Shear structure

1 Shear structures with one set of planar defects -shear structure 

At the beginning of 1950, Professor A. Magneli s group in Sweden started a systematic study of the crystal structures of the oxides of transition metal elements such as Ti, V, Mo, and W, mainly by X-ray diffraction techniques. 

As a result, they confirmed the existence of the homologous compounds expressed by V O2 i, Ti Oj -1 etc. (n = 2, 3, 4.) and also predicted that the crystal structure of these compounds could be derived from a mother structure, rutile . 

Structure to be rutile. T his prediction was verified by the structure determinations of TijOg and V O2 i. These compounds are called Magncli phases after the main investigator, and similar compounds have been discovered. 

Anderson and Hyde proposed the structural principle for these compounds, by which the daughter structure (they called shear structure ), i.e. Ti O2 i and V O2 i, etc., can be derived from the mother structure as follows  

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However, a detailed model for the defect structure is probably considerably more complex than that predicted by the ideal, dilute solution model. For higher-defect concentration (e.g., more than 1%) the defect structure would involve association of defects with formation of defect complexes and clusters and formation of shear structures or microdomains with ordered defect. The thermodynamics, defect structure, and charge transfer in doped LaCo03 have been reviewed recently [84],... 

Nb02.5 and this was described in detail in elegant papers by Anderson (1970, 1973). Ordered phases are based on shear structures, with parallel CS planes (double crystallographic shear) separating the blocks of the ReOs lattice. Ternary and intergrowth block structures have been discovered by extensive HRTEM... 

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... 

Next, the rutile (TiO2) based shear structure is discussed. In the rutile-type structure (tetragonal), the metal and oxygen occupy the following positions as shown in Fig. 2.8 ... 

Figures 2.11(a), (b), and (c) show the structures in the vicinity of the shear planes for the shear operations (121) [0il], (132) [011], and (011) [0il] (APB). The structure near the shear plane for the shear operation (132)1 [Oil] can be regarded as being composed of (121) [0il] and (011) [011] (APB), i.e. alternate stacking of (121) shear structure and (Oil) APB. Generally the shear operation (/jk/) [Oil] can be resolved into its components ...

Fig. 2.11 Structure in the vicinity of the shear plane for the shear operation (/ifcZ) [011] of a TiOj-type structure, (a) (121)5[0il] (b) (132) [0il] (c) (011)2[0il] the structure obtained after only operation (3) on TiO2 without the elimination of an oxygen-only plane. The structure is called an APB (anti phase boundary) or twin structure, and is similar to the shear structure of (110)j[li0] of ReOj (see Fig. 2.6(a)). Note the atomic arrangement in the zones framed by dotted lines (see also Fig. 2.113).

Compounds with shear structure in transition metal oxides (a) Ti-O, V-O, l i(Cr) ()... 

Shear structure with two sets of planar defects—block structure... 

Fig. 2.19 Example of a process for deriving a block structure by a successive shear operation on ReOj-type structure (100)5[i0i] -> (010)5[0ii]. (a) Shear plane of (100) of ReOj-type. The mark O denotes the oxygens to be removed, (b) Derivation of a one-dimensional block (shear) structure by the shear operation (100)j[101]. (c) Block structure [3 x 4) obtained by a successive shear operation of (010)1(011] on structure (b). This type of block structure is characterized by edge-sharing of all octahedra at the corners of the blocks with four adjacent blocks.

Though many kinds of block structures have been recognized so far, we have described only the structural principle and typical examples of block structures. The mechanism of the shear structure formation, and the redox processes of these oxides are of interest in solid state chemistry, and detailed and comprehensive descriptions can be found in the literature. ... 

On the other hand, two types of shear structure based on rutile appear in the TijOj-TiOj, (>20,1102, and systems. For the... 

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