Question

Packing sequence of ABCABCAB.... is observed in

• A. Simple cubic
• B. Body centered cubic
• C. Face centered cubic
• D. Hexagonally close packed structure

Hint:

A simple way to remember the stacking sequences:

• ABAB... (2-layer repeat) \(\rightarrow\) HCP (Hexagonal)

• ABCABC... (3-layer repeat) \(\rightarrow\) FCC (Cubic)

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The question refers to the stacking sequence of close-packed atomic planes in a crystal structure. Close-packed structures (FCC and HCP) are formed by stacking layers of atoms in the most efficient way. The way each subsequent layer is placed relative to the one below defines the overall crystal structure.

Step 2: Detailed Explanation:
Let's consider the stacking of close-packed planes. A single close-packed layer of atoms is denoted as 'A'. The next layer can be placed in one of two possible sets of hollows. Let's say we place it in the 'B' sites. So we have an 'AB' stack.
Now, for the third layer, there are two possibilities:

• Place the third layer directly above the first layer (in 'A' sites): This creates a stacking sequence of ...ABABAB... This sequence results in the

Hexagonally Close-Packed (HCP) structure.

• Place the third layer in the remaining set of hollows ('C' sites), which are not directly above 'A' or 'B': This creates a stacking sequence of ...ABCABC... The fourth layer is then placed directly above the first 'A' layer, repeating the sequence. This stacking results in the

Face-Centered Cubic (FCC) structure.
Simple cubic and body-centered cubic structures are not close-packed, and their atomic arrangement cannot be described by these simple stacking sequences of close-packed planes.
Therefore, the sequence ...ABCABCAB... is characteristic of the Face-Centered Cubic (FCC) structure.

Step 3: Final Answer:
The stacking sequence of close-packed planes as ...ABCABC... corresponds to the Face-Centered Cubic (FCC) crystal structure.