Defects superstructure

Schwartz D K, Viswanathan R and Zasadzinski JAN 1993 Commensurate defect superstructures in a Langmuir-Blodgett film Phys. Rev. Lett. 70 1267... 

Several superstructures and defect superstructures based on sphalerite and on wurtzite have been described. The tI16-FeCuS2 (chalcopyrite) type structure (tetragonal, a = 525 pm, c = 1032 pm, c/a = 1.966), for instance, is a superstructure of sphalerite in which the two metals adopt ordered positions. The superstructure cell corresponds to two sphalerite cells stacked in the c direction. The cfla ratio is nearly 1. The oP16-BeSiN2 type structure is another example which similarly corresponds to the wurtzite-type structure. The degenerate structures of sphalerite and wurtzite (when, for instance, both Zn and S are replaced by C) correspond to the previously described cF8-diamond-type structure and, respectively, to the hP4-hexagonal diamond or lonsdaleite, which is very rare compared with the cubic, more common, gem diamond. The unit cell dimensions of lonsdaleite (prepared at 13 GPa and 1000°C) are a = 252 pm, c = 412 pm, c/a = 1.635 (compare with ZnS wurtzite). 

The NiAs structure is an important reference type because of its several (filled-up and defect) derivative structures. If atoms are left out of the metal layers in an ordered way, defects superstructures are obtained the Cdl2 type is obtained when all the metal atoms are omitted in alternate layers. On the contrary other derivative structures (filled-up structures) may be obtained by adding atoms in the same layers as... 

The influence of defect clusters or extended defects in the sense of this section on the mobility of structure elements of a crystal or, more generally, on the reactivity of solids has not yet been explored and is certainly an important field of future research. It is understood, however, that beside shear planes, defect superstructures, or defect clusters, there are also free point defects present in the crystal lattice of compounds with extended ranges of homogeneity. 

Aside from the superstructures mentioned, other superstructures with other enlargement factors for the unit cell are known, as well as superstructures of wurtzite. Defect structures,... 

Barth, J. V., Bume, H., Ertl, G., and Behm, R. J. (1990). Scanning tunneling microscopy observations on the reconstructed Au(lll) surface Atomic structure, long-range superstructure, rotational domains, and surface defects. Phys. Rev. B 42, 9307-9318. 

By means of HREM and electron diffraction a variety of intergrowth defects have been characterized in both the single and double layer Tl-systems. In some (Tl,Ba) systems weak superstructure modulations have been reported (47)(48)(58). 

Figure 5.12 (a) Ordered defects in monoclinic TiOfTij gOj g) (b) ordered defects in (orthorhombic) nonstoichiometric TiOj l (c) coherent intergrowth of (a) and (b) along the (120) planes of rocksalt structure. Lines indicate unit cell faces of the superstructures. (After Anderson, 1984.)... 

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. 

Approaches to artificial ion channels have, for instance, made use of macrocyclic units [6.72,6.74] (see also below), of peptide [8.183-8.185] and cyclic peptide [8.186] components, of non-peptidic polymers [8.187] and of various amphiphilic molecules [6.11, 8.188, 8.189]. The properties of such molecules incorporated in bilayer membranes may be studied by techniques such as ion conductance [6.69], patch-clamp [8.190] or NMR [8.191, 8.192] measurements. However, the nature of the superstructure formed and the mechanism of ion passage (carrier, channel, pore, defect) are difficult to determine and often remain a matter of conjecture. 

Two models have been developed to describe the superstructure found in these salts. In the Kobayashi et al. model, cation ordering in the channels adjacent to the [Pt(C204)2] chain is responsible for the development of the superstructure and the 3D modulation of the Pt atom chain.79 On the other hand, Bertinotti and coworkers have proposed that the chains are fragmented into micro-domains by periodic intrinsic defects associated with polarons.78 82 Three orthogonal deformation modes, one longitudinal and two transverse, are present in each chain and a fourth mode corresponds to a global sliding of the molecular column. 

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