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Nitride is a chemical compound formed when nitrogen bonds with an element that has lower electronegativity, such as boron, silicon and most metals. Nitrides are classified into four different groups based on the nature of bonding, which are Ionic or saline nitrides, Covalent binary nitrides, Transition metal nitrides and Molecule forming nitrides.
The Ionic nitrides form when nitrogen bonds with alkali metals, alkaline earth metals and the metals of group 3. These compounds, which contain the nitride ion (N3-), are unstable and react with water to produce ammonia and the metal hydroxide. Hexagonal boron nitride, which has the formula BN, is prepared by heating boron trichloride, BCl3, in an excess of ammonia at 750 degC (1,400 degF).
Boron nitride and carbon nitride are both examples of covalent binary nitrides. The hexagonal form, which is similar to graphite, has a layered structure that consists of planar six-membered rings that alternate between boron and nitrogen. These atoms are offset so that a boron atom in one layer is directly above a nitrogen atom in an adjacent layer, which makes boron nitride isoelectronic with elemental carbon.
The interstitial nitrides that form with transition metals are the largest group of nitrides. These nitrides, which have the general chemical formulas MNmN, M2NM2N and M4NM4N, resemble interstitial carbides in that nitrogen atoms fill the gaps or “interstices” between tightly packed metal atoms. These substances are refractory, hard and often opaque materials with metallic lustres and strong conductivities. Silicon nitride and titanium nitride are used for the cutting of materials and for hard coatings, while aluminum nitride, which has piezoelectric properties, is used in small surface acoustic wave sensors.