Oxygen superstructures

Adsorption. Oxygen is adsorbed on clean tungsten surfaces in a variety of atomic and molecular states. At low temperature (<0 °C), oxygen is adsorbed molecularly, but at room temperature this adsorption is a precursor state to the atomic adsorption. A covered surface shows an ordered oxygen superstructure. If the temperature is increased, a more extensive coverage occurs and oxide-like structures are formed. The surface layer can be described as adsorbed oxide. 

In the real structures the oxygen atoms are added at specific sites aligned along the b axis of the structure (Fig. 8.7b). A number of ordered superstructures over this composition range form when samples are carefully annealed. 

Bi2Sr2CaCu2Og but containing a commensurate modulated superstructure has been prepared and characterized (19) and gives further insight into the structures of the Bi-O layers. This is discussed in further detail below. A material containing three copper-oxygen sheets, with ideal formula Bi2Sr2Ca2Cu3O10, can also be made. The Tc s of the one, two, and three copper-sheet compounds are 10, 85, and 110 K, respectively. 

Figure 6.1. (d) Perovskite transformations of LaNi03 thermogravimetriac analyses (TGA) data in air (broken curve) and in oxygen (b) ED of anion-deficient LaNiOs in [100], showing superstructure due to anion vacancy ordering arrowed. (After Gai and Rao Z. Naturforsch. a 30). 

Perovskites constitute an important class of inorganic solids and it would be instructive to survey the variety of defect structures exhibited by oxides of this family. Nonstoichiometry in perovskite oxides can arise from cation deficiency (in A or B site), oxygen deficiency or oxygen excess. Some intergrowth structures formed by oxides of perovskite and related structures were mentioned in the previous section and in this section we shall be mainly concerned with defect ordering and superstructures exhibited by these oxides. 

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