Flattening of the acoustic phonon branches in the clathrate Ba₈Ga₁₆Ge₃₀

In the search for high-performance thermoelectric materials, phonon-glass electron crystal (PGEC) materials such as clathrates have drawn attention due to having both glass-like low phonon thermal conductivity and crystal-like high electrical conductivity [1]. Ba₈Ga₁₆Ge₃₀ (BGG) has a guest Ba atom trapped inside Ga/Ge cage structures is known for avoided crossings between acoustic phonons and the flat guest atom modes proposed to be the source of the low lattice thermal conductivity [2,3]. Ga/Ge site disorder with Ga and Ge exchanging places in different unit cells has also been reported [4]. We used time-of-flight neutron scattering to measure the complete phonon spectrum in a large single crystal of BGG and compared these results with predictions of density functional theory. Calculations assuming the structure where Ga/Ge atoms occupy their nominal sites, as well as a disordered configuration showed that the latter agrees much better with the data. Disorder strongly reduces phonon group velocities despite nearly identical masses of Ga and Ge, which accounts for exceptionally low thermal conductivity. Our work points at a new path towards optimizing thermoelectrics.

References:

[1] Slack, G. A. CRC Handbook of Thermoelectrics Vol. 407 (1995).

[2] J. Dong et al., Phys. Rev. Lett. 86, 2361–2364 (2001).

[3] M. Christensen et al., Nature Materials 7, pages 811–815 (2008).

[4] A. Bentien et al., Phys. Rev. B 71, 144107 (2005).