Structure intergrowth

There are several metal oxides exhibiting well-defined recurrent intergrowth structures 

If the ABO3 perovskite structure is cut parallel to the (110) plane, slabs of the compositions are obtained if these slabs are stacked, an extra sheet of A 

Essentials of Inorganic Materials Synthesis, First Edition. C.N.R. Rao and Kanishka Biswas. 2015 John Wiley Sons, Inc. Published 2015 by John Wiley Sons, Inc. 

TABLE 14.5.1 Ordered intergrowth Structure Forming Homologous Series 

NajCa Nbp o HRTEM and X-ray diffraction show that an ordered 

White and Hyde extended their observations to the manganohumite family (Mg is replaced by Mn in humite) and leucophoenicite 3Mn2SiO4-Mn(OH)2 and obtained fruitful results from the viewpoint of the micro-twin structure. 

The structure of so-called hexagonal ferrite is based on the spinel (MgAljOJ structure. Arrangement of oxygen in spinel is CCP, and Mg and 

The stacking of oxygen layers along the c-axis is as follows  

The block T stands for the layer stacking of ABAB, ACAC, BCBC, BABA, CACA, and CBCB. The structure is designated by the notation Similarly the structure of the W phase is expressed as S2RSJR [(c /z ) J. The widths of the blocks S, R, T (length of the c-axis of each block) are about 5.0, 6.6, and 9.6 A, respectively. 

As mentioned above, many phases have been found on the line MY (Kohn and co-workershave contributed greatly to these findings). These compounds, generally expressed as M Y , have been revealed to have the intergrowth structure of M and Y. Table 2.5 shows some of the phases that have been confirmed. As seen from the table, many combinations of M and Y for a fixed (m, n) pair can exist as stable phases. For instance the compound 

The stacking sequence of the AOj layers in the compounds in this section is (cc... chh). The hh motif signifies a triplet of face-sharing octahedra, separated by (p-1) sheets of apex-sharing octahedra. There are two major groups, firstly those with a full complement of cations, and an approximate composition of and sec- 

The off-stoichiometric phase 5H—BajIrCo Oj with a packing sequence 

The same (cc...chh) stacking sequence described earlier is found in an extensive series of cation-deficient hexagonal perovskite phases with a general formula A B with n generally taking values of 4 and more. These are formed by many large A cations, typically Ca, Sr, Ba, La, Pr and Nd, and medium-sized B cations, such as Mg , Cd , B , AF, Ga , Cr, Fe, TF , TF , ZF, Sn,  

The overall symmetry of the stmcture is related to the value of n in the series formula. In cases where n is a [(multiple of 3)-1], the stmcture has a Bravais hexagonal lattice, while if n is given by a [(multiple of 3)] or a [(multiple of 3)+1], the structure conforms to a Bravais rhombohedral lattice. 

Note that in some of these phases, the octahedra in the layers may be tilted in opposite directions in successive layers, similar to the octahedral tilts found in the ABXj perovskites (Chapter 1), a feature not emphasised in the figures. 

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On the intergrowth structure of zeolite crystals as revealed by wide field and confocal fluorescence microscopy of the template removal processes... 

Figure 3. Exploded and normal representation of the proposed intergrowth structures of the zeolite crystals under study a) CrAPO-5 (front subunits are not shown) b) SAPO-34 c) SAPO-5 (front subunits are not shown) and d) ZSM-5.

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. 

Unlike crystals that are packed with identical unit cells in 3D space, aperiodic crystals lack such units. So far, aperiodic crystals include not only quasiperiodic crystals, but also crystals in which incommensurable modulations or intergrowth structures (or composites) occur [14], That is to say, quasiperiodicity is only one of the aperiodicities. So what is quasiperiodicity Simply speaking, a structure is classified to be quasiperiodic if it is aperiodic and exhibits self-similarity upon inflation and deflation by tau (x = 1.618, the golden mean). By this, one recognizes the fact that objects with perfect fivefold symmetry can exist in the 3D space however, no 3D space groups are available to build or to interpret such structures. 

Strictly related to this kind of description are the concepts of Recombination Structures and of Intergrowth Structure Series, to be discussed in the following see 3.9.4. 

Grey, I.E. Li, C. Watts, J.A. (1983) Hydrothermal synthesis of goethite-rutile intergrowth structures and their relationship to pseudorutile. Am. Min. 68 981-988 Griffith, W.P. (1974) Raman spectroscopy of minerals. In Farmer,V.C. (ed.) The infrared spectra of minerals. Min. Soc. London Monograph 4 119-136... 

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. 

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. 

Fig. 2.27 Ideal structure of H-Nb2O5 ([3 4], t[3 x 5] ), based on the basic units [B] and [C]. Note that the U and L sheets differ in both composition and the dimension of the blocks. Therefore this structure can be regarded as a kind of intergrowth structure.

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