Microscopic view of disorder in the ordered phase of superionic RbAg4I5

Mössbauer spectroscopy in ${}_{}{}^{129}{}_{}{}^{}\mathrm{I}$ was employed to verify the existence and investigate the properties of local disorder in the "ordered" state of the superionic conductor Rb${\mathrm{Ag}}_4$ ${\mathrm{I}}_5$. Studies were conducted in the temperature range of 4-180 K covering the low-temperature $\gamma$ phase ($T<\mathrm{}122\mathrm{}$ K) and part of the high-conductivity, disordered $\beta$ phase. The absorption spectra at all temperatures were analyzed with two iodine sites, characterized by their quadrupole splitting and isomer-shift values and a third site with a nonsplit single line. It was found that the electric field gradient (efg) at the ${}_{}{}^{129}{}_{}{}^{}\mathrm{I}$ nucleus, produced by the neighboring Ag ions, has a continuous linear temperature dependence down to 4.2 K. The hyperfine constants and the Debye-Waller factor showed no discontinuity at $T_c$. The anomalous existence of the single line has been attributed to a fast fluctuating efg due to local hopping of the Ag ions. This dynamic disorder is present even at low temperatures where (8-10)% of Ag ions are locally hopping, and its occurrence increases drastically at $T>\mathrm{}{T}_c$. A nonharmonic Debye-Waller factor was observed from which a linear temperature-dependent Debye temperature could be derived.