AuCu structure

PtRu nanoparticles can be prepared by w/o reverse micro-emulsions of water/Triton X-lOO/propanol-2/cyclo-hexane [105]. The bimetallic nanoparticles were characterized by XPS and other techniques. The XPS analysis revealed the presence of Pt and Ru metal as well as some oxide of ruthenium. Hills et al. [169] studied preparation of Pt/Ru bimetallic nanoparticles via a seeded reductive condensation of one metal precursor onto pre-supported nanoparticles of a second metal. XPS and other analytical data indicated that the preparation method provided fully alloyed bimetallic nanoparticles instead of core/shell structure. AgAu and AuCu bimetallic nanoparticles of various compositions with diameters ca. 3 nm, prepared in chloroform, exhibited characteristic XPS spectra of alloy structures [84]. 

Even when complete miscibility is possible in the solid state, ordered structures will be favored at suitable compositions if the atoms have different sizes. For example copper atoms are smaller than gold atoms (radii 127.8 and 144.2 pm) copper and gold form mixed crystals of any composition, but ordered alloys are formed with the compositions AuCu and AuCu3 (Fig. 15.1). The degree of order is temperature dependent with increasing temperatures the order decreases continuously. Therefore, there is no phase transition with a well-defined transition temperature. This can be seen in the temperature dependence of the specific heat (Fig. 15.2). Because of the form of the curve, this kind of order-disorder transformation is also called a A type transformation it is observed in many solid-state transformations. 

The structures of the ordered alloys AuCu and AuCu3. At higher temperatures they are transformed to alloys which have all atomic positions statistically occupied by the Cu and Au atoms... 

Because strings of the same atoms come to be adjacent when these layers are stacked, alternating layers of atoms of one kind each are formed. These layers are planar in AuCu they are inclined relative to the plane of the paper in the unit cell (Fig. 15.1) they are parallel to the base plane. The layers of equal atoms are undulated in the other two structure types. 

This structure can be considered a superstructure of the AuCu(I) type with 1N atom inserted in an octahedral interstice. This structure, as the previously described cP5-Fe4N type, can be considered an interstitial ordered phase. 

A special case of long-period structure to be considered is the oI40-AuCu(II) type structure which has ID substitutional and displacive modulations (Fig. 3.41). We must first mention that ordering of the Au-Cu face-centred cubic (cF4-Cu type) solid solution, having a 50-50 atomic composition, re-distributes Cu and Au atoms... 

Figure 3.41. The oI40-AuCu(II) structure. This superstructure contains 10, slightly distorted, tP4-AuCu( I) pseudo-cells. The long-period ordering corresponds to a periodic shift of the structure (every five cells along the orthorhombic Yaxis) by />. (at I c) in the % c plane. The anti-phase domain contains 5 AuCu(I) pseudo-cells.

Remarks on the alloy crystal chemistry of the 4th group metals. Selected groups of isostructural phases, pertaining to simple common structural types have been collected in Table 5.25. A number of them (for instance CsCl, AuCu types, Laves phases, AuCu3 type) correspond to more or less extended solid solution... 

Several 1 1 compounds with CsCl, AuCu-type structures are formed especially with metals close to the end of the transition series and with semi-metals and non-metals of the 15th and 16th groups. 

Rh and Ir alloys. A choice of formulae, composition ranges and structure types observed in selected intermediate phases of the Rh and Ir alloys is shown in Table 5.48b. Notice in the Rh (Ir) regions of the systems, several CaCu5-, AuCu3-, Cu2Mg-type phases and, in the central parts of the diagrams, CsCl-type (andNiAs-and AuCu-type) solid solutions phases. 

Figure 7.26. Section sequence parallel to the base plane of the tP2-AuCu (I) type structure. A tP4 pseudo-cell is outlined by dotted lines.

A special type of modulated superstructure (long-period superstructure) is known for AuCu (AuCul) and results in the oI40-AuCu (II) type. This antiphase-domain structure has been discussed in 3.11.1 (Fig. 3.41). 

This structure can be described as a tetragonal distortion of the AuCu3-type structure. It may also be considered a variant of the previously described AuCu( I) type (compare with its tP4 pseudo-cell). 

The substitution of bromide for ammonia in [Au(NH3)4]3+ occurs in a stepwise fashion to give [AuBr(NH3)3]2+, fra/w-[AuBr2(NH3)2]+, [AuBr3(NH3)] and [AuBr4], and the corresponding second-order rate constants follow the sequence ki < k2 k3 complex trans-[AuBr2(NH3)2]Br can be isolated from partially reacted mixtures.570 The complex [AuC13(NH3)] has been isolated in a different way by pyrolysis of NH4[AuCU], and its structure has been determined.119... 

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