Rhombohedral BN

The atomic layers from c-BN (ABCABC) have to rearrange into an ABAB stacking sequence of h-BN during the solid state phase conversion. A possible mechanism would be the intermediate formation of the rhombohedral BN phase (r-BN) with ABCABC stacking. The r-BN phase is structurally related to the hexagonal phase, but only differs in the d-values (h-BN d = 6.66 A r-BN d = 10.0 A) of the layers (Fig. 5b). Subsequently the rhombohedral phase is transformed into the hexagonal modification at the reaction temperatures [10]. 

Rhombohedral BN (rBN) forms in the fusion product of KCN and Na2B407 and by deposition at 1500°C from hexagonal BN vapor originally formed at 2100°C in a graphite resistance tube furnace. Products collect on a pitted carbon film. ... 

Fig. 4-1. Structural relationship between the layered phases and the fourfold-coordinated structures considered by [14] a) Rhombohedral BN to p-BN transition b) a-BN to y-BN transition.

The reaction of KCN with B2O3 at 1100°C leads to a rhombohedral BN modification which is an ordered /3-graphite version [57a]. 

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). 

The thermodynamically most stable polymorph of boron is the /3-rhombohedral modification which has a much more complex structure with 105 B atoms in the unit cell (no 1014.5 pm, a 65.28°). The basic unit can be thought of as a central Bn icosahedron surrounded by an icosahedron of icosahedra this can be visualized as 12 of the B7 units in Fig. 6.1b arranged so that the apex atoms form the central Bn surrounded by 12 radially disposed pentagonal dishes to give the Bg4 unit shown in Fig. 6.3a. The 12 half-icosahedra are then completed by means of 2 complicated Bjo subunits per unit cell,... 

The rhombohedral (r-BN) structure is similar to the h-BN phase but the atomic layers sequence is ABC ABC. It was reported that r-BN is formed during conversion of c-BN into h-BN [22] (Fig. 4). 

It is interesting that most of the NbSe2 nanotubes contain more than 10 layers with one or two containing a smaller number of layers. Some of the nanotubes exhibit a different type of stacking due to the presence of different polytypes just as in BN nanotubes where the local rhombohedral stacking occurs within the hexagonal phase.44... 

A rhombohedral boron carbide Bi3C2 results from the pyrolysis of BBr3-CHLt-Hz mixtures on Ta or BN substrates at 900—1800°C. It has the crystal-chemical composition Bi2(CBC), i.e. Bi2 icosahedra and linear CBC chains.149 Excess carbon up to a resultant formula of B13C3 can be accommodated in the structure. 

Fig. 12.5 Part of one layer of the infinite lattice of a-rhombohedral boron, showing the Bn-icosahedral building blocks which are covalently linked to give a rigid, infinite lattice.

Controlled pyrolysis of a BBr3-CH4 H2 mixture over a BN surface at 1550—1650 C produces a rhombohedral boron carbide B13C2. This is believed to contain icosahedral B12 units and linear CBC chains. 

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. 

---