Intermediate valence in rare-earth materials (invited)

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Abstract
The magnetic and other properties of compounds in which rare-earth elements occur with an intermediate valence are reviewed. A distinction between ’’metallic’’ and ’’covalent’’ systems is suggested. Strong similarities are found between mixed-valent materials, particularly of the metallic kind, and transition-metal or actinide systems which are reasonably well understood in terms of an itinerant-electron model. The parallel between intermediate valence and itinerant magnetism extends to dynamic properties as measured by quasielastic neutron scattering and X-ray photoemission spectroscopy. It is argued that photoemission spectra provide no information of the time-dependent properties of the initial state and therefore cannot provide an upper limit for the frequency of valence fluctuations.