Compression Crystal structure

An example of research in the micromechanics of shock compression of solids is the study of rate-dependent plasticity and its relationship to crystal structure, crystal orientation, and the fundamental unit of plasticity, the dislocation. The majority of data on high-rate plastic flow in shock-compressed solids is in the form of ... 

Analysis of the volumetric effects indicates that as a result of such mechanical activation, iron and manganese are concentrated in the extended part of the crystal, while tantalum and niobium are predominantly collected in the compressed part of the distorted crystal structure. It is interesting to note that this effect is more pronounced in the case of tantalite than it is for columbite, due to the higher rigidity of the former. Akimov and Chernyak [452] concluded that the effect of redistribution of the ions might cause the selective predominant dissolution of iron and manganese during the interaction with sulfuric acid and other acids. 

Just as an example, the X-ray diffraction patterns of compression moulded samples of PVDF, poly(vinylfluoride), and of some VDF-VF copolymers of different compositions are shown in Fig. 17 [90]. The degrees of crystallinity of the copolymer samples (40-50%) are high and analogous to those of the homopolymer samples. This indicates a nearly perfect isomorphism between the VF and VDF monomeric units [90, 96], The diffraction patterns and the crystal structures of the copolymers are similar to those of PVF, which are in turn similar to the X-ray pattern and crystalline structure of the P form of PVDF. On the contrary, the X-ray pattern of a PVDF sample crystallized under the same conditions (Fig. 17 a) is completely different, that is typical of the non-piezoelectric a form [90]. 

The volume defect is somewhat more difficult to visualize in two dimensions. Let us suppose that a line defect has appeared while the crystal structure was forming. This would be a situation similar to that already shown in 3.1.3. where aline defect was shown. The compression-tension area of the defect has a definitive effect upon the growing crystal and causes it to deform around the line defect. This is shown in the following diagram ... 

Dopamine /3-hydroxylase (D/3H) is a copper-containing glycoprotein that hydroxylates dopamine at the benzylic position to norepinephrine.84 During the attempted crystallization of the bis(hydroxide)-bridged dicopper(II) dimer, a side product was subsequently isolated (complex (63)), revealing intramolecular hydroxylation at a formally benzylic position of the tris(imidazo-lyl)phosphine ligand.85 The copper(II) center has an axially compressed TBP structure. 

Figure 5.2 (a) Electron density contour map of the CI2 molecule (see Chapter 6) showing that the chlorine atoms in a CI2 molecule are not portions of spheres rather, the atoms are slightly flattened at the ends of the molecule. So the molecule has two van der Waals radii a smaller van der Waals radius, r2 = 190 pm, in the direction of the bond axis and a larger radius, r =215 pm, in the perpendicular direction, (b) Portion of the crystal structure of solid chlorine showing the packing of CI2 molecules in the (100) plane. In the solid the two contact distances ry + ry and ry + r2 have the values 342 pm and 328 pm, so the two radii are r 1 = 171 pm and r2 = 157, pm which are appreciably smaller than the radii for the free CI2 molecule showing that the molecule is compressed by the intermolecular forces in the solid state. 

When the stress (compressive) rises to a value approaching G/10 near the Debye temperature, motion of gliding dislocations tends to be replaced by the formation of phase transformation dislocations. The crystal structure then transforms to a new one of greater density. This occurs when the compressive stress (the hardness number) equals the energy band gap density (gap/molecular volume). 

In relatively recent years, it has been found that that indentations made in covalent crystals at temperatures below their Debye temperatures often result from crystal structure changes, as well as from plastic deformation via dislocation activity. Thus, indentation hardness numbers may provide better critical parameters for structural stability than pressure cell studies because indentation involves a combination of shear and hydrostatic compression and a phase transformation involves both of these quantities. 

The (TTFPh)2.5[Au(C6F5)2Cl2], (TTFPh)[Au(C6F5)2I2] and (TTF)[Au(C6F3H2)2 Cl2] salts have been prepared by electrocrystallization [53]. Their crystal structures have not been determined but their stoichiometries have been estimated through elemental analysis. Their room temperature conductivities, as measured on compressed pellets, are 2 x 10 1 x 10-3, and 2 x 10 6 S cm-1, and their EPR line... 

Matsubayashi et al. revealed donor abilities of the unsymmetrical diimine-dithiolene complexes [11-14]. The unsymmetrical complexes provided cation radical salts with various anions including I3, Br3 and TCNQ by use of chemical oxidation [11-14]. The electrical resistivities of the cation radical salts measured with their compressed pellets at room temperature are summarized in Table 1. The electrical resistivities of the dmit complexes were very high. The cation radical salts of the CgH4Sg-complexes, which have the BEDT-TTF moiety [22, 23], exhibited lower resistivity than those of dmit complexes, except for [(Bu-pia)Pt(CgH4Sg)] salts. However, crystal structures of these salts were not reported, and details of their electrical properties and electronic states were not discussed based on their crystal structures. 

The crystal structure of D-gulono-1,4-lactone (2) has been determined,42 and is closely approximated by formula 38. Interestingly, the conformation of the side chain in 38 differs from that in crystalline L-ascorbic acid,1 very probably because of different steric interactions between the 3- and 5-hydroxyl groups in 2 (that is, 38) and 6. Bridgman43 measured the compressibility of gulono-1,4-lactone crystals. 

Figure 1,13 Expansion and/or compression effects on crystal structures (planar section). A = shared edge B = shared corner. From R. M. Hazen and L. W. Finger, Comparative Crystal Chemistry, copyright 1982 by John Wiley and Sons. Reprinted by permission of John Wiley Sons.

Hazen and Finger (1979) extended equation 1.110 to mean polyhedral compressibility (mean compressibility of a given coordination polyhedron within a crystal structure), suggesting that it is related to the charge of ions in the polyhedron through an ionicity factor, analogous to what we have already seen for thermal expansion—i.e.. 

Hazen R. M. and Finger L. W. (1978). Crystal structure and compressibilities of pyrope and grossular to 60 kbar. Amer. Mineral, 63 297-303. 

Silvery-white, soft maUeable metal exists in two aUotropic forms an alpha hexagonal from and a beta form that has body-centered cubic crystal structure the alpha allotrope converts to beta modification at 868°C paramagnetic density 7.004 g/cm compressibility 3.0x10 cm /kg melts at 1024°C vaporizes at 3027°C vapor pressure 400 torr at 2870°C electrical resistivity 65x10 ohm-cm (as measured on polycrystalline wire at 25°C) Young s modulus 3.79xl0 ii dynes/cm2 Poisson s ratio 0.306 thermal neutron cross section 46 barns. 

---