Structure and properties of single crystalline CaMg2Bi2, EuMg2Bi2, and YbMg2Bi2

Thermoelectric Properties of Zn-Doped YbMg1.85-xZnxBi1.98

Bi-based YbMg2Bi1.98 Zintl compounds represent promising thermoelectric materials. Precise composition and appropriate doping are of great importance for this complex semiconductor. Here, the influence of Zn substitution for Mg on the microstructure and thermoelectric properties of p-type YbMg1.85-xZnxBi1.98 (x = 0, 0.05, 0.08, 0.13, 0.23) was investigated. Polycrystalline samples were prepared using induction melting and densified with spark plasma sintering. X-ray diffraction confirmed that the major phase of the samples possesses the trigonal CaAl2Si2-type crystal structure, and SEM/EDS indicated the presence of minor secondary phases. The electrical conductivity increases and the lattice thermal conductivity decreases with more Zn doping in YbMg1.85-xZnxBi1.98, whereas the Seebeck coefficient has a large reduction. The band gap decreases with increasing Zn concentration and leads to bipolar conduction, resulting in an increase in the thermal conductivity at higher temperatures. Figure of merit ZT values of 0.51 and 0.49 were found for the samples with x = 0 and 0.05 at 773 K, respectively. The maximum amount of Zn doping is suggested to be less than x = 0.1.

Mechanism of Thermoelectric Performance Enhancement in CaMg2 Bi2 -Based Materials with Different Cation Site Doping

Chemical bonds determine electron and phonon transport in solids. Tailoring chemical bonding in thermoelectric materials causes desirable or compromise thermoelectric transport properties. In this work, taking an example of CaMg2 Bi2 with covalent and ionic bonds, density functional theory calculations uncover that element Zn, respectively, replacing Ca and Mg sites cause the weakness of ionic and covalent bonding. Electrically, Zn doping at both Ca and Mg sites increases carrier concentration, while the former leads to higher carrier concentration than that of the latter because of its lower vacancy formation energy. Both doping types increase density-of-state effective mass but their mechanisms are different. The Zn doping Ca site induces resonance level in valence band and Zn doping Mg site promotes orbital alignment. Thermally, point defect and the change of phonon dispersion introduced by doping result in pronounced reduction of lattice thermal conductivity. Finally, combining with the further increase of carrier concentration caused by Na doping and the modulation of band structure and the decrease of lattice thermal conductivity caused by Ba doping, a high figure-of-merit ZT of 1.1 at 823 K in Zn doping Ca sample is realized, which is competitive in 1-2-2 Zintl phase thermoelectric systems.

Dirac semimetal phase and switching of band inversion in XMg2Bi2 (X = Ba and Sr)

Topological Dirac semimetals (TDSs) offer an excellent opportunity to realize outstanding physical properties distinct from those of topological insulators. Since TDSs verified so far have their own problems such as high reactivity in the atmosphere and difficulty in controlling topological phases via chemical substitution, it is highly desirable to find a new material platform of TDSs. By angle-resolved photoemission spectroscopy combined with first-principles band-structure calculations, we show that ternary compound BaMg₂Bi₂ is a TDS with a simple Dirac-band crossing around the Brillouin-zone center protected by the C₃ symmetry of crystal. We also found that isostructural SrMg₂Bi₂ is an ordinary insulator characterized by the absence of band inversion due to the reduction of spin–orbit coupling. Thus, XMg₂Bi₂ (X = Sr, Ba, etc.) serves as a useful platform to study the interplay among crystal symmetry, spin–orbit coupling, and topological phase transition around the TDS phase.

Weak localization and electron-phonon interaction in layered Zintl phase SrIn2P2single crystal

Recently, the Zintl phase SrIn₂P₂single crystal was proposed to be a topological insulator candidate under lattice strain. Here, we report systematic electrical transport studies on the unstrained layered SrIn₂P₂single crystals. The resistance presents a minimum value aroundT_c= 136 K and then increases remarkably at low temperature. Distinct negative magnetoresistance belowT_c, combined with the anomalous resistance, implies the carriers are weak localized at low temperature due to strong quantum coherence. Further analysis based on three-dimensional weak localization (WL) model suggests that the electron-phonon interaction dominates the phase decoherence process. Moreover, Hall measurements indicate that the transport properties are mainly dominated by hole-type carriers, and the WL effect is obviously affected by the carrier transport. These findings not only provide us a promising platform for the fundamental physical research but also open up a new route for exploring the potential electronic applications.

Sr9Mg4.45(1)Bi9 and Sr9Mg4.42(1)Sb9: Mg-Containing Zintl Phases with Low Thermal Conductivity

Zintl phases with nominal 9-4-9 formulas are very interesting for their potential applications as thermoelectric materials. However, the formation of such phases usually requires divalent transition metals, for example, Zn, Mn, and Cd, which are covalently bonded to the pnictogen atoms. In this report, for the first time, two Mg-containing compounds with such structures as Sr₉Mg_4.45(1)Bi₉ and Sr₉Mg_4.42(1)Sb₉ were synthesized and their structures were determined by the single-crystal X-ray diffraction method. Both title compounds crystallize in the orthorhombic space group Pnma and are isostructural with Ca₉Mn_4.41(1)Sb₉, which features complex polyanion structures compared to the classical 9-4-9 phases. For Sr₉Mg_4.45(1)Bi₉, its low thermal conductivity, combined with its high electrical conductivity and moderate Seebeck coefficient, leads to a decent figure of merit of 0.57 at 773 K, which obviously prevails in the unoptimized 9-4-9 phases. The discovery of such Mg-containing 9-4-9 phases is very significant, as the discovery not only enriches the structure map of the well-known 9-4-9 family but also provides very valuable thermoelectric candidates surely deserving of more in-depth investigation.

Achieving High Thermoelectric Performance in Rare-Earth Element-Free CaMg2Bi2 with High Carrier Mobility and Ultralow Lattice Thermal Conductivity

CaMg₂Bi₂-based compounds, a kind of the representative compounds of Zintl phases, have uniquely inherent layered structure and hence are considered to be potential thermoelectric materials. Generally, alloying is a traditional and effective way to reduce the lattice thermal conductivity through the mass and strain field fluctuation between host and guest atoms. The cation sites have very few contributions to the band structure around the fermi level; thus, cation substitution may have negligible influence on the electric transport properties. What is more, widespread application of thermoelectric materials not only desires high ZT value but also calls for low-cost and environmentally benign constituent elements. Here, Ba substitution on cation site achieves a sharp reduction in lattice thermal conductivity through enhanced point defects scattering without the obvious sacrifice of high carrier mobility, and thus improves thermoelectric properties. Then, by combining further enhanced phonon scattering caused by isoelectronic substitution of Zn on the Mg site, an extraordinarily low lattice thermal conductivity of 0.51 W m⁻¹ K⁻¹ at 873 K is achieved in (Ca_0.75Ba_0.25)_0.995Na_0.005Mg_1.95Zn_0.05Bi_1.98 alloy, approaching the amorphous limit. Such maintenance of high mobility and realization of ultralow lattice thermal conductivity synergistically result in broadly improvement of the quality factor β. Finally, a maximum ZT of 1.25 at 873 K and the corresponding ZT_ave up to 0.85 from 300 K to 873 K have been obtained for the same composition, meanwhile possessing temperature independent compatibility factor. To our knowledge, the current ZT_ave exceeds all the reported values in AMg₂Bi₂-based compounds so far. Furthermore, the low-cost and environment-friendly characteristic plus excellent thermoelectric performance also make the present Zintl phase CaMg₂Bi₂ more competitive in practical application.

Enhanced Thermoelectric Properties of p-Type CaMg2Bi2 via a Synergistic Effect Originated from Zn and Alkali-Metal Co-doping

Bi-based Zintl phase CaMg₂Bi₂ is a promising thermoelectric material. Here, we report that the high-concentration point defects induced by equivalent Zn doping on the Mg site significantly enhance phonon scattering and then suppress lattice thermal conductivity by 50% at room temperature. Subsequently, partial substitution of divalent calcium ions with alkali-ion doping (Li, Na, K) not only optimizes the electrical transport properties by increasing the carrier concentration but also further reduces the lattice thermal conductivity through crystal disorder. Finally, the synergistic effect of Zn and Li co-doping leads to a high ZT of ∼1.0 at 873 K and an average ZT of 0.6 between 300 and 873 K for Ca_0.995Li_0.005Mg_1.9Zn_0.1Bi_1.98. This work demonstrates an instructive method to reduce the lattice thermal conductivity via doping at the Mg site, which has never been reported in the CaMg₂Bi₂ system. Moreover, high-performance Ca_0.995Li_0.005Mg_1.9Zn_0.1Bi_1.98 alloy does not contain any toxic elements and expensive rare earth elements, which is of great significance for the development of environment-friendly thermoelectric materials.

Chemical bonding origin of the unexpected isotropic physical properties in thermoelectric Mg3Sb2 and related materials

The Mg₃Sb₂ structure is currently being intensely scrutinized due to its outstanding thermoelectric properties. Usually, it is described as a layered Zintl phase with a clear distinction between covalent [Mg₂Sb₂]²⁻ layers and ionic Mg²⁺ layers. Based on the quantitative chemical bonding analysis, we unravel instead that Mg₃Sb₂ exhibits a nearly isotropic three-dimensional bonding network with the interlayer and intralayer bonds being mostly ionic and surprisingly similar, which results in the nearly isotropic structural and thermal properties. The isotropic three-dimensional bonding network is found to be broadly applicable to many Mg-containing compounds with the CaAl₂Si₂-type structure. Intriguingly, a parameter based on the electron density can be used as an indicator measuring the anisotropy of lattice thermal conductivity in Mg₃Sb₂-related structures. This work extends our understanding of structure and properties based on chemical bonding analysis, and it will guide the search for and design of materials with tailored anisotropic properties.

The outstanding thermoelectric Mg₃Sb₂ is currently being intensely scrutinized. Here, the authors reveal a nearly isotropic three-dimensional chemical bonding network as the origin of isotropic physical properties in Mg₃Sb₂, indicating the breakdown of the simple layered Zintl phase view.

Crystal chemistry and thermoelectric transport of layered AM2X2compounds

This review highlights the chemical diversity and transport properties of AM₂X₂Zintl compounds and strategies to achieve a high thermoelectric figure of merit.

Higher thermoelectric performance of Zintl phases (Eu0.5Yb0.5)1-xCaxMg2Bi2 by band engineering and strain fluctuation

Complex Zintl phases, especially antimony (Sb)-based YbZn0.4Cd1.6Sb2 with figure-of-merit (ZT) of ∼1.2 at 700 K, are good candidates as thermoelectric materials because of their intrinsic "electron-crystal, phonon-glass" nature. Here, we report the rarely studied p-type bismuth (Bi)-based Zintl phases (Ca,Yb,Eu)Mg2Bi2 with a record thermoelectric performance. Phase-pure EuMg2Bi2 is successfully prepared with suppressed bipolar effect to reach ZT ∼ 1. Further partial substitution of Eu by Ca and Yb enhanced ZT to ∼1.3 for Eu0.2Yb0.2Ca0.6Mg2Bi2 at 873 K. Density-functional theory (DFT) simulation indicates the alloying has no effect on the valence band, but does affect the conduction band. Such band engineering results in good p-type thermoelectric properties with high carrier mobility. Using transmission electron microscopy, various types of strains are observed and are believed to be due to atomic mass and size fluctuations. Point defects, strain, dislocations, and nanostructures jointly contribute to phonon scattering, confirmed by the semiclassical theoretical calculations based on a modified Debye-Callaway model of lattice thermal conductivity. This work indicates Bi-based (Ca,Yb,Eu)Mg2Bi2 is better than the Sb-based Zintl phases.

Designing high-performance layered thermoelectric materials through orbital engineering

Thermoelectric technology, which possesses potential application in recycling industrial waste heat as energy, calls for novel high-performance materials. The systematic exploration of novel thermoelectric materials with excellent electronic transport properties is severely hindered by limited insight into the underlying bonding orbitals of atomic structures. Here we propose a simple yet successful strategy to discover and design high-performance layered thermoelectric materials through minimizing the crystal field splitting energy of orbitals to realize high orbital degeneracy. The approach naturally leads to design maps for optimizing the thermoelectric power factor through forming solid solutions and biaxial strain. Using this approach, we predict a series of potential thermoelectric candidates from layered CaAl2Si2-type Zintl compounds. Several of them contain nontoxic, low-cost and earth-abundant elements. Moreover, the approach can be extended to several other non-cubic materials, thereby substantially accelerating the screening and design of new thermoelectric materials.

Investigation of the bipolar effect in the thermoelectric material CaMg2Bi2 using a first-principles study

An effective carrier concentration n_eff is proposed to evaluate the bipolar effect, and the results show good consistency with measured data.