Nitrides Of the group 13 metals

Boron nitride, BN, is a robust (sublimation point = 2603 K), chemically rather inert compound which is used as a ceramic material (e.g. in crucible manufacture). Preparative routes include the high-temperature reactions of borax with [NH₄]⁺Cl, B₂O₃ with NH₃, and B(OH)₃ with [NH₄]⁺Cl. High-purity boron nitride can be made by reacting NH₃ with BF₃ or BCl₃. The common form of boron nitride has an ordered layer structure containing hexagonal rings (Figure 1.1). The layers are arranged so that a B atom in one layer lies directly over an N atom in the next, and so on. The B^_N distances within a layer are much shorter than those between layers (Figure 1.1) and, in Table 1.1, it is compared with those in other B^_N species. The B^_N bonds are shorter than in adducts such as Me₃N.BBr₃ in which a single boron– nitrogen bond can be assigned, and imply the presence of π-bonding in BN resulting from overlap between orthogonal N 2p (occupied) and B 2p (vacant) orbitals. The interlayer distance of 330pm is consistent with van der Waals interactions, and boron nitride acts as a good lubricant, thus resembling graphite. Unlike graphite, BN is white and an insulator. This difference can be interpreted in terms of band theory  with the band gap in boron nitride being considerably greater than that in graphite because of the polarity of the B^_N bond.

     Heating the layered form of BN at ≈ 2000K and >50 kbar pressure in the presence of catalytic amounts of Li₃N or Mg₃N₂ converts it to a more dense polymorph, cubic-BN, with the zinc blende structure.

Table 1.1 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 π-bonding within the layers of the latter. Structurally, the cubic form of BN resembles diamond 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 lattice is formed by compression of the layered form at ≈12 kbar.

   Of the group 13 metals, only Al reacts directly with N₂ (at 1020 K) to form a nitride; AlN has a wurtzite lattice and is hydrolysed to NH₃ by hot dilute alkali. Gallium and indium nitrides also crystallize with the wurtzite structure, and are more reactive than their B or Al counterparts. The importance of the group 13 metal nitrides, and of the related MP, MAs and MSb (M = Al, Ga, In) compounds, lies in their applications in the semiconductor industry (see also Section 18.4).

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