CuAu structure

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). 

We present the phase diagrams of the RPM and screened Coulomb particles and show the experimental observations of the CuAu structure. Our simulations consist of N spheres with a diameter of a = 2fl in a volume V, half of which carry a positive charge Ze and the other half a negative charge -Ze. The particles interact via the screened Coulomb potential (Equation 8.11), and we define a reduced temperature... 

FIGU RE 8.13 Unit cells of (a) CsCl, (b) tetragonal, and (c) CuAu structures, where the light and dark spheres have opposite charges. In (d), the tetragonal cell of the CuAu structure is highlighted. (From Hynninen AP,... 

We showed that the two phase diagrams are qualitatively similar, and more importantly, that both contain a novel solid phase, which is analogous to the CuAu structure. We also observed the Cu Au structure in our experiments with oppositely charged coUoids, which can be seen as an experimental realization of screened Coulomb particles. 

Hynninen AP, Leunissen ME, van Blaaderen A, and Dijkstra M. 2006. CuAu structure in the restricted primitive model and oppositely charged colloids. Physical Review Letters 96 018303. 

Fig. 19. Schematic drawing of an antiphase boundary in the CuAu structure. The Au atoms are represented by open circles and the Cu atoms by closed circles. At the antiphase boundary where the atoms exchange positions, the atoms may be either Au or Cu as represented by the symbol The upward vertical direction is [001], while [100] is normal to the plane of the paper. The antiphase boundary shown is on an (001) plane.

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]). 

Surface phenomena related to specific crystal growth conditions may lead to surface atomic ordering in alloys. This order may be preserved in the bulk of the alloy resulting in thermodynamically metastable superstructures, most commonly the CuPt and CuAu structures. 

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. 

Structure tP4 (CuAu) is ordered with respect to an underlying face-centred cubic lattice, so that it takes the Jensen symbol 12/12. The CuAu lattice does show, however, a small tetragonal distortion since the ordering of the copper and gold atoms on alternate (100) layers breaks the cubic symmetry. Zinc blende (cF8(ZnS)) and wurtzite (hP4(ZnS)) are ordered structures with respect to underlying cubic and hexagonal diamond lattices respectively. Since both lattices are four-fold tetrahedrally coordinated, differing only in... 

The alloys just considered are substitutional solid solutions. Interstitial solid solutions are alloys with small atoms, for example, H, C, N, and O, in the interstitial sites, usually O and T sites. Some alloys have random distribution (disordered) if the melt is quenched but become ordered if heated and annealed or if cooled slowly. An example is the 1 1 alloy CuAu. The disordered structure is ccp, and the ordered structure is also ccp, except alternate layers parallel to a cell face contain Cu or Au. 

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. 

Treatment of napelline with dilute mineral acid results in rapid isomerization to isonapelline (42, 43). The structure of isonapelline was represented as 36 with undetermined stereochemistry at C-16. Recently, we established the stereochemistry of the C-16 methyl group in the related alkaloid, cuau-chichicine, an acid-catalyzed rearrangement product of garryfoline. By analogy, the acid-catalyzed rearrangement of napelline to isonapelline should result in a C-16 /1-methyl group in isonapelline. Thus structure 37 can be assigned to isonapelline. 

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).

However, it may be noted that this is the combination which occurs in the related commensurate layer structure CuAu II, discussed below... 

As a simple (indeed over-simplified) example, we take (as did Clapp) the case of the alloy CuAu we employ an essentially geometrical treatment of strain and ignore electronic effects (the effect of apbs on the Brillouin zones of the alloy, which controls the scale of the final periodicity). CuAu I has a simple superstructure of the cubic-close-packed (c.c.p.) arrangement of metal atoms in pure Cu or Au metals. In the parent, f.c.c. unit cell alternate (001) A layers of atoms contain exclusively Au and exclusively Cu (Fig. 23). Au is larger than Cu and hence, pmely in terms of size effects, the Cu layers must be under tension and the Au layers under compression if the layers are to be perfectly commensurate (as they are). The size effect is in fact seen, for the structure is metrically as well as symmetrically tetragonal the (now distorted) f.c.c. unit cell is face-centred tetragonal, with da = 0.93s instead of 1.000. In the CuAu II structure this strain is relieved (in one direction only ) by the introduction of apbs at every fifth cube plane normal to the layers (Fig. 24). 

Fig. 24. The structure of CuAu H projected on (010). Large circles = Au, small circles = Cu open...

Extrapolating, one might expect that non-commensurability in two directions could be relieved by introducing two sets of apbs normal to those directions - giving column structures - even in CuAu. But we know of no such case... 

Turning to other ordered phases of the Cu-Au system, we again find that the (100) surface terminations are compatible with both Au segregation and ordering. The Llg and LI2 structures of CuAu and AU3CU, respectively, allow a pure Au termination, and thus segregation of Au (cf. Fig. 8). Thus, in spite of the moderate to weak ordering tendencies (Tj ans m resp.),... 

The electronic structure of the Cu 100 -c(2x2)-Pd, Au and Mn systems have been probed by ARUPS. Wang et al. [16] have compared valence band photo-emission spectra using synchrotron radiation from the Cu 100 -c(2x2)-Au surface alloy with a CusAullOO bulk alloy terminated by a mixed c(2x2) CuAu monolayer. A surface-induced Au d-band narrowing of 0.45 eV was foimd for the Cu 100 -c(2x2)-Au surface alloy, despite the smaller lattice... 

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