The sulphides of titanium

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.). 

Sulphide S Layer sequence Fractional site occupancy between successive S layers Reference 

An extraordinary variety of solid phases consisting of sulphur combined with more than one kind of metal is found in the mineral world. As in oxides isomorphous replacement is widespread, leading to random non-stoichiometric compounds, but we may recognize three main types of compound. If for simplicity we describe the bonds A-S and B—S in a compound Aj BjjS as essentially ionic or essentially covalent we might expect to find three combinations  

In (a) and (c) there would be no great difference between the characters of the A-S and B—S bonds in a particular compound, while in (b) the B and S atoms form a covalent complex which may be finite or infinite in one, two, or three dimensions. By analogy with oxides we should describe (a) and (c) as complex sulphides and (b) as thio-salts. Compounds of type (c) are not found in oxy-compounds, and moreover the criterion for isomorphous replacement is different from that applicable to complex oxides because of the more ionic character of the bonding in the latter. In ionic compounds the possibility of isomorphous replacement depends largely on ionic radius, and the chemical properties of a particular ion are of minor importance. So we find the following ions replacing one another in oxide structures Fe, Mg , Mn , Zn, in positions of octahedral coordination, while Na more often replaces Ca (which has approximately the same size) than K , to which it is more closely related chemically. In sulphides, on the other hand, the criterion is the formation of the same number of directed bonds, and we find atoms such as Cu, Fe, Mo, Sn, Ag, and Hg replacing Zn in zinc-blende and closely related structures. 

Obviously this naive classification is too simple to accommodate all known compounds, and because of its basis it has the disadvantage of prejudging the bond type. An essentially geometrical classification based on known crystal structures would, however, be of the same general type. Class (a) includes structures like those of complex oxides (see Table 17.9) but will tend to merge into class (c) as bond character changes from ionic to covalent or covalent-metallic. In class (a) the ions 

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