Nitrides—Structures and crystal growth

Abstract

Considerable progress has been made recently in the study of nitrides. In this review, nitrides are classified according to cation coordination number, and crystal growth is presented as a function of the medium (solid, liquid or gas) in which it takes place. The results are not very spectacular if compared with those obtained with oxides. In addition to the difficulties normally encountered in crystal growth must be superimposed those due to nitride chemistry. However, several techniques, as yet inadequately investigated, look promising.Nitrogen combines with a large number of elements and forms nitrides with most of them. Nitrogen atoms, having an electron affinity of 3eV, are very reactive and are able to form very high energy bonds, particularly so in the N2 molecule. The reactivity of nitrogen depends therefore on whether it is involved in the reaction in the molecular or the atomic state. In the first case, reaction occurs easily, as for example in the formation of nitrogenyl complexes; in the second case, the large enthalpy of dissociation of nitrogen (226 kcal mole-1) allows reactions to take place only at high temperature, which explains the relative chemical inertness traditionally attributed to nitrogen.With electropositive elements — e.g. those of groups I and IIa — the nitrides have a marked ionic character and contain N3- ions. Nitrides of most other elements are essentially covalent, except for those of the transition metals, which are interstitial in character.