TABLE OF CONTENTS The group of rare-earth elements comprise the series from lanthanum (atomic number 57) to lutetium (atomic number 71), where the electronic configuration (EC) gradually changes from [Xe]4 f 05 d 16 s 2 to [Xe]4 f 145 d 16 s 2, respectively (see Localized 4f and 5f Moments: Magnetism). Light elements of this series like cerium, elements with a nearly half-filled 4 f shell like samarium and europium, or with an almost filled 4 f shell like thulium and ytterbium may, however, deviate from such a well-defined EC with integer valency. As a result, the ions exhibit intermediate valence (IV), representing a homogeneous quantum mechanical mixture of two consecutive integer valence configurations. Not only are rare-earth systems known to show such valence instabilities, but about 40 elements (3 d, 4 d, 5 d, 5 f ) and their compounds can emerge in noninteger valency.
A necessary condition for nonintegral valence to occur is that two bounding states of the rare earth, i.e., 4 f n(5 d6 s) m and 4 f n ? 1(5 d6 s) m + 1, are nearly degenerate. The system may then lower its energy by adopting a mixture of both EC and an additional degree of freedom is acquired. On each 4 f site charge fluctuations appear between these configurations on a typical time scale ? cf. Experiments probing such...
