AuCus, ordering

A good example is given by intermetallic compounds with the AuCus crystal structure the interactinide spacing are very large (4 A or over) and they should all be magnetic whereas less than half of the compounds studied thus far order magnetically. 

Fig. 13-2 Atom arrangements on a 100 plane, disordered and ordered AuCus.

At low temperatures, the long-range order in AuCus is virtually perfect but, as Tc is approached, some randomness sets in. This departure from perfect order can be described by means of the long-range order parameter S, defined as follows ... 

Fig. 13-4 Variation of the long-range order parameter S with temperature, for AuCus and CuZn. (AuCua from Keating and Warren [13.2] CuZn data from Chipman and Warren [13.3]).

Figure 13-4 indicates how the degree of long-range order in CuZn varies with the temperature. The order parameter for CuZn decreases continuously to zero as T approaches T, whereas for AuCus it remains fairly high right up to and... 

We have already seen that the intensity of a superlattice line from an ordered solid solution is much lower than that of a fundamental line. Will it ever be so low that the line cannot be detected We can make an approximate estimate by ignoring the variation in multiplicity factor and Lorentz-polarization factor from line to line, and assuming that the relative integrated intensities of a superlattice and fundamental line are given by their relative F values. For fully ordered AuCus, for example, we find from Eqs. (13-1) that... 

Alloys. Many alloys have structures in which the atoms of the different metals are ordered. Hence they behave hke compotmds (e.g., AuCus and CujZug). CsAu has the NaCl structure (see Figure 3) and behaves almost as if it were ionic, Cs Au. This corresponds with the low ionization energy... 

Binary Alloys. The material Au-20wt%Ag is used for low-voltage electrical contacts. Gold-copper alloys form the ordered phases AusCu [60748-60-9], AuCu [12006-51-8], and AuCus [12044-96-1]. Gold-nickel alloys decompose into gold-rich and nickel-rich solid... 

Introduction of symmetry in otherwise asymmetrical structure is also found in super lattices discovered in 1923 in AuCus aUoj and foimd later to exist in a number of alloys below a temperature known as critical temperature and they are PtCus, FeNis, MnNi3, and (MnFe) Nis alloys. Ordinarily an alloy of say A and B elements exists in solid solutions wherein the atoms of A and B are arranged randomly in the interstitials. This is the state of affairs in the alloys other than those mentioned above. In these alloys, the random structures are available at an elevated temperature, and when they are cooled down below a particular temperature called critical temperature, an ordered state happens wherein a particular set of lattice sites are occupied periodically by say A atoms and the other particular sites by B atoms. The solution is then said to be ordered and the lattice thus constituted is known by super lattice. This is a sort of disorder-order transformation and is manifested by an extra reflection in X-ray diffraction pattern. This is an important phenomena not only because of the fact that this ordered state exhibits different physical and chemical properties, but it is also an example of asymmetry to symmetry transformation. The long range order that exist in the super lattice of AuCus alloys can be explained as follows ... 

In AuCus alloys, the occupancy probability for a particular lattice site say by Au atoms is 1/4, then for Cu atoms it will be 3/4 because of the composition, and the unit cell for the disordered and ordered structures will look as given in Fig. 10.3. 

At AnXa stoichiometry there are four families of binary intermetallic compounds, which represent different stacking schemes of the close-packed layer shown in Fig. 19.6(a) [34]. All stacking variants yield crystal structures in which a CN 12 polyhedron consists entirely of X atoms surrounding the An atom. The structures that occur are the AuCus, TiNis, MgCdj, and PuAls types. The AuCus-type structure, ordered fee, is the most prevalent, occurring particularly when the actinides combine with Rh and Pd the other three structure types are ordered variants of the hep structure. 

XPD experiments on the valence levels of aluminum metal have shown that, in the DOS hmit, the same diflraction pattern is formed as by close-lying core levels, even though the valence electrons in this metal are quasi free and thus delocahzed [78]. This equivalence of the valence and core-level angular patterns may be used as a key to disentangle partial densities of states of compounds or alloys, which has been demonstrated for the ordered alloy AuCus [79]. Similar information can also... 

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