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 ${\mathrm{Sr}}_8{\mathrm{Ga}}_{16}{\mathrm{Si}}_{30-x}{\mathrm{Ge}}_x$, 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 $10.446\text{to}10.726\mathrm{\AA }$, and the localized vibrational energies of the Sr guest ions in the tetrakaidecahedron (dodecahedron) cages decrease from $59\left(120\right)\mathrm{K}\text{to}35\left(90\right)\mathrm{K}$. Furthermore, the lattice thermal conductivity at low temperatures is largely suppressed. In fact, a crystalline peak found at $15\mathrm{K}$ 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.

• Received 23 December 2006

DOI:https://doi.org/10.1103/PhysRevB.75.195210