Structure, Phase Composition and Thermoelectric Properties of YbxCo4Sb12 and Their Dependence on Synthesis Method

Kurzfassung

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 an about 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 723K is observed for the sample with a refined Yb content of x2a = 0.159(3), synthesized by the ampoule route.