Tetragonal space groups

Formula Chemical class Chemical type Pb,(C03)Ci, Anhydrous carbonate with hydroxyl or halogen (AB),(X03>Z, Crystal system Mineral group Space group Tetragonal Phosgenite P4/mbm... 

Kea.tlte, Keatite has been prepared (65) by the crystallisation of amorphous precipitated silica ia a hydrothermal bomb from dilute alkah hydroxide or carbonate solutions at 380—585°C and 35—120 MPa (345—1180 atm). The stmcture (66) is tetragonal. There are 12 Si02 units ia the unit cell ttg = 745 pm and Cg = 8604 pm the space group is P42. Keatite has a negative volumetric expansion coefficient from 20—550°C. It is unchanged by beating at 1100°C, but is transformed completely to cristobahte ia three hours at 1620°C. 

Inspired by experimental observations on bundles of carbon nanotubes, calculations of the electronic structure have also been carried out on arrays of (6,6) armchair nanotubes to determine the crystalline structure of the arrays, the relative orientation of adjacent nanotubes, and the optimal spacing between them. Figure 5 shows one tetragonal and two hexagonal arrays that were considered, with space group symmetries P42/mmc P6/mmni Dh,), and P6/mcc... 

The mixed halide structure, for which the composition was refined to [(Zr6B)Clii.47(2)ll,, 53], crystallizes in the tetragonal space group P42/mnm, contrary to the only-chloride parent-type, which crystallizes orthorhombically [17]. This increase of symmetry is achieved through a random iodine substitution of 19.1(3)% of the Cl4 -site. Thereby the Zr3CF -units become planar, as can be seen from Fig. 5.5. 

The similarity of the structures of rutile, CaCl2 and marcasite also comes to light by comparison of their crystal structure data (Table 17.2). The space groups of CaCl2 and marcasite (both Pnnm) are subgroups of the space group of rutile. The tetragonal sym-... 

The intensely yellow compounds 10 and 11 crystallize in the tetragonal space group PA/nnc. The oval Ei0Lii6 cluster skeleton (E = P, As) is topologically best described as a twofold capped Archimedean antiprism formed by the 10 E atoms, of which each of the 16 triangular faces are occupied by a lithium center (Fig. 12) (21). 

On the basis of these experimental observations, the model of packing reported in Figure 2.26 (space group I4 ) has been proposed.33 153 The low symmetry of the packing comes from the fact that two of the four chains included in the tetragonal unit cell are independent, that is, are not related by... 

The minimum amount of information needed to specify a crystal structure is the unit cell type, that is, cubic, tetragonal, and so on, the unit cell parameters, and the positions of all of the atoms in the unit cell. The atomic contents of the unit cell are a simple multiple, Z, of the composition of the material. The value of Z is equal to the number of formula units of the solid in the unit cell. Atom positions are expressed in terms of three coordinates, x, y, and z. These are taken as fractions of a, b, and c, the unit cell sides, say and j. The x, y, and z coordinates are plotted with respect to the unit cell axes, not to a Cartesian set of axes. The space group describes the symmetry of the unit cell, and although it is not mandatory when specifying a structure, its use considerably shortens the list of atomic positions that must be specified in order to built the structure. 

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