Nitrides: Transition Metal Solid-State Chemistry

Together with the transition metal carbides and borides, the transition metal nitrides belong to a family of materials that features an unusual combination of outstanding properties, among which are exceptional hardness, high melting point, metallic luster with sometimes vivid colors, and simple metallic structures combined with excellent electrical and thermal conductivities. This combination of these properties has attracted considerable attention and has resulted in numerous technical applications.1–4 Lengauer5 has compiled several data and features of transition metal nitrides and carbonitrides. Many of the nitride phases closely resemble metallic alloys, with broad ranges of homogeneity, particularly if the structure of the metallic host lattice is one of the types encountered in typical metals. They often form solid solutions with structurally related compounds, such as other transition metal nitrides or carbides, and are easily wetted by liquid metals of the iron group. The chemical stability of transition nitrides is considerable; they are not readily attacked by dilute acids except by oxidizing acids and hydrofluoric acid or alkaline solutions. The thermal stability is determined by their free energies of formation; the stability decreases with increasing group number. While TiN, ZrN, and HfN can be melted without decomposition at ambient pressure, the nitrides of the other groups decompose by liberating nitrogen before melting points are reached. Table 1 gives a brief survey of the most important properties of the well-characterized binary transition metal nitrides with a maximum nitrogen content of 50 mol% N. It should, however,