Hexagonal crystalline form

The theoretical endproduct of the dehydration of phosphoric acid is phosphorus (V) oxide, which has three crystalline forms with known structures (65,66,128,180) and two amorphous forms (135). All of them contain only tertiary P04 tctrahedra and represent the nonacidic final product of the series of cross-linked or ultra phosphates. Only the readily volatile hexagonal crystalline form has been studied in any detail chemically. This form consists of P401() molecules with four P04 tetrahedra joined together (47). Formula (53)]. When this form of the oxide is treated with water or... 

While the isolated water molecule is one of the simplest in Nature, the condensed phases of H O reveal many complex features that still elude complete understanding . The proton-disordered hexagonal crystalline form of water, ice 4, for which several aspects of structure-properties relationship remain unclear, is an example of such a phase. 

A boron-containing rmcleating agent is used with a polyarylene sulfide. Boron nitride was foimd to be particularly beneficial, especially in its hexagonal crystalline form. The crystallization properties of PAS are significantly improved. ... 

Tellurium belongs to group 16 (formerly called VIB) of the periodic table, which also includes oxygen, sulfur, and selenium. Its atomic number is 52, atomic weight 127.6, melting point 449.8°C, and boiling point 989.9°C. Elemental tellurium is a silvery white, metallic, lustrous solid that is quite brittle. Tellurium is known to exist in two allotropic forms the hexagonal crystalline form and an amorphous powder. Chemical properties of tellurium resemble that of selenium and sulfur. However, it is more metallic than these elements. Tellurium exhibits variable valence states from 2- to 6+ the most common ones are 2-, 4+, and 6+. Elemental tellurium is insoluble in water,... 

Silicon nitride can be obtained as an amorphous material or in two hexagonal crystalline forms, a and P, the latter being the high-temperature form. An irreversible transformation from a to >0 occurs at 1600°C. The material has excellent overall properties such as ... 

PP is a semicrystalline polymer whose physical, optical, and mechanical properties are determined to a great degree by its crystallinity, crystal form, and crystal structure. Under different crystallization conditions, it can form a, p, y, 5, and quasi-hexagonal crystalline forms, among which a and P are the most common. 

Properties. The physical properties of magnesium hydroxide are Hsted in Table 8. The crystalline form of magnesium hydroxide is uniaxial hexagonal platelets (Fig. 4). Magnesium hydroxide begins to decompose thermally above 350°C, and the last traces of water are driven off at higher temperatures to yield magnesia. 

Polymorphism. Many crystalline polyolefins, particularly polymers of a-olefins with linear alkyl groups, can exist in several polymorphic modifications. The type of polymorph depends on crystallisa tion conditions. Isotactic PB can exist in five crystal forms form I (twinned hexagonal), form II (tetragonal), form III (orthorhombic), form P (untwinned hexagonal), and form IP (37—39). The crystal stmctures and thermal parameters of the first three forms are given in Table 3. Form II is formed when a PB resin crystallises from the melt. Over time, it is spontaneously transformed into the thermodynamically stable form I at room temperature, the transition takes about one week to complete. Forms P, IP, and III of PB are rare they can be formed when the polymer crystallises from solution at low temperature or under pressure (38). Syndiotactic PB exists in two crystalline forms, I and II (35). Form I comes into shape during crystallisation from the melt (very slow process) and form II is produced by stretching form-1 crystalline specimens (35). 

Electron diffraction studies indicate that phosphoms pentoxide vapor consists of P O q molecules. The vapor usually condenses to the hexagonal crystalline modification but under rapid cooling can be condensed to an amorphous soHd (P-form). The Hquid obtained by melting the stable orthorhombic modification cools to form a glass which is the P-form. The Hquid obtained from the H modification also can be supercooled to a glass. 

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. 

The electronic stmcture of cobalt is [Ar] 3i/4A. At room temperature the crystalline stmcture of the a (or s) form, is close-packed hexagonal (cph) and lattice parameters are a = 0.2501 nm and c = 0.4066 nm. Above approximately 417°C, a face-centered cubic (fee) aHotrope, the y (or P) form, having a lattice parameter a = 0.3544 nm, becomes the stable crystalline form. The mechanism of the aHotropic transformation has been well described (5,10—12). Cobalt is magnetic up to 1123°C and at room temperature the magnetic moment is parallel to the ( -direction. Physical properties are Hsted in Table 2. 

When water freezes the crystalline form adopted depends upon the detailed conditions employed. At least nine structurally distinct forms of ice are known and the phase relations between them are summarized in Fig. 14.9. Thus, when liquid or gaseous water crystallizes at atmospheric pressure normal hexagonal ice If, forms, but at very low temperatures (—120° to — 140°) the vapour condenses to the cubic form, ice Ic. The relation between these structures is the same as that between the tridymite and cristobalite forms of SiOa (p. 342), though in both forms of ice the protons are disordered. 

Tellurium has only one crystalline form and this is composed of a network of spiral chains similar to those in hexagonal Se (Fig. 16.1c and d). Although the intra-chain Te-Te distance of 284 pm and the c dimension of the crystal (593 pm) are both substantially greater than for Scjt (as expected), nevertheless the closest interatomic distance between chains is almost identical for the 2 elements (Te Te 350 pm). Accordingly the elements form a continuous range of solid solutions in which there is a random... 

For instance, in the three crystalline forms (a, 3, y) of i-PP the chains are always in the conformation of threefold helix (s(3/l)l symmetry) but are packed in different ways in monoclinic [24], hexagonal [25], and orthorhombic [26] unit cells, respectively. The X-ray diffraction spectra of unoriented samples in the crystalline forms a, P, y are reported in Fig. 6. 

Another interesting example is the melt crystallization of s-PS. For the case of rapid cooling from the melt, the hexagonal a form is obtained [7-9], while for low cooling rates or for isothermal crystallizations, the crystalline form which is... 

Boron nitride has two crystalline forms, hexagonal (h-BN) and cubic (c-BN), with much different properties. Hexagonal BN is the more important and has many industrial applications. Its structure is similar to that of graphite which it resembles in many ways. It has a very large anisotropy in the crystal with resulting anisotropic properties. 

We examine briefly some specific instances starting with syndiotactic polypropylene (sPP). Aside from the already discussed hexagonal mesophase which can be obtained both drawing fibers and under quiescent conditions, this polymer presents four crystalline forms phases I [73] and II [74-76] where chains adopt the (T2G2)n helical conformation, forms III [30] and... 

The commonest crystalline forms of carbon, cubic diamond and hexagonal graphite, are classical examples of allotropy that arc found in every chemistry textbook. Both diamond and graphite also exist in two minor crystallographic forms hexagonal diamond and rhombohedral graphite. To these must be added carbynes and Fullerenes, both of which are crystalline carbon forms. FulleTenes are sometimes referred to as the third allotrope of carbon. However, since Fullerenes were discovered more recently than carbynes, they are... 

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