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Several other materials with close-packed structures are now known to exhibit similar complicated close-packings and it is necessary to point out to students that the hcp and ccp structures are not the only close-packed structures which occur in nature. The mechanism of the formation of long-period polytype structures, with a periodicity much larger than the range of any known atomic forces, has posed a problem in solid-state physics which has yet to be answered satisfactorily. For a detailed account of polytype structures observed in different materials and the different theories put forward to explain their formation, the reader is referred to a book by Verma and Krishna3 and to more recent review articles23,24,25,26.
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