Crystal structure space groups and

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). 

Crystal structure space groups, numbers of structures (N) and percentages... 

The crystal structure (space group I4icd) consists of a helical polymer of BH4Be and BH4 units with two hydrogen bridges between B and Be [29[. So far, experimental data are not available but first principle calculations suggest a decomposition temperature of —111 °C at 0.1 MPa [30]. 

The powder pattern of Na3AlHfi shows significant similarities to the pattern of cryolite, Na3AlF6 [142, 143]. Rormebro et al. performed structure refinements and confirmed the isotypic structures of sodium hexaaluminate and cryolite [144]. The crystals structure (space group P2i/n) consists of isolated distorted [AlHg] octa-hedra (Figure 5.10). 

FIGURE 5-12. (b) Calcite crystal structure. Space group R3c, hexagonal unit cell dimensions a = 6 = 4.9898, c = 17.060 A. View down the unique axis, c, and (c) view perpendicular to this, with the unique axis pointing up. 

The cubic fluorite crystal structure (space group Fm3 m) can be described as an fee array of cations in which all the tet interstices are filled with anions and the oet sites are empty. Alternatively, as shown in Figure 2.3, the ionic arrangement can be viewed as a simple cubic array of anions with cations occupying alternate cube centers. 

Crystal structure (space group, unit cell dimensiort, atomic positions, molecular packing, and disorder)... 

Such an exercise can be carried out at varying levels of scrutiny. The traditional approach is to analyse manually several crystal structures and decide whether they are similar or not. The problem in such a complex and detailed analysis is that there are always minor differences between any two structures and the decision as to what is important and what is not is, in the end, quite subjective. Inspection of the crystallographic parameters can obscure the focus and need not always be helpful. Conversely, crystals with different crystal symmetries, space groups and unit cell parameters may be structurally quite similar. For these and related reasons, manual comparison of complete crystal structures is not practical. Some simpHfication is necessary. 

Crystal structure (space group, crystal system, and structure type) (tables and figures). 

RBe take the CaBe-type crystal structure (space group Pm3m), where rare earth atoms and Be octahedra are arranged... 

The room-temperature tetragonal structure was first solved by Atoji and Medrud for LaC2 and takes the CaC2-type crystal structure (space group I4/mmm). The high-temperature cubic phase structure is that of the KCN-type (space group Fm3m). RC2 has been reported to form for the rare earth elements of R = Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu. 

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