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B4: Thermal management and dynamics
Copper selenide attracted particular interest due to a high ionic conductivity and highest value of thermoelectric figure of merit among the bulk materials [1, 2]. The characteristic feature of lattice dynamics of Cu2-xSe compounds is the presence of low-energy phonon modes similar to observed in other fast ionic conductors.
The phonon dispersion curves in Cu1.85Se and Cu1.96Se were measured by neutron spectroscopy at temperatures 300 K and 450 K. It has been found that TA [100], TA [111] and TA1 [110] acoustic branches demonstrate an optic-like behaviour and strong broadening of phonon peaks at wavevectors q> 0.5. Experimental results are compared with density functional theoretical calculations showing the presence of unstable soft modes. These modes are related to ordering of Cu atoms observed in α-Cu1.8Se at 300 K 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 explains the strong broadening of acoustic phonons at q > 0.5 and optic modes in the energy range 10-15 meV related mostly to Cu vibrations. Observed strong phonon damping may be responsible for the low values of lattice component of thermal conductivity in this material.
References
1. H Liu, et al. Nature Materials, http://dx.doi.org/10.1038/nmat3273
2. X. Xing-Xing, et al. Chin. Phys. B Vol. 20, No. 8 (2011) 087201