Thermal expansion of silver iodide-silver molybdate glasses at low temperatures

Ionic glasses obtained combining silver iodide and silver molybdate are characterized by quite low values of the glass transition temperature $Tg$ around 320–350 K, by high values of the dc ionic conductivity even at room temperature and by a peculiar behavior of the mechanical response at ultrasonic frequencies. In fact, at temperatures well below their glass transition temperature, these glasses exhibit an intense peak of acoustic attenuation well described by two different and almost overlapping relaxational contributions. Considering also that negative thermal expansion has been reported for some molybdate crystalline compounds, we have investigated in this work the thermal expansion of two silver iodomolybdate glasses $(AgI)1−x(Ag2MoO4)x$ for $x=0.25$ and $x=0.33$ in a wide temperature range (4.2–300 K) from cryogenic temperatures up to some 20 K below $Tg$ using a precision capacitance dilatometer aiming to understand whether the expansivity shows some possible fingerprint corresponding to the above-mentioned mechanical response. Two different measuring methods, a quasiadiabatic and a continuous one, have been used for the thermal expansion measurements. The results are discussed in comparison with the information obtained from previous investigations based on the extended x-ray absorption fine structure (EXAFS) technique and with the behavior of other ionic glasses.