AuCu, ordering

Insulin, a small protein of molecular mass 6000 daltons, is composed of two chains designated A and B. There are no reduced cysteine residues in insulin, but it contains three essential disulfide bonds two that crosslink the A and B chains, and one internal to the A chain to stabilize the overall tertiary stmcture. These disulfide bonds are cleaved in the presence of excess AuX4, leaving A and B chains that have cysteine residues that have become oxidized to sulfonic adds [119]. With smaller amounts of AuX4, a single disulfide bond will be attacked to form sulfinic acid [119]. The reaction is second order for AuCU while AuBr4 reacts too quickly for accurate monitoring. 

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... 

A first group of superstructures, described in several paragraphs of this chapter and of Chapter 7, must be mentioned these include the types tP2-AuCu(I), cP4-AuCu3 and tP4-Ti3Cu which can be considered face-centred cubic-based substitutional ordered superstructures. 

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.

Arrange the following complex ions in order of increasing stability [AuFJ-, [AuCU. AuBr>J-. (AulJ-. [Au(CN)2r- Provide a ralionale for your senes. 

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... 

Any departure from perfect long-range order in a superlattice causes the superlattice lines to become weaker. It may be shown [G.30] that the structure factors of partially ordered AuCu, are given by... 

Before considering the ordering transformation in AuCu, which is rather complex, we might examine the behavior of j5-brass. This alloy is stable at room temperature over a composition range of about 46 to almost 50 atomic percent zinc, and so may be represented fairly closely by the formula CuZn. At high temperatures its structure is, statistically, body-centered cubic, with the copper and zinc atoms distributed at random. Below a critical temperature of about 460°C, ordering occurs the cell corners are then occupied only by copper atoms and the cell centers only by zinc atoms, as indicated in Fig. 13-6. The ordered alloy therefore has the CsCl structure and its Bravais lattice is simple cubic. Other alloys which have the same ordered structure are CuBe, CuPd, and FeCo. 

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