Defects intergrowth

Curiously, the thallium cuprates often exhibit intergrowth defects contrary to the bismuth cuprates, whose structures are very similar. 

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). 

The figure of merit should change dramatically between composition phases. Moreover, within each phase a, the figure of merit should also vary with y = X - Xo, due to crystallinity effects such as crystallite size, intergrowths, defects, and faulting (van Dover et aL, 1998). In addition, the noncomposition variables should also affect the measured figure of merit. The noncomposition variables are denoted by the fi-dimensional vector z, with... 

Electron diffraction patterns were taken from many crystal fragments of Ga-2434. Some typical patterns are shown in Fig. 1.3. From the reflection conditions (h - - k = 2n for hkO, I = 2n for Okl, hOl and 00/, h = 2n for 00 and k = 2n for 0 0) the space group is uniquely determined to be Pccn (56). The lattice parameters are a = 5.46, 6 = 5.54 and c = 51.3A, being in agreement with those obtained from X-ray diffraction. Weak diffuse streaks along [010] in Fig. 1.3(a) must be due to stacking faults, which have been observed locally as intergrowth defects. 

It was proposed that a further nucleation process occurs at the interface between the central and outward components, making the boundary between them defect-rich. These discontinuities in the crystalline structure and in the porous network are not sufficiently large to be directly noticeable by optical microscopy or SEM [18], nevertheless it allows us to visualize the internal intergrowth structure. 

In the previous sections composition variation has been attributed more or less to point defects and extensions of the point defect concept. In this section structures that can be considered to be built from slabs of one or more parent structures are described. They are frequently found in mineral specimens, and the piecemeal way in which early examples were discovered has led to a number of more or less synonymic terms for their description, including intergrowth phases, composite structures, polysynthetic twinned phases, polysomatic phases, and tropochemical cell-twinned phases. In general, they are all considered to be modular structures. 

Modular structures are those that can be considered to be built from slabs of one or more parent structures. Slabs can be sections from just one parent phase, as in many perovskite-related structures and CS phases, or they can come from two or more parent structures, as in the mica-pyroxene intergrowths. Some of these crystals possess enormous unit cells, of some hundreds of nanometers in length. In many materials the slab thicknesses may vary widely, in which case the slab boundaries will not fall on a regular lattice and form planar defects. 

Extended defects are primarily composed of linear dislocations, shear planes, and intergrowth phenomena. Figure 4.1 A and B, for example, show two types of linear dislocation an edge dislocation and a screw dislocation. 

EM plays a crucial role in the development of thermodynamic data, especially for defective solids, multi-phase solids and solids with coexisting intergrowth structures. These microstructural details, which are essential to catalytic properties, cannot be revealed readily by other diffraction methods which tend to average structural information. The formation of anion vacancies in catalytic reactions and the resulting extended defects are described here, from which an improved understanding of the formation of CS planes and their role in catalysis can be obtained. These general results are applicable to other CS structures. 

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.)... 

In the various intergrowth systems examined (see Table 5.3) there is no evidence for the presence of point defects. The origin of long-range periodicity in the complex recurrent intergrowth systems is, however, intriguing. The importance of elastic forces in the formation of polytypes, shear structures and infinitely adaptive structures was... 

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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