Cage-size control of guest vibration and thermal conductivity in Sr8 Ga16 Si30-x Gex

We present a systematic study of thermal conductivity, specific heat, electrical resistivity, thermopower, and x-ray diffraction measurements performed on single-crystalline samples of the pseudoquaternary type-I clathrate system Sr8 Ga16 Si30-x Gex, in the full range of 0≤x≤30. All the samples show metallic behavior with n -type majority carriers. However, the thermal conductivity and specific heat strongly depend on x. Upon increasing x from 0 to 30, the lattice parameter increases by 3%, from 10.446 to 10.726 Å, and the localized vibrational energies of the Sr guest ions in the tetrakaidecahedron (dodecahedron) cages decrease from 59 (120) K to 35 (90) K. Furthermore, the lattice thermal conductivity at low temperatures is largely suppressed. In fact, a crystalline peak found at 15 K for x=0 gradually decreases and disappears for x≥20, evolving into the anomalous glasslike behavior observed for x=30. It is found that the increase of the free space for the Sr guest motion directly correlates with a continuous transition from quasi-on-center harmonic vibration to off-center anharmonic vibration, with consequent increase in the coupling strength between the guest's low-energy modes and the cage's acoustic phonon modes.