Group lattice structure

The predominantly ionic alkali metal sulfides M2S (Li, Na, K, Rb, Cs) adopt the antifluorite structure (p. 118) in which each S atom is surrounded by a cube of 8 M and each M by a tetrahedron of S. The alkaline earth sulfides MS (Mg, Ca, Sr, Ba) adopt the NaCl-type 6 6 structure (p. 242) as do many other monosulfides of rather less basic metals (M = Pb, Mn, La, Ce, Pr, Nd, Sm, Eu, Tb, Ho, Th, U, Pu). However, many metals in the later transition element groups show substantial trends to increasing covalency leading either to lower coordination numbers or to layer-lattice structures. Thus MS (Be, Zn, Cd, Hg) adopt the 4 4 zinc blende structure (p. 1210) and ZnS, CdS and MnS also crystallize in the 4 4 wurtzite modification (p. 1210). In both of these structures both M and S are tetrahedrally coordinated, whereas PtS, which also has 4 4... 

Throughout this paper we use the Strukturbericht labels T3, DSg,, 44 for Ti, TisSia, and Si phases, respectively. The symmetry of the DSg structure corresponds to the (P6a/mcm) space group. The structure is hexagonal with lattice constants" a = 14.039 a.u. and c=9.712 a.u. The special coordinates (in fractions of the lattice constants) are... 

Three different crystalline forms of anhydrous copper(II) formate have been identified [1033] and these three variations provide a convenient group of reactants for an investigation of the influence of lattice structure on the kinetics and mechanisms of the decomposition. Erofe ev and Kravchuck [1034] showed that kinetic characteristics for the decompositions of two of these forms were appreciably different, an effect attributed to different relative dispositions of the cations in the two reactant structures. 

There is one other factor contributing to the overall symmetries of the lattice structure. This factor involves the local symmetry of the atomic groups which actually form the structure. Examples are the "solid-state building blocks" given above, e.g.- the tetrahedon - the group, P043-, and... 

The result is that Factor III of 2.2.6. given above imposes further symmetry restrictions on the 32 point groups and we obtain a total of 231 space groups. We do not intend to delve further into this aspect of lattice contributions to crystal structure of solids, and the factors which cause them to vary in form. It is sufficient to know that they exist. Having covered the essential parts of lattice structure, we will elucidate how one goes about determining the structure for a given solid. 

A group of crystals show diffraction patterns in which two or more 3D lattices having periods commensurate or incommensurate to each other may be recognized. In other words, the crystal consists of two or more interpenetrating substructures (two or more different atom sets) with different periods at least along one direction (see Fig. 3.42). Names such as composite crystals, vernier structures, misfit-layer structures, and chimney-ladder structures have been used for this group of structures. 

The members of this group possess a layered-lattice structure, and this is reflected in the platy habit of their crystals and in the perfect cleavage (OOOlj parallel to the layers. The minerals are flexible and soft (2-25). Limited isomorphism is foimd between Mg and Mn. The small number of available analyses, however, influences the record of compositional varia tion in the species. 

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