Copper selenide: soft phonon modes and superionic phase transition

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Australian Institute of Physics
This paper reports lattice dynamical measurements of Cu1.8Se superionic conductor having structure of the superionic α-phase at ambient temperature. Cu2δSe is a mixed ionic-electronic conductor with a superionic transition at 414K in stoichiometric compound Cu2Se. At room temperature the superionic α -phase exists in the composition range from δ = 0.15 to 0.25. The important features of the Cu1.8Se compound is the ordering of Cu atoms observed at ambient temperature [1] which is described as “disordered” α- phase in the literature and presence of low-energy transverse acoustic (TA) modes [2]. Measurements of phonon dispersion curves were performed with the new triple-axis spectrometer, TAIPAN, at the OPAL reactor [3]. We found that TA [100], TA [111] and TA1 [110] phonon branches demonstrate a decrease in frequency at wavevectors q/qm > 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 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/qm > 0.5.
Copper selenides, Copper, Selenides, Ambient temperature, Crystal lattices, Crystal structure, Stoichiometry, OPAL Reactor, Phonons, Ions
Danilkin, S. A., Yethiraj, M., Kearley, G. J. (2010). Copper selenide: soft phonon modes and superionic phase transition. Paper presented to the 34th Annual Condensed Matter and Materials Meeting 2010, Waiheke Island Resort, Waiheke, Auckland, New Zealand 2 - 5 February 2010. Retrieved from: