Modifications crystal structures

White powder. M.p. 2680°C, b.p. 4300°C d 5.73. Exists in several modifications.Crystal structure tetragonalandmonoclinic. 

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 ...

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 "... 

In all cases, broad diffuse reflections are observed in the high interface distance range of X-ray powder diffraction patterns. The presence of such diffuse reflection is related to a high-order distortion in the crystal structure. The intensity of the diffuse reflections drops, the closer the valencies of the cations contained in the compound are. Such compounds characterizing by similar type of crystal structure also have approximately the same type of IR absorption spectra [261]. Compounds with rock-salt-type structures with disordered ion distributions display a practically continuous absorption in the range of 900-400 cm 1 (see Fig. 44, curves 1 - 4). However, the transition into a tetragonal phase or cubic modification, characterized by the entry of the ions into certain positions in the compound, generates discrete bands in the IR absorption spectra (see Fig. 44, curves 5 - 8). 

In general, increasing the temperature within the stability range of a single crystal structure modification leads to a smooth change in all three parameters of vibration spectra frequency, half-width and intensity. The dependency of the frequency (wave number) on the temperature is usually related to variations in bond lengths and force constants [370] the half-width of the band represents parameters of the particles Brownian motion [371] and the intensity of the bands is related to characteristics of the chemical bonds [372]. 

The presence of the foreign cation stabilizes the crystal structure of a - Mn02 compounds. This manganese dioxide modification (more exactly it is not a real MnOz modification, since the structure contains a considerable proportion of foreign atoms) can be heated to relatively high temperatures (300 - 400 °C) without destruction of the lattice. Although Thackeray et al. reported the synthesis of cation-and water- free a - MnOz [49, 50J, which is reported to be stable up to 300 °C without destruction of the [2 x 2] tunnel structure, it is commonly believed that a small,... 

Although the phase which appears to be very stable for plutonium has not been observed in other An02 S03 H20 systems, phases of identical composition have been observed for Zr, Hf and Ce. The crystal structure of the zirconium compound Zr2(0H)2-(SOO 3 (H20) i,, is well known 05). One very interesting feature of the M02 S03 H20 systems for Zr, Hf and Co is that there are a large number of phases which have been observed. Some of these correspond to phases which are known for Th, U and Np. For zirconium, a series of basic sulfates is known to include Zr2(0H)2-(SOi,) 3 (H20)i, and two modifications of Zr(0H)2S0i, as the major constituents (5). Other basic sulfates such as Zr(OH)2S0if,H20,... 

Fig. 7a,b. Crystal structures of the two modifications A and B of PCH9 a crystal packing in PCH9A along [010] b crystal packing in PCH9B along [100]. (Reprinted from [70])... 

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]. 

However, in the case of multimetallic catalysts, the problem of the stability of the surface layer is cmcial. Preferential dissolution of one metal is possible, leading to a modification of the nature and therefore the properties of the electrocatalyst. Changes in the size and crystal structure of nanoparticles are also possible, and should be checked. All these problems of ageing are crucial for applications in fuel cells. 

The pH dependence of the rate of modification shows that the pKa of Asp-32 is less than 3.165 It is seen in the high-resolution crystal structures that the carboxyl groups of the two aspartate residues are hydrogen-bonded to each other. This is similar to the ionization of maleic acid, which has pKa values of 1.9 and 6.2 (equation 16.30). 

No element shows as many different structures as sulfur. Crystal structures are known for the following forms S6, S7 (four modifications), Sg (three modifications), S10, S6-S10, Sn, S12, S13, S14, S15, S18 (two forms), S20, (Fig. 11.2). Many of them can be separated by chromatography from solutions that were obtained by extraction of quenched... 

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). 

Four-coordinate complexes with monodentate ligands have also been reported. The X-ray crystal structures of three modifications of the compound [Au(PPh3)4]BPh4 have been determined, none of which shows the expected simple tetrahedral geometry.2540 Thus, the chloroform solvate... 

A number of chemical elements, mainly oxygen and carbon but also others, such as tin, phosphorus, and sulfur, occur naturally in more than one form. The various forms differ from one another in their physical properties and also, less frequently, in some of their chemical properties. The characteristic of some elements to exist in two or more modifications is known as allotropy, and the different modifications of each element are known as its allotropes. The phenomenon of allotropy is generally attributed to dissimilarities in the way the component atoms bond to each other in each allotrope either variation in the number of atoms bonded to form a molecule, as in the allotropes oxygen and ozone, or to differences in the crystal structure of solids such as graphite and diamond, the allotropes of carbon. 

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