Off-center rattling modes and glasslike thermal conductivity in the type-I clathrate Ba₈Ga₁₆Sn₃₀

Type-I clathrate Ba₈Ga₁₆Sn₃₀ is unique for showing glasslike behavior in the lattice thermal conductivity κL irrespective of the charge carrier type. For better understanding the relation between this behavior and guest rattling, polarized Raman-scattering measurements have been performed on carrier-tuned single crystals. The appearance of a symmetry-forbidden mode in the Eg symmetry spectrum indicates that the Ba atoms are rotating among the off-center positions in the tetrakaidecahedron. On cooling from 300 to 4 K, the energies of Eg and T2g guest modes decrease by 15% and 25%, respectively. This drastic decrease originates from the strongly anharmonic potential with a quartic term of the atomic displacement. The energy of 14 cm^-1 for the T2g mode at 4 K is the lowest among intermetallic clathrates. These results for Ba₈Ga ₁₆Sn₃₀ are compared with those for type-I Eu ₈Ga₁₆Ge₃₀ and Sr₈Ga ₁₆Ge₃₀ whose κL (T) show similar glasslike behavior with a plateau. The comparison between three systems indicates that the guest rattling energy is proportional to the temperature range of the plateau in κL (T).