Space groups hexagonal

Urea forms an isomorphous series of crystalline non-stoichiometric inclusion compounds with n-alkanes and their derivatives (including alcohols, esters, ethers, aldehydes, ketones, carboxylic acids, amines, nitriles, thioalcohols, and thioethers), provided that their main chain contains six or more carbon atoms. 

The crystal structure of cesium iron fluoride, Cs3Fe2F9, comprises a packing of binuclear face-sharing octahedral FeiF - units and Cs+ ions. With a = 634.7(1), c = 1480.5(3) pm and Z = 2, the locations of the atoms and the Fe2Fg anion are shown in the following table  

Atom/Unit Wyckoff position Site symmetry X y Z 

The structure of the Fe2Fg anion and the crystal packing in Cs3Fe2F9 are shown in Fig. 9.6.24. 

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A more complex example may be represented by TaSe2 its modification called 2H-TaSe2 is hexagonal (space group P6 mmc, with two formula units in the unit cell). This layered compound shows a displacive 2D modulation (defined by two vectors) its symmetry may be therefore described in terms of a supergroup in a 5D superspace (Janner and Janssen 1980). A general point is therefore denoted by the 5 parameters x,y,z, t, u, and a position vector by the five components xa + yb + zc + td + ue of the superspace, with a, b, c basis in the position space and d and e in the internal subspace . 

Hexagonal, space group P63lmmc, N. 194 Atomic positions ... 

UHg2, hP3,AlB derivative structural type Hexagonal, space group P6/mmm, N. 191. 

NdPtSb, hP6 andLiGaGe, hP6, structures (AlB2-derivative types) Both structures are hexagonal, space group P63me, N. 186. with the following atomic positions ... 

Zr fAl3, hP7, structural type (a tetrahedrally close-packed phase) Hexagonal, space group P6, N. 174. 

Fluorapatite (FA) corresponds to the chemical formula Caio(P04)eF2 and crystallises in the hexagonal space group PGs/m, with Z = 1 and unit-cell parameters a = b = 9.367 A and c = 6.884 A [1] (Fig. 2). From a structural viewpoint, fluorapatite is often considered as a crystalline model for other apatites and is seen as a reference apatitic array [2]. It is one of the very first apatite structures to have been solved. It has been thoroughly studied since the 1930s [3] and is well documented in the literature. In particular, Sudarsanan et al. [1] reported the single crystal refinement of X-ray diffraction (XRD) data, and the detailed description of atomic positions and local symmetry is fully available [4,5],... 

Fig. 4.2 Unil cells of two zinc sulfiide (2 2) structures circles in order of decreasing size are and Zn (a) wurtzite, hexagonal, space group P6 mc (b) zinc blende, cubic, space group F43/it. [From Ladd, M. F. C. Structure and Bonding in Solid State Chemistry, Wiley New York. 1979. Reproduced with permission.]...

Fig. 7.8 Stereoview of the unit cell of the cadmum iodide, Cdl2, structure type hexagonal, space group P2nA Large circles, I small circles, Cd. (From Ladd, M. F. C. Structure und Bonding in Solid Stole Chemistry, Ellis Norwood Chicester. 1979.

Figure 3.3 Idealized models of mesostmctured M41S materials (a) MCM-41 (2D hexagonal, space group pbnun ). (b) MCM-48 (cubic, space group Ia- id), and (c) MCM-50 (lamellar, space group pi).

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