Microscopic view of disorder in the ordered phase of superionic RbAg4I5

Mössbauer spectroscopy in I129 was employed to verify the existence and investigate the properties of local disorder in the "ordered" state of the superionic conductor RbAg4I5. Studies were conducted in the temperature range of 4-180 K covering the low-temperature phase (T<122 K) and part of the high-conductivity, disordered 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 I129 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 Tc. 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>Tc. A nonharmonic Debye-Waller factor was observed from which a linear temperature-dependent Debye temperature could be derived.