Point Defects in Intermetallic Compounds

Several attempts have been made recently to calculate from first principles the electronic structure of point defects in intermetallic compounds (and from this their... 

A variety of experimental methods have been used to elucidate the properties of point defects in intermetallic compounds. The specific examples given below are relevant to understanding the irradiation behavior of these compounds. More detailed information on point defects in intermetallic compounds can be found in Chapter 23 by de Novion in this volume. 

The most extensive studies of point defects in intermetallic compounds have been performed in ordered alloys. In such alloys both irradiation-induced disordering and irradiation-enhanced ordering can occur, as will be discussed in Section 3 (also see the review by Schulson, 1979). The principal objective of many of these studies was to attempt to elucidate the mechanisms responsible for the irradiation-induced... 

With the establishment of suitable potentials for interatomic compounds (see Chapter 23 by de Novion in this volume) and large memory and fast computers (particularly for molecular-dynamics calculations), more detailed theoretical information is now emerging on point defects in intermetallic compounds. Several examples are given below. 

There is increasing experimental evidence for the superlattice ordering of vacant sites or interstitial atoms as a result of interactions between them. Superlattice ordering of point defects has been found in metal halides, oxides, sulphides, carbides and other systems, and the relation between such ordering and nonstoichiometry has been reviewed extensively (Anderson, 1974, 1984 Anderson Tilley, 1974). Superlattice ordering of point defects is also found in alloys and in some intermetallic compounds (Gleiter, 1983). We shall examine the features of some typical systems to illustrate this phenomenon, which has minimized the relevance of isolated point defects in many of the chemically interesting solids. 

Point defects have a significant role in intermetallic compounds, as they control many properties of technological importance, such as atomic diffusion, high-temperature creep and other mechanical properties, sintering, behavior under irradiation, and in particular irradiation-induced crystalline-to-amorphous transitions. Introduction of point defects by irradiation has even allowed one to obtain an ordered phase (FeNi) in a system where it was hindered by the low atomic mobility (Neel et al., 1964 Koczak et ai, 1971). 

In intermetallic compounds, the production of point defects by electron irradiation has generally been studied by electrical resistivity measurements. This has the advantage that the specific resistivity of a Frenkel pair is one to two orders of magnitude larger than that of an antisite defect. 

Up to now, the main progress in simulating point-defect properties in intermetallic compounds has been made by semiempirical methods. The first detailed papers in this sense concern the A15-structure superconductors (Moseev et al., 1983, 1986 Welch et al., 1984). Although these authors used very simplified pair potentials, qualitative results were obtained, which are discussed in more detail in Section 9.1. 

Up to now, we have discussed mostly model intermetallic compounds with simple crystal structures (generally cubic LI2, B2,. . . ) and containing two metal species. We shall now present briefly some properties of point defects in more exotic systems, of considerable interest the A15 superconductors, transition-metal carbides and nitrides, and III-V semiconductors (e.g. GaAs). 

The possible point-defect structures, migration properties,. . . are so numerous in intermetallic compounds that experiment alone cannot solve the problem. Unfortunately, theory here is still in its infancy. For example, the commonly used Miedema and bond-breaking semiempirical models to estimate point-defect formation energies are quite contradictory. It is only recently, since the 1980s, that more sophisticated theoretical methods have been developed and seem to be able to predict point-defect structures and properties with some accuracy. Great progress can be expected from the combined use of Monte-Carlo and molecular-dynamics simulations (Rey-Losada et al., 1993). 

Some good papers have been published recently. Unfortunately the corresponding experimental data are most often lacking. The point-defect properties calculated from the electronic structure will have to be integrated in a proper thermodynamic theory. Such knowledge will also allow study in important fields that are practically unexplored up to now in intermetallic compounds point defect-impurity interaction, point defect-dislocation interaction, and consequences on the mechanical properties, etc. Considerable work is still required. 

Compounds exhibiting large positive deviations from stoichiometry may be treated in the same manner. In this case, the possible defects are M vacancies, X interstitials, and X substitutionals. Here again, a different functional relationship between activity and 5 is obtained for each type of defect as can be seen from Eqs. (33), (36), and (42). An example of the use of these equations to deduce the nature of the defects is given for the intermetallic compound AuZn. From Zn activity measurements as a function of composition, equilibrium constants were calculated for each point defect. AuZn has the CsCl-type structure where a = 6 and s = 1. The results are shown in Table 3. It can be seen that d change with composition... 

---