Pseudogap and anharmonic phonon behavior in Ba8Ga16Ge30: An NMR study

Ali A. Sirusi and Joseph H. Ross, Jr.

J. Chem. Phys. 145, 054702 (2016).

We have performed 69Ga, 71Ga, and 137Ba NMR on Ba8Ga16 Ge 30, a clathrate semiconductor which has been of considerable interest due to its large figure of merit for thermoelectric applications. In measurements from 4 K to 450 K, we used measurements on the two Ga nuclei to separate the magnetic and electric quadrupole hyperfine contributions and thereby gain information about the metallic and phonon behavior. The results show the presence of a pseudogap in the Ga electronic states within the conduction band, superposed upon a large Ba contribution to the conduction band. Meanwhile the phonon contributions to the Ga relaxation rates are large and increase more rapidly with temperature than typical semiconductors. These results provide evidence for enhanced anharmonicity of the propagative phonon modes over a wide range, providing experimental evidence for enhanced phonon-phonon scattering as a mechanism for the reduced thermal conductivity.

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