H Fukuoka

Publications (6)3.69 Total impact

• Source

  Available from: Tessui Nakagawa

  Article: Simultaneous structure and carrier tuning of dimorphic clathrate Ba 8 Ga 16 Sn 30

  K Suekuni, M A Avila, K Umeo, H Fukuoka, S Yamanaka, T Nakagawa, T Takabatake
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  ABSTRACT: We report structural, transport, and thermal properties of carrier-tuned Ba 8 Ga 16 Sn 30 single crystals with the type-1 clathrate structure phase, demonstrating that Ba 8 Ga 16 Sn 30 is a unique thermoelectric clathrate material wherein both the structure type and the carrier type are tunable. The results are compared with the properties of the better known type-8 structure phase and of A 8 Ga 16 Ge 30 A =Sr,Eu. Differential thermal analysis and powder x-ray diffraction show that both phases are stable up to their virtually identical melting point of 520 3 ° C. Refinements of single-crystal x-ray diffraction data indicate that the Ba2 guest ion in the tetrakaidecahedron occupies the off-center 24k sites which are 0.43– 0.44 Å away from the centered 6d site. The temperature-linear coefficient of the specific heat is 29 mJ/ mol K 2 for both n-and p-type carriers in the phase, four times larger than that for the phase, suggesting contributions from tunneling of the Ba2 guest ions between off-center minima. Analysis of specific heat with a soft-potential model SPM gives a charac-teristic energy of 20 K for the Ba2 vibration, significantly lower than 50 K for the phase and in fact the lowest among type-1 clathrates. The lattice thermal conductivities L for the phase with both charge carrier types are very similar and show a glasslike temperature dependence. This behavior in L T is also described using SPM analysis, where it is found that the coupling strength between guest modes and acoustic phonons for the phase is significantly larger than that for Sr 8 Ga 16 Ge 30 .
  Physical Review B 06/2008; 77(23):235119. · 3.69 Impact Factor
• Source

  Available from: arxiv.org

  Article: Glasslike vs. crystalline thermal conductivity in carrier-tuned Ba8Ga16X30 clathrates (X = Ge, Sn)

  M. A. Avila, K. Suekuni, K. Umeo, H Fukuoka, S Yamanaka, T Takabatake
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  ABSTRACT: The present controversy over the origin of glasslike thermal conductivity observed in certain crystalline materials is addressed by studies on single-crystal x-ray diffraction, thermal conductivity k(T) and specific heat Cp(T) of carrier-tuned Ba8Ga16X30 (X = Ge, Sn) clathrates. These crystals show radically different low-temperature k(T) behaviors depending on whether their charge carriers are electrons or holes, displaying the usual crystalline peak in the former case and an anomalous glasslike plateau in the latter. In contrast, Cp(T) above 4 K and the general structural properties are essentially insensitive to carrier tuning. We analyze these combined results within the framework of a Tunneling/Resonant/Rayleigh scatterings model, and conclude that the evolution from crystalline to glasslike k(T) is accompanied by an increase both in the effective density of tunnelling states and in the resonant scattering level, while neither one of these contributions can solely account for the observed changes in the full temperature range. This suggests that the most relevant factor which determines crystalline or glasslike behavior is the coupling strength between the guest vibrational modes and the frameworks with different charge carriers. Comment: 8 pages, 4 figures, 4 tables, submitted to Phys. Rev. B
  06/2006;
• Article: Electronic structure and vibrational properties of Ba8Si46,Ba8AgnSi46-n , and Ba8AunSi46-n

  J. S. Tse, T. Iitaka, T. Kume, H. Shimizu, K. Parlinski, H. Fukuoka, S. Yamanaka
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  ABSTRACT: The electronic and vibrational structures of Ba8Si46 and Ba8MnSi46-n , where some of the framework Si were replaced by metal ( M=Ag or Au) have been studied. Theoretical calculations show that the nature of chemical bonding of the substituted clathrates and their vibrational properties are fundamentally different from Ba8Si46 . Low-frequency vibrations associated with the motions of Ba in the large cages and the framework metals have been identified from their Raman spectra. A surprisingly large contribution of the low-frequency modes, particularly those which arise from the Ba vibrations in the large cages, to the electron-phonon coupling parameter in Ba8Si46 was found.
  Physical Review B - PHYS REV B. 01/2005; 72(15).
• Source

  Available from: ArXiv

  Article: Structural, transport, and thermal properties of single crystalline type-VIII clathrate Ba8Ga16Sn30

  D. Huo, T Sakata, T. Sasakawa, M. A. Avila, M. Tsubota, F. Iga, H Fukuoka, S Yamanaka, S Aoyagi, T Takabatake
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  ABSTRACT: We report the electrical resistivity, Hall coefficient, thermoelectric power, specific heat, and thermal conductivity on single crystals of the type-VIII clathrate Ba8Ga16Sn30 grown from Sn-flux. Negative S and R_H over a wide temperature range indicate that electrons dominate electrical transport properties. Both rho(T) and S(T) show typical behavior of a heavily doped semiconductor. The absolute value of S increases monotonically to 243 uV/K with increasing temperature up to 550 K. The large S may originate from the low carrier concentration n=3.7x10^19 cm^(-3). Hall mobility u_H shows a maximum of 62 cm^2/Vs around 70 K. The analysis of temperature dependence of u_H suggests a crossover of dominant scattering mechanism from ionized impurity to acoustic phonon scattering with increasing temperature. The existence of local vibration modes of Ba atoms in cages composed of Ga and Sn atoms is evidenced by analysis of experimental data of structural refinement and specific heat, which give an Einstein temperature of 50 K and a Debye temperature of 200 K. This local vibration of Ba atoms should be responsible for the low thermal conductivity (1.1 W/m K at 150 K). The potential of type-VIII clathrate compounds for thermoelectric application is discussed. Comment: 6 pages, 6 figures, submitted to Phys. Rev. B
  09/2004;
• Article: Structural, transport, and thermal properties of the single-crystalline type-VIII clathrate Ba_ {8} Ga_ {16} Sn_ {30}

  D. Huo, T. Sakata, T. Sasakawa, M. A. Avila, M. Tsubota, F. Iga, H. Fukuoka, S. Yamanaka, S. Aoyagi, T. Takabatake
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  ABSTRACT: We report the electrical resistivity ρ, Hall coefficient RH, thermoelectric power S, specific heat C, and thermal conductivity κ on single crystals of the type-VIII clathrate Ba8Ga16Sn30 grown from Sn-flux. Negative S and RH over a wide temperature range indicate that electrons dominate electrical transport properties. Both ρ(T) and S(T) show typical behavior of a heavily doped semiconductor. The absolute value of S increases monotonically to 243 μV∕K with increasing temperature up to 550 K. The large S may originate from the low carrier concentration n=3.7×1019 cm−3. Hall mobility μH shows a maximum of 62 cm2∕V s around 70 K. The analysis of temperature dependence of μH suggests a crossover of a dominant scattering mechanism from ionized impurity to acoustic phonon scattering with increasing temperature. The existence of local vibration modes of Ba atoms in cages composed of Ga and Sn atoms is evidenced by analysis of experimental data of structural refinement and specific heat, which give an Einstein temperature of 50 K and a Debye temperature of 200 K. This local vibration of Ba atoms should be responsible for the low thermal conductivity (1.1 W∕m K at 150 K). The potential of type-VIII clathrate compounds for thermoelectric application is discussed.
  Phys. Rev. B. 71(7).
• Article: Glasslike versus crystalline thermal conductivity in carrier-tuned Ba_ {8} Ga_ {16} X_ {30} clathrates (X= Ge, Sn)

  M. A. Avila, K. Suekuni, K. Umeo, H. Fukuoka, S. Yamanaka, T. Takabatake
  [show abstract] [hide abstract]
  ABSTRACT: The present controversy over the origin of glasslike thermal conductivity observed in certain crystalline materials is addressed by studies on single-crystal x-ray diffraction, thermal conductivity κ(T), and specific heat Cp(T) of carrier-tuned Ba8Ga16X30 (X=Ge,Sn) clathrates. These crystals show radically different low-temperature κ(T) behaviors depending on whether their charge carriers are electrons or holes, displaying the usual crystalline peak in the former case and an anomalous glasslike plateau in the latter. In contrast, Cp(T) above 4 K and the general structural properties are essentially insensitive to carrier tuning. We analyze these combined results within the framework of tunneling scattering, resonant scattering, and Rayleigh scattering models, and conclude that the evolution from crystalline to glasslike κ(T) is accompanied by an increase both in the effective density of tunneling states and in the resonant scattering level, while neither one of these contributions can solely account for the observed changes in the full temperature range. This suggests that the most relevant factor that determines crystalline or glasslike behavior is the coupling strength between the guest vibrational modes and the frameworks with different charge carriers.
  Phys. Rev. B. 74(12).
• Article: Interplay between thermoelectric and structural properties of type-I clathrate K_ {8} Ga_ {8} Sn_ {38} single crystals

  T. Tanaka, T. Onimaru, K. Suekuni, S. Mano, H. Fukuoka, S. Yamanaka, T. Takabatake
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  ABSTRACT: We report structural, transport, and thermal properties of type-I clathrate K8Ga8Sn38 single crystals grown by the self-flux method. Single-crystal x-ray diffraction analysis confirmed that the guest K+ ion locates on the center in the tetrakaidecahedron composed of Ga and Sn atoms. The thermopower is largely negative, −200 μV/K at room temperature, irrespective of the flux used during growth (Ga or Sn). The thermal conductivity κ(T) exhibits a large peak at 14 K. These observations in K8Ga8Sn38 are contrasting with the splitting of the guest site and the glasslike behavior in κ(T) reported for type-I Ba8Ga16Sn30 although the free space for the guest is almost the same in both compounds. The electrostatic potential for the tetrakaidecahedron was calculated using the occupation probabilities of Ga ions in the three sites on the cage. It is found that the off-centered state is stabilized for the Ba2+ ions in Ba8Ga16Sn30 by the partial occupation of Ga anion in the 16i site while the on-center state for K8Ga8Sn38 is stabilized by the strongly preferred occupation of Ga anions in the 6c site. We conclude that the charge distribution on the cage is crucial for the splitting of the guest site into off-center positions in the tetrakaidecahedron of the type-I clathrate.
  Phys. Rev. B. 81(16).