Phonon dispersion in superionic copper selenide - observation of soft phonon modes in superionic phase transition.

This paper reports lattice dynamical measurements of Cu1.8Se superionic conductor in the superionic α-phase at ambient temperature. Measurements of phonon dispersion curves were performed with the new triple-axis spectrometer, TAIPAN, at the OPAL reactor. We found that TA [100], TA [111] and TA1 [110] phonon branches demonstrate a decrease in frequency at wavevectors q > 0.5 rather than the flattening observed previously. Results are compared with calculated density functional theoretical calculations showing the presence of unstable soft mode related to ordering of Cu atoms observed in Cu1.8Se at room temperature followed by α–β phase transition at a lower temperature. Superstructure arising from the ordering causes effects similar to the folding of the Brillouin zone, although phonon intensities at new Brillouin zone centres are weak. The coupling of low-energy phonon modes with displacement of mobile ions can explain the strong damping of phonons at q > 0.5 observed in the experiment