Incommensurate superstructures

Figure 15. Arrangement of the Mn - O layers and separating sheets according to Giovanoli [3]. The layer structure can be (a) completely ordered or (d) completely disordered (turbostratic disorder). The cases (h) and (c) represent situation between the two extremes, (b) Disorder of the interlayer atoms or molecules but an ordered stacking of the Mn - O layers with constant layer distance, (c) Disorder of the interlayer atoms and an incommensurate shift of the complete Mn - O sheet within the layer plane, resulting in an incommensurate superstructure along the r -direction (perpendicular to the layer) and in a diffuse distribution of the electron density in this layer, resulting in a lower contribution of this layer to the 0 0 / reflections. (Adapted from Ref. [47]).

Similar to the ID organic metals, the superconductivity in ET salts is highly sensitive to disorder. This is obvious from the low-pressure phase of /d-(ET)2l3 where Tc is suppressed by the incommensurate superstructure already mentioned, or from the absence of superconductivity in /3-(ET)2l2Br due to anion disorder [128,131]. Systematic investigations of the composition dependence of Tc in /3-(ET)2(l3)i-x(IBr2)x showed a rapid suppression of superconductivity for concentrations only slightly away from x = 1 and x = 0 [232]. This is contrary to the behavior found usually in superconductors and... 

An incommensurate superstructure is a superstructure derived from a basic structure without a proportional relation of dimensions to the basic unit cell. This type of superstructure is often found in mixed oxides. A good example is Bi203-based solid solutions. 

Some incommensurate superstructures are developed from lattice distortion instead of partial ordering of cations. One example is the superconducting oxide Bi2+)tSr2Cai jtCu20g, 6 [43, 44]. 

General speaking, a crystal defect is a type of microstructure in which the parent crystal structure is locally disturbed. Such a microstructure carmot be described by a superstructure, even by an incommensurate superstructure. Since the defect areas are usually very small, they may not be detected by XRD and neutron diffraction methods. HRTEM is the most powerful technique to detect them and to investigate their relation with the parent crystals. The defects can greatly change the properties of an oxide. Some examples of the common defects in oxides are given in the following sub-sections. 

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