Rhombohedral crystal lattice structure

The first complete description of crystalline Se was reported by Donohue et al. in 1961 on the basis of an X-ray diffraction study [64]. The rhombohedral structure was verified, and the molecular symmetry was ascertained to be D d. Since cyc/o-hexasulfur decomposes rapidly under X-ray irradiation at standard temperature-pressure (STP) conditions Steidel et al. reinvestigated the molecular and crystal structure at 183 K giving results with a higher accuracy [65]. The molecular and crystal lattice parameters are summarized in Tables 1 and 2. 

Boron carbide crystallizes in the trigonal-rhombohedral space group R3m the unit cell is shown in Figure 4.5. The structure is usually described as an arrangement of distorted icosahedra located at the nodes of a rhombohedral Bravais lattice. Parallel to the space diagonal, which is the c-axis in hexagonal notation, a linear chain of three... 

The structures of the two rhombohedral forms of elemental boron (Table 5) are of interest in illustrating what can happen when icosahedra are packed into an infinite three-dimensional lattice. In these rhombohedral structures the local symmetry of a Bj2 icosahedron is reduced from //, to Dsd because of the loss of the 5-fold rotation axis when packing icosahedra into a crystal lattice. The 12 vertices of an icosahedron, which are all equivalent under //, local symmetry, are split under iXd local symmetry into two nonequivalent sets of six vertices each (Figure 19a). The six rhombohedral vertices (labeled R in Figure 19a) define the directions of the rhombohedral axes. The six equatorial vertices (labeled E in Figure 19a) lie in a staggered belt around the equator of the... 

The crystal structures and lattice parameters of rare-earth bromides are rather well known (Brown 1968). The tribromides of lanthanum to praseodymium inclusive possess the hexagonal UClj-type structure (CN = 9). The tribromides of neodymium to europium inclusive possess the orthorhombic PuBrs-type structure (CN = 8), while those of gadolinium to lutetium inclusive as well as those of scandium and yttrium possess the rhombohedral FeClj-type structure (CN=6). 

Fig. 5 shows data from a simulation of TIP4P water that is confined on both sides by a rhombohedral mercury crystal with (111) surface structure. Bosio et al. [135] deduce from their X-ray studies that a solid o-mercury lattice with a larger lattice constant in the z direction may be used as a good structural model for liquid mercury. Thus, the mercury phase was modeled as a rigid crystal in order to simplify the simulations. The surface of such a crystal shows rather low corrugation. 

Rhombic prism lattice, 8 114t Rhombohedral structure, of ferroelectric crystals, 11 95, 96 Rhombohedron lattice, 8 114t Rhomboidal symmetry, 8 114t Rhone-Poulenc process, 24 482, 485 Rhovanil extra pure vanillin, 25 548t, 549-550... 

The calcium ion is of such a size that it may enter 6-fold coordination to produce the rhombohedral carbonate, calcite, or it may enter 9-fold coordination to form the orthorhombic carbonate, aragonite. Cations larger than Ca2+, e.g., Sr2+, Ba2+, Pb2+, and Ra2 only form orthorhombic carbonates (at earth surface conditions) which are not, of course, isomorphous with calcite. Therefore these cations are incapable of isomorphous substitution in calcite, but may participate in isodimorphous or "forced isomorphous" substitution (21). Isodimorphous substitution occurs when an ion "adapts" to a crystal structure different from its own by occupying the lattice site of the appropriate major ion in that structure. For example, Sr2+ may substitute for Ca2 in the rhombohedral lattice of calcite even though SrC03, strontianite, forms an orthorhombic lattice. Note that the coordination of Sr2 to the carbonate groups in each of these structures is quite different. Very limited miscibility normally characterizes such substitution. 

Only fourteen space lattices, called Bravais lattices, are possible for the seven crystal systems (Fig. 328). Designations are P (primitive), / (body-centered), F (face-centered),34 C pace-centered in one set of laces), and R (rhombohedral) Thus our monoclinic structure P2Jc belongs to the monoclinic crystal system and has a primitive Bravais lattice. 

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