Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Defect and filled-up derivative structures

Derived structures may also be formed by the ordered introduction of vacant sites. As an example, consider the hP3-CdI2 type structure (see Chapter 7) which can be related to the hP4-NiAs type structure in which the set of equivalent points 0,0,0 and 0, 0, M is considered as being subdivided into two groups (each of one site) 0,0,0 (occupied by one atomic species) and 0, 0, M (vacant). We can therefore regard the hP3-CdI2 type structure as a defect derivative form of the hP4-NiAs type (see 7.4.2.4.3). Similar considerations maybe extended to include (besides substitution and subtraction) ordered addition of atoms. In this case stuffed or filled-up derivative [Pg.155]

This filled-up superstructure may therefore be described in terms of the occupation by N of an interstice (centred in Vi, A, Vi) of a Cu-type (or AuCu3-type) structure. The N atom is octahedrally surrounded by 6 Fe atoms. This structure could also be described as a deficient NaCl-type derivative structure the Fe atoms are in the same positions as the Na atoms in NaCl and one out of the four Cl positions is occupied by the N atoms. [Pg.156]

As a footnote to these observations, we also have to mention that frequently structural distortions (axial ratio and/or inter-axial angle variations) accompany the formation of derivative structures, especially because of the ordered distribution of atoms of different sizes or of vacant sites. [Pg.156]

Several cubic structures, therefore, in which (besides 0, 0, 0 0, K, M M, 0, M M, M, 0) one or more of the reported coordinate groups are occupied could be considered as filled-up derivatives of the cubic close-packed structures. The NaCl, CaF2, ZnS (sphalerite), AgMgAs and Li3Bi-type structures could, therefore, be included in this family of derivative structures. For this purpose, however, it may be useful to note that the radii of small spheres which fit exactly into tetrahedral and octahedral holes are, respectively, 0.225. and 0.414... if the radius of the close-packed spheres is 1.0. For a given phase pertaining to one of the aforementioned types (NaCl, ZnS, etc.) if the stated dimensional conditions are not fulfilled, alternative descriptions of the structure may be more convenient than the reported derivation schemes. [Pg.157]

Notice, moreover, that one face-centred cubic cell of atoms X in which all the interstices are occupied (the octahedral by X and the tetrahedral by Z atoms) is equivalent to a block of 8 XZ, CsCl-type cells (see Fig. 3.31). This relationship (and others with other structures such as Li3Bi and MnCu2Al) should be kept in mind when considering, for instance, phase transformations occurring in ordering processes. [Pg.157]




SEARCH



Defect structure

Derivative Structure

Filled structure

Filling defect

Structural defects

Structural derivation

© 2024 chempedia.info