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  • 1.
    Eklöf, Daniel
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Inorganic and Structural Chemistry.
    Fischer, A.
    Wu, Y.
    Scheidt, E. -W
    Scherer, W.
    Haussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Transport properties of the ii v semiconductor znsb2013In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 1, no 4, p. 1407-1414Article in journal (Refereed)
    Abstract [en]

    The intermetallic compound ZnSb is an electron poor (II-V) semiconductor with interesting thermoelectric properties. Electrical resistivity, thermopower and thermal conductivity were measured on single crystalline and various polycrystalline specimens. The work establishes the presence of impurity band conduction as an intrinsic phenomenon of ZnSb. The impurity band governs electrical transport properties at temperatures up to 300-400 K after which ZnSb becomes an intrinsic conductor. Furthermore this work establishes an inherently low lattice thermal conductivity of ZnSb, which is comparable to the state-of-the- art thermoelectric material PbTe. It is argued that the impurity band relates to the presence of Zn defects and the low thermal conductivity to the electron-poor bonding properties of ZnSb.

  • 2. Fischer, A.
    et al.
    Scheidt, E. -W.
    Scherer, W.
    Benson, D. E.
    Wu, Y.
    Eklöf, Daniel
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Haussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Thermal and vibrational properties of thermoelectric ZnSb: Exploring the origin of low thermal conductivity2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 22, article id 224309Article in journal (Refereed)
    Abstract [en]

    The intermetallic compound ZnSb is an interesting thermoelectric material largely due to its low lattice thermal conductivity. The origin of the low thermal conductivity has so far been speculative. Using multitemperature single crystal x-ray diffraction (9-400 K) and powder x-ray diffraction (300-725 K) measurements, we characterized the volume expansion and the evolution of structural properties with temperature and identified an increasingly anharmonic behavior of the Zn atoms. From a combination of Raman spectroscopy and first principles calculations of phonons, we consolidate the presence of low-energy optic modes with wave numbers below 60 cm(-1). Heat capacity measurements between 2 and 400 K can be well described by a Debye-Einstein model containing one Debye and two Einstein contributions with temperatures Theta(D) = 195 K, Theta(E1) = 78 K, and Theta(E2) = 277K as well as a significant contribution due to anharmonicity above 150 K. The presence of a multitude of weakly dispersed low-energy optical modes (which couple with the acoustic, heat carrying phonons) combined with anharmonic thermal behavior provides an effective mechanism for low lattice thermal conductivity. The peculiar vibrational properties of ZnSb are attributed to its chemical bonding properties, which are characterized by multicenter bonded structural entities. We argue that the proposed mechanism to explain the low lattice thermal conductivity of ZnSb might also control the thermoelectric properties of other electron poor semiconductors, such as Zn4Sb3, CdSb, Cd4Sb3, Cd13-xInyZn10, and Zn5Sb4In2-delta.

  • 3. Fischer, Andreas
    et al.
    Eklöf, Daniel
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Benson, Daryn E.
    Wu, Yang
    Scheidt, Ernst-Wilhelm
    Scherer, Wolfgang
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Synthesis, Structure, and Properties of the Electron-Poor II-V Semiconductor ZnAs2014In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 53, no 16, p. 8691-8699Article in journal (Refereed)
    Abstract [en]

    ZnAs was synthesized at 6 GPa and 1273 K utilizing multianvil highpressure techniques and structurally characterized by single-crystal and powder X-ray 7 diffraction (space group Pbca (No. 61), a = 5.6768(2) angstrom, b = 7.2796(2) angstrom, c = 7.5593(2) angstrom, Z = 8). The compound is isostructural to ZnSb (CdSb type) and displays multicenter bonded rhomboid rings Zn2As2, which are connected to each other by classical two-center, two-electron bonds. At ambient pressure ZnAs is metastable with respect to Zn3As2 and ZnAs2. When heating at a rate of 10 K/min decomposition takes place at similar to 700 K. Diffuse reflectance measurements reveal a band gap of 0.9 eV. Electrical resistivity, thermopower, and thermal conductivity were measured in the temperature range of 2-400 K and compared to thermoelectric ZnSb. The room temperature values of the resistivity and thermopower are similar to 1 Omega cm and +27 mu V/K, respectively. These values are considerably higher and lower, respectively, compared to Zn Sb. Above 150 K the thermal conductivity attains low values, around 2 W/m.K, which is similar to that of ZnSb. The heat capacity of ZnAs was measured between 2 and 300 K and partitioned into a Debye and two Einstein contributions with temperatures of theta(D) = 234 K, theta(E1) = 95 K, and theta(E2) = 353 K. Heat capacity and thermal conductivity of ZnSb and ZnAs show very similar features, which possibly relates to their common electron-poor bonding properties.

  • 4. Gebresenbut, Girma Hailu
    et al.
    Tamura, Ryuji
    Eklöf, Daniel
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Gomez, Cesar Pay
    Syntheses optimization, structural and thermoelectric properties of 1/1 Tsai-type quasicrystal approximants in RE-Au-SM systems (RE = Yb, Gd and SM = Si, Ge)2013In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 25, no 13, p. 135402-Article in journal (Refereed)
    Abstract [en]

    Yb-Cd (Tsai-type) quasicrystals constitute the largest icosahedral quasicrystal family where Yb can be replaced by other rare earth elements (RE) and Cd by pairs of p- and d-block elements. YbCd6 is a prototype 1/1 Tsai-type approximant phase which has a similar local structure to the Yb-Cd quasicrystal. In this study, the syntheses of Yb15.78Au65.22Ge19.00, Gd14.34Au67.16Ge18.5 and Gd14.19Au69.87Si15.94 Tsai-type 1/1 quasicrystal approximants are optimized using the self-flux technique. The crystal structures of the compounds are refined by collecting single crystal x-ray diffraction data. The structural refinements indicated that the compounds are essentially isostructural with some differences at their cluster centers. The basic polyhedral cluster unit in all the three compounds can be described by concentric shells of icosahedra symmetry and of disordered tetrahedra and/or a rare earth atom at the cluster center. Furthermore, the thermoelectric properties of the compounds are probed and their dimensionless figures of merit are calculated at different temperatures. A significant difference is observed in their thermoelectric properties, which could arise due to the slight difference in their crystal structure and chemical composition, as we move from Ge to Si and/or Gd to Yb. Therefore, this study shows the systematic effect of the chemical substitution of structurally similar materials on their thermoelectric properties.

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