Nitrides

compounds of nitrogen with more electropositive elements, mainly metals. The following groups of nitrides are distinguished on the basis of structure and properties:

(1) The saltlike nitrides of metals of Groups I and II of the Mendeleev periodic table; they are easily decomposed by water with the formation of ammonia:

Mg₃N₂ + 6H₂O = 3Mg(OH)₂ + 2NH₃

(2) The covalent nitrides of nonmetals, and also of aluminum, gallium, indium, and thallium. These nitrides, particularly AlN, BN, and Si₃N₄, are extremely stable chemically, refractory, and heat-resistant and are dielectrics or semiconductors. Among this group, boron nitride is especially important.

(3) Metal-like nitrides of the transition metals—the largest group of nitrides. Their structure is characterized by the introduction of nitrogen atoms into the crystal lattice of the metal. These nitrides often do not obey the rules of formal valence and are nonstoichiometric compounds (ZrN, Mn₄N, and W₂N) with broad regions of homogeneity, within which a substantial change in their properties takes place. These nitrides are similar to metals in many aspects. They have high electrical and thermal conductance and are refractory (for example, TiN and HfN melt at 3200° and 3380°C, respectively). They differ from metals in their great hardness, brittleness, and lack of plasticity. Metal-like nitrides are chemically highly stable.

Nitrides are formed on the surface of metals under the action of nitrogen or ammonia at 500°–900°C. The nitride coatings impart increased hardness, durability, and corrosion-resistance to metal parts. Parts made from nitrides are used in high-temperature technology, in the construction of gas turbines, in the power industry, and in space technology. Some metal-like nitrides are superconductors (for example, NbN and MoN are superconducting at 15.6°K and 12°K, respectively). The semiconductor and electrical-insulation properties of nitrides are used in semiconductor technology.