When nitrogen is combined with an element with a lower electronegativity than nitrogen (such as boron, silicon and other metals), it is called a nitride. These compounds have a large range of properties and applications.
Nitrides can be categorized into three different groups, ionic, interstitial and covalent. Examples of ionic nitrides are Mg3N2 and Be3N2, which are stable at high temperatures but decompose at low ones.
Interstitial nitrides are chemically inert and exhibit very little reactivity, but they can be used to make crucibles or other high-temperature vessels. They are also refractory, meaning that they can be exposed to high heat and still remain solid.
Boron nitride is a crystalline, colourless material that can be prepared by heating boron trichloride, BCl3, in an excess of ammonia at 750 degC (1,400 degF). It is similar to graphite in that hexagonal boron nitride is layered with planar rings of boron and nitrogen atoms stacked on top of each other.
Aluminum nitride, AlN, is a polymorph of aluminium that has a diamond-like wurtzite structure. It is a member of the III-V nitride semiconductor group and has found many interesting applications.
Covalent nitrides are also formed in nature from nitrogen combining with elements of equal or lower electronegativity, such as boron or silicon. These covalent nitrides have a wide range of properties and are important materials for research purposes.
There are two principal methods of synthesis of nitrides. The first is by direct reaction of the elements (usually at elevated temperature), such as in the synthesis of calcium nitride shown here. The second is by the loss of ammonia by thermal decomposition of a metal amide, such as in the synthesis of barium nitride.