Zn 4 Sb 3 -based compounds constitute a lead-free material family with a best thermoelectric figure of merit (zT) in the midtemperature range. Unlike being a stoichiometric compound, the mutual solubility of Zn and Sb elicits rich microstructures and the structural instability of Zn 4 Sb 3 . The solubility limit and neighboring phase relations are crucial for the delicate balance between the thermoelectric performance and phase stability of Zn 4 Sb 3 . In this work, we constructed the isothermal section of the Zn−Sb−In ternary phase diagram, built the zT map near the β-Zn 4 Sb 3 phase region, and showed that the formation of multiscale microstructures has a profound impact on the electronic and phonon transport properties and phase stability. A high-zT zone was identified near the phase boundary between the twophase InSb + Zn 5 Sb 4 In 2 and the three-phase Zn 4 Sb 3 + InSb + Zn 5 Sb 4 In 2 regions. A sample with a nominal composition of Zn 3.8 In 0.2 Sb 3 exhibited an ultralow κ L of 0.2 (W m −1 K −1 ), an enhanced PF of 1.75 (mW m −1 K −2 ), and a remarkable zT value of 1.8 at 698 K. These state-of-the-art thermoelectric properties were attributed to the simultaneous enhancement in phonon scattering and the carrier energy-filtering effect in a unique hierarchical microstructure, in which InSb nanoprecipitates are dispersed in Zn 5 Sb 4 In 2 coarse grains, and the latter are embedded in the host matrix In−Zn 4 Sb 3 . These results opened an avenue for environmentally friendly cost-effective midtemperature thermoelectric materials.

Alongside the evolution of technology timeline, environmental protection and energy sustainability significantly guide the research feast toward the high-performance green energy resources and technologies. [1,2] Alternatives to the fossil fuels that emit harmful greenhouse gases become desired, in which the thermoelectric (TE) materials have played an inevitable role since the 1960s. [3] One of TE technology features is the waste heat recovery via the Seebeck effect, [4] which allows the conversion between thermal energy and electricity. Such a TE generator produces precious electrical energy from any arbitrary interfaces where a temperature gradient exists, simultaneously boosting energy usage efficiency while eases global warming.Many TE materials are being explored for power generation applications, such as GeTe, [5] PbTe, [6,7] Bi 2 Te 3 , [8] and silicides. [9] Moreover, the TE cooler, which utilizes the Peltier effect, [4] emerges as a vital spot-cooling device assembled by all-solid-state materials. With the advantages of refrigerant-free and size-minimization, the TE cooler exhibits the advantages of refrigerant-free and size-minimization, which can be used as the next-generation cooling technology. [10] After more than 60 years of development, the practical application and potential coverage of TE technology are comprehensive. Nevertheless, a TE material's thermal-to-electric efficiency is still required to be boosted. In general, the TE figure-of-merit zT ¼ ðS 2 σÞT=κ positively relates to the performance of TE materials, in which the S is the Seebeck coefficient, σ ¼ ρ À1 refers to the electrical conductivity, and κ ¼ κ e þ κ L þ κ b comprises the total thermal conductivity κ, lattice thermal conductivity κ, and bipolar thermal conductivity κ b , respectively. Most importantly, the S, σ, and κ e have the carrier concentration n H as a common factor, which correlates and depends on each other. Therefore, the counterbalance between the thermal and electrical transport properties is essential to attain high zT values, which could be fulfilled by band structure engineering, [11] carrier optimization, [12,13] filtering effect, [14] , and so on. In parallel, the reduction in κ L also paves the way toward highperformance TE materials, mainly accomplished by all-scale defect engineering [15] and alloying effect. [16] Those imperfections introduce multiscale roadblocks, such as the interstitial/ antisite defect, dislocations, nanoprecipitates, and grain boundaries, aiming to impede the phonons with different wavelengths.Countless efforts bring the breakthroughs of zT value in succession. The peak zT grows less than unity in the 1960s to greater than 2.5 after 2015, [17] whose progress is slow yet steadily. Complex TE materials with various dopants are the primary targets, while different approaches can be adopted. [18,19] The

Thermoelectric (TE) generators have come a long way since the first commercial apparatus launched in the 1950s. Since then, the β‐Zn4Sb3 has manifested its potential as a cost‐effective and environmentally friendly TE generator compared with the tellurium‐bearing TE materials. Although the β‐Zn4Sb3 features an intrinsically low thermal conductivity κ, it suffers from a long‐lasting structural instability issue arising from the highly mobile zinc ions. Herein, the dilute Ga dopant gives rise to the aliovalent substitution, lowers the mobile zinc ions, and optimizes the hole carrier concentration nH simultaneously. Meanwhile, the formation of nano‐moiré fringes suggests the modulated distribution of point defect that results from soluble Ga in a β‐Zn4Sb3 lattice, which elicits an ultralow lattice thermal conductivity κL = 0.2 W m−1 K−1 in a (Zn0.992Ga0.008)4Sb3 alloy. Hence, a fully dense β‐Zn4Sb3 incorporated with the dilute Ga doping reveals superior structural stability with a peak zT > 1.4 at 623 K. In this work, the aliovalent dilute doping coupled with phase diagram engineering optimizes the fluxes of moving electrons and charged ions, which stabilizes the single‐phase β‐Zn4Sb3 while boosting the TE performance at the mid‐temperature region. The synergistic strategies endow the ionic crystals with a thermodynamic route, which opens up a new category for high‐performance and thermal robust TE alloys.

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