Magnesium silicide and related alloys are attractive for thermoelectric applications due to their low toxicity, thermal stability, low density, relative abundance and low cost of production. Earlier work on the synthesis of Mg₂Siviahigh energy ball milling resulted in incomplete product formation, oxide impurities, and contamination from milling media. Here we present an improved solid-state synthesis of n-type magnesium silicide using the mechanochemical technique of high energy ball milling of the elements followed by high pressure sintering using hot uniaxial compaction. This robust synthetic method permits a detailed investigation of thermoelectric properties as a function of Bi doping. The thermoelectric properties of Mg₂Si_1−xBi_x (0 ≤ x ≤ 0.021) samples are characterized from 300 K to 775 K. These results are analyzed within a single parabolic band (SPB) model to determine the effective conduction band parameters and identify regimes of non-SPB behavior.

Motivated by excellent thermoelectric performance in the well-known Yb-based Zintl compounds Yb₁₄MnSb₁₁ and YbZn_2−xMn_xSb₂, this study investigates the thermoelectric properties of Yb₉Mn_4.2Sb₉. Unlike most transition metal containing Zintl phases, Yb₉Mn_4.2Sb₉ contains a partially occupied Mn site and thus does not have a valence-precise stoichiometry. Samples were synthesized by direct ball milling of the elements, followed by hot pressing. Consistent with previous reports, X-ray diffraction and wavelength dispersive spectroscopy confirmed a narrow composition range near Yb₉Mn_4.2Sb₉. High temperature measurements of the electronic properties of Yb₉Mn_4.2Sb₉ indicate that it is a degenerate p-type semiconductor with a band gap sufficiently large for high temperature thermoelectric applications. Hall measurements reveal that Yb₉Mn_4.2Sb₉ has a high extrinsic carrier concentration (∼10²⁰ h⁺ cm⁻³), which is due to the deviation from the theoretical “Zintl composition” of Yb₉Mn_4.5Sb₉. The measured carrier concentration coincides with the optimum concentration predicted using a single parabolic band model. Measurements of the thermal diffusivity and heat capacity reveal an extremely low, temperature-independent lattice thermal conductivity in this compound (κ_L < 0.4 W mK⁻¹), which is due to both the large unit cell size (44 atoms per primitive cell) and substantial disorder on the Mn site. This favorable combination of optimized electronic properties and low lattice thermal conductivity leads to a promising figure of merit at high temperature (zT = 0.7 at 950 K).

Yb ₁₄ MnSb ₁₁ and Yb ₁₄ MgSb ₁₁ are among the best p-type high-temperature (>1200 K) thermoelectric materials, yet other compounds of this Ca ₁₄ AlSb ₁₁ structure type have not matched their stability and efficiency. First-principles computations show that the features in the electronic structures that have been identified to lead to high thermoelectric performances are present in Yb ₁₄ ZnSb ₁₁ , which has been presumed to be a poor thermoelectric material. We show that the previously reported low power factor of Yb ₁₄ ZnSb ₁₁ is not intrinsic and is due to the presence of a Yb ₉ Zn _{4+ x} Sb ₉ impurity uniquely present in the Zn system. Phase-pure Yb ₁₄ ZnSb ₁₁ synthesized through a route avoiding the impurity formation reveals its exceptional high-temperature thermoelectric properties, reaching a peak zT of 1.2 at 1175 K. Beyond Yb ₁₄ ZnSb ₁₁ , the favorable band structure features for thermoelectric performance are universal among the Ca ₁₄ AlSb ₁₁ structure type, opening the possibility for high-performance thermoelectric materials.

Yb₁₄MnSb₁₁ is a p-type high temperature thermoelectric material that has been shown to have a peak zT of 1.3 at 1273 K and stable lifetime testing at that temperature for over 1500 h by NASA. Yb₁₄MgSb₁₁ is a structural analog, but the highest temperature thermoelectric properties have not yet been reported. Yb₁₄MgSb₁₁ has been prepared in an environmentally friendly route employing metal hydrides to provide phase pure samples with excellent control of stoichiometry. We present a comparative study employing either MgH₂ or YbH₂ as a reactive precursor that also facilitates milling of the elements. High purity compositions are synthesized, and their high temperature thermoelectric properties were measured on dense pellets. Temperature-dependent thermoelectric properties were measured from 300 to 1273 K. Yb₁₄MgSb₁₁ exhibited a peak zT = 1.2 at 1273 K due to an appreciable power factor and low-lattice thermal conductivity. Carrier concentration and hall mobility were also measured from 300 to 1275 K and ranged from 5.3 × 10²⁰ to 1.3 × 10²¹ cm⁻³ and from 4.7 to 0.7 cm² V⁻¹ S⁻¹, respectively.