Materials Science And Engineering Practice Exam

Which unit cell type is associated with the stacking sequence ABCABCABCABC in its close-packed layers?

Body Centered Cubic Face Centered Cubic Hexagonal Close-Packed Simple Cubic

Correct Answer:

Face Centered Cubic

Explanation:

Stacking of close-packed layers determines the type of close-packed crystal you have. When the second layer sits in the holes of the first, the third layer can occupy a different hole, producing a three-layer repeating sequence: ABCABCABC. This three-step repeating pattern is the hallmark of cubic close packing, where the close-packed planes are the {111} planes in a face-centered cubic lattice. The unit cell that hosts this arrangement is the face-centered cubic structure, with atoms at the corners and at the centers of the faces, giving the characteristic dense packing along the [111] direction. In contrast, hexagonal close packing shows an ABAB stacking, not ABC, and the simple cubic and body-centered cubic structures do not exhibit this close-packed layering pattern.

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