Hexagonal layered structure

Figure 6.27 Comparison of the hexagonal layer structures of BN and graphite. In BN the atoms of one layer are located directly above the atoms of adjacent layers with B - N contacts in graphite the C atoms in one layer are located above interstices in the adjacent layer and are directly above atoms in alternate layers only.

X-Ray diffraction showed that the molybdenum disulfide powder used in this experiment has a hexagonal layer structure. A remarkable feature of such layer compounds is that the powder is composed of small single crystal particles. In view of these facts, an interesting question arises as to whether... 

In general, the cadmium halides show in their crystal structure the relation between polarizing effect and si/e of anion. The tluoride has tile smallest and least polarizable anion of Ihe lour and forms a cubic structure, while the mure polarizable heavy halides have hexagonal layer structures, increasingly covalent and al increasing distances apart in inxler down tire periodic table, in solution the halides exhibit anomalous thermal and transport properties, due primarily to the presence of complex ions, such as CDlr and CdBr r. especially in concentrated solutions or those containing excess halide ions. 

Hexagonal layer structure of the 1 1 complex of melamine and cyanuric acid. Three kinds of assembly in lower dimensions are possible (a) linear tape, (b) crinkled tape, and (c) rosette. 

There seems to be even less structural similarity for many other metal halides as the crystalline systems are compared with the molecules in the vapor phase. Aluminum trichloride, e.g., crystallizes in a hexagonal layer structure. Upon melting, and then, upon evaporation at relatively low temperatures, dimeric molecules are formed. At higher temperatures they dissociate into monomers (Figure 9-58) [107], The coordination number decreases from 6 to 4 and then to 3 in this process. However, at closer scrutiny, even the dimeric aluminum trichloride molecules can be derived from the crystal structure. Figure 9-59 shows another representation of crystalline aluminum trichloride which facilitates the identification of the dimeric units. A further example is chromium dichloride illustrated in Figure 9-60. The small oligomers in its vapor have structures [108] that are closely related to the solid structure [109], Correlation between the molecular composition of the vapor and their source crystal has been established for some metal halides [110],... 

Ga2S green prisms GaS hexagonal layered structure, Ga2Se ... 

The electron microscopic analysis revealed that the hexaaluminate powder prepared by hydrolysis of the alkoxides crystallized in plate-like microcrystals.The size of the powder is 30 nm in thickness and 200 nm in diameter. The plate-like crystal was also observed for the sample derived from powder mixture however, the sample contains granular-shaped crystals and amorphous regions. The plate-like shape is characteristic of the hexaaluminate microcrystals due to their hexagonal layered structure. The granular crystals and amorphous regions observed in the calcined powder mixture are the unreacted particle or the intermediate phases. The composition of each microcrystal is not uniform in the sample from the powder mixture. This means that the bulk diffusion significantly limits the phase formation of hexaaluminate in the case of powder mixing. 

Initially, BN was synthesized by the pyrolysis of boric acid and urea in the presence of ammonia [27]. In this reaction, boric acid presumably reacts with urea to form a urea-boric acid complex gel, which on pyrolysis in an ammonia atmosphere results in the formation of BN [28]. Precursors for BN recently received the greatest attention, with emphasis on borazine-derived polymers. It has been recognized that borazine (1) can form polymeric chains of B and N, which can form BN ceramics on thermal pyrolysis [29]. The polymerization gives a graphitelike hexagonal layered structure (2) ... 

There seems to be even less structural similarity for many other metal halides when the crystalline systems are compared with the molecules in the vapor phase. Aluminum trichloride, for example, crystallizes in a hexagonal layer structure. Upon melting and then evaporation at relatively low temperatures, dimeric molecules are formed. At higher temperatures, they dissociate into monomers (Figure 9-60) [9-60]. The coordination number decreases from six to four and then to three in this process. 

Densities above 95% have been achieved by axial hot-pressing at pressures exceeding 20 MPa and temperatures above 1800°C. The microstructures consist typically of particles of > 20 qm in size. Another problem is related to the hexagonal layered structure of the AlB2-type borides. Because of the strong anisotropic behavior of the physical properties, especially of the coefficients of thermal expansion, the... 

F L Vogel and J N Zemel of the University of Pennsylvania have observed that hexagonal layered structures form when cellulosic materials are charred and that the relatively low temperature of our process suggests diffusion by an interstitial mechanism (4) ... 

It would seem that the order of hexagonal layered structure would show in X-ray diffraction and other studies that showed no long-range order for our materials. The lack of long-range order imparts glasslike properties and poor electrical conductivity except when metallic aluminum is partially exposed in the surface ... 

Figure 10.1 Possible atomic arrangements of crystalline carbon (a) diamond tetrahydral, cyclohexane chair, crystalline structure, (b) graphite planar hexagonal layered structure (c) three-dimensional quasicrystalline turbostatic structure. Biomedical carbons have a turbostatic structure. 

Co (II) [A] (3d) Simple compounds With hydroxide and nitrite / -Co(OH)2 300--450 5.2 + 20 a.c. hexagonal layer structure of Cdlj type determined from Curie-Weiss law antiferromagnetic, n=12.3 K magnetization measured at 4.2 K 66T1, 66M15, Fig. 250, 251... 

GeSb (metastable) tetrag. NaCl type [377] SnP(r) hexagonal layer structure ( = SnP3 )... 

Figure 2. (A) Ball-stick structure model of hexagonal layered structure LiM02 (M = Mn, Co, or Ni) and (B) unit cell of LiM02 (M = Mn, Co, or Ni).

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