CuAU type

The solidphase fullerite of any composition has been produced experimentally [13-17]. It is naturally to assume that solid solution of fullerite. So, it can be proposed that the sites of first and second type, corresponding to the 4>i, d>2 fullerenes, respectively, can interchange their role during the ordering process. Figure 2 illustrates such superctructures of fullerite with sc and fee lattices of B1 structure (NaCl type) and Ll0 structure (CuAu type), respectively. 

Figure 2. The elementary cells of the fullerite crystal lattice of sc phase (a) of B1 structure (NaCl type) and fee phase (b) of Ll0 structure (CuAu type).

The titanium aluminide TiAl - often designated as y phase - crystallizes with the tetragonal LIq structure (CuAu-type) which is shown in Fig. 1. The LI o structure results from ordering in the f.c.c. lattice (Al), i.e. it is basically a cubic structure which is tetragonally distorted because of the particular stacking of the atom planes, as is seen in Fig. 1. The ratio of the lattice parameters c and a is cja = 1.015 at the stoichiometric composition and the density is 3.76 g/cm (Kim and Dimiduk, 1991), whereas for TiAl-base alloys the range 3.7-3.9 g/cm is given (see Table 2). This density is still lower than that of TijAl and has made the titanium aluminides most attractive for materials developments. 

The ten most commonly occurring structure types in order of frequency are NaCl, CsCl, CrB, FeB, NiAs, CuAu, cubic ZnS, MnP, hexagonal ZnS, and FeSi respectively. Structures cF8 (NaCl) and cP2 (CsCl) are ordered with respect to underlying simple cubic and body-centred cubic lattices respectively, as is clear from Figs 1.10(a) and 1.11(a). The Na, G sites and Cs, Cl sites are, therefore, six-fold octahedrally coordinated and fourteen-fold rhombic dodecahedrally coordinated, respectively, as indicated by the Jensen symbols 6/6 and 14/14. 

Five common ordered structures (A) L20-type CuZn, (B) L12-type Cu3Au, (C) L1o-type CuAu, (D) D03-type Fe3AI, and (E) DO-ig-type Mg3Cd. From W. F. Hosford, Physical Metallurgy (Boca Raton, FL CRC Press, 2004), p. 97, figure 5.6. 

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). 

Only three stannides of RSn composition with known structure form in the systems R-Sn. They are LaSn, EuSn (CrB strueture type) and YbSn (CuAu structure type). The structure of the remaining RSn compounds (R=Pr, Nd, Tb) is unknown (table 1). 

Either the CuAu structure in which ordering of the alloyed atoms occurs on (100)-type planes or the CuPt ordering on (111) type planes is observed in virtually all epitaxial pseudobinary III-V alloys (see Figure 6.8).[9] Occasional ordering on other planes is also found. The extent of ordering is generally described in terms of an order parameter, p (see Equation 6.8), which ranges from P > -1 for a completely ordered phase [Pab->1 and x O.5 for Equation 6.8], to zero (a completely random alloy) to +1 (a perfectly phase separated structure with no AB units [Pab x]). 

It is important to emphasize again that this is not normally an equilibrium structure in the bulk of the solid. Calculations show that a random bulk alloy is typically favored over the CuPt or CuAu organization. Ordering of this type in alloys is a result of the growth process and is directly tied to the surface structure as growth occurs. This is why sometimes one order is found, sometimes another, and sometimes no order is present. 

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