ANOMALOUS TEMPERATURE BEHAVIOR OF IMPURITIES IN LEAD AND SILVER HOSTS

XAFS and Mossbauer effect measurements were made on impurities that lo wer the bulk melting temperature, T(m), of Pb and Ag. In addition XAFS measurements were made on Tl impurities that raise T(m) of Pb. The sp ectral intensity of the Mossbauer signal in Pb varies as expected from harmonic vibrations of the 1 at.% Sn Mossbauer atoms below a temperat ure of about 140K. Above this temperature the spectral intensity drops rapidly which is attributed to a localized hopping associated with a high entropy density. In Ag hosts a similar behavior occurs above 900K . The XAFS signal from Hg impurities in Pb, which like Sn lowers T(m), starts deviating from vibrational behavior above 400K where the appar ent coordination number decreases. This is also interpreted as a local ized hopping. However, Tl, which raises T(m), shows the normal vibrati onal behavior in XAFS measurements in a Pb host. XAFS measurements on Ga impurities in Ag, which also lower T(m) of bulk Ag, show only expec ted anharmonic behavior up to 900K. All of these results suggest a liq uid-like high entropy region within a radius of somewhat more than an atomic spacing around impurities that lower T(m). These high entropy, or ''premelted'', regions could act as nucleation sites for three dime nsional melting and should dominate over both grain boundaries and fre e surfaces in nucleating bulk melting in large single crystals.