Rare-earth compounds with mixed valencies

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Abstract
This paper reviews the properties of certain rare-earth compounds in which the 4f band has mixed valency, notably SmB6 and the high-pressure forms of SmS, SmSe and SmTe. The metal-insulator transitions of the last three materials under pressure are discussed. It is suggested that the low-pressure form of SmS is an excitonic insulator. In SmB6 and high-pressure SmS a very small gap separates occupied from unoccupied states, this in our view being due to hybridization of 4f and 5d bands. The electrical properties are discussed; if kT is greater than the gap energy, then the gap does not affect the metallic behaviour. Finally metallic compounds such as CeAl3 are described, in which there is no magnetic ordering at low temperatures, and it is suggested that this must always occur if the Kondo temperature is higher than the RKKY interaction. In this case, as in compounds with mixed valency, the Fermi energy will pass through the 4f band, and there is a very large enhancement of the effective mass. The relationship to the side-band model is discussed.