Structure space groups

LaNis has the CaCu, structure, space group P6/mmm [26] the hexagonal metal lattice is shown in Fig. 4. The crystal structure of LaNi5D7 has been determined 127, 28] and is illustrated in Fig. 5. There are three types of interstitial D sites La2Ni4 tetrahedra, and NiA tetrahedra. The unit cell is doubled along the oaxis because of the formation of a superlattice which is a consequence of long-range correlations between occupied and unoccupied... 

Another contribution is represented by an investigation of a cubic thallium cluster phase of the Bergmann type Na13(TlA.Cdi A.)27 (0.24 < x <0.33) (Li and Corbett 2004). For this phase too the body centred cubic structure (space group Im 3, a = 1587-1599 pm) may be described in terms of multiple endo-hedral concentric shells of atoms around the cell positions 0, 0, 0, and 14,14,14. The subsequent shells in every unit are an icosahedron (formed by mixed Cd-Tl atoms), a pentagonal dodecahedron (20 Na atoms), a larger icosahedron (12 Cd atoms) these are surrounded by a truncated icosahedron (60 mixed Cd-Tl atoms) and then by a 24 vertices Na polyhedron. Every atom in the last two shells is shared with those of like shells in adjacent units. A view of the unit cell is shown in Fig. 4.38. According to Li and Corbett (2004), it may be described as an electron-poor Zintl phase. A systematic description of condensed metal clusters was reported by Simon (1981). 

Elansari et al. [201] developed a novel method of synthesizing alkali metal hydrides Na, KH, RbH, and CsH by reactive mechanical milling of pure alkaline metals under hydrogen pressure up to 30 bars in a planetary mill (Retsch PM 400). The reaction proceeds in 16 h and gives 3-15 g of very pure alkali metal hydride with FCC crystal structure (space group Fm3m). 

Both compounds crystallize with the cadmium diiodide structure (space group P3ml) as previously reported on polycrystalline samples.3 For platinum disulfide, ao = 3.542(1) A and c0 = 5.043(1) A, and for platinum ditelluride, a0 = 4.023(1) A and c0 = 5.220(3) A. Direct chemical analysis for the component elements was not carried out. Instead, precision density and unit-cell determinations were performed to characterize the samples. The densities of both compounds as determined by a hydrostatic technique with heptadecafluorodeca-hydro-l-(trifluoromethyl)naphthalene as the density fluid4 indicated that they are slightly deficient in platinum. For platinum disulfide, = 7.86 g/cm3 and Pmeas = 7.7(1) gm/cm3, and for platinum ditelluride, p = 10.2 gm/cm3 and Pmeas = 9.8(1) gm/cm3. In a typical experiment an emission spectrum of the platinum disulfide showed that phosphorus was present in less than 5 ppm. A mass spectroscopic examination of the platinum ditelluride revealed a small doping by sulfur (less than 0.4%) and traces of chlorine and phosphorus (less than 100 ppm). 

Stishovite. Stishovite was first prepared (68) in the laboratory in 1961 at 1200—1400°C and pressures >16 GPa (158,000 atm). It was subsequendy discovered, along with natural coesite, in the Arizona meteor crater. It has been suggested that these minerals are geological indicators of meteorite impact structures. Stishovite (p = 4.35 g/cm3) is the densest known phase of silica. The structure, space group P42/nmn, is similar to that of... 

Among the numerous ternary rare-earth borocarbides the compounds RB2C2 with the LaB2C2-type structure (space group P4/mbm) consist of R layers and covalently bonded B-C networks alternatively stacked along the tetragonal c-axis (Bauer and Bars... 

GdNi crystallizes in the orthorhombic CrB type of structure (space group Cmcm). The Gd atoms are located on the 4c sites with point symmetry mm (0, yGd % 0.14, ). The Ni atoms also occupy the 4c sites with yNi 0.43 (see e.g. Buschow (1980)). It is interesting to note that among the RNi compounds there exists a second type of orthorhombic structure. Only the compounds with R from La to Gd crystallize in the CrB type. The compounds with R from Dy to Tm and Y show the less symmetric FeB type with space group Pnma (see e.g. Burzo et al. (1990)). As reported by Blanco et al. (1992) TbNi crystallizes in a monoclinic intermediate structure. Figure 26 shows the orthorhombic unit cell and the atomic arrangement of the CrB type of structure. (The shown orthorhombic cell is not the... 

Like the RNi compounds, the RPt compounds crystallize in two different orthorhombic structures. The heavy RPt (R = Gd to Tm) show the FeB type of structure (space group Pnma), whereas the light RPt (R = La to Nd) show the more symmetric CrB type of structure with space group Cmcm (Dwight et al. 1965 Roy et al. 1978). The Gd atoms occupy the 4c-sites (x, y — 3, z, point symmetry m) of the space group Pnma with... 

The monocarbides of the transition metals of Sc, Ti, V, Cr, Hf, Ta, Y, Nb and Zr form the NaCl or B1 structure (Space group Fm3m). This structure is based on the/cc lattice with two atoms per primitive unit cell, with both the metal atoms and carbon atoms forming fee sub-lattices. 

Tables 2.1 and 2.2a provide details of the geometry of the hydrate cages (number of cavities per unit cell, average cavity radius) and crystal cell structures (space group, lattice parameters, cell formula, atomic positions), respectively. Table 2.2b lists the atomic coordinates for structures I, II, and H.

Presented band structure calculations have been done by computer code SOLID 2000 [4,5], The lattice parameters of MgB2 (hexagonal structure, space group P6mmm), with the fraction coordinates of the unit cell atoms Mg = (0,0,0) B1 = (1/3,2/3,1/2) B2 = (2/3,1/3,1/2), have been optimized in a good agreement with the experiment, a/ Equilibrium - undistorted geometry... 

Crystallographic data Crystal structure Space group Lattice constant... 

The lanthanide trichlorides of La to Eu have a hexagonal crystal structure (space group Pf>- /m) at room temperature. For GdCb the hexagonal as well as the orthorhombic structures have been reported at room temperature. There is some uncertainty which of these is the... 

The triiodides of La to Nd have an orthorhombic structure (space group Ccmm), the triiodides of Sm to Lu have a hexagonal/rhombohedral structure (space group A3). High temperature transformations have been reported for the lanthanide triiodides from Ndfo to Dyfo. The orthorhombic NdL probably transforms into the rhombohedral structure (Dworkin and Bredig,... 

Another interesting example is a crystalline to amorphous phase transition in Eu(OH)3 (Chen et al., 1994a) at room temperature. The initial structure is the UCI3 -type structure (space group P63 /m ) which was confirmed by X-ray diffraction. Figure 11 shows that broad bands appear under pressure and completely replace the former sharp lines at around 5.5 GPa. The... 

Further X-ray diffraction work revealed a second-order phase transition at 150 K, attributed to an orientational order-disorder transition of the K+-NH3 pair at the octahedral site [36]. The low-temperature orthorhombic structure (space group Fddd) is derived by doubling the lattice constants of the high-tem-perature phase along all three axes, with the K+-NH3 pairs orienting along the [110] direction in an antiferroelectric fashion. 

Ia-3d), 32 and MCM-50 (with a lamellar structure, space group p2), where MCM stands for Mobile Composition of Matter (Fig. 3.3). 33... 

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