Oxides hexagonal

There exists quite a number of hexagonal oxidic perovskites 183, 332), but there seem to be only three types in the case of ternary fluorides. Their occurrence again clearly depends on the tolerance factor wich thus proves to be useful in classifying the hexagonal perovskites also. After having described their structures in detail they will be further discussed under a common point of view. 

The structure of the hexagonal oxide perovskite BaRuOa, recently described by Donohue et al. 84), is also adapted by the ternary fluoride CsCoFs 11). The positional parameters (not listed above) are almost the same in both compounds. 

As intermediate products of thermal decomposition of hydrated R carbonates or oxalates, oxide carbonates R2O2CO3 are obtained for practically all Rs (Petru et al. 1966, Sawyer et al. 1972). Several metastable phases with defined crystal structures are formed as a function of temperature, before hexagonal (type-II) R2O2CO3 appears, with crystal structure (Christensen 1970, Attfield and Ferey 1989) [La202C03 P6 /mmc 407.55(6), 1595.7(1)] related to that of the hexagonal oxides of the early lanthanides. For R=La, Pr, Nd, Sm, and Gd, these oxide carbonates are relatively stable and can be isolated as single... 

The method has severe limitations for systems where gradients on near-atomic scale are important (as in the protein folding process or in bilayer membranes that contain only two molecules in a separated phase), but is extremely powerful for (co)polymer mixtures and solutions [147, 148, 149]. As an example Fig. 6 gives a snapshot in the process of self-organisation of a polypropylene oxide-ethylene oxide copolymer PL64 in aqueous solution on its way from a completely homogeneous initial distribution to a hexagonal structure. 

The metal has a bright silvery metallic luster. Neodymium is one of the more reactive rare-earth metals and quickly tarnishes in air, forming an oxide that spalls off and exposes metal to oxidation. The metal, therefore, should be kept under light mineral oil or sealed in a plastic material. Neodymium exists in two allotropic forms, with a transformation from a double hexagonal to a body-centered cubic structure taking place at 863oC. 

The many commercially attractive properties of acetal resins are due in large part to the inherent high crystallinity of the base polymers. Values reported for percentage crystallinity (x ray, density) range from 60 to 77%. The lower values are typical of copolymer. Poly oxymethylene most commonly crystallizes in a hexagonal unit cell (9) with the polymer chains in a 9/5 helix (10,11). An orthorhombic unit cell has also been reported (9). The oxyethylene units in copolymers of trioxane and ethylene oxide can be incorporated in the crystal lattice (12). The nominal value of the melting point of homopolymer is 175°C, that of the copolymer is 165°C. Other thermal properties, which depend substantially on the crystallization or melting of the polymer, are Hsted in Table 1. See also reference 13. 

Iron(II) hydroxide [18624-44-7], Fe(OH)2, is prepared by precipitation of an iron(II) salt solution by strong base in the absence of air. It occurs as pale green, hexagonal crystals or a white amorphous powder. It is practically insoluble in water, fairly soluble in ammonium salt solutions, and soluble in acids and in concentrated NaOH solution. It is slowly oxidized by air. Conversion to Fe203 atH20 is eventually complete. 

Perchlorates. Iron(II) perchlorate hexahydrate [13922-23-8], Fe(C10 2 6H20, is prepared by dissolving iron in cold, dilute perchloric acid or by dissolving FeS in perchloric acid. It crystallizes in hygroscopic, light green hexagonal prisms which are stable in dry air and extremely soluble (0.978 g/mL H2O at 0°C) in water and alcohol. It is susceptible to air oxidation in aqueous solution and decomposes above 100°C. Yellow iron(III) perchlorate... 

Benzil. Ben il [134-81-6] (diphenylethanedione) is a yellow soHd that crystallines from alcohol in hexagonal prisms. Ben nil can be prepared by the oxidation of bennoin [579-44-2] (2-hydroxy-2-phenylacetophenone) (298,299), which is itself prepared by the self-condensation of bennaldehyde (300). Ben nil is commercially produced in Japan and is used as a uv resin curing sensitizer (301). It has also been suggested as a chigger repeUant (302). 

Silver nitrate forms colorless, rhombic crystals. It is dimorphic and changes to the hexagonal rhombohedral form at 159.8°C. It melts at 212°C to a yellowish Hquid which solidifies to a white, crystalline mass on cooling. An alchemical name, lunar caustic, is stiU appHed to this fused salt. In the presence of a trace of nitric acid, silver nitrate is stable to 350°C. It decomposes at 440°C to metallic silver, nitrogen, and nitrogen oxides. Solutions of silver nitrate are usually acidic, having a pH of 3.6—4.6. Silver nitrate is soluble in ethanol and acetone. 

Strontium carbonate is a colorless or white crystalline soHd having a rhombic stmcture below 926°C and a hexagonal stmcture above this temperature. It has a specific gravity of 3.70, a melting point of 1497°C at 6 MPa (60 atm), and it decomposes to the oxide on heating at 1340°C. It is insoluble in water but reacts with acids, and is soluble in solutions of ammonium salts. 

Properties. Thallium is grayish white, heavy, and soft. When freshly cut, it has a metallic luster that quickly dulls to a bluish gray tinge like that of lead. A heavy oxide cmst forms on the metal surface when in contact with air for several days. The metal has a close-packed hexagonal lattice below 230°C, at which point it is transformed to a body-centered cubic lattice. At high pressures, thallium transforms to a face-centered cubic form. The triple point between the three phases is at 110°C and 3000 MPa (30 kbar). The physical properties of thallium are summarized in Table 1. 

Titanium Triiodlde. Titanium triiodide is a violet crystalline soHd having a hexagonal unit cell (146). The crystals oxidize rapidly in air but are stable under vacuum up to 300°C above that temperature, disproportionation to the diiodide and tetraiodide begins (147). 

The diboride has a hexagonal stmcture and melts at 3245°C (136) it is considered to have the best oxidation resistance of all the refractory hard metals. The dodecaboride has a cubic stmcture. 

Hexagonal boron nitride is relatively stable in oxygen or chlorine up to 700°C, probably because of a protective surface layer of boric oxide. It is attacked by steam at 900°C, and rapidly by hot alkaU or fused alkaU carbonates. It is attacked slowly by many acids as well as alcohols (to form borate esters), acetone, and carbon tetrachloride. It is not wetted by most molten metals or many molten glasses. 

Preparation. Hexagonal boron nitride can be prepared by heating boric oxide with ammonia, or by heating boric oxide, boric acid, or its salts with ammonium chloride, alkaU cyanides, or calcium cyanamide at atmospheric pressure. Elemental nitrogen does not react with boric oxide even in the presence of carbon, though it does react with elemental boron at high temperatures. Boron nitride obtained from the reaction of boron trichloride or boron trifluoride with ammonia is easily purified. 

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