Question

Given an alloy of Cu, Ag, and Au in which Cu atoms constitute the CCP arrangement. If the hypothetical formula of the alloy is $C{u}_{4}A{g}_{3}Au$. What are the probable locations of Ag and Au atoms?

• A. Ag - all Tetrahedral voids; Au - all Octahedral voids
• B. Ag - 3/8th Tetrahedral voids; Au - 1/4th Octahedral voids
• C. Ag - 1/2 Octahedral voids; Au - 1/2 Tetrahedral voids
• D. Ag - all Octahedral voids; Au - all tetrahedral voids

Answer 1

The correct option is A Ag - all Tetrahedral voids; Au - all Octahedral voids

Given that, $Cu$ atoms forms CCP arrangement which is analogous to FCC type arrangement.So,

Number of atoms of $Cu$ present in unit cell $=4$

So,

Number of octahedral voids $=$ Number of atoms in FCC arrangement $=4$

Number of tetrahedral voids $=2\times$ Number of octahedral voids

$=2\times 4=8$

The given hypothetical formula is $C{u}_{4}A{g}_{3}Au$

This is possible if ${\left(\frac{1}{4}\right)}^{th}$ of the octahedral voids and ${\left(\frac{3}{2}\right)}^{th}$ of the tetrahedral voids are occupied.