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Question
Third layer of spheres is added to second layer so as to form hcp or ccp structure. What is the difference between the addition of third layer to form these hexagonal close-packed structures?
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Solution
- The first one-dimensional row of a hexagonal closed-packed (hcp) structure has depressions between neighbouring atoms.
- When a second row is organised so that spheres fit in the depressions, a staggered pattern results. If row 1 is A, row 2 is B.
- When the third row is staggered and in contact with the second row, an A-type layout is created.
- Similarly, the spheres in the fourth row can be organised as a B-layer. This results in the layers being given to the ABAB type. This produces a hexagonal close packing (hcp) structure.
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