Lattice instability and elastic dispersion due to the rattling motion in the type-I clathrate Ba_8Ga_<16>Sn_<30>

Ishii Isao

Suetomi Yasuhiko

Fujita Takahiro K.

Suekuni Koichiro

Tanaka Tomoo

Takabatake Toshiro

Suzuki Takashi

Marcos A. Avila

Physical Review B

To investigate the off-center rattling motion and its charge-carrier dependence in type-I clathrate compounds, we carried out ultrasonic measurements on type-I Ba8Ga16Sn30 and a reference compound, K8Ga8Sn38. We found elastic softening of C44 originating from a lattice instability due to the off-center rattling motion of Ba atom in Ba8Ga16Sn30. Elastic softening below 1 K suggests that the lattice instability remains at very low temperatures. We also found ultrasonic dispersion which has no mode selectivity. No-mode-selective ultrasonic dispersion in Ba8Ga16Sn30 would be caused by a strong electron-phonon coupling. No charge-carrier dependence is observed between n-type and p-type Ba8Ga16Sn30. The significant softening on the bulk modulus in Ba8Ga16Sn30 contrasts to the continuous hardening in K8Ga8Sn38, indicating the central role of the rattling motion in the softening.

Physics [ 420 ]

eng

American Physical Society

(c) 2012 American Physical Society

[ISSN] 1098-0121

[DOI] 10.1103/PhysRevB.85.085101

[NCID] AA11187113

[DOI] http://dx.doi.org/10.1103/PhysRevB.85.085101