Structural Complexity and High Thermoelectric Performance of the Zintl Phase: Yb₂₁Mn₄Sb₁₈

Abstract

Thermoelectric materials are a unique class of compounds that can recycle energy through conversion of heat into electrical energy. A new 21–4–18 Zintl phase has been discovered in the Yb–Mn–Sb system with high performance in the mid-to-high temperature regime. The efficiency of the Yb21Mn4Sb18 results mainly from its large Seebeck coefficient (∼290 μV K–1 at 650 K) and extremely low thermal conductivity (∼0.4 W m–1 K–1). The complex crystal structure has been studied through single crystal X-ray diffraction, synchrotron powder X-ray diffraction, and pair distribution function (PDF) analysis using time-of-flight neutron diffraction revealing positional disorder on several sites. Electronic structure calculations of the band structure and the partial spin-density of states reveal that states near the Fermi level are contributed mostly by the Mn and Sb atoms that participate in the [Mn4Sb10]22– motif of the structure. The band structure confirms the p-type semiconducting nature of this material. The optimization of the hole carrier concentration was tuned according to a single parabolic band model through Na doping on the Yb site (Yb21–x Na x Mn4Sb18, x = 0, 0.2, 0.4) showing an improvement in zT over the whole temperature range. A maximum zT ≈ 0.8 at 800 K is obtained for the x = 0.4 sample and increases the ZTavg from 0.34 to 0.49 (over the entire temperature range) compared to the undoped sample.

Crystal structure characterization and electronic structure of a rare-earth-containing Zintl phase in the Yb–Al–Sb family: Yb3AlSb3

“…All materials were handled in an argon-filled dry box with <1 ppm H 2 O. The crystals were small, black, and reflective, and were discovered serendipitously from attempts at growing an Al analog of Yb 21 Mn 4 Sb 18 (He et al, 2019). Stoichiometric amounts of Yb pieces (Metall Rare Earth Ltd, 99.99%), Al shots (Alfa Aesar, 99.999%), Sb shots (Alfa Aesar, 99.999%), and Pb shots (Alfa Aesar, 99.99%) were loaded with the ratio Yb 21 Al 4 -Sb 18 Pb 172 , for a total of 5 g, into an alumina crucible set (5 ml) (Canfield et al, 2016).…”

Manipulation of Defects for High‐Performance Thermoelectric PbTe‐Based Alloys

“…This leads to an improvement in electronic performance (power factor, PF ¼ S 2 /ρ). Such a strategy has realized significant zT breakthroughs in a variety of thermoelectric materials including Mg 2 Si, [5,6] CoSb 3 , [7] half-Heusler, [8,9] IV-VI semiconductors, [3,[10][11][12][13][14][15][16][17] Zintl phases, [18][19][20][21] and elemental Te. [4] Alternatively, κ L is the only independent parameter determining zT, and thereby κ L minimization becomes an equally important strategy for zT enhancements.…”

The impact of site selectivity and disorder on the thermoelectric properties of Yb21Mn4Sb18 solid solutions: Yb21Mn4−xCdxSb18 and Yb21−yCayMn4Sb18

“…14 Sodium doping improves the ZT avg and brings the thermoelectric compatibility factor(s) of the 21-4-18 phase closer to other top performing thermoelectric materials making it more desirable for practical applications. 12 Since thermoelectric devices depend on segmentation of different materials, maximal efficiencies are only achieved if the compatibility factors of said materials are similar. Building on our previous work, herein we present the structure and thermoelectric properties of the Yb 21 Mn 4-x Cd x Sb 18 (x = 0, 0.5, 1.0, 1.5) and Yb 21-y Ca y Mn 4 Sb 18 (y = 3, 6, 9, 10.5) solid solutions.…”

“…Yb 21 Mn 4 Sb 18 is a half metallic compound with Mn 2+ having a half filled d band. 12 Cd was chosen as it has a filled d orbital and as such was expected to remove the half metallicity to improve the electrical transport. Ca substitution was expected to increase electron donation to the clusters and thereby increase the Seebeck coefficients further.…”

“…We believe that the Cd site preference for the terminal sites of the tetramer unit arise from their intrinsically softer, longer bonds. The terminal sites are directly next to an octahedrally coordinated Yb atom (Yb11) which has previously been shown to exhibit rattling and disorder 12 Substituting Ca on the rare earth site (Yb) did not result in any site preference (see Figure 3b). Overall, the Ca atoms are spread fairly evenly on all of the 11 sites which is probably due to the similarity in their 6-coordinate ionic radii (Ca: 1.00 Å vs. Yb: 1.02 Å) and bonding.…”