Antistructure defects

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... 

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. 

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... 

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 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. 

In these cases, the total number of sites remains constant, as it also does in the formation of antistructure defects ... 

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