Defects ordered point

Doping — is the controlled addition of a relatively small amount of foreign component (dopant) to solid materials in order to change their properties, or the process of adding impurity atoms. As a rule, dopant ions or atoms are incorporated into the crystal lattice of host material. The dopant ions are often considered as point defects (see - defects in solids). Depending on the type of host lattice and dopant, the incorporation of foreign species may lead to creation of - electronic defects, other point... 

Let us consider the system of electric dipoles and other defects randomly distributed in the film paraelectric phase. Similarly to the random field model for bulk relaxor ferroelectrics [83], this phase is called Burns reference phase. For example, the relaxor ferroelectric Pbo,92Lao,osZro,65Tio,3503 (PLZT) (where La ions are the main sources of random field) is known to have the Burns phase simply as the paraelectric phase of PbZro,65Tio,3s03 (PZT). Latter phase exists at T > Tj, Td is so-called Burns temperature and Td = 1), where Tc is transition temperature form paraelectric to ferroelectric phase in PZT. The indirect interaction of electric dipoles via soft phonon mode of a host crystal tends to order the system and so to generate the ferroelectric phase in it [84]. However, the direct interaction of dipoles and other defects like point charges, try to disorder a system, transforming it into relaxor ferroelectric. 

Point-defect ordering (e.g., vacancy-dopant pairs) leads to interesting complications. Preparation conditions themselves (e.g., oxygen partial pressure and temperature in oxides) thermodynamically define and control this defect content and structure. It is important to realize that point defects are thermodynamically allowed and defined they are not anomalous in the least. Therefore, undoped, high-purity compounds may exhibit sizable nonstoichiometry due to intrinsic point defects. Doping (intentional addition of an impurity) allows one to precisely control the point-defect content and nonstoichiometry and, thereby, the properties. Transport properties are influenced by the point defects. Electrical conduction (hole or electron transport) and solid state diffusion of atoms generally vary with the quantity and type of point defects. 

Order or disorder of defects is a matter of a fine balance between the configurational entropy of the defects and the exothermic enthalpy gained in association of certain point defects to form various t) es of larger defect agglomerations. Systems with relatively small defect concentrations are entropy controlled and consist of randomly distributed point defects. As concentration increases the enthalpy takes control and leads successively to formation of larger defect entities randomised associated defects or defect clusters, point defects assimilated into randomised stmcture elements of a new phase, superlattice ordering and extended defects, shear planes, and discrete intermediate phases. 

Issues associated with order occupy a large area of study for crystalline matter [1, 7, 8]. For nearly perfect crystals, one can have systems with defects such as point defects and extended defects such as dislocations and grain... 

As in crystals, defects in liquid crystals can be classified as point, line or wall defects. Dislocations are a feature of liquid crystal phases where tliere is translational order, since tliese are line defects in tliis lattice order. Unlike crystals, tliere is a type of line defect unique to liquid crystals tenned disclination [39]. A disclination is a discontinuity of orientation of tire director field. 

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