The sulphides of chromium

The Cr-S system is much more complex than it was originally thought to be. 

Between the compositions CrS and Cr2S3 there are three definite solid phases  

In all the trigonal Cr sulphides Cr has six S neighbours at 2-42-2-46 A but there are also Cr-Cr bonds of length approximately 2-80 A, that is, there are ionic Cr-S bonds but also metal-metal bonds. 

All these sulphides except CrS have structures intermediate between the NiAs and Cdl2 (C6) structures. In Cr7Sg there are random vacancies in alternate metal layers of the NiAs structure, while the others have ordered vacancies in alternate metal layers. The proportions of occupied metal sites between c.p. layers are  

The patterns of vacant metal sites in 7283 (trigonal and rhombohedral forms) and CrsSg are shown in Fig. 17.6 Cr3Si is also of this type. A further sulphide, CrsSg, has been produced under pressure it also has a structure of the same general type, as also does the isostructural VsSg.  

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