Raman spectroscopic evidence of a continuous order-disorder transition in superionic α-AgI

A careful determination of the temperature dependence of the integrated Raman scattering intensity and spectral shape of superionic AgI is reported from 150 to 500^∘{}C. The Raman intensity is found to vary according to the law expected for first-order scattering up to 320^∘{}C; for higher temperatures the intensity decreases by more than one order of magnitude over a temperature range of about 60^∘{}C. Currently a temperature dependence of the Raman normalized spectral shape was also observed. The data are interpreted on the basis of the possibility of an order-disorder-like transition, which takes place in the same temperature region in which specific-heat anomalies have been reported.