Polymorphism boron nitride

Fig. ft.1-51 BN. Heat capacities for four polymorphic boron nitride modifications. Data taken from [1.50]... 

N. Dubrovinskaia, V. L. Solozhenko, N. Miyajima, V. Dmitriev, O. O. Kurakevych, and L.Dubrovinsky, Superhard Nanocomposite of Dense Polymorphs of Boron Nitride Noncarbon Material has Reached Diamond Hardness , Appl. Phys. Lett., 90,101912 (2007). 

Boron nitride (BN) is a versatile ceramic that is found and used in various crystallographic and geometrical forms. Although there are chemical similarities, the conditions for the synthesis and fabrication of each form (hexagonal, cubic, polymorphs of BN, fibers, two-dimensional forms and composites) have different requirements and are... 

Fig. 12.18 Part of the layer structure of the common polymorph of boron nitride, BN. Hexagonal rings in adjacent layers lie over one another so that B and N atoms are eclipsed. This is emphasized by the yellow lines.

Heating the layered form of BN at rj2000 K and >50 kbar pressure in the presence of catalytic amounts of Li3N or MgsN2 converts it to a more dense polymorph, cubic-BN, with the zinc blende structure (see Section 6.11). Table 13.2 shows that the B—N bond distance in cubic-BN is similar to those in R3N-BR3 adducts and longer than in the layered form of boron nitride this further supports the existence of 7r-bonding within the layers of the latter. 

Structurally, the cubic form of BN resembles diamond (Figure 6.19) and the two materials are almost equally hard. Crystalline cubic BN is called borazon and is used as an abrasive. A third polymorph of boron nitride with a wurtzite-type structure is formed by compression of the layered form at 12 kbar. 

The above-mentioned phase transitions conform to the Le Chatelier principle, the sample volume decreasing under high pressure. They are not basically different Irom those observed in the static method, under conditions of thermodynamic equilibrium. There is, however, a class of anomalous phase transitions, which occur only in dynamic experiments and in which the shock compression gives rise to lower densities. The first of such phases was obtained in 1965 by shock treatment of the turbostratic BN [224] the new phase differed from both the graphite-Uke (/i-BN) like (c-BN) polymorphs of boron nitride and was named E-BN (E standing for the explosion phase ). Later, it appeared that the lattice parameters of E-BN are nearly identical to one of the phases of fullerene Ceo [225, 226], viz. a = 11.14, ft = 8.06, c = 7.40 A for E-BN, cf. a= 11.16, = 8.17, c = 7.58 A for Qo, with similar densities of 2.50 g/cm. Thus, the BN-fullerene was obtained by explosion (though not recognized as such) some 25 years before the carbon fuUerene was identified. Later on. 

Pokropivny VV, Smolyar AS, Pokropivny AV (2007) Ruid synthesis and structure of a new boron nitride polymorph—hyperdiamond fulborenite B12N12 (E phase). Phys Solid State... 

Boron nitride [10043-11-5], chemical formula BN, exists as three different poly-morphs alpha-boron nitride (a-BN), a soft and ductile polymorph (p = 2280 kg.m" and m.p. = 2700°C)... 

Boron nitride shows many similarities to carbon owing to its isoelectronic valence electron structure that is, both compounds possess an equal sum of eight valence electrons (4 -i- 4 in carbon 3 -r 5 in boron nitride) and an equal sum of (two) atoms each (C2 BN). Consequently, boron nitride shares with carbon a multiple of polymorphic modifications with very similar properties (Table 11.5). 

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...

Figure 77.73 Phase relations of boron nitride polymorphs. Solid lines A and B according to Corrigan and Bundy (1975) chain lines according to Tani et al. (1975). The triple point w-BN/h-BN/c-BN is approximately at 1780 K/4.85 CPa and 1725 K/5.5CPa, respectively. The activation...

BORON NITRIDE. BN. A highly refractory material with physical and chemical properties similar to carbon. Graphite-like (g-BN), wurzite (w-BN) and zinc blende (z-BN) are known polymorphs of BN corresponding to the graphite (hexagonal) and diamond (cubic) structures. Transformation of g-BN to w-BN occurs at pressures above 12 GPa at relatively low temperature (230°C). 

Some borides have industrial applications, for example titanium diboride TiBa or lanthanum hexaboride LaBe, and some boron compounds are conventionally classified among borides, including boron carbide B4C and boron nitride BN, a material which has three polymorphs, including two isostructural carbon polymorphs graphite and diamond. 

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