Nanoscopic fluctuational dynamic model of anomalous temperature and concentration dependences in the Sn Mössbauer isomer shift in Ag-Sn alloys

A nanoscopic fluctuational dynamic model is suggested for observed anomalous reversible changes of the M?ssbauer isomer shift (ARCOMIS) found recently in Ag-Sn alloys not affected by long ageing. The model considers fluctuation-induced nanoscale large transient atomic displacements (LTADs) and material disordering. They cause electron localization, taking place simultaneously and permanently in the nanometre vicinities of M?ssbauer impurity atoms in the entire alloy. This causes an additional enhancement in the electron charge density |?f(0)|2 at the M?ssbauer nuclei, leading to the ARCOMIS at temperature Ts. These phenomena are generated by persistent sequences of nanoscale short-lived (picosecond) large energy fluctuations (SLEFs) of atomic particles of the peak thermal energy 0p>?E>>kT, which occur simultaneously and constantly in the material. The model leads to the following results. (i)?A SLEF-mediated dynamic mechanism for the ARCOMIS occurring at relatively low temperatures Ts is proposed. (ii)?The temperature Ts(Ag-Sn)?500?K is calculated, in good agreement with observations. (iii)?The narrow temperature interval ?T 3% is suggested. (vi)?An explanation is offered for the absence of the ARCOMIS in SnO2 material. (vii)?Similar results are obtained for the Au-Sn alloys not affected by long ageing, in good agreement with tentative experimental data.