Transition crystal structures

The disadvantages of using AN in rocket propellants and pyrolants are its hygroscopic nature and its crystal structure transitions. The crystal transformations from phase 1 to phase V occur with decreasing temperature as follows ... 

Silver Sulfide. Silver sulfide, Ag2S, forms as a finely divided black precipitate when solutions or suspensions of most silver salts are treated with an alkaline sulfide solution or hydrogen sulfide. Silver sulfide has a dimorphic crystal structure. Transition from the rhombic (acanthite) to the cubic (argentite) form occurs at 175°C. Both crystal structures are found in nature. 

The nature of 4f electrons in lanthanides and their compounds, either localized or itinerant, is responsible for most of their physical and chemical properties. Localized states corresponding to tightly bound electron shells or to narrow bands of correlated electrons near the Fermi level are observed for all lanthanides. Pressure has a striking effect on the electronic structure, which, in turn, induces structural changes. For instance, crystal structure transitions from hexagonal closed packed (hep) samarium type double... 

The solid solution LaAg Ini- crystallizes with a CsCl type stmcture (Balster et al., 1975). This pseudobinary system undergoes a martensitic (displacive) crystal structure transition. At low temperature the X-ray powder patterns of polycrystalline samples show line splitting corresponding to a cubic-to-tetragonal transformation. The indium concentration 1 - x and thus the electron count per formula unit has a large influence on the transition temperature. This structural phase transition is revealed also in the temperature dependence of the electrical resistivity. For indium concentrations above 5% the curves show a pronounced hysteresis behavior. 

Around 900 K, McEwen and Touborg observed anomalies in both and with hysteresis of some 15 K. We attribute these to a rhombohedral - hep crystal structure transition detected by X-ray diffraction. Mardon and Koch (1970) found such a transition, in polycrystalline Sm, at temperatures ranging from 710 to 870 K - higher temperatures being associated with higher purity material (also see ch. 2 section 6.2). 

For instance, nitrogenation modulates the crystal structures, saturation magnetization (Ms), and Curie temperature (7c) of Fe films [4, 5], substantially. An addition of a small amount of N raises the Ms value of the a"-FeiftN2 phase by 25 % of that of pure Fe (2.22 b)- Further doping of N lowers the Ms value associated with crystal structure transition from s, y, to the paramagnetic phase of amorphous FeN [6]. Nitrogenation also raises the Ms value and the Tc of rare earth I-ferromagnet (Co and Fe) considerably [7]. The Ms of the R(Fe, Co) alloys is increased by 30-40 % relative to their parent alloys [7]. 

Statistical copolymers of ET and ethylene naphthalene -2,6-dicarboxylate (EN) units present yet another example of isodimorphic crystallization [86]. Both X-ray diffraction and thermal data, shown in Figure 11.9, indicate that as the concentration of ET units increases, the copolymer undergoes a crystal structure transition from... 

Satellite transition MAS NMR provides an alternative method for detennining the interactions. The intensity envelope of the spimiing sidebands are dominated by site A2 (using the crystal structure nomenclature) which has the smallest Cq, resulting in the intensity for the transitions of this site being spread over the smallest... 

The first technique is very intuitive. Out of the few proteins that could be crystallized in a number of different conformations, adenylate kinase is probably the best-studied example. By combining nine observed crystal structures and interpolating between them, a movie was constructed that visualized a hypothetical path of its hinge-bending transition (jVonrhein et al. 1995]). 

To provide a rational framework in terms of which the student can become familiar with these concepts, we shall organize our discussion of the crystal-liquid transition in terms of thermodynamic, kinetic, and structural perspectives. Likewise, we shall discuss the glass-liquid transition in terms of thermodynamic and mechanistic principles. Every now and then, however, to impart a little flavor of the real world, we shall make reference to such complications as the prior history of the sample, which can also play a role in the solid behavior of a polymer. 

Some materials undergo transitions from one crystal structure to another as a function of temperature and pressure. Sets of Raman spectra, collected at various temperatures or pressures through the transition often provide useftil information on the mechanism of the phase change first or second order, order/disorder, soft mode, etc. 

Because of the low glass transition temperature it is not possible to make clear film, stable at room temperature, by quenching. Some improvement in clarity may be obtained by cold rolling as this tends to dispose the crystal structure into layers (see Chapter 6). 

Crystallography is a very broad science, stretching from crystal-structure determination to crystal physics (especially the systematic study and mathematical analysis of anisotropy), crystal chemistry and the geometrical study of phase transitions in the solid state, and stretching to the prediction of crystal structures from first principles this last is very active nowadays and is entirely dependent on recent advances in the electron theory of solids. There is also a flourishing field of applied crystallography, encompassing such skills as the determination of preferred orientations, alias textures, in polycrystalline assemblies. It would be fair to say that... 

Figure 7.6. A filled. skutterudite antimonide crystal structure. A transition niclal atom (Fc or Co) at the centre of each octahedron is bonded to antimony atoms at each corner. The rare earth atoms (small spheres) are located in cages made by eight octahedra. The large thermal motion of rattling of the rare earth atoms in their cages is believed be responsible for the strikingly low thermal conductivity of these materials (Sales 1997).

The X-ray crystal structures of many of these complexes have now been determined representative examples are. shown in Fig. 4.11 from which it is clear that, at least for the larger cations, coordinative saturation and bond rhrectionality are far less significant factors than in many transition element complexes. Further interest in these ligands stems from their use in biochemical modelling since they sometimes mimic the behaviour of naturally occurring, neutral, macrocydic antibiotics such as valinomycin, monactin, nonactin, nigericin... 

A limited number of non-transition-metal derivatives of thiophene will be considered in this subsection. There are no short-range contacts between the lithium atoms originating from the (LiO)6 cores and the sulfur atoms in [Li—O—EMc2 (2-C4H3S)]6 (E = C, Si) (97OM5032), and evidence for Tr-interactions can be found in the X-ray crystal structures of these compounds. Theoretical computations show that a- (S ) Li" " interactions are weak, whereas Tr-Li" contributions are considerable, in accord with the general reasoning on the electronic characteristics of uncomplexed thiophene. 

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