ANOMALOUS CONCENTRATION-DEPENDENCE OF THE SN-119 MOSSBAUER ISOMER-SHIFT OF AG-SN ALLOYS

The position of the E-gamma approximate to 24 keV Mossbauer resonant a bsorption line of Sn-119 was measured at various temperatures and conc entrations of substitutional Sn impurities in the alpha-phase of the A g-Sn alloy. Whereas 1.0 and 2.0 at.% Sn alloys showed the expected tem perature dependence of the line shift (LS), 4.0 and 8.0 at.% Sn alloys showed several anomalous features. After aging at room temperature (R T) for several months, these alloys showed hysteresis of the LS from R T up to about 500 K. At about 520 K, the unusual slope transforms to i ts expected value but continues with an increased isomer shift (IS). F urther thermal cycles resulted in the expected slope of the LS with no measurable hysteresis, except for the transition region around 520 K above which the IS increase persisted. The experiments indicate that a fter thermal cycling the high-concentration alloys are in an intermedi ate state below about 500 K, which becomes metastable at RT as shown b y the changes with aging. The transition to the increased-IS state at T approximate to 520 K, however, is independent of the thermal history . A discussion of the observed phenomena is presented, which is based on thermally induced changes in the Sn-Sn distribution (short-range or der) that preserve the long-range order of the alloy.