Structure, Phase Composition, and Thermoelectric Properties of Yb_xCo₄Sb₁₂ and Their Dependence on Synthesis Method

Abstract

We present a combined microscopic and macroscopic study of Yb x Co4Sb12 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.

Electronic Structure and Phase Stability of Yb-Filled CoSb3 Skutterudite Thermoelectrics from First-Principles

“…[66][67][68][69] In experiments, a phase of Yb x Co 4 Sb 12 with a maximum in Yb content is typically achieved without evidence of Yb ordering or separation into Yb-rich and Yb-poor phases; this phase often coexists with the binary impurity phases CoSb 2 and YbSb 2 . 42,44,[70][71][72][73] This behavior is qualitatively consistent with our computational findings. We note that samples whose preparation involves ball milling may exhibit a inhomogeneous Yb distribution due to non-equilibrium effects, however.…”

“…to explicitly indicate an empty void, the formula becomes 1−x R x Co 4 Sb 12 . We note that the upper limit of x, i.e., the filling fraction limit (FFL), is lower than 1 in practice, [41][42][43][44] but we consider the full crystallographic range of 0 ≤ x ≤ 1.…”

High thermoelectric figure of merit in well optimized YbyCo4Sb12

“…This measured FCL ylim = 0.41 is in fair agreement with the FCL ylim = 0.3 theoretically predicted 29,30 . Tang et al 25 showed that the FCL in YbyCo4Sb12 increases with the synthesis temperature from ylim ~ 0.1 at 773 K to ylim ~ 0.5 at 1073 K. It explains why in the literature, the reported FCL is ylim = 0.26 -0.29 when synthesized at 993 K 24,26,31 and why ylim = 0.41 at 1073 K can be found in the present work. Nonetheless, there is a +20 % discrepancy between the value ylim = 0.48 determined from EPMA data in samples synthesized at 1073 K by authors of ref.…”

“…The heavier atomic mass and the larger filling concentration limit (FCL) of the Yb atoms indeed lead to lower values of the lattice thermal conductivity (λL = 1.7 -1.8 W m -1 K -1 at 300 K in Yb0.2-0.3Co4Sb12) [24][25][26][27] than in Ba0.25Co4Sb12 (λL = 3.0 W m -1 K -1 at 300 K) 28 and in Ce0.15Co4Sb12 (λL = 2.3 W m -1 K -1 at 300 K) 3,4 , respectively. The FCL of Yb in YbyCo4Sb12 had been theoretically predicted to be ylim = 0.3 29,30 but was latter experimentally revised to the nonconsensual values ylim = 0.26 24 , ylim = 0.29 26,31 or ylim = 0.48 25,27 , depending on the synthesis temperature 25 or method 27,31 . The large Seebeck coefficient occurring in AyCo4Sb12 has been related to a large density of states effective mass, which remains independent on the nature of the filler atom 32 and increases with the electron concentration [32][33][34] from ~ 1.5 to 3.5me (me is the bare electron mass).…”

A review of CoSb3-based skutterudite thermoelectric materials

“…Although the TE performance of CoSb 3 has been greatly improved by the single atom filling [34,74,75,82,84,86,88,[90][91][92][93][94][95][96][97][98][99][100][101][102][103], the TE properties of single-filled CoSb 3 skutterudites can be further improved by doping other elements at Co or Sb sites [111,116,117]. Recently, Trivedi et al [117] found that the ZT value of single element Dy-filled CoSb 3 skutterudites can be significantly enhanced by the microstructure and doping effect of Ni.…”