B. E. Douglas, D. H. McDaniel, J. J. Alexander, Concepts and Models of Inorganic Chemistry, 3rd edition, John Wiley & Sons, New York, 1994, page 249.
 
 

Problem 5.4

Which of the following are not possible close-packing schemes?
 
(a) ABCABC… (d) ABCBC…
(b) ABAC… (e) ABBA…
(c) ABABC… (f) ABCCAB…

 

Answer:

(a) ABCABC is a cubic closest-packed lattice.

(b) ABAC is closest-packed; the C layer is still in the "crevices' of the adjacent A layer but the other set of valleys compared to the B layer.

(c) ABABC is closest-packed; this combines and alternates the hcp and ccp packing schemes.

(d) ABCBC is closest-packed; it is the same as ABABC.

(e) ABBA is not closet-packed; the adjacent B layers are not in valleys.

(f) ABCCAB is not closet-packed; the adjacent C layers are not in valleys.