Hydrides of intermetallic compounds

Abstract

Aspects of the progress over the recent years on hydrides of intermetallic compounds are reviewed with emphasis on structure, stability, solid-state properties, catalysis, and kinetics. Some new routes to an understanding of hydride phenomenology are indicated. Generally speaking hydrides represent but one special aspect of intermetallic compounds. They are, however, unique as model systems for questions concerning the stability of intermetallics in general, as thermodynamic data become more readily available during synthesis than is the case with most other metallic systems. Stability criteria are exemplified on model systems chosen to represen instructive or extreme cases. Hydrides are, moreover, unique with respect to the unusual properties of hydrogen as a “metallic” component, especially concerning such characteristics as mass (light), size (ranging from average to exceedingly small as a proton) and electronegativity (relatively high and comparable to B). This promises a wealth of special solid-state properties concerning magnetism, electrical conductivity or superconductivity. Some selected examples in this direction are discussed with emphasis on questions pertaining to the electronic state and the stability of the hydrides. The kinetics of compound (hydride) formation are often extraordinarily fast around ambient conditions opening this facet of intermetallics to rigorous study. Also intriguing new catalytic activity has been identified with metal hydrides implicating in one way or another the special surface properties of metal hydrides. Finally, metal hydrides are gaining in importance with respect to technological applications within a projected hydrogen economy (H or heat storage, electrochemical cells, etc.). Many of the fundamental aspects of metal hybrides have therefore become of practical relevance.