2. When a solid solution becomes \"ordered\", a new compound has formed. Conside

Question

2. When a solid solution becomes "ordered", a new compound has formed. Consider a mixture of Au-Cu "ordered", a new compound has fomed. with a 1:3 atomic ratio. At high temperatures the atoms substitute for each other randomly in a disordered way. At lower temperatures they have preferred positions in the crystal as shown below. O Gold atom Copper atom "Average gold copper atom (a) Disordered (b) Ordered (a) which structure corresponds to an intermetallic compound and which one a solid solution? (b) Based on the Hume-Rothery Rules, speculate on the possible driving forces behind the tendency for ordering in this system. (c) Speculate on hether you would expect a mixture of AuAgs to show a greater or lesser tendency toward ordering in this system and explain your rationale (use physical data available in the textbook) (d) what is the Bravais lattice type of the solid when it is (a) ordered (b) disordered?

Explanation / Answer

(a) At higher temperature, the mixture of Au and Cu will be in liquid state and will have a disordered arrangement. Hence the structure (a) will correspond to an intermetallic compound . At lower temperature, the mixture of Au and Cu will exist in solid state and will have an ordered arrangement. Hence structure (b) will correspond to a solid solution.

(b) Since this is an example of substitutional solid solution, according to Hume - Rothery rule, the following are the possible driving forces behind the tendency for ordering in this system

1.The atomic radius of the solute and solvent atoms must not differ by more tham 15%

2.The solute and the solvent must have similar crystal structure.

3.Also the solute and the solvent should have similar electronegativity values and valency.

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