CdI2 is known to be a strongly polytypic substance. More than 140 polytypes of CdI2 have been reported. The crystal structure of only a small number of them have reported. This is because the X-ray photographs of most of them streaking and arching redering them difficult or unsuitable for detailed structural analysis.CdI2 has a layer strcture. The basic unit of CdI2-polytypes consists of two hexagonal closed-packed iodine layers with the small Cd ions sandwiched between them. Varius stacking arrangements of these layers can produce different polytypes with the same or different c dimensions perpendicular to the layers. You must go through the literature of the structures of different polytypes of CdI2. Many of these structures have been published in the IUCR Journal Acta Cryst.
The structure of polytypes are reported in the following way:
Polytyp 20H_p:
The detailed structure of this polytype is as follows:
Space group P3ml
Cell dimensions a = b = 4.24, c = 68.35Å
Zhdanov symbol (22)_4 1111
ABC sequence [(AyB) (Calpha(B)]_4 (AyB) (AyB)
Atomic coordinates:
6 iodine atoms at 0, 0, nIz
nl = 0, 8, 16, 24, 32, 36
10 iodine atoms at 7,$,n2z
n2=2,6,10,14, 18,22,26,30,34,38
4 iodine atoms at 1⁄2,2,n3z
n3 = 4, 12, 20, 28
4 cadmium atoms at 0, 0,n4z
n4=5,13,21,29
6 cadmium atoms at 1⁄3,2/3, n_5 z
n5=1,9,17,25,33,37
where z = 1/40.
(there many mistakes due to copying from a Journal page)
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