Diffraction methods crystal systems

As has been shown by the X-ray diffraction method the parent metals (i.e. Pd or Ni), the a-phase, and /3-phase all have the same type of crystal lattice, namely face centered cubic of the NaCl type. However, the /9-phase exhibits a significant expansion of the lattice in comparison with the metal itself. Extensive X-ray structural studies of the Pd-H system have been carried out by Owen and Williams (14), and on the Ni-H system by Janko (8), Majchrzak (15), and Janko and Pielaszek (16). The relevant details arc to be found in the references cited. It should be emphasized here, however, that at moderate temperatures palladium and nickel hydrides have lattices of the NaCl type with parameters respectively 3.6% and 6% larger than those of the parent metals. Within the limits of the solid solution the metal lattice expands also with increased hydrogen concentration, but the lattice parameter does not depart significantly from that of the pure metal (for palladium at least up to about 100°C). 

The Ca-Cu system has been reexamined using thermal analysis and x-ray diffraction methods an independent study of the CaCuj-Cu section has also been completed. The resultant phase diagram, although similar to that in ref. 3 at the Cu-rich end, differs markedly for Ca-rich alloys. Supporting evidence for the modifications has been obtained from the Ca-Mg-Cu ternaiy system. Three intermediate compounds are formed in the system CaCuj (950 C) melts congruently, whereas CajCu (488 C) and CaCu (567°C) are formed in peritectic reactions. Single-crystal x-ray diffraction studies verify the stoichiometry of CajCu and examine the polymorphism of CaCu. ... 

Confinement effects may also be employed to characterize the nucleation and growth of porous materials [211]. The underlying mechanisms of self-assembly and crystallization of these complex heterogeneous systems may be traced by solid state NMR methods well before their detection by diffraction methods. 

The basic modem data describing the atomic stmcture of matter have been obtained by the using of diffraction methods - X-ray, neutron and electron diffraction. All three radiations are used not only for the stmcture analysis of various natural and synthetic crystals - inorganic, metallic, organic, biological crystals but also for the analysis of other condensed states of matter - quasicrystals, incommensurate phases, and partly disordered system, namely, for high-molecular polymers, liquid crystals, amorphous substances and liquids, and isolated molecules in vapours or gases. This tremendous... 

Even a small protein consists of many hundreds of atoms. The primary aim of a structure determination is to place each of these atoms in space by determining the coordinates of each atom relative to a fixed coordinate system. The only techniques that can provide detailed information of this type are based on diffraction methods, and X-ray diffraction is the primary method. We must consider the meaning of the structures derived from these studies. Now X-ray diffraction can only be used to determine structures in crystals. One is, however, really concerned with the structure of proteins in solution, and it is therefore necessary to examine the difference between structure in the solid state and the solution state. We consider differences in general between these two states, and then differences in specific cases. To perform structural studies in solution, spectroscopic methods must be used. These methods are quite different from diffraction methods, being concerned with specific absorption or emission... 

However, diffraction methods have severe drawbacks. Disordered crystals are often difficult to tackle. If the disorder is of dynamic nature, e.g. arising from small-or large-amplitude motions in the crystal, the use of devices for variable-temperature measurements is compulsory and can also yield very useful information (see below for some examples) on the existence of enantiotropic systems related by phase transitions. In some, not frequent, cases the crystals are sufficiently robust to be used for direct phase transition measurements on the diffractometer. Figure 3 shows an example of multiple diffraction data sets collected on the same specimen... 

In terms of the structural features that are probed with various analytical methods, solid state nuclear magnetic resonance (SSNMR) may be looked upon as representing a middle ground between IR spectroscopy and X-ray powder diffraction methods. The former provides a measure of essentially molecular parameters, mainly the strengths of bonds as represented by characteristic frequencies, while the latter reflect the periodic nature of the structure of the solid. For polymorphs differences in molecular environment and/or molecular conformation may be reflected in changes in the IR spectrum. The differences in crystal structure that define a polymorphic system are clearly reflected in changes in the X-ray powder diffraction. Details on changes in molecular conformation or in molecular environment can only be determined from full crystal structure analyses as discussed in Section 4.4. 

Safonov et al. (331) determined the liquidus surface of the ternary In-Te-Cl system by DTA, X-ray diffraction, and crystal optical methods. Only one ternary compound, InTeCl, exists. The crystallization field of InTeCl occupies 6% of the diagram, which demonstrates the considerable thermodynamic stability of this compound. InTeCl melts congruently at 453°C. It forms part of the two pseudobinary systems In Tes-InCls and InCl-Te (82). Whereas the first consists of the two eutectic parts In Tes-InTeCl and InTeCl InCls, the latter is more complicated. It is composed of the monotectic system InTeCl-InCl and the eutectic system Te-InTeCl, where tellurium forms a solid solution with InTeCl containing from 100 to 82 atom% of Te at the eutectic temperature (82). 

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