Boron-stabilized crystal structure

Cubic Phase of Boron Nitride c-BN. The cubic phase of boron nitride (c-BN) is one of the hardest materials, second only to diamond and with similar crystal structure. It is the first example of a new material theoretically predicted and then synthesized in laboratory. From automated synthesis a microcrystalline phase of cubic boron nitride is recovered at ambient conditions in a metastable state, providing the basic material for a wide range of cutting and grinding applications. Synthetic polycrystalline diamonds and nitrides are principally used as abrasives but in spite of the greater hardness of diamond, its employment as a superabrasive is limited by a relatively low chemical and thermal stability. Cubic boron nitride, on the contrary, has only half the hardness of diamond but an extremely high thermal stability and inertness. 

The more stable boron chelates can be isolated even from aqueous solution, whereas those of lower stabilities are only accessible from non-aqueous media. Catechol- and inositol-borates (3, 5 and 6) possesses a well-defined monomeric structure,75 whereas those obtained from monosaccharides and alditols are polymeric.121 A crystal structure determination122 has been carried out for sodium scyUo-inositol diborate (6). 

Kinetically stabilized silabenzene 32, the crystal structure of which has been discussed in Section 7.14.3, undergoes 1,2- and 1,4-addition reactions similar to those of the boron analogs, for example, with water to afford 1-hydroxy-silacyclohexadienes 81 and 82 (Scheme 4) <20050M6141>. Reaction of 32 with phenylacetylene results in two addition products, the silabarrelene derivative 83 formed by a [4+2] cycloaddition, and the 1,2-addition product 84. 

It is very often difficult to establish the existence and composition of borides owing to one or more of the following reasons. The great reactivity of boron toward, for instance, the crucible material leads to the occurrence of impurities in the products. The impurities are often said to stabilize a structure type, while in reality a ternary compound has been formed although unintentionally. It is also difficult to analyze chemically or by microprobe the boron content owing to low accuracy of the methods. Indeed, in many cases a single-crystal-structure determination is, together with complementary structural information obtained by other... 

For instance, attempts at isolating donor-free alkynylboranes have been hampered by spontaneous polymerization. The first donor-free tris(alkynyl)borane, tris(3,3-dimethyl-l-butynyl)borane (30), has only recently been obtained by reaction of deprotonated 3,3-dimethyl-l-butyne with boron trichloride at —78°C in pentane (equation 32). The alkynylborane (30) was fully characterized by multinuclear NMR spectroscopy, mass spectrometry, and X-ray crystallography. The NMR signal of (30) shows a remarkable shift of 5 = 48, which is upfield from typical shifts of alkyl-and alkenylboranes. The crystal structure of (30) shows similar C-C bond lengths, but significantly shorter B-C bonds in comparison to those observed for the donor-stabilized complexes (30) D indicative of a small degree of p - p interactions between the sp -boron atom and the sp-hybridized carbon. Weak r-overlap was further confirmed by ab initio calculations. It is this r-interaction that has been exploited for the development of nonlinear optical (NLO) materials based on alkynylboranes as described in Section 7.1. ... 

Magnets based on the Nd-Fe-B system are basically metallic, and they are prepared by metallurgical techniques. The teclmique for introducing boron atoms into the structure is basically that of alloy making. Except for second-phase formation, the boron atoms naturally find their sites in the crystal structure during the formation of the 2-14-1 phase as a result of thermodynamic stability. 

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