Intermetallic compounds, CsCl structures

Introduction. A number of common structures, ideally corresponding to a 1 1 stoichiometry, are presented in this chapter. Some of them are not specifically characteristic of intermetallic compounds only. The CsCl and NaCl types, for instance, are observed for several kinds of chemical compounds (from typical ionic to metallic phases). Notice that for a number of prototypes a few derivative structures have also been considered and described, underlining crystal analogies and relationships even if with a change in the reference stoichiometry. 

Some intermetallic compounds have the same structures as those of simple polar compounds. Quite a few AM type intermetallic compounds have the NaCl (3 2PO, Section 5.1.1) structure, but usually for those differing significantly in electronegativities. Table 5.1 includes many compounds of the types MP, MAs, MSb, and MBi. The NiAs structure (2-2PO, Section 5.2.1) is found for a few MAs and MSb compounds (Table 5.5), the MSn compounds of Fe, Ni, Cu, Pd, Pt, Rh, and Au, and MnBi, NiBi, PtBi, and RhBi. The ZnS structures (CN 4) are not usually encountered for intermetallic compounds. The compounds of Al, Ga, and In with P, As, and Sb have the zinc blende (ZnS, 3 2PT, Section 6.1.1) structure. These are semiconductors or insulators. Because the bcc structure is common for metals, it is not surprising that many 1 1 intermetallic compounds have the CsCl structure (3 2PTOT, Section 7.2.1). A few of these intermetallic compounds are included in Table 7.1 a more extensive list is given in Table 9.1. 

Table 9.1. Intermetallic compounds with the CsCl structure (3 2PTOT).

In the CsCl structure, the coordination number of either kind of ion is eight, and the interionic distance is [ (3)1/2]a. Two principal kinds of compounds crystallize with the CsCl structure. In one group are the halides of the largest univalent ions, and in the other are intermetallic compounds. Some examples are listed in Table 10.3.3. 

The structures of some intermetallic compounds are formed from the stacking of blocks, each resembling a unit cell of the CsCl type. For example, the unit cell of Cr2Al may be regarded as a stack of three blocks, as shown in Fig. 10.3.3(b). 

Band structure calculations have improved considerably in the past 20 years and give a more detailed picture of the character of the conduction electrons at different sites in intermetallic compounds to be compared with the NMR results see, e.g., examples for CsCl-type intermetallic compounds by Belakhovsky et al. (1972, 1973) and Seipler et al. (1977). 

In this section we present experimental results on the temperature dependence of elastic constants for intermetallic rare-earth compounds in which magnetoelastic effects due to the presence of crystal fields are dominant. There are systematic studies of these effects for given structures across the rare-earth series. Examples are the rare-earth monopnictides, especially the rare-earth antimonides (RSb), the rare-earth dialuminides (RAlj) and rare-earth compounds with the CsCl structure. From such experiments one obtains the single-ion magnetoelastic coupling constants gj. across the series and in a few cases the quadrupolar coupling constant gf [eq. (38)] too. The case of a cooperative Jahn-TeUer effect will be treated separately in sect. 2.4.3. The examples presented here can be explained mostly with the single-ion strain susceptibility Xr [ <1- (35) instead of eq. 

The authors of [23] demonstrated the power of the linear augmented plane wave method (LAPW) for intermetallic compounds. They calculated lattice constants, electronic structure, and elastic moduli in SbY (the NaCl type structure), CoAl (the CsCl structure), and Nbir (the CuAu structure). The predicted bulk moduli are within 7% of experimental values (14—16th rows of Table 9.1). 

The metal atom substructure may also differ completely from the intermetallic structure, i.e., the intermetallic compound may suffer a reconstruction during formation of the metal hydride. On hydrogenation of ZrCo (CsCl type structure), a hydride ZrCoHs with a completely rearrange ZrCo substructure (CrB type) is formed. The reverse reconstruction is found for EuPd (CrB type structure), which... 

Band structure calculations are needed in order to determine the shape of the Fermi surface in intermetallics. The RKKY model would then have a more respectable foundation if applied to a compound whose Fermi surface is known to be approximately spherical and if the model could be explicitly avoided for the cases where the Fermi surface is distinctly nonspherical. As already mentioned in subsection 2.1 modifications of the RKKY model for general Fermi surface shapes are extremely difficult to apply. Band structure calculations up till now have been confined to the more symmetrical lattices. As a consequence, a very large fraction of such calculations involves the CsCl structure. As a first approximation the objects of the investigations contain no magnetic components, in order to find the undisturbed state of the Fermi surface. When... 

NMR work on the magnetically ordered state of other intermetallic compounds is summarized in table 18.11. Many of these compounds have the CsCl structure, and in general a major objective has been to understand the dependence of the s-f exchange interaction on conduction electron concentration. 

The structures of intermetallic phases are often designated in the literature by Strukturbericht and Pearson symbols instead of by the space group. The Strukturbericht method for classifying metallic structures consists of a capital letter followed by a number and a subscript. The A-series were supposed to be elemental structures (with an exception for A-15) the B-series, AB compounds the C-series, AB2 compounds, etc. The number and subscript following the letter designated the lattice. Examples are A1 for fee, A2 for bcc, Aa for bet, A3 for hep, A4 for diamond, etc., up to A20. For the AB binary systems, B1 is the NaCl structure, B2 is the CsCl structure, B3 is zinc blende, B4 is wurtzite, etc. up to B37. C designates the AB2 compoimds, D the A Bm compounds, etc. 

The NiAl intermetallic phases are of particular importance because of their use in strengthening Ni-based alloys, especially the superalloys used in high-temperature gas turbine blades (the Ni-Al phase diagram is shovm in Figure 12.12). Nickel aluminide (Ni-Al) is the simplest of these compounds. It is described by the Strukturbericht symbol B2, which tells us it has the CsCl structure, and the Pearson symbol cP2 tells us we have a cubic lattice with two atoms per unit cell. Therefore NiAl has Ni atoms on the comers of a cube and an A1 atom in the center (or vice versa). (Note this is not a bcc structure because the atom in the center is not the same as those on the comers.) The width of this phase in the phase diagram tells us that some solid solution is possible within the phase. 

The same behavior was found for a number of other group VIII-IIIA intermetallic compounds with the B2 (cP2) structure (CsCl type) (Neumann et al., 1976), in particular FeAl (Ho and Dodd, 1978), CoAl (Bradley and Seager, 1939 Wachtel et al., 1973 Fleischer, 1993), CoGa (Berner et al., 1975 Van Ommen et al., 1981), and NiGa (Donaldson and Rawlings, 1976 Seybolt and Westbrook, 1964 Wasilewski et al., 1968), as well as for a few other B2-structure compounds (see review by Chang and Neumann, 1982). 

The intermetallic compound FeAl exists with the CsCl structure (bcc). Ordered, stoichiometric as possible, FeAl exhibits a very small susceptibility, does not exhibit magnetic order, and Mossbauer studies on annealed FeAl have shown that no magnetic moment is associated with the iron atoms. When some of the A1 atoms are replaced by Fe atoms, so that Fe atoms now have Fe NN, local moments on Fe atoms with Fe NN develop. Crushing or plastic deformation also produces FeNN (see Section 1.5.2). The experiments with nonstoichio-metric FeAl indicate that Fe atoms with eight Fe NN have a moment of 1 )Ub and at low temperature, Fei 1AI0.9 orders ferromagnetically. 

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... 

Deformation potential coupling constants are of the order of fip, (Ziman 1960). To observe deformation potential effects in the temperature dependence of elastic constants several conditions have to be met as discussed above dpA(,(0) must be large and - Eq has to be of the order of k T. This excludes normal metals and only d-band metals with rather narrow bands can exhibit this behavior. Typical examples have been given above. In intermetallic rare-earth compounds simple density of states arguments show why elastic constant effects can be observed only for CsCl-type and Th3P4-type materials. In table 4 electronic specific heat values are listed for various rare earth compounds. This is an updated list of a previous work, see Liithi et al. (1982). This table indicates that monopnictides and monochalcogenides have smaller values of y than CsCl- and Th3P4-structure materials, i.e., the 5d band of the former structure is more hybridized than in the latter. 

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