Enhanced thermoelectric performance of heavy-fermion compounds YbTM2Zn20 (TM = Co, Rh, Ir) at low temperatures

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Science Advances  31 May 2019:
Vol. 5, no. 5, eaaw6183
DOI: 10.1126/sciadv.aaw6183


  • Fig. 1 Crystal structure and partial structures of YbTM2Zn20.

    (A) Single crystals of YbCo2Zn20 synthesized by flux growth method. (B) [111] directional view of the unit cell of YbTM2Zn20, suggesting kagome lattice formed by Yb and TM atoms. (C) Polyhedron representation of the unit cell. The coordination between Yb atoms and the TM cages with (D) Yb as the center and (E) TM cage as the center. (F) Frank-Kasper polyhedron formed by Yb and Zn atoms.

  • Fig. 2 Temperature-dependent Seebeck coefficient of YbCo2Zn20 (circle), YbRh2Zn20 (up-triangle), and YbIr2Zn20 (down-triangle).

    The dashed line indicates S = 0, and the solid line is an extrapolation of the S of YbIr2Zn20 at higher temperature. Inset: Temperature-dependent electrical resistivity of YbCo2Zn20 (circle), YbRh2Zn20 (up-triangle), and YbIr2Zn20 (down-triangle).

  • Fig. 3 Temperature-dependent heat capacity of YbTM2Zn20.

    (A) YbCo2Zn20. (B) YbRh2Zn20. (C) YbIr2Zn20. Solid lines representing the fitting using a combined Debye-Einstein model: CP=CD(TθD)30θD/Tx4ex(ex1)2dx+CE(θET)2eθE/T(eθE/T1)2, where CD and CE are constants containing numbers of oscillators and degrees of freedom, respectively (28). The insets show Cp/T versus T2 data at low temperatures, with the solid lines representing the Cp/T = γ + βT2 relation.

  • Fig. 4 Temperature-dependent thermal conductivity of YbTM2Zn20.

    Temperature-dependent thermal conductivity (A) and lattice thermal conductivity (B) for YbCo2Zn20 (circle), YbRh2Zn20 (up-triangle), and YbIr2Zn20 (down-triangle). The solid lines are fits based on Eqs. 1 and 2 described in the text.

  • Fig. 5 Temperature-dependent power factor and thermoelectric figure of merit of YbTM2Zn20.

    Temperature-dependent (A) PF and (B) ZT of YbCo2Zn20 (circle), YbRh2Zn20 (up-triangle), and YbIr2Zn20 (down-triangle). The insets in (A) and (B) are the PF and ZT values of several well-known thermoelectric materials at 35 K plotted together with parent compounds.


  • Table 1 Structural diameters from single-crystal XRD and fitting parameters of κL using the Debye model as described in the text.

    The results from fitting Cp based on the combined Debye-Einstein model are in parentheses.

    Yb Wyckoff site8a8a8a
    Zn1 Wyckoff site16c16c16c
    Zn2 Wyckoff site96g96g96g
    Yb-Zn1 (Å)3.0299(1)3.0633(1)3.0662(1)
    Yb-Zn2 (Å)3.0873(4)3.0994(3)3.0928(4)
    Zn1-Zn2 (Å)2.9859(3)3.0075(3)3.0059(4)
    Zn2-Zn2 (Å)2.6010(4)/
    Cage dimension (Å)5.9113(4)5.9549(4)5.9515(5)
    Uiso of Yb (Å)0.00781(8)0.00688(9)0.00881(10)
    θD (K)239(265)228(250)224(240)
    v (m/s)391938263813
    A (10−42 s3)8.812.417.5
    B (10−18 s·K−1)
    C (1033 s−3)7.217.616
    ω0 (THz)
    θE (K)72(76)91(95)115(110)

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