Antistructure

Anti-Frenkel Anti-Structure Vacancy- AntiStructure Xi Vx + Mx Vm -h Mx... 

The notion of point defects in an otherwise perfect crystal dates from the classical papers by Frenkel88 and by Schottky and Wagner.75 86 The perfect lattice is thermodynamically unstable with respect to a lattice in which a certain number of atoms are removed from normal lattice sites to the surface (vacancy disorder) or in which a certain number of atoms are transferred from the surface to interstitial positions inside the crystal (interstitial disorder). These forms of disorder can occur in many elemental solids and compounds. The formation of equal numbers of vacant lattice sites in both M and X sublattices of a compound M0Xft is called Schottky disorder. In compounds in which M and X occupy different sublattices in the perfect crystal there is also the possibility of antistructure disorder in which small numbers of M and X atoms are interchanged. These three sorts of disorder can be combined to give three hybrid types of disorder in crystalline compounds. The most important of these is Frenkel disorder, in which equal numbers of vacancies and interstitials of the same kind of atom are formed in a compound. The possibility of Schottky-antistructure disorder (in which a vacancy is formed by... 

The alternative approach to structures is an inversion of the traditional one we concentrate on a packed array of cations, into the interstices of which the anions are inserted, rather than putting cations into a packed anion array. Even in conventional terms the logic of the new approach can be justified, since many structures have their own antitypes (and even more show a partial structure/ antistructure relation). 

We feel that it is very important not to draw conclusions about bonding just from the observation of the occurrence of a particular structure. It is well known that the same structure can serve for crystals of presumably different bond types Mg2Sn and Li20, CuZn and CsCl are examples of pairs of compounds with the same well-known structures. Less well-known perhaps but more important in the present context is the wide occurrence of anti-structures in which cation and anion positions are interchanged. In Table 1 a number of structure-antistructure pairs is given, mainly to emphasize how wide-spread the phenomenon is, and that it encompasses both simple and complex structures. 

Also of interest is the occurrence of the same partial structure as both an anion and a metal atom array even when two complete structures are not antistructures of each other. 

Structure Compound with structure Compound with antistructure... 

Table 2. Examples of partial antistructure in which anions and metal atoms have the same arrangement ...

Isostructural compounds include some other lanthanide and actinide sesquioxides and oxide-chalcogenides such as La202X (X = S or Se) as well as Th2N2X (X = O, S, Se), Th2NOX (X = P, As) and analogous uranium compounds. In all cases the third anion is the one in octahedral coordination. (Antistructures include N2Li2Zr.)... 

Tables 3.1, 3.2 and 3.3 compiled by Povarennykh (1963) specify the initial data accepted for the calculation of hardness from formulae (3.5) and (3.6). As the ratio WJWa increases, the coefficient a decreases (Table 3.1). For compounds with ratios inverse to those given in the table, i.e., for compounds having a so-called antistructure, the coefficient a will be exactly the same, e.g., 1/2 and 2/1. In both cases, x — 80. The link attenuation coefficient / varies over a relatively narrow range, usually between 0.7 and 1.0 (Table 3.2). This coefficient requires the state of lattice linkage to be considered in each case, and like coefficient a it depends on the type of compound involved. For various types of compounds, the values of the coefficient / may be lower taking as an example minerals in the pyrite and skutterudite group, they are as follows for compounds 2/2—0.60, for 3/3—0.48 and for 4/4—0.39. The values of the coefficient y grow proportionally with coordination number (Table 3.3). The constancy of the coefficient y depends on the constancy of the coordination number which is influenced by the valence ratio of electropositive and electronegative atoms. Lattice spacings, state of chemical bonds and electron-shell structure, and for complex compounds, also the degree of action of the remain-...

Methylphenyldimethoxysilane is used as a stabiliser (antistructuring additive) in the production of rubber compounds based on silicone elastomers and highly active fillers. Introducing up to 10% (weight) of methylphenyldimethoxysilane into a rubber mixture improves the physicochemical properties of vulcanised rubbers and helps to preserve the technological characteristics of the compounds in storage. 

If these compounds crystallized in their antistructure, which is a 2H—MoSa structure with all octahedral holes filled, semiconductivity would be feasible by assuming the cation in trigonal-prismatic coordination to be divalent and the one in octahedral coordination to be tetra-valent. 

Formally the phases HfSb2(h), ThAs2(h),. . . UP2,. . . belong to the Cu2Sb type too, since they crystallize in the antistructure. Chemically, however, they are binary PbFCl-type compounds. This is reflected also by the axial ratio which is near 2. Therefore, we have discussed these phases together with their ternary analogues. 

A variety of defect formation mechanisms (lattice disorder) are known. Classical cases include the - Schottky and -> Frenkel mechanisms. For the Schottky defects, an anion vacancy and a cation vacancy are formed in an ionic crystal due to replacing two atoms at the surface. The Frenkel defect involves one atom displaced from its lattice site into an interstitial position, which is normally empty. The Schottky and Frenkel defects are both stoichiometric, i.e., can be formed without a change in the crystal composition. The structural disorder, characteristic of -> superionics (fast -> ion conductors), relates to crystals where the stoichiometric number of mobile ions is significantly lower than the number of positions available for these ions. Examples of structurally disordered solids are -> f-alumina, -> NASICON, and d-phase of - bismuth oxide. The antistructural disorder, typical for - intermetallic and essentially covalent phases, appears due to mixing of atoms between their regular sites. In many cases important for practice, the defects are formed to compensate charge of dopant ions due to the crystal electroneutrality rule (doping-induced disorder) (see also -> electroneutrality condition). 

A technology of Sr2FeMoObui (SFMO) nanosized films deposition by ion-beam sputtering is described. Optimization of deposition conditions on formation of structural ly-perfect SFMO double perovskite films is presented. Several problems arise with the use of the ion-beam sputtering method concerning the films inhomogeneity, the presence of multiple phases and Femo and Mope antistructural defects. It is shown that they are solved by means of complex selection of parameters substrate temperature, deposition rate and subsequent thermal processing. 

CS3O has been described as crystallizing in the antistructure type. (29) Although this structure model has now 30) been refined on the basis of diffractometer data down to R = 0,044, the model definitely does not account for all observations. Thus the temperature factors remain unusually large even at 130 K. Strong... 

The pyrochlore-type compounds, where the crystal structure is usually considered as a cation-ordered fluorite derivative with % vacant oxygen site per fluorite formula unit, constitute another large family of oxygen anion conductors [9, 33, 41—43, 84—88]. The unoccupied sites provide pathways for oxygen migration furthermore, the pyrochlore structure may tolerate formation of cation and anion vacancies, doping in both cation sublattices, and antistructural cation disorder. Regardless of these factors. 

The current view of these and of the off-stoichiometric (Fe0 5 Co0 5), >,Ti y alloys seems to be that they can be correctly represented by an itinerant part which is well-described in terms of the SEW model of weak-itinerant ferromagnetism but that there is additional moment formation associated with Fe or Co atoms on Ti sites (antistructure atoms) in the region of 0 < x < 0.35 leading to localized effects (Buis et al. 1981b). Parviainen (1982) has shown that the observation of a negative... 

Stoichiometric reaction is one in which no mass is transferred across the crystal boundaries. The three most common stoichiometric defects are Schottky defects, Frenkel defects, and antistructure disorder or misplaced atoms. 

Antistructure disorder or misplaced atoms. These are sites where one type of atom is found at a site normally occupied by another. This defect does not occur in ionic ceramics, but it has been postulated to occur in covalent ceramics like SiC. The notation for such a defect would be Si or C j, and the corresponding defect reaction is... 

TiAl-base alloys are in the range 160-180 GPa which is only 10-20% lower than that of the superalloys (see Table 2). Recently, it has been found by ab initio calculations that deviations from stoichiometry are due to accommodated antistructure atoms, i.e. constitutional disorder, instead of vacancies in the sublattices, and that the concentration of thermal vacancies is comparatively low because of the high formation energy (Fu and Yoo, 1993). The self-diffusion of Ti in TiAl has been studied (Kroll etal., 1992). 

Diffusion in NijAl has been studied by few investigators - in particular Chou and Chou (1985) and Hoshino et al. (1988) -and has been reviewed and discussed with respect to mechanisms and defects (Bakker, 1984 Wever et al., 1989 Stoloff, 1989). The constitutional defects are antistructure atoms on both sides of stoichiometry, i.e. Al on Ni sites and Ni on Al sites, and the concentration of constitutional, i.e. ather-mal, vacancies is very small. The vacancy content of 6 x 10 at the melting temperature and the vacancy formation enthalpy of 1.60 eV correspond to the respective values for Ni, i.e. the vacancy behavior of NijAI is similar to that of pure metals (Schaefer et al., 1992). The diffusion of Ni in NijAl is not very different from that in pure Ni and at high temperatures it is insensitive to deviations from stoichiometry. The diffusion of Al in NijAl is less well studied because a tracer is not readily available. Defects may interact with dissolved third elements which affects diffusion. In particular vacancies interact with B which is needed for ductilization , and this leads to a complex dependence of the Ni diffusion coefficient on the Al and B content of NijAl (Hoshino etal., 1988). Data for the diffusion of the third elements, Co, Cr, or Ti, in Nij Al are available (Minamino etal., 1992). 

Polyferrocenylsilane block copolymers in which the blocks are immiscible (which is generally the case) would be expected to self-assemble to form phase-separated organometallic domains in the solid state. Based on the classical behavior of organic block copolymers, thin films of polyferrocene diblock copolymers would be expected to form domains such as spheres, cylinders, double diamonds (or gyro-ids) (or their antistructures), or lamellae (Chapter 1, Section 1.2.5). The preferred domain structure would be expected to be controlled by the ratio of the blocks, their degree of immiscibility (as defined by the Flory-Hu ins interaction parameter x), and the overall molecular weight of the block copolymer [159]. 

Uses Silicone chemical antistructuring (crepe hardening) additive for filled silicone... 

There is an asymmetry to Pauling s rule in that it treats cations and anions differently. This is neither in the spirit of the bond valence method, nor satisfying when one considers the large number of structures and antistructures [3]. In some instances it leads to predictions that are probably wrong. For this reason it is found useful to restate Pauling s rule in a different (and not entirely equivalent) way as follows ... 

The lattices of the same stoichiometry in which the anions and the cations change reciprocally their places, are being defined through the antistructure object. 

We start by extending the formulation of the quadratic L-qroups in terms of chain complexes to the quadratic L-qroups of rings with antistructure in the sense of Wall (5). 

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