Cage-size control of guest vibration and thermal conductivity in Sr8Ga16Si30-xGex

Abstract

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 Sr8Ga16Si30-xGex, 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 A, and the localized vibrational energies of the Sr guest ions in the tetrakaidekahedron (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 glass-like 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 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.

7 pages, 7 figures, submitted to PRB

References in corpus (5)

• Glass-Like Heat Conduction in High-Mobility Crystalline Semiconductors
• Glasslike vs. crystalline thermal conductivity in carrier-tuned Ba8Ga16X30 clathrates (X = Ge, Sn)
• Four-well tunneling states and elastic response of clathrates
• Possible mechanism for achieving glass-like thermal conductivities in crystals with off-center atoms
• Tunable charge carriers and thermoelectricity of single-crystal Ba8Ga16Sn30

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