Crystalline modification

This is formed when arsenic burns in air (cf. phosphorus which gives P4O10). It can exist in two crystalline modifications the stable one at room temperature, which also occurs naturally as arsenolite, has an octahedral form. Solid arsenic(III) oxide is easily reduced, for example by heating with charcoal, when arsenic deposits as a black shiny solid on the cooler parts of the tube. 

Iodine monochloride [7790-99-0] ICl, mol wt 162.38, 78.16% I, is a black crystalline soHd or a reddish brown Hquid. SoHd ICl exists ia two crystalline modifications the a-form, as stable mby-red needles, d = 3.86 g/mL and mp 27.3°C and as metastable brownish red platelets, d = 3.66 g/mL, mp 13.9°C and bp 100°C (dec). Iodine monochloride is used as a halogenation catalyst and as an analytical reagent (Wij s solution) to determine iodine values of fats and oils (see Fats and fatty oils). ICl is prepared by direct reaction of iodine and Hquid chlorine. Aqueous solutions ate obtained by treating a suspension of iodine ia moderately strong hydrochloric acid with chlorine gas or iodic acid (118,119). 

PMP can also crystallise in several crystalline forms (28). Form I is produced during crystallisation from the melt. This is the only crystalline form present in commercially manufactured articles. Other crystalline modifications ate formed when the polymer is crystallised from solution (28). 

Sodium Pyrophosphates. Known pyrophosphate compounds in the Na20—H2O—P20 system are given in Table 10. Commercially important sodium pyrophosphates include tetrasodium pyrophosphate (TSPP), Na4P20y, and disodium pyrophosphate, Na2H2P20y, commonly referred to as sodium acid pyrophosphate (SAPP). These are prepared industrially by thermal dehydration of disodium and monosodium orthophosphate, respectively. Tetrasodium pyrophosphate exists in five crystalline modifications, only one of which is stable at room temperature. 

Several aHotropes of black phosphoms have also been reported (2). These include one amorphous and three crystalline modifications. At increasing pressures and temperatures reaching above 1200 MPa (12 kbar) and several hundred degrees, a series of black phosphoms modifications are formed that are characterized by even higher densities (2.70 g/cm ). These include orthorhombic, rhombohedral, and cubic varieties. The black forms have lower reactivity and solubiUty than red phosphoms. 

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. 

Results obtained at high temperatures indicate that the solubihties of the crystalline modifications of sihca are in the order tridymite > cristobahte > quartz, an order that parallels to some extent the chemical reactivity of these forms. Lower values for solubihty of crystalline as compared to amorphous sihca are consistent with the free-energy differences between them. 

Crystalline Silica. Sihca exists in a variety of polymorphic crystalline forms (23,41—43), in amorphous modifications, and as a Hquid. The Hterature on crystalline modifications is to some degree controversial. According to the conventional view of the polymorphism of siHca, there are three main forms at atmospheric pressure quart2, stable below about 870°C tridymite, stable from about 870—1470°C and cristobaHte, stable from about 1470°C to the melting point at about 1723°C. In all of these forms, the stmctures are based on SiO tetrahedra linked in such a way that every oxygen atom is shared between two siHcon atoms. The stmctures, however, are quite different in detail. In addition, there are other forms of siHca that are not stable at atmospheric pressure, including that of stishovite, in which the coordination number of siHcon is six rather than four. 

Constants and Chemical Properties. The constants of sulfur are presented in Table 1. Two freezing points ate given for each of the two crystalline modifications. When the Hquid phase consists solely of octatomic sulfur rings, the temperature ranges at which the various modifications form are called the ideal freezing points. The temperatures at which the crystalline forms are in equiHbtium with Hquid sulfur containing equiHbtium amounts of... 

Although 16 different crystalline modifications have been identified (24,25), the a-pentahydrate is the stable form below 48°C. Solutions of sodium thiosulfate in the absence of seed crystals can be easily supercooled below their normal crystallisation temperatures. The dotted line extension of the dihydrate phase in Figure 1 is an indication that, if supercooling takes place below this line, solutions normally giving the pentahydrate may form the dihydrate [36989-90-9] s1ste2id. 

Antimony(Ill) fluoride may be prepared by treating antimony trioxide or trichloride with hydrofluoric acid. Pure SbF is then obtained by carefully evaporating all of the water from the cmde product, which is subsequently sublimed. SbF does not hydrolyze as readily as do the other antimony ttihahdes. When heated in open air at 100°C, a crystalline soHd quickly forms of composition Sb302(0H)2F3, which, upon further heating, is transformed into antimony oxide fluoride [11083-22-0] SbOF. This compound may also be prepared by heating 1 1 mixtures of Sb203 and SbF. There are three known crystalline modifications. 

The name boric acid is usually associated with orthoboric acid, which is the only commercially important form of boric acid and is found ia nature as the mineral sassoflte. Three crystalline modifications of metaboric acid also exist. AH these forms of boric acid can be regarded as hydrates of boric oxide and formulated as B2O3 3H20 for orthoboric acid and B2O3 H20 for metaboric acid. 

When Li metal is cold-worked it transforms from body-centred cubic to cubic close-packed in which each atom is surrounded by 12 others in twinned cuboctahedral coordination below 78 K the stable crystalline modification is hexagonal dose-packed in which each lithium atom has 12 nearest neighbours in the form of a cuboctahedron. This very high coordination... 

Figure 6.4 Crystal structure of ar-tetragonal boron. This was originally thought to be B50 (4Bi2 + 2B) but is now known to be either B50C2 or B50N2 in which the 2C (or 2N) occupy the 2(b) positions the remaining 2B are distributed statistically at other vacant sites in the lattice. Note that this reformulation solves three problems which attended the description of the or-tetragonal phase as a crystalline modification of pure B ...

Partial dehydration of B(OH)3 above 100° yields metaboric acid HBO2 which can exist in several crystalline modifications ... 

The LiCl is removed and the filtrate, if left at this stage, soon deposits an intractable etherate of variable composition. To avoid this, the solution is worked up with an excess of LiAlH4 and some added LiBITi in the presence of a large excess of benzene under reflux at 76-79°C. Crystals of o -A1H3 soon form. Slight variations in the conditions lead to other crystalline modifications of unsolvated AIH3, 6 of which have been identified. 

The binary oxides and hydroxides of Ga, In and T1 have been much less extensively studied. The Ga system is somewhat similar to the Al system and a diagram summarizing the transformations in the systems is in Fig. 7.13. In general the a- and y-series have the same structure as their Al counterparts. )3-Ga203 is the most stable crystalline modification (mp 1740°) it has a unique crystal structure with the oxide ions in distorted ccp and Ga " in distorted tetrahedral and octahedral sites. The structure appears to owe its stability to these distortions and, because of the lower coordination of half the Ga ", the density is 10% less than for the a-(corundum-type) form. This preference of Ga "... 

S7 is known in four crystalline modifications one of these, obtained by crystallization from CS2 at —78 , rapidly disintegrates to a powder at room temperature, but an X-ray study at —110° showed it to consist of cyclo-S molecules with... 

Tammann has, by a large amount of experimental evidence, apparently refuted the hypothesis of a critical point of fusion— against which of course there is no a priori objection. In some cases (e.g., with ice) new crystalline modifications appear when the temperature and pressure are modified, and in all cases the pair of relations L = 0, Ar = 0 (cf. 89) were never simultaneously satisfied. 

It is important to note that, for important sub-cases of case /), which will be discussed in more detail in Sect. 2.4, there is a low extent of disorder entropy effects, if any, are small and changes of the lattice dimensions are absent or small. These particular disordered forms are not considered as mesomorphic. In such cases, the limiting models which are fully ordered or fully disordered may be designated respectively as ordered or disordered crystalline modifications, if their consideration is useful for the structural description of a polymeric material. Note... 

Recently, it has been shown [1071] that CoC204 2 H20 exists in two crystalline modifications, a and 3. Taskinen et al. [1072] prepared anhydrous cobalt oxalate of different particle sizes by dehydration of the (3 (coarser grained) phase and the a/(3 mixture (finer grained). The coarser grained preparation decomposed at 590—700 K with a sigmoid a—time curve fitted by the Avrami—Erofe ev equation [eqn. (6), n = 2] and below and above 625 K, E values were 150 and 57 kJ mole-1, respectively. Reaction of the fine preparation obeyed eqn. (6) (n = 3) and below and above 665 K, values of E were 120 and 59 kJ mole-1, respectively. Catalytic properties of the products of decomposition of cobalt oxalate have been investigated [1073]. 

Several of the claimed crystalline modifications of boron are not truly boron modifications but consist of phase mixtures or B-rich compounds formed under favorable kinetic conditions. The existence of the a- and /3-rhombohedral (a-rh and... 

In 1933, Bernal and Crowfoot [1] reported on the solid state polymorphism of p-azoxyanisole. They found two crystalline modifications of this compound, a stable yellow form and an unstable white polymorph. Krigbaum et al. [31 reexamined the crystal structure of the stable yellow form. The compound shows an imbricated structure which is the basic packing required for nematic behaviour according to Gray [132]. 

Ice I is one of at least nine polymorphic forms of ice. Ices II to VII are crystalline modifications of various types, formed at high pressures ice VIII is a low-temperature modification of ice VII. Many of these polymorphs exist metastably at liquid nitrogen temperature and atmospheric pressure, and hence it has been possible to study their structures without undue difficulty. In addition to these crystalline polymorphs, so-called vitreous ice has been found within the low-temperature field of ice I. It is not a polymorph, however, since it is a glass, i.e. a highly supercooled liquid. It is formed when water vapour condenses on surfaces cooled to below — 160°C. 

It is not appropriate in this chapter to give a detailed review of the solid-state behaviour of water in its various crystalline modifications. However, there are some general structures which are relevant and worth highlighting. Firstly, water molecules in these various solids have dimensions and bond angles which do not differ much from those of an isolated water molecule. Secondly, the number of nearest neighbours to which each individual molecule is hydrogen-bonded remains four, regardless of the ice polymorph. 

Droit s original 4 1 5 phase has been studied by X-ray diffraction (Nowacki Silverman, 1961, 1962) and found to have a rhombohedral layer structure. The 1 1 1 phase was also found to have a layer structure, which consisted of pseudohexagonal layers of zinc atoms separated by ordered layers comprising oxygen and chlorine atoms (Feitknecht, Ostwald Forsberg, 1959). This fundamental structure was apparently found for both of the crystalline modifications in which this phase has been found to occur, namely the monoclinic and the orthorhombic (Sorrell, 1977). 

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