Boron nitride rhombohedral

Rhombohedral boron nitride can be prepared by heating a mixture of NaBH and NH Cl rapidly to 750—1000°C (15). The presence of NaCl appears to favor the rhombohedral form. 

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). 

Poly crystalline boron nitride films, with a structure similar to rhombohedral boron carbide and a ratio of boron to nitrogen of 3 1, were produced by hot-filament CVD. This work indicates the possible existence of other boron-nitride structures. 

Boron Nitride, Metal Borides, and Related Spedes.— Alo.o6BeB3 05, Le. BeB3 belongs to the space group P6/mmm, and contains B12 icosahedra and other polyhedral units of Be and B atoms. The linkages between the polyhedra resemble those in /3-rhombohedral boron. Aluminium atoms occupy interstitial... 

Boron compounds with nonmetals, i.e., boron hydrides, carbides, nitrides, oxides, silicides, and arsenides, show simple atomic structures. For example, boron nitride (BN) can be found in layered hexagonal, rhombohedral, and turbostratic or denser cubic and wurtzite-like structures, as well as in the form of nanotubes and fullerenes. Boron compounds with metalloids also differ from borides by electronic properties being semiconductors or wide-gap insulators. 

Crystalline boron-rich boron nitrides of the compositions B25N up to B53N as well as amorphous phases of the composition B3N and B5N have also been synthesized by CVD methods [120,121]. The formation of a rhombohedral structure of the boron-rich nitride, B4N, is described in [122]. The films deposited at 1200°C on hBN substrates by a tungsten hot filament assisted vapor-phase reaction... 

The field of transformation of other BN phases (i.e., a-BN Including rhombohedral and turbostratic BN, y-BN) into cubic boron nitride (P-BN) has been the subject of numerous reviews within the past four years. Perhaps the most comprehensive review of p-BN formation, structure, and properties was compiled by Rapoport [1]. Another review is given in English [2], and there are reviews in French [3], Japanese [4 to 8], Russian [9 to 12], and German [13]. In... 

Figure 11.11 Units cells of (a) Hexagonal sheet-type (h-BN) (b) Rhombohedral sheet-type (r-BN) (c) Wurtzite-type (w-BN) (d) Zincblende-type (z-BN) polymorphic modifications of boron nitride. The lengths of...

Sato, T., Ishii, T., and Setaka, N. (1982) Formation of cubic boron nitride from rhombohedral boron nitride by explosive shock compression, f Am. Ceram. Soc.,... 

The B-N system is accepted ITom [2003Rec]. Only one intermediate phase BN exists in this system. Boron nitride has four crystalline stractural modifications cubic (cBN), wurtzite (wBN), hexagonal (ABN) and rhombohedral (rBN). Except for that, there are two other ordered BN phases EBN, obtained by explosion (E) of a mixture of ABN and aBN, compressed ABN attributable to a monoclinic lattice distortion of ABN and two disordered BN phases turbostratic BN (tBN) and amorphous BN (uBN). 

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