Crystal structure rhombohedral

Graphite exists in two forms alpha and beta. These have identical physical properties, except for their crystal structure. Naturally occurring graphites are reported to contain as much as 30% of the rhombohedral (beta) form, whereas synthetic materials contain only the alpha form. The hexagonal alpha type can be converted to the beta by mechanical treatment, and the beta form reverts to the alpha on heating it above lOOOoC. 

Crystal Structure. Sihcon carbide may crystalline in the cubic, hexagonal, or rhombohedral stmcture. There is a broad temperature range where these stmctures may form. The hexagonal and rhombohedral stmcture designated as the a-form (noncubic) may crystalline in a large number of polytypes. 

The materials for solid solutions of transition elements in -rh boron are prepared by arc melting the component elements or by solid-state diffusion of the metal into /3-rhombohedral (/3-rh) boron. Compositions as determined by crystal structure and electron microprobe analyses together with the unit cell dimensions are given in Table 1. The volume of the unit cell (VT ) increases when the solid solution is formed. As illustrated in Fig. 1, V increases nearly linearly with metal content for the solid solution of Cu in /3-rh boron. In addition to the elements listed in Table 1, the expansion of the unit cell exceeds 7.0 X 10 pm for saturated solid solutions " of Ti, V, Co, Ni, As, Se and Hf in /3-rh boron, whereas the increase is smaller for the remaining elements. The solubility of these elements does not exceed a few tenths at %. The microhardness of the solid solution increases with V, . Boron is a brittle material, indicating the accommodation of transition-element atoms in the /3-rh boron structure is associated with an increase in the cohesion energy of the solid. 

Boron is as unusual in its structures as it is in its chemical behavior. Sixteen boron modifications have been described, but most of them have not been well characterized. Many samples assumed to have consisted only of boron were possibly boron-rich borides (many of which are known, e.g. YB66). An established structure is that of rhombohedral a-B12 (the subscript number designates the number of atoms per unit cell). The crystal structures of three further forms are known, tetragonal -B50, rhombohedral J3-B105 and rhombohedral j3-B 320, but probably boron-rich borides were studied. a-B50 should be formulated B48X2. It consists of B12 icosahedra that are linked by tetrahedrally coordinated X atoms. These atoms are presumably C or N atoms (B, C and N can hardly be distinguished by X-ray diffraction). 

A crystal structure analysis proved SeF4-NbF5 to have the same unit-cell dimensions as SeF4-TaFs. The atomic arrangement in the rhombohedral crystals is shown in Fig. 2 and is consistent with the ionic formulation (SeF3)+(NbF6) , with, however, substantial fluorine... 

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. 

The nearly cubic, rhombohedral unit cell of Mo6PbS8 is shown in Fig. 4.30. The bonding between different clusters in the crystal structure of the Chevrel phase Mo6PbS8 is indicated in Fig. 4.31. 

Figure 4.31. Crystal structure of the Chevrel phase Mo6PbS8. Portions of four rhombohedral unit cells with one common Pb atom (black) are shown. The bonding between different clusters is suggested by the Mo—S inter-cluster links.

The commonest habits for hematite crystals are rhombohedral, platy and rounded (Fig. 4.19). The plates vary in thickness and can be round, hexagonal or of irregular shape. Under hydrothermal conditions, these three morphologies predominate successively as the temperature decreases (Rosier, 1983). The principal forms are given in Table 4.1. Hematite twins on the 001 and the 102 planes. The crystal structure of hematite has a less directional effect on crystal habit than does that of goethite and for this reason, the habit of hematite is readily modified. A variety of morphologies has been synthesized, but in most cases, the crystal faces that enclose the crystals have not been identified. 

Pinkish-red translucent crystals hexagonal-rhombohedral structure refractive index 1.597 density 3.70 g/cm hardness 3.8 Mohs decomposes above 200°C slightly soluble in water Ksp2.24xl0-ip soluble in dilute acids. 

Hard yellow metal exhibits two crystals forms an alpha form having a rhombohedral crystal structure at ordinary temperatures the alpha form converts to a body-centered cubic beta form at 917°C density 7.52 g/cm (alpha form) and 7.40 g/cm (beta form) melts at 1,074°C vaporizes at 1,791°C ignites in air at 150°C electrical resistivity 94 microhm-cm at 25°C thermal neutron absorption cross section 5,600 barns insoluble in water soluble in acid. 

Ternary fluorides Cs2MeF4 on the other hand, containing the large cesium ions, are known of several transition element ions Me +. They crystallize in rhombohedral structures with large sizes of the unit cells z = 14) 8). Detailed information about the coordination of Me and Cs in these compounds is not yet available. 

The a-rhombohedral form of boron has the simplest crystal structure with slightly deformed cubic close packing. At 1200°C a-rhombohedral boron degrades, and at 1500°C converts to p-rhombohedral boron, which is the most thermodynamically stable form. The unit cell has 104 boron atoms, a central B12 icosahedron, and 12 pentagonal pyramids of boron atom directed outward. Twenty additional boron atoms complete a complex coordination (2). 

Fig. 4.5 Crystal structures ofjwo forms of ciilcium carbonate (a) unit cell of calcitc. rhombohedral. space group R3c (b) unit cel of aragonite, orthorhombic, space group Pcmn Circles in decreasing order of size are oxygen, calcium, and carbon. [From Ladd. M. F. C Structure and Bonduiy in Solid State Chemistry Wiley New York, 1979. Reproduced with permission.]...

Multiple Melting Points A compound may have different crystal structures (i.e., solid phases). For example, carbon tetrachloride has three known solid phases at atmospheric pressure la (face-centered cubic), lb (rhombohedral), and II (monoclinic). Ia and lb melt at temperatures some 5K apart [3]. Multiple melting points have been reported for a large set of compounds, such as many of those listed in the Merck Index [4], Dearden and Rahman improved a structure-melting point correlation for substituted anilines by excluding two outliers on the ground that their Tm values were inadequate, due to different crystalline forms [5]. 

SAMARIUM. [CAS 7440-19-9]. Chemical element symbol Sm, at. no. 62, at. wt. 150.35, fifth in the Lanthanide Series in the periodic table, mp 1,073°C, bp l,79l°C, density 7.520 g/cm3 (20 C). Elemental samarium has a rhombohedral crystal structure at 25DC. The pure metallic samarium is silver-gray in color, retaining a luster in dry air, but only moderately stable in moist air, with formation of an adherent oxide. When pure, the metal is soft and malleable, but must be worked and fabricated under an inert gas atmosphere. Finely divided samarium as well as chips from working are... 

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