Impurity-induced local disorder in the ordered state of the superionic conductorβ-Ag2Se

Mössbauer studies of ${}_{}{}^{119}{}_{}{}^{}\mathrm{Sn}$ impurities dissolved in the superionic conductor ${\mathrm{Ag}}_2$Se show a dramatic decrease in the resonance absorption intensity 56 K below the normal superionic transition temperature $T_c=406\mathrm{}$ K. This effect is found to be independent of impurity concentration (0.5-2 at.%) and is reversible with temperature. A mechanism is suggested which relates this new phenomenon to the onset of a local order-disorder phase transition brought about by the fast hopping of ${\mathrm{Ag}}^{+}$ ions surrounding the impurity Sn atom. Differential thermal analysis of the same samples showed that this "local melting" does not affect the transition temperature of the bulk. The present observations demonstrate that local disorder induced by the Sn impurity coexists with the bulk ordered phase far below $T_c$.