Interstitial compounds

The field of interstitial compounds is a mature one, with an enormous literature. Nevertheless, a question as fundamental as the nature of their electronic structure remains a topic for lively (sometimes acrimonious) debate today. Moreover, industry has yet to tap more than a few of the myriad technical possibilities of these materials. The most commonly exploited property is their great hardness: tons of the interstitial compound tungsten carbide are produced annually for use in grinding and cutting tools. But other properties have seen little commercial application as yet. To cite a few: as a class, they are refractory, yet metallic, and they often have abnormally low work functions, making them excellent electron emitters. This combination suggests, for example, utility as magnetohydrodynamic channel electrodes. Some interstitial compounds, such as NbN, are superconducting, with transition temperatures second only to such champions as ${\mathrm{Nb}}_3\mathrm{Sn}.$ Tungsten carbide and related compounds are moderately good catalysts, able to operate in chemically hostile environments, as may be encountered, for example, in fuel cells. Many are respectable ferro‐ and antiferromagnets. As we will try to indicate, this class of compounds poses fascinating possibilities for the technologist and real challenges for the researcher. Applications of these materials derive from their hardness, high melting points and, in some cases, catalytic and magnetic properties and superconductivity; yet their electronic structure is still something of a mystery.