Boron nitrides synthesized directly from the elements at high pressures and temperatures Page: 4 of 16
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Many important materials composed of the elements, boron, carbon, nitrogen, oxygen,
aluminum, and silicon are hard and have covalently-bonded three-dimensional structures.
Examples include diamond and cubic boron nitride, c-BN. The networks of these materials
contrast with softer molecular forms of the same elements like graphite, N2 and 02. Although
they have strong intramolecular covalent bonds, only weak van der Waals attractions operate
between molecules. Localizing electrons in molecular covalent bonds minimizes their potential
energy, makes the bonds extremely stable, and acts as a barrier to reactions leading to network
structures. Applying pressure raises the kinetic energies of the electrons by p', where p is the
density. This is faster than the potential energy increases; it grows as p ". High pressures are,
therefore, inimical to local covalent bonds. At high densities, electrons eventually delocalize,
converting molecular solids to more closely packed network structures and eventually to metals.
Many experiments support these ideas. Most unsaturated organic molecules polymerize at
pressures of the order of 10 GPa.' Layered covalent solids like graphite and h-BN transform at
high pressures to dense, three-dimensional network materials, diamond and c-BN.2 The proposed
superhard material,3 R-C3N4, also may have a covalently bonded network structure. Many
diatomic molecules, halogen, and inert gases even metallize at the pressures of 100 GPa. s
However, large activation barriers often limit the rates of achieving these stable high pressure
products. Synthetic diamond grows only at substantially higher than the equilibrium transition
temperature6 or under kinetic control.
The standard enthalpies of formation in Table I show the stabilities of some first- and
second-row oxides, nitrides, and carbides. Even the boron-nitrogen reaction leading to h-BN is
very exothermic. In contrast, formation of many hydrides or a theoretical nitrogen polymer
consume energy. Despite the thermodynamic stability of the product, boron and nitrogen do not
react with each other at ambient temperature even at 50 GPa, the highest pressure of our
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Nicol, M.; Yoo, C.S.; Akella, J. & Cynn, H. Boron nitrides synthesized directly from the elements at high pressures and temperatures, article, November 1, 1996; California. (digital.library.unt.edu/ark:/67531/metadc676583/m1/4/: accessed December 14, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.