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 iodo-molybdate 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 abovementioned mechanical response. Two different measuring methods, a quasi-adiabatic 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 EXAFS technique, and with the behavior of other ionic glasses.