Crystal Structure and Defects

A rather common case for unstrained crystals, the jumper , is that of a molecular resonance performing a random walk within a set of preferred frequencies. In its simplest version, the two-state jumper, the molecular resonance visits only two frequencies, i.e. it is coupled to a single TLS. The two-frequency case is rather rare in pentacene/p-terphenyl (one case in a few tens). The more frequent multistate jumpers usually explore a large number of frequencies, which may be explained by a coupling of the molecule to several TLS s. 

When the number of perturbing defects in the molecular surroundings is large, either because the molecule is very close to an extended crystal fault, or because the crystal is extremely disordered, the number of preferred frequencies is so large that it becomes impossible to recognize a preferred set. Then, the number of jumps between two scans of the laser is large, and the trajectory looks like a onedimensional random walk, or a spectral diffusion. Such a pattern appears for jumping molecules in strained and disordered crystals at the end of an optical fiber. 

Spectral diffusion behaviors differ deeply from molecule to molecule. For example, one molecule, the creeper (Fig. 4) was found in [41] to drift steadily over 1.6 hours towards the line center. The wandering of this molecule presented small discontinuous jumps in addition to the drift, suggesting a driven random walker. This drift might result from a structural relaxation of the strained crystal over a long timescale. Within the more specific model of interaction of the molecule with a 

The study of single pentacene molecules in p-terphenyl shows the value of this method for investigating spectral diffusion in solids. The evolution of microscopic defects can be followed in real time, revealing a wide variety of possible local environments of molecules and of correlated behaviors for the molecular optical line. 

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The reaction rates of the thermal reduction and the reoxidation by CO2 are increased by high oxygen anion conductivity and high surface areas. Oxygen anion conductivity is a function of temperature, crystal structure, and defects. Because cycling results in stoichiometric gas-solid reactions, the gas-solid interface can be a crucial parameter depending on the reaction conditions. Whether gas-solid, intraparticle mass transfer, or surface chemical processes are rate-limiting is primarily determined by the reaction temperature and gas flow rates. 

The morphology, electronic conductivity, exposed specific surface area, crystal structure, and defect chemistry (cation distribution and oxidation states) of the manganese oxides considerably influence the performance of pseudocapacitor (capacitance, cycle life, and charge/discharge rate).153,156, l57 173,iso, 186,188 ese fea-... 

At the beginning of the century, nobody knew that a small proportion of atoms in a crystal are routinely missing, even less that this was not a mailer of accident but of thermodynamic equilibrium. The recognition in the 1920s that such vacancies had to exist in equilibrium was due to a school of statistical thermodynamicians such as the Russian Frenkel and the Germans Jost, Wagner and Schollky. That, moreover, as we know now, is only one kind of point defect an atom removed for whatever reason from its lattice site can be inserted into a small gap in the crystal structure, and then it becomes an interstitial . Moreover, in insulating crystals a point defect is apt to be associated with a local excess or deficiency of electrons. 

So far the structure of pure metals has been discussed with reference to bulk characteristics and continuous crystals. However, corrosion is essentially a surface phenomenon and it is necessary to consider how the structure and defects already described interact with free surfaces. At this stage it is convenient to consider only a film-free metal surface, although of course in most corrosion phenomena the presence of surface films is of the utmost importance. Furthermore, it is at free surfaces that the hard sphere model of metals... 

Table 2. Crystal Structures and Boron Coordination of Platinum Metal Borides WITH Isolated B Atoms (Owing to Defect Boron Sublattice)...

Tlhe importance of zeolites in research on heterogeneous catalysis is A based mainly on the fact that the structure of the active surface is a defined part of the crystal structure and does not represent a more or less severe lattice defect as most catalyst surfaces do. The crystal structure, and therefore the structure of the zeolite surface, can be determined by x-ray diffraction. Knowledge of this structure allows the construction of simple models of the distribution of electric fields in the holes of the zeolite by which wide ranges of experimental results can be explained, as is shown by the pioneering work of Barrer 1-5) and Kiselev 6-9) on calculation of the heats of adsorption of various substances. 

An extension of the kinetic theory on cases when a mechanical pressure interacts with kinetic processes inside solid volume and on interfaces has wide application interests. The elastic deformations in solid are presented from influence of external forces and from presence of internal defects of crystal structure point defects (vacancy, intersite atoms, complexes of atoms, etc.), extended defects (dislocations and inner interfaces in polycrystals), and three-dimensional defects (heterophases crystals, polycrystals). 

The essential difference between the traditional concept of a crystal structure and crystalline polymers is that the former is a single crystal whilst the polymer is polycrystalline. A single crystal means a crystalline particle grown without interruption from a single nucleus and relatively free from defects. The term polycrystallinity refers to a state in which clusters of single crystals are involved, developed from the more or less simultaneous growth of many nuclei. 

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