ANOMALOUS TEMPERATURE BEHAVIOR OF SN IMPURITIES

Sn impurities in Pb and Ag hosts have been investigated by Mossbauer e ffect and in Pb by x-ray-absorption fine-structure (XAFS) studies. The Sn atoms are dissolved up to at least 2 at. % in Pb and up to at leas t 8 at. % in Ag for the temperature ranges investigated. The concentra tion limit for Sn-Sn interactions is 1 at. % for Pb and 2 at. % for Ag as determined experimentally by lowering the Sn concentration until n o appreciable change occurs in the Mossbauer effect. XAFS measurements verify that the Sn impurities in Pb are dissolved and predominantly a t substitutional sites. For both hosts the temperature dependence of t he spectral intensities of isolated Sn impurities below a temperature T0 is as expected for vibrating about a lattice site. Above T0 the Mos sbauer spectral intensity exhibits a greatly increased rate of drop-of f with temperature without appreciable broadening. This drop-off is to o steep to be explained by ordinary anharmonic effects and can be expl ained by a liquidlike rapid hopping of the Sn, localized about a latti ce site. Higher-entropy-density regions of radii somewhat more than an atomic spacing surround such impurities, and can act as nucleation si tes for three-dimensional melting.