The observation of crystal defects

A small complication arises from the presence of crystal defects, which at normal concentrations result in a small residual EFG such that the average Cq is not identically 0. Quadrupolar nuclei in all nominally cubic materials experience small EFG s due to charged defects (e.g., Abragam, pp. 237-241) and that observed for AIPO4 P-cristobalite is smaller than typically observed for crystals such as... 

A similar effect occurs in highly chiral nematic Hquid crystals. In a narrow temperature range (seldom wider than 1°C) between the chiral nematic phase and the isotropic Hquid phase, up to three phases are stable in which a cubic lattice of defects (where the director is not defined) exist in a compHcated, orientationaHy ordered twisted stmcture (11). Again, the introduction of these defects allows the bulk of the Hquid crystal to adopt a chiral stmcture which is energetically more favorable than both the chiral nematic and isotropic phases. The distance between defects is hundreds of nanometers, so these phases reflect light just as crystals reflect x-rays. They are called the blue phases because the first phases of this type observed reflected light in the blue part of the spectmm. The arrangement of defects possesses body-centered cubic symmetry for one blue phase, simple cubic symmetry for another blue phase, and seems to be amorphous for a third blue phase. 

Among the newer probes now being developed, spectroscopic observations of crystals in the elastic-plastic regime hold promise for limited development of atomic level physical descriptions of local defects [91S02]. It is yet to be determined how generally this probe can be applied to solids. The electrochemical probe appears to have considerable potential to describe shock-compressed matter from a radically different perspective. 

The formation of surface defects of a crystal lattice. It was observed while using crystal compounds of transition metals as catalysts [e.g. as was shown by Arlman (171, 173), for a TiCl3 surface defects appear on the lateral faces of the crystal]. In this case low surface concentration of the propagation centers should be expected, as is illustrated in the case of polymerization by titanium dichloride (158). The observed... 

In very pure crystals, the number of intrinsic defects may be greater than the number of defects due to impurities, especially at high temperatures. Under these circumstances, the value of D0 will be influenced by the intrinsic defect population and may contribute to the observed value of the activation energy. 

We turn first to the (4 + 4) photodimerization of anthracenes, which has been most extensively studied in this context. In many anthracenes it has been possible to show that in the starting crystals defects are present at which the structure is appropriate for formation of the observed dimer in others it has been argued that the presence of such defects is very plausible. The weakness of this interpretation, at this stage, is that in no case has it yet proved possible to establish that the reaction indeed occurs at these defect sites. 

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