Like Boron Nitride Modifications

Diamond-Like Boron Nitride Modifications Cubic Boron Nitride with Sphalerite Structure (P-BN)... 

Ca3(BN2)2 is readily formed when (distilled) calcium metal is melted in the presence of (layer-type) boron nitride. This reaction provides some insight on how alkaline-earth metals like calcium may act as a catalyst in the phase transformation of layered a-BN into its cubic modification. Instead of metals, nowadays alkaline-earth (Ca, Sr, Ba) nitridoborates can be used as a flux catalyst in high-pressure and high-temperature transformation reactions to produce cubic boron nitride [15]. 

The traditional method for the preparation of boron nitride is by the fusion of urea with boric acid in an atmosphere of ammonia at 750 °C.54 The product from these reactions is hexagonal boron nitride with a layer structure like that of graphite. Unlike graphite, it is colorless and is not an electronic conductor. Conversion of the hexagonal form to a cubic modification requires heating at 1,800 °C at 85,000 atmospheres pressure. 

In graphite, which can be considered as a giant two-dimensional molecule from the series of condensed rings, the bonding between the separate layers is very weak, being due, as in molecular lattices, to Van der Waals-London interaction. The now infinite system of n electrons results in metallic conduction, only, however, in the plane of the rings Boron nitride has perhaps also a diamond-like form as well as the common graphite-like modification (p. 235). 

Boron nitride (BN) exists in a hexagonal modification with a graphite-like crystal structure and a cubic high temperature/high pressure modification with a diamondlike structure. 

Figure 4.12. Two modifications of boron nitride, which is isoelectronic with carbon (a) layered hexagonal a-BN, and (b) cubic ) -BN with a sphalerite structure. Both are electrical insulators, but like carbon they are either very soft or very hard.

Adadurov, G.A., Ananin, A.V., Bavina, T.V., and Breusov, O.N. (1982) Method of obtaining diamond and/or diamond-like modification of boron nitride. G.B. Patent 2,090,239. 

So it is not by chance that boron and nitrogen form boron nitride, one modification of which is similar to graphite regarding structure and properties. Due to its color, it is called white graphite. like common black graphite, it can be transformed to a cubic modification at high temperature and pressure. Cubic boron nitride is the second hardest substance known, after diamond. 

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