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  Ryll, B.; Schmitz, A.; de Boor, J.; Franz, A.; Whitfield, P.S.; Reehuis, M.; Hoser, A.; Müller, E.; Habicht, K.; Fritsch, K.: Structure, Phase Composition, and Thermoelectric Properties of Yb_xCo₄Sb₁₂ and Their Dependence on Synthesis Method. ACS Applied Energy Materials 1 (2018), p. 113-122

10.1021/acsaem.7b00015
Open Accesn Version

Abstract:
We present a combined microscopic and macroscopic study of YbxCo4Sb12 skutterudites for a range of nominal filling fractions, 0.15 < x < 0.75. The samples were synthesized using two different methods - a melt−quench−annealing route in evacuated quartz ampoules and a non-equilibrium ball-mill route - for which we directly compare the crystal structure and phase composition as well as the thermoelectric properties. Rietveld refinements of high-quality neutron powder diffraction data reveal about a 30−40% smaller Yb occupancy on the crystallographic 2a site than nominally expected for both synthesis routes. We observe a maximum filling fraction of at least 0.439(7) for a sample synthesized by the ball-mill routine, exceeding theoretical predictions of the filling fraction limit of 0.2−0.3. A single secondary phase of CoSb2 is observed in ball-mill-synthesized samples, while two secondary phases, CoSb2 and YbSb2, are detected for samples prepared by the ampoule route. A detrimental influence of the secondary phases on the thermoelectric properties is observed for secondary-phase fractions larger than 8 wt % regardless of the kind of secondary phase. The largest figure of merit of all samples with a ZT ∼ 1.0 at 723 K is observed for the sample with a refined Yb content of x(2a) = 0.159(3), synthesized by the ampoule route.