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

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–xNaxMn4Sb18, 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.