Boron hexagonal

Yoo C S, Akella J, Nicol M and Cynn H 1997 Direct elementary synthesis of hexagonal and cubic boron nitrides at high pressures and temperatures Phys. Rev. B 56 140... 

Lithium Nitride. Lithium nitride [26134-62-3], Li N, is prepared from the strongly exothermic direct reaction of lithium and nitrogen. The reaction proceeds to completion even when the temperature is kept below the melting point of lithium metal. The lithium ion is extremely mobile in the hexagonal lattice resulting in one of the highest known soHd ionic conductivities. Lithium nitride in combination with other compounds is used as a catalyst for the conversion of hexagonal boron nitride to the cubic form. The properties of lithium nitride have been extensively reviewed (66). 

Properties. Under nitrogen pressure hexagonal boron nitride melts at about 3000°C but sublimes at about 2500°C at atmospheric pressure. Despite the high melting point, the substance is mechanically weak because of the relatively easy sliding of the sheets of rings past one another (3). The theoretical density is 2.27 g/mL and the resistivity is about 10 H-cm. 

Hexagonal boron nitride is relatively stable in oxygen or chlorine up to 700°C, probably because of a protective surface layer of boric oxide. It is attacked by steam at 900°C, and rapidly by hot alkaU or fused alkaU carbonates. It is attacked slowly by many acids as well as alcohols (to form borate esters), acetone, and carbon tetrachloride. It is not wetted by most molten metals or many molten glasses. 

Preparation. Hexagonal boron nitride can be prepared by heating boric oxide with ammonia, or by heating boric oxide, boric acid, or its salts with ammonium chloride, alkaU cyanides, or calcium cyanamide at atmospheric pressure. Elemental nitrogen does not react with boric oxide even in the presence of carbon, though it does react with elemental boron at high temperatures. Boron nitride obtained from the reaction of boron trichloride or boron trifluoride with ammonia is easily purified. 

The cubic 2inc blende form of boron nitride is usually prepared from the hexagonal or rhombohedral form at high (4—6 GPa (40—60 kbar)) pressures and temperatures (1400—1700°C). The reaction is accelerated by lithium or alkaline-earth nitrides or amides, which are the best catalysts, and form intermediate Hquid compounds with BN, which are molten under synthesis conditions (11,16). Many other substances can aid the transformation. At higher pressures (6—13 GPa) the cubic or wurt2itic forms are obtained without catalysts (17). 

Borazine, B3N3Hft, a compound that has been called inorganic benzene because of its similar hexagonal structure (but with alternating B and N atoms in place of C atoms), is the basis of a large class of boron—nitrogen compounds. Write its Lewis structure and predict the composition of the hybrid orbitals used by each B and N atom. 

The unit cell of MgB2 has six boron atoms in the center of a hexagonal array of magnesium atoms. The superconductivity appears to stem from the high-energy vibrational modes of the planes of boron atoms that extend throughout the crystal. 

Its structure resembles that of graphite, but the latter s flat planes of carbon hexagons are replaced in boron nitride by planes of hexagons of alternating B and N atoms (Fig. 14.27). Unlike graphite, boron nitride is white and does not conduct... 

FIGURE 14.27 (a) The structure of hexagonal boron nitride, BN, resembles that of graphite, consisting of flat planes of hexagons of alternating B and N atoms (in place of C atoms but, as shown for two adjacent layers in part (b), the planes are stacked differently, with each B atom directly over an N atom and vice-versa (compare with Fig. 14.29). Note that (to make them distinguishable) the B atoms in the top layer are red and the N atoms blue. 

In Fig. 14.27 we see that the planes in hexagonal boron nitride take positions in which the B atoms are located directly over N atoms, whereas in graphite (Fig. 14.29 ), the carbon atoms are offset. Explain this difference in structure between the two substances. 

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. 

Summary of Characteristics and Properties of CVD Hexagonal Boron Nitride... 

The applications of hexagonal boron nitride form an important market, mostly as powder for lubricants and additives. Many of these applications are produced by CVD. 

Cubic boron nitride is obtained from hexagonal boron nitride at high pressure and temperature in the presence of lithium nitride as a catalyst. It is almost as hard as diamond and has superior chemical resistance and a much higher oxidation threshold.Efforts to... 

Planar hexagonal boron layers are also found in a lower boride structure Pr5, Co2+xBg (0 < X < 1), where B and Co atoms are substituting each other to some extent. 

Molybdenum disulhde (M0S2), graphite, hexagonal boron nitride, and boric acid are examples of lamella materials commonly applied as solid lubricants. The self-lubricating nature of the materials results from the lamella crystalline structure that can shear easily to provide low friction. Some of these materials used to be added to oils and greases in powder forms to enhance their lubricity. Attention has been shifted in recent years to the production and use of nanosize particles of M0S2, WS2, and graphite to be dispersed in liquid lubricants, which yields substantial decreases in friction and wear. 

Structures of the lanthanide nitridoborates appear as layered structures with approximate hexagonal arrangements of metal atoms, and typical coordination preferences of anions. As in many metal nitrides, the nitride ion prefers an octahedral environment such as in lanthanum nitride (LaN). As a terminal constituent of a BNx anion, the nitrogen atom prefers a six-fold environment, such as B-N Lns, where Ln atoms form a square pyramid around N. Boron is typically surrounded by a trigonal prismatic arrangement of lanthanide atoms, as in many metal borides (Fig. 8.10). All known structures of lanthanide nitridoborates compromise these coordination patterns. 

There is great interest in developing molecular precursors for boron-nitrogen polymers and boron nitride solid state materials, and one general procedure is described in this report. Combinations of B-trichloroborazene and hexamethyldisilazane lead to formation of a gel which, upon thermolysis, gives hexagonal boron nitride. The BN has been characterized by infrared spectroscopy, x-ray powder diffraction and transmission electron microscopy. 

Boron nitride may be obtained in three primary crystalline modifications (2) a, j3, and y. The most commonly encountered a form has a graphitic structure (hexagonal cell, a = 2.504 A, c = 6.661 A). For many years, this modification has been prepared from combinations of cheap boron and nitrogen containing reagents, e.g. B(0H)3 and (NH2)C0, B(0H)3, C and N2 or KBH4 and NH4C1 (3-5). More... 

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