Mixed-valence compounds

In a number of rare-earth compounds, the atomic-like $f$ levels and the wide $s-d$ band coexist at the Fermi level. Such compounds are being referred to as mixed-valence compounds. These compounds have a variety of unique thermal and magnetic properties. In this article, we review some salient experimental results in such compounds and discuss a theoretical framework in which they may be understood. Special emphasis is given to the Samarium chalcogenides, on which extensive experimental results are available. The review is not meant to be a comprehensive survey of theoretical and experimental results. Rather it reflects the interests of the author. The hope is that the basic unresolved issues in the theory of mixed-valence compounds are pointed out so as to stimulate further experimental and theoretical work.