 

Naturally, the number of molecules with carbon-iodine bonds is smallest among the halogen compounds and this also applies to gas-phase and crystal structures for such derivatives. Furthermore, gas-phase studies of iodine compounds are hampered by their low volatility and require elevated temperatures in most cases. 

Composition, Atomic %D Phases and Intensity Phase and Crystal Structure Lattice Constants Interatomic Distances, A. Vol. of Unit Cell, Calcd. 70-24 Density, Cc. G./Cc. 

Mootz, D. and Staben, D., Clathrate hydrates of tetramethylammonium hydroxide - new phases and crystal-structures. Z. Naturforsch. B - Chem. Sci. 47, 263-274 (1992). 

Figure 23.5 is a typical TEM image of a Ft catalyst on a carbon support with polymer electrolyte. The Ft particles, polymer electrolyte, carbon support for the Ft, and voids can be identified clearly. Using EDX and electron diffraction, the elements of the phases and crystal structure can also be determined. TEM has been widely used to measure the catalyst particle size and surface area, to determine Ft dissolution and migration into the membrane, to analyze the catalyst layer structures and catalyst alloy phases, and to identify MEA failures [32-39]. 

Therefore, these materials are attracting research interest, especially in designing novel elastomers with unique combinations of aramid and soft segments, and in improving the synthetic routes and the structure analyses. So far, the phase and crystal structures of these materials have not been fully described and understood. Consequently, the understanding of the microstructures would be a key of great importance to enhance the unique properties of thermoplastic aramid elastomers. 

Ahrens, T.J., Anderson, D.L., and Ringwood, A.E. (1969), Equation of State and Crystal Structures of High-Pressure Phases of Shocked Silicates and Oxides, Rev. Geophys. 7, 667-707. 

The crystal structures of the borides of the rare earth metals (M g) are describedand phase equilibria in ternary and higher order systems containing rare earths and B, including information on structures, magnetic and electrical properties as well as low-T phase equilibria, are available. Phase equilibria and crystal structure in binary and ternary systems containing an actinide metal and B are... 

Cyc/o-Undecasulfur Su was first prepared in 1982 and vibrational spectra served to identify this orthorhombic allotrope as a new phase of elemental sulfur [160]. Later, the molecular and crystal structures were determined by X-ray diffraction [161, 162]. The Sn molecules are of C2 symmetry but occupy sites of Cl symmetry. The vibrational spectra show signals for the SS stretching modes between 410 and 480 cm and the bending, torsion and lattice vibrations below 290 cm [160, 162]. For a detailed list of wavenumbers, see [160]. The vibrational spectra of solid Sn are shown in Fig. 23. 

The synthesis and crystal structure of the peptide nucleic acid (PNA) monomer 25 having cyanuric acid as nucleobase have been described. Monomer 25 can be directly used for the solid phase synthesis of PNA oligomers . 

Since excellent reviews on block copolymer crystallization have been published recently [43,44], we have concentrated in this paper on aspects that have not been previously considered in these references. In particular, previous reviews have focused mostly on AB diblock copolymers with one crystal-lizable block, and particular emphasis has been placed in the phase behavior, crystal structure, morphology and chain orientation within MD structures. In this review, we will concentrate on aspects such as thermal properties and their relationship to the block copolymer morphology. Furthermore, the nucleation, crystallization and morphology of more complex materials like double-crystalline AB diblock copolymers and ABC triblock copolymers with one or two crystallizable blocks will be considered in detail. 

Three dimensional X-ray diffraction analysis has been employed to elucidate the molecular and crystal structure of Copper Phthalocyanine Blue ((3-modifica-tion). In all modifications, the planar and almost square phthalocyanine molecules are arranged like rolls of coins, i.e., in one dimensional stacks. The modifications vary only in terms of how these stacks are arranged relative to each other. One important aspect is the angle between staple axis and molecular plane. The a-phase features an angle of 24.7°, while the stacks in the -modification deviate by as much as 45.8° [13]. 

Remarks on the crystal chemistry of the alloys of the 3rd group metals. A large number of intermediate phases have been identified in the binary alloys formed by the rare earth metals and actinides with several elements. A short illustrative list is shown in Tables 5.19 and 5.20. Compounds of a few selected rare earth metals and actinides have been considered in order to show some frequent stoichiometries and crystal structure types. The existence of a number of analogies among the different metals considered and the formation of some isostructural series of compounds may be noticed. 

Remarks on the alloy crystal chemistry of the 7th group metals. A short list of phases, and corresponding structural prototypes, formed in the alloys of Mn, Tc and Re is shown in Table 5.40. 

Liquid Crystalline Polymers. One class of polymers that requires some special attention from a structural standpoint is liquid crystalline polymers, or LCPs. Liquid crystalline polymers are nonisotropic materials that are composed of long molecules parallel to each other in large clusters and that have properties intermediate between those of crystalline solids and liquids. Because they are neither completely liquids nor solids, LCPs are called mesophase (intermediate phase) materials. These mesophase materials have liquid-like properties, so that they can flow but under certain conditions, they also have long-range order and crystal structures. Because they are liquid-like, LCPs have a translational degree of freedom that most solid crystals we have described so far do not have. That is, crystals have three-dimensional order, whereas LCPs have only one- or two-dimensional order. Nevertheless, they are called crystals, and we shall treat them as such in this section. 

Three-Phase Transformations in Binary Systems. Although this chapter focuses on the equilibrium between phases in binary component systems, we have already seen that in the case of a entectic point, phase transformations that occur over minute temperature fluctuations can be represented on phase diagrams as well. These transformations are known as three-phase transformations, becanse they involve three distinct phases that coexist at the transformation temperature. Then-characteristic shapes as they occnr in binary component phase diagrams are summarized in Table 2.3. Here, the Greek letters a, f), y, and so on, designate solid phases, and L designates the liquid phase. Subscripts differentiate between immiscible phases of different compositions. For example, Lj and Ljj are immiscible liquids, and a and a are allotropic solid phases (different crystal structures). 

K. L. Komarek, ed., Hafnium Physico-Chemical Properties of Its Compounds and Alloys, International Atomic Energy Agency, Vienna, 1981, pp. 11,13,14, 16. Covers thermochemical properties, phase diagrams, crystal structure, and density data on hafnium, hafnium compounds, and alloys. 

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