Boronates structure

The materials for solid solutions of transition elements in j3-rh boron are prepared by arc melting the component elements or by solid-state diffusion of the metal into /3-rhombohedral (/3-rh) boron. Compositions as determined by erystal structure and electron microprobe analyses together with the unit cell dimensions are given in Table 1. The volume of the unit cell (V ) increases when the solid solution is formed. As illustrated in Fig. 1, V increases nearly linearly with metal content for the solid solution of Cu in /3-rh boron. In addition to the elements listed in Table 1, the expansion of the unit cell exceeds 7.0 X 10 pm for saturated solid solutions " of Ti, V, (2o, Ni, As, Se and Hf in /3-rh boron, whereas the increase is smaller for the remaining elements. The solubility of these elements does not exceed a few tenths at %. The microhardness of the solid solution increases with V . Boron is a brittle material, indicating the accommodation of transition-element atoms in the -rh boron structure is associated with an increase in the cohesion energy of the solid. 

Nuclear y-ray resonance spectra of solid solutions of Fe and Co in /3-rh boron give inconclusive results, although the large isomer shifts as compared to Fe metal indicate that the accommodation of Fe atoms in the boron structure is associated with changes in the electronic state. The magnitudes of the shifts are... 

Other single-crystal x-ray diffraction studies of transition element dopants in jS-rh boron are based on the results of a refinement of the /3-rh boron structure that establishes the occurrence of four new low-occupancy (3.7, 6.6, 6.8 and 8.5%) B positions in addition to the earlier known ones. The dopant elements studied, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Hf and Ta, do not enter B positions in the framework, but they enter the Al, A2, D and E positions. In some cases the doping elements have been studied at several concentrations for each element and for different cooling rates. The percentage occupancies of certain positions are eorrelated with the atomie sizes of the dopants. The bond distances between the polyhedra are shorter than those within the polyhedra. The mechanism of doping for some cases is denoted displacive, rather than interstitial or substitutional, because of competing interactions between the six different partially occupied B positions and dopant atoms. 

Compound LII, on the other hand, can be made readily. It can have either the planar tricovalent boron structure or the "triptych tetra-covalent structure. In the latter structure the nitrogen is attached to boron and should be considerably less basic and nucleophilic than usual. It does in fact react unusually slowly with methyl iodide and with acids. The neutralization reaction with acids in water is not only slow but of zero order with respect to the acid. It is believed to have a rate-determining transformation from the triptych to the more basic form as the first step. 

As a final example we consider the icosahedral B12H122- dianion, whose relevance to higher extended boron structures may be motivated in the following terms. 

Tetrameric hydrazinoboranes exhibit a polyhedral cage structure involving tetra-coordinated boron (structure 18) which was confirmed by X-ray studies 203>. 

The anion [BgHj -has the boron structural framework shown below for clarity, none of the protons are shown. Metallation of this anion gives nido-[(cpJRhBgHB]- The proton-decoupled nB COSY spectrum of this metallated product is shown here. What is the structure of the product, and what information is missing ... 

The boron so formed resembled other amorphous borons in appearance and was found to be similar in structure to jS-rhombohedral boron. Individual particles were random in shape, consisting mainly of platelets between 200 and 7500 A in diameter. The presence of regular dodecagonal platelets of boron was also observed, in the product, which had typical diameters of 7000 A and a unit cell lattice constant of 30 A. This unit cell dimension is considerably larger than any previously reported form of boron and was considered to be a further modification of known boron structures. 

B13P2 The crystal structure of B13P2 has determined to be the a-rhombohedral boron structure with two phosphorus atoms (partly replaced hy boron) in a unit cell from x-ray diffraction. Maeda et al. [24] measured P 3p DOS of B13P2 hy 7 Kp K-M2,z) x-ray emission spectroscopy. They also calculated the P Zp DOS using three... 

Nearly all syntheses of zeolites and microporous aluminophosphates have limitations to gel composition and other parameters. For example, some zeolites with special compositions such as high-silica Y zeolite and low-silica ZSM-5 cannot be directly synthesized. A secondary framework modification is necessary for their preparation. For instance, dealuminization, isomorphous substitution of extraneous silicon for aluminum, and removal of the sodium process in Y zeolite are necessary to prepare ultra-stable zeolite Y (USY) isomorphous replacement of framework atoms of boron with aluminum in a presynthesized silicon-boron structure is often used to prepare some specific aluminosilicate zeolites that cannot be directly synthesized, such as Al-SSZ-24 (AFI) and Al-CIT-1. Secondary synthesis (post-treatment) will be discussed in detail in Chapter 6. 

In all boron three-dimensional (3D) structures, most of the atoms are members of the almost regular atomic triangles and form 6 bonds (five intra- and one inter-icosahedral bonds). This circumstance leads to the possibility to synthesize bare boron structures with triangular two-dimensional (2D) lattices, e.g., nanotubular boron in the form of cylindrically rolled fiat or buckling surfaces (Boustani et al. 1999). 

The results of calculations are summarized in Table 3.1. Let us note main features that are revealed. First, the concentration of B atoms in all 3D boron modifications (four crystalline and one amorphous) is almost the same (1.23-1.37) x 10 cm-. Second, the concentration of boron atoms in certain boron compounds can be comparable (e.g., BjOj) to that in structures of el ental boron or even exceed it (e.g., layered BN). This result once again underlines the effect of large voids in 3D boron structures. Third, the concentration of atoms in bundles of smaU-diameter boron nanotubes is expected to be higher than in solid-state structures, near the limit achievable in hypothetical close-packed crystal. These estimates imply that physical-technical parameters of boron-containing materials designed for neutron shielding should be evaluated in the range n = (1-25) x cm-. ... 

Kobayashi, M. et al. (1995) Rietveld analysis of LiB13 with p-rhombohedral boron structure. J. Alloys Compd., 221 (1-2), 120-124. 

We have been studying non-siloxsine silicon polymers to obtain thermally stable and mechanically strong material. Ethynylene-silylene polymers have been found with excellent properties [8]. In addition, we have tried to introduce boron structures into silicon polymers to get further durability. 

A flammability test under a 50 % oxygen atmosphere was also carried out. The vinyl polymer 1, ferrocene 6b and benzene polymers 7 are burned out when attacked with flame. The carborane hybrids 3a, 3b are both ignited but extinguished soon in the case of the higher (y + z) contents. The introduction of carborane brings non-flammability to the hybrid polymers along with thermal and mechanieal strength. Transformation of the boron structure to boron oxide was observed around... 

S. Johansson, J. A. Schweitz, H. Westberg and M Boman, Microfabrication of three-dimensional boron structures by laser chemical processing, J. Appl. Phys. 72 [12], 5956-5973 (1993). 

The basic boron structural element is the icosahedron, i.e., a polyhedron having twenty faces, twelve equivalent vertices and I2J, symmetry, forming a cage of twelve atoms shown in Fig. 7.7.P1 To form a boron crystal, diese icosahedra combine in a ihombohedral configuration, i.e., a geom cal pattern with axes of equal length and equal axial angles (but not 90 ) (Fig. 7.8). 

In this and in other experiments using NaBH4 with aldehydes and ketones, there are two steps (1) acyl addition of the hydride and (2) aqueous acid workup. This result is entirely consistent with the acyl addition reactions discussed in Chapter 18. There has never been evidence for an intermediate, so the alkoxide product is formed directly. With knowledge that no intermediate is formed when 9 is transformed to 11, this experiment must be explained by examining a transition state. The requisite transition state brings a B-H unit of sodium borohydride into close proximity to the polarized C=0 unit of 2-butanone due to electrostatic attraction between the 6- H and the 6-1- C of the carbonyl. The hydride of the borohydride is transferred to the 6-1- carbon of the carbonyl, breaking the ji-bond with transfer of those electrons to the boron. Structure 10 represents the requisite four-centered transition state. 

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