Anomalous concentration dependence of the ¹¹⁹Sn Mössbauer isomer shift of Ag-Sn alloys

The position of the E_γ ≈ 24 keV Mössbauer resonant absorption line of ¹¹⁹Sn was measured at various temperatures and concentrations of substitutional Sn impurities in the α-phase of the Ag-Sn alloy. Whereas 1.0 and 2.0 at.% Sn alloys showed the expected temperature dependence of the line shift (LS), 4.0 and 8.0 at.% Sn alloys showed several anomalous features. After aging at room temperature (RT) for several months, these alloys showed hysteresis of the LS from RT up to about 500 K. At about 520 K, the unusual slope transforms to its expected value but continues with an increased isomer shift (IS). Further 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 after thermal cycling the high-concentration alloys are in an intermediate state below about 500 K, which becomes metastable at RT as shown by the changes with aging. The transition to the increased-IS state at T ≈ 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 order) that preserve the long-range order of the alloy.