Antistructure atoms

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... 

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). 

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... 

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. 

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

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-...

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). 

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... 

The solid being stoichiometric, the ratio of the number of atoms (B/A) must remain constant. In addition, as the ratio of sites (B sites/A sites) should also remain constant, we must thus have the simultaneous presence of at least two types of defects. This whole of two defects found simultaneously is called a disorder. We can see, according to the list of defects described earlier, that theoretically there exist six classes of disorders with two defects. Among these classes, we can distinguish two groups the symmetrical disorders, which utilize the two sub-lattices of A and B, and the asymmetrical disorders, which utilize only one of the two sublattices of A or B. In fact, in practice, only four types of disorders are known. Two are symmetrical Schotlky disorder and antistructure disorder. The other two disorders are asymmetrical Frenkel disorder and S. A. disorder. 

The antistructure disorder is the simultaneous presence of two types of exchanged atoms Ag and The exchanges are simple because they do not modify stoichiometry (see section 2.S.2.2.3). The well and the source of the disorder are local. We encounter this type of disorder especially if the two chemical species have close properties (comparable volumes, close electronegativities), for example, intermetallic compounds. 

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