Structures based on more complex

A few examples are known of crystal structures based on more complex nets. Borocarbides MB2C2 are formed by scandium and the rare-earth metals, and they consist of layers of composition B2C2 interleaved with metal atoms. In the compounds of the 4f metals the layer is the 4 8 layer, the pattern of atoms being similar to that of Fig. 3.27(b), the open and shaded circles now representing B and C atoms. The scandium compound is of special interest as the only example at present known of the layer consisting of equal numbers of 5-gons and 7-gons. 

Structures based on more complex 4-connected nets... 

Apart from the extreme compounds TiS3 and TijS the sulphides of Ti include TiS2 and TiS, with the simple (h.c.p.) Cdl2 and NlAs structures, and a series of phases with compositions intermediate between those of the disulphide and monosulphide (Table 17.8). These phases have structures based on more complex c.p. sequences in which Ti atoms occupy octahedral interstices. Certain layers of metal atom sites are fully (or almost fully) occupied while others are only partially occupied (at random)-contrast the Cr sulphides Cr7Sg, CrgSg, and Cr3S4, in which the packing of the S atoms remains the same (h.c.p.). 

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