Seminars Archive

Rare earth valence transitions as a mechanism for zero or negative thermal expansion: the case of YbGaGe

Abstract
Changes in rare earth valence have been proposed as the underlying mechanism for recent observations of zero or negative thermal expansion in a number of systems. We have determined the ytterbium valence as a function of temperature in the reported near zero thermal expansion material YbGaGe using x-ray photoemission at various incident photon energies. The Yb 3d, 4d and 4f levels, which directly yield the Yb valence, have been measured. Careful analysis enabled the clear separation of surface and bulk contributions. Resonant photoemission at the 4d-4f absorption edge was used to enhance the low contribution of the Yb3+ component. Contrary to the initially proposed Yb valence transition we find no change in the valence from room temperature down to 115 K. Our planned and ongoing investigations of the related systems Yb8Ge3Sb5, Yb2.75C60 and Sm2.75C60 are also discussed.