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Phonon-Glass Electron Crystal (PGEC) materials such as clathrates have both glass-like low phonon thermal conductivity and crystal-like high electrical conductivity [1]. This property is of interest due to potential applications in thermoelectric devices. Ba₈Ga₁₆Ge_30, one of the clathrate compounds, has guest atom Ba trapped inside the Ga/Ge cage structure. There are theoretical [2] as well as experimental investigations on the avoided crossings of acoustic phonons and the flat guest atom modes, which is presumed to be the reason behind its low lattice thermal conductivity [1]. We measured a complete phonon spectrum in several hundred Brillouin zones using inelastic time-of-flight neutron scattering to better understand the origin of low thermal conductivity of Ba₈Ga₁₆Ge₃₀. Analysis of the data using the Phonon Explorer software revealed that slope of the longitudinal acoustic branch along the [h 0 0] direction is anomalously reduced and close to that of the transverse acoustic mode, which will decrease the sound velocity and, as a consequence, reduce the thermal conductivity.

[1] M. Christensen et al, Nature Materials 7, 811–815 (2008).
[2] J. Dong et al, Phys. Rev. Lett. 86, 2361–2364 (2001).