Crystalline structures impurity states

SOLID. Matter in its most highly concentrated form, i.e., the atoms or molecules are much more closely packed than in gases or liquids and thus more resistant to deformation. The normal condition of the solid state is crystalline structure—the orderly arrangement of the constituent atoms of a substance in a frame work called a lattice, See also Crystal. Crystals are of many types and normally have defects and impurities that profoundly affect their applications, as in semiconductors, The geometric structure of... 

This formulation has been confirmed by a study of trityl-uridine, a substance first isolated (in a highly impure state) by Bredereck" in 1932 and assumed by him, without direct experimental evidence, to be 6-trityl-uridine. Levene and Tipson - succeeded in separating the crude material into two crystalline substances, one a monotrityl- and the other a ditrityl-uridine. Furthermore they found that treatment of the monotrityl-uridine with trityl chloride gives the ditrityl derivative. The structure of the monotrityl-uridine was determined as follows. On... 

Light-induced processes are described quite differently in molecular photochemistry and solid-state photophysics. In photochemistry one is used to an atomistic picture in which the arrangement of the atoms in the structure of a single molecule determines the electronic levels and thus the photochemical behavior. In contrast, the electronic levels of a solid are determined by the infinite periodicity of the atomic sequence in the crystal lattice. This leads to a basic concept according to which the solid can be treated as a dielectric continuum. Atomistic irregularities in the crystalline structure, such as lattice defects or impurities, are treated as perturbations of the spatially independent states in the energy bands. 

White et al. [ 109] report that crystalline structure has little effect on the initiation of EMs by a shock wave, van der Heijden et al. [110], using the example of RDX, HMX and HNIW, state that the following crystal parameters play a role in determining the sensitivity towards a shock stimulus (a) internal product quality, (b) mean particle size, (c) surface smoothness/shape of the explosive particle. Like the impact sensitivity, also the shock sensitivity is affected by the density defect content (dislocations, grain boundaries, voids, impurities, inclusions). The shock initiation tests with HMX, for example, have clearly demonstrated a relationship between the average crystal density and shock initiation pressure. The findings from this area are very important for practice, because a modification of quality and crystal shape of, for example, RDX can give a product with increased resistance to impact and shock (I-RDX [110]). 

Some surfactants show crystal polymorphism. Transitions between different crystalline structures are observed as sharp endothermic peaks on DSC thermograms. Frequently, crystal-crystal transitions are not reproducible because impurities or thermal treatment can promote the formation of metastable states. Visual observations are recommended to distinguish crystal-crystal transitions from melting. 

It may seem surprising to apply thermal equilibrium concepts to amorphous silicon, because the amorphous phase of a solid is not the equilibrium phase. However, a subset of bonding states may be in equilibrium even if the structure as a whole is not in its lowest energy state. The attainment of equilibrium is prevented by bonding constraints on the atomic structure. The collective motion of many atoms is required to achieve long range crystalline order and the topological constraints are formidable. On the other hand the transformation of point defects requires the cooperation of only a few atoms. Therefore any partial thermal equilibrium may be expected at point defects or impurities. 

Independent of the impurity the solid state structure is influenced by the preparation high crystallinity (< )-60%) by reaction in bulk under normal pressure or v uum combined with additional heating amorphous products by bulk reaction under high pressure or reaction in solution Polymers with uniform end groups are prepared from TCB, m i67) pmdA >. 

---