Anomalous temperature behavior of impurities in lead and silver hosts

XAFS and Mossbauer effect measurements were made on impurities that lower the bulk melting temperature, T_m, of Pb and Ag. In addition XAFS measurements were made on T1 impurities that raise T_mof Pb. The spectral intensity of the Mossbauer signal in Pb varies as expected from harmonic vibrations of the 1 at.% Sn Mossbauer atoms below a temperature 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 apparent coordination number decreases. This is also interpreted as a localized hopping. However, Tl, which raises T_m, shows the normal vibrational 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 expected anharmonic behavior up to 900K. All of these results suggest a liquid-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 dimensional melting and should dominate over both grain boundaries and free surfaces in nucleating bulk melting in large single crystals.