GeTe experiences phase transition between cubic and rhombohedral through distortion along the [111] direction. Cubic GeTe shares the similarity of a two-valence-band structure (high-energy L and low-energy Σ bands) with other cubic IV–VI semiconductors such as PbTe, SnTe, and PbSe, and all show a high thermoelectric performance due to a high band degeneracy. Very recently, the two valence bands were found to switch in energy in rhombohedral GeTe and to be split due to symmetry-breaking of the crystal structure. This enables the overall band degeneracy to be manipulated either by the control of symmetry-induced degeneracy or by the design of energy-aligned orbital degeneracy. Here, we show Sb-doping for optimizing carrier concentration and manipulating the degree of rhombohedral lattice distortion to maximize the band degeneracy and then electronic performance. In addition, Sb-doping significantly promotes the solubility of PbTe, enhancing the scattering of phonons by Ge/Pb substitutional defects for minimizing the lattice thermal conductivity. This successfully realizes a superior thermoelectric figure of merit, zT of >2 in both rhombohedral and cubic GeTe, demonstrating these alloys as top candidates for thermoelectric applications at T < 800 K. This work further sheds light on the importance of crystal structure symmetry manipulation for advancing thermoelectrics.

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As a typical class of Zintl thermoelectrics, AB2C2 (A = Eu, Yb, Ba, Ca, Mg; B = Zn, Cd, Mg, and C = Sb, Bi) compounds have shown a superior thermoelectric performance, largely stemming from the existence of multiple transporting bands in both conduction types. Being similar to many III–V and elemental semiconductors, the transport of holes in AB2C2 Zintls usually involves multiple valence bands with extrema at the Brillouin zone center Γ. However, these valence bands, originating from different orbitals, are unnecessarily aligned in energy due to the crystal field splitting. Formation of solid solutions between constituent compounds having opposite arrangements in energy of band orbitals is believed to be particularly helpful for thermoelectric enhancements, because orbital alignment increases band degeneracy while alloy defects scatter phonons. These effects are simultaneously realized in this work, where the p orbitals of anions in YbCd2–x Zn x Sb2 alloys are well-aligned for maximizing the electronic performance, and meanwhile high-concentration Cd/Zn substitutions are introduced for minimizing the lattice thermal conductivity. As a result, a significantly enhanced thermoelectric figure of merit, zT ∼ 1.3, is achieved, being a record among AB2C2 Zintls in p-type. This work demonstrates not only YbCd2–x Zn x Sb2 alloys as efficient thermoelectrics but also orbital alignment as an effective strategy for advancing thermoelectrics.

Semiconducting cubic group IV monotellurides, including PbTe and SnTe, have historically led most of the advancements in thermoelectrics. Recently, noncubic ones such as GeTe and MnTe have also shown to be promising, which motivates the current work focusing on the thermoelectric properties of MnGeTe 2 , a derivative compound of noncubic GeTe and MnTe but crystalizing in a cubic structure. This compound intrinsically comes with a carrier concentration as high as~3.6×10 21 cm −3 , indicating the existence of highconcentration cation vacancies due to Ge-precipitation. This intrinsic carrier concentration is much higher than that needed for thermoelectric applications but can be successfully decreased to~9×10 20 cm −3 for MnGe 0.9 Bi 0.1 Te 2 at room temperature. Such a broad carrier concentration not only offers a full assessment of its electronic transport properties according to a single parabolic band model with acoustic scattering, but also enables an optimization for thermoelectric power factor. The low lattice thermal conductivity of~1.2 W m −1 K −1 or lower in the entire temperature range, can be understood by the highly disordered cations and cation vacancies. A peak zT approaching 1.0 at 850 K was achieved in materials at an optimal carrier concentration of~9×10 20 cm −3 , an isotropic cubic structure as well as a Vickers hardness of >200 H V , strongly indicating MnGeTe 2 as a promising thermoelectric material.

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Alloying EuCd2Sb2 with EuZn2Sb2 enables a valence band alignment and a reduction in lattice thermal conductivity resulting in a substantial thermoelectric improvement.

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