Compounds with Crystallographic Shear

Obviously, Ba2YCu4Og corresponds to the case m = 0, and Ba4Y2Cu7014+ to the case m = n = 1. In Ba4Y2Cu7014+g the double chains parallel to the b-axis alternate, along c, with single chains identical to those present in Ba2YCusOe+g. These single chains 

1 HCuOik- (n-WCo). ICuOikl) 1 HCuOik (n-1HCak ICuOikl] 1 (ICuO.) 1) 

Parameters In the first line of each entry refer to the refinement at 295 K and those In the second line refer to the refinement at 10 K. 

Space group Bmab Is a different setting of the conventional space group Cmca. Bmab Is used here to permit direct comparison with the parameters of the room temperature phase. 

The compound (La ggSrg jg) CuO has practically the same positional parameters as (La Ba j JCuO and lattice parameters  

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From a structural point of view, Pb2Sr2YCu808 can be oxidized to Pb2Sr2YCu809 with the layers (Cu) becoming (CuO). The formation of the oxidized compound, with a layer (CuO) in its structure, would make possible the existence of compounds with crystallographic shears of the type discussed before in the case of Ba CugOy. No oxidation experiments have been carried out so far, however. No other possible compounds of this homologous series have been reported so far. 

At present this group is represented only by its type mineral, cannizzarite, and by a number of Pb-Bi sulphosalts which are CC compounds (Chap. 6), with layers modified by crystallographic shear or in other ways. 

IR-11.6.1 Introduction Several special problems of nomenclature for non-stoichiometric phases have arisen with the improvements in the precision with which their structures can be determined. Thus, there are references to homologous series, non-commensurate and semi-commensurate structures, Vernier structures, crystallographic shear phases, Wadsley defects, chemical twinned phases, infinitely adaptive phases and modulated structures. Many of the phases that fall into these classes have no observable composition ranges although they have complex structures and formulae an example is Mo17047. These phases, despite their complex formulae, are essentially stoichiometric and possession of a complex formula must not be taken as an indication of a non-stoichiometric compound (cf. Section IR-11.1.2). 

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