Wyckoff sites

According to Parthe et al. (1993), a standardization procedure is necessary in the presentation of the relevant data characteristic of a crystal structure (see also Parthe and Gelato 1984). A convenient description of the structure types is then possible using the Wyckoff sequence (the letters of the occupied Wyckoff sites). This allows a finer classification of structure types and offers suggestions not only for recognizing isotypic structures but also possible structural relationships like substitution, formation of vacancy or filled-in structure variants. 

The space group is R3c (D j) with two CaCOs entities in the unit cell. The Ca + ions are located at Wyckoff sites (a) (Hahn, 1987) with symmetry 32, the carbon atoms are found at (f ) sites (symmetry 3), and the oxygen atoms are at (c) sites. The structure... 

In the structural pattern of the clathrate-I type, the framework is formed by three positions called hereafter as position 1 (Wyckoff site 6c, % 0 Vi), 2 (Wyckoff site 16/, XXX, X x 0.18) and 3 (space group Pm3n, Wyckoff site 24 k, Oyz, y X 0.30, z 0.11). The interatomic distances are usually close to the sum of the covalent radii of the constituting atoms. The average distance between the... 

The stracture of CGZA crystal can be indexed to the cubic system. For the final acceptable refinement, which relates to a lower R factor, the Al " ions occupy the A sites, the larger Zr" ions occupy the D sites, and the largest Ca " and Gd " ions occupy the L sites with a ratio of 2 1 [16]. The crystallographic data and stracture refinement for the CGZA crystal are listed in Table 9.1 [16]. The 24c, 16a, and 24d WyckofF sites are fully occupied by Ca VGd, Zr", and Al, respectively. Based on the effective ionic radii (r) of the cations, Ce " (r = 1.14 A for CN = 8) is expected to preferably occupy the sites of Gd [16]. 

The result from Mossbauer spectroscopy is that tin in a-SnF2 has a stereoactive lone pair of electrons. Is the presence of a stereoactive lone pair compatible with the tin Wyckoff sites determined by Bergerhoff [99] That is the question. While there is no symmetry constrains on the general 8f site that can, therefore, accommodate any site distortion, including the presence of a lone pair, compatibility between the presence of a stereoactive lone pair and the local symmetry of the two special sites (4b and 4e) proposed by Bergerhoff must be investigated. A lone pair is axial and can, therefore, be accommodated on a site with axial symmetry, such as a rotation axis, provided some conditions are met. In BergerhofPs... 

The stannides with the structures of the binary compounds and their superstructures form the second group (table 7). The formation of the superstructures of the ternary stannides is connected with the change of the number of the components and ordered placement of their atoms in Wyckoff sites which are occupied in the binary compounds by one component. In most cases, M and Sn are the ordered atoms, and only in the superstructures of the TiAs type (RPt2Sn stannides) and CrasCg (phases III) do the R and Sn atoms order. The structure of the RNi2Sn (MnCu2Al type) compound can be considered as both a phase of insertion and as a superstructure. 

Ant] studied the effect of Ta additions on the order-disorder transition temperatures of (Fe,Ta)74Al26 alloys by means of X-ray diffraction and calorimetry techniques. Their results show that Ta increases DO3 —> B2 transition temperature. Also, X-ray diffraction data show that Ta atom occupies predominantly the 4(h) Wyckoff sites (1/2, 1/2, 1/2) in DO3 stracture. Based on a systematic study involving several early transition elements, [1995Ant] argued that the site preference of Ta has no strong correlation with atomic... 

The garnet structure has the general structure A3B2C3O12 where A, B, and C are cations [111]. Garnets crystallize in the Ia3d space group. There are four free parameters within the structure, the lattice parameter a, and three positional parameters x, y, and z for the anion on the 96h Wyckoff site. The crystal structure is shown in Fig. 24 and is drawn in such a way to highlight the octahedra and tetrahedra present in the structure. 

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