Zintl phases for thermoelectric devices

When van der Waals Met Kagome: A 2D Antimonide with a Vanadium-Kagome Network

2D materials showcase unconventional properties emerging from quantum confinement effects. In this work, a "soft chemical" route allows for the deintercalation of K+ from the layered antimonide KV6Sb6, resulting in the discovery of a new metastable 2D-Kagome antimonide K0.1(1)V6Sb6 with a van der Waals gap of 3.2 Å. The structure of K0.1(1)V6Sb6 was determined via the synergistic techniques, including X-ray pair distribution function analysis, advanced transmission electron microscopy, and density functional theory calculations. The K0.1(1)V6Sb6 compound crystallizes in the monoclinic space group C2/m (a = 9.57(2) Å, b = 5.502(8) Å, c = 10.23(2) Å, β = 97.6(2)°, Z = 2). The [V6Sb6] layers in K0.1(1)V6Sb6 are retained upon deintercalation and closely resemble the layers in the parent compound, yet deintercalation results in a relative shift of the adjacent [V6Sb6] layers. The magnetic properties of the K0.1(1)V6Sb6 phase in the 2-300 K range are comparable to those of KV6Sb6 and another Kagome antimonide KV3Sb5, consistent with nearly temperature-independent paramagnetism. Electronic band structure calculation suggests a nontrivial band topology with flat bands and opening of band crossing afforded by deintercalation. Transport property measurements reveal a metallic nature for K0.1(1)V6Sb6 and a low thermal conductivity of 0.6 W K-1 m-1 at 300 K. Additionally, ion exchange in KV6Sb6 via a solvothermal route leads to a successful partial exchange of K+ with A+ (A = Na, Rb, and Cs). This study highlights the tunability of the layered structure of the KV6Sb6 compound, providing a rich playground for the realization of new 2D materials.

Synthesis, Crystal and Electronic Structures, and Magnetic and Electrical Transport Properties of Bismuthides NdZn0.6Bi2 and (La0.5RE0.5)Zn0.6Bi2 (RE = Pr or Nd)

Bismuth is a good constituent element for many quantum materials due to its large atomic number, 6s26p3 orbitals, and strong spin-orbital coupling. In this work, three new bismuthides, NdZn0.6Bi2, (La0.5Pr0.5)Zn0.6Bi2, and (La0.5Nd0.5)Zn0.6Bi2, were grown by a metal flux method, and their crystal structures were accurately determined by single-crystal X-ray diffraction. These new bismuthides belong to the RE-T-Pn2 (RE = La-Lu, T = Mn, Fe, Co, Ni, or Zn, and Pn = P, As, Sb, or Bi) family, are isostructural, and crystallize in the HfCuSi2 structure type. The bismuth elements have two possible oxidation states, Bi3- and Bi-, which were studied by X-ray photoelectron spectroscopy (XPS). Two binding energy peaks of 155.91 and 161.23 eV were observed for Bi atoms within NdZn0.6Bi2, and similar binding energy peaks were detected in NdBi and LiBi. XPS also confirmed the trivalent nature of Nd, which was further verified by magnetic measurements. Additionally, magnetic measurements revealed that NdZn0.6Bi2 exhibits an antiferromagnetic transition around 3 K, while the mixed-cation compounds do not show any magnetic transition down to 2 K. Electronic transport measurements reveal weak magnetoresistance in all three compounds, with a maximum value of ∼25% at 2 K and 9 T for (La0.5Nd0.5)Zn0.6Bi2.

A DFT Computational Study of Type-I Clathrates A8Sn46-x (A = Cs or NH4, x = 0 or 2)

Semiconducting clathrates have attracted considerable interest in the field of thermoelectric materials. We report here a computational study on the crystal structure, the enthalpy of formation, and the physical properties of the following type-I clathrates: (a) experimentally studied Cs8Sn44 and hypothetical Cs8Sn46 and (b) hypothetical (NH4)8Sn46-x (x = 0 or 2). The ab initio VASP calculations for the nominal stoichiometries include the geometry optimization of the initial structural models, enthalpies of formation, and the electronic and phonon density of states. Comparison of the chemical bonding of the structural models is performed via the electron localization function. The results show that the presence and distribution of defects in the Sn framework for both Cs8Sn46-x and (NH4)8Sn46-x systems significantly alters the formation energy and its electrical properties, ranging from metallic to semiconducting behavior. In particular, one defect per six-membered Sn ring in a 3D spiro-network is the thermodynamically preferred configuration that results in the Cs8Sn44 and (NH4)8Sn44 stoichiometries with narrow-band gap semiconducting behavior. Moreover, the rotation of the ammonium cation in the polyhedral cavities is an interesting feature that may promote the use of ammonium or other small molecular cations as guests in clathrates for thermoelectric applications; this is due to the decrease in the lattice thermal conductivity.

In-Plane Overdamping and Out-Plane Localized Vibration Contribute to Ultralow Lattice Thermal Conductivity of Zintl Phase KCdSb

Zintl phases typically exhibit low lattice thermal conductivity, which are extensively investigated as promising thermoelectric candidates. While the significance of Zintl anionic frameworks in electronic transport properties is widely recognized, their roles in thermal transport properties have often been overlooked. This study delves into KCdSb as a representative case, where the [CdSb4/4]- tetrahedrons not only impact charge transfer but also phonon transport. The phonon velocity and mean free path, are heavily influenced by the bonding distance and strength of the Zintl anions Cd and Sb, considering the three acoustic branches arising from their vibrations. Furthermore, the weakly bound Zintl cation K exhibits localized vibration behaviors, resulting in strong coupling between the high-lying acoustic branch and the low-lying optical branch, further impeding phonon diffusion. The calculations reveal that grain boundaries also contribute to the low lattice thermal conductivity of KCdSb through medium-frequency phonon scattering. These combined factors create a glass-like thermal transport behavior, which is advantageous for improving the thermoelectric merit of zT. Notably, a maximum zT of 0.6 is achieved for K0.84Na0.16CdSb at 712 K. The study offers both intrinsic and extrinsic strategies for developing high-efficiency thermoelectric Zintl materials with extremely low lattice thermal conductivity.

Polytelluride square planar chain-induced anharmonicity results in ultralow thermal conductivity and high thermoelectric efficiency in Al2Te5 monolayers

Two-dimensional (2D) metal chalcogenides provide rich ground for the development of nanoscale thermoelectrics, although achieving optimal thermoelectric efficiency is still a challenge. Here, we leverage the unique chemistry of tellurium (Te), renowned for its hypervalent bonding and catenation abilities, to tackle this challenge as manifested in Al2Te3 and Al2Te5 monolayers. While the former forms a straightforward covalent Al-Te network, the latter adopts a more intricate bonding mechanism, enabled by eccentric features of Te chemistry, to maintain charge balance. In Al2Te5, a square planar chain (SPC) known as polytelluride [Te3]2- is neutralized by the covalently bonded [Al2Te2]2+ framework. The hypervalent nature of Te results in bizarre Born effective charges of 7 and -4 for adjacent Te atoms within the square planar chain, a feature that induces significant anharmonicity in Al2Te5 monolayers. Enhanced anharmonic lattice vibrations and the accordion pattern bestow glass-like, strongly anisotropic thermal conductivity to the Al2Te5 monolayer. The calculated κL values of 1.8 and 0.5 W m-1 K-1 along the a- and b-axes at 600 K are one order of magnitude lower than those of Al2Te3, and even lower than monolayers that contain heavy cations like Bi2Te3. Moreover, the tellurium chain dominates the valence band maximum and conduction band minimum of Al2Te5, leading to a high valley degeneracy of 10, and thus a high power factor and figure of merit (zT). Using rigorous first-principles calculations of electron relaxation time, the estimated hole-doped and electron-doped zT of, respectively, 1.5 and 0.5 at 600 K is achieved for Al2Te5. The pioneering zT of Al2Te5 compared to that of Al2Te3 is rooted merely in its amorphous-like lattice thermal transport and its polytelluride chain. These findings underscore the importance of aluminum telluride and polymeric-based inorganic compounds as practical and cost-effective thermoelectric materials, pending further experimental validation.

Zintl Phase versus Covalent Metal: Chemical Bonding in Silicon Dumbbells of Ca5Si3 and CaSi3

Silicon dumbbells constitute identifiable anionic molecular species in Zintl phases and so-called covalent metals holding units with homopolar bonding inside a metallic framework. Based on electron-precise Ca5Si3 and metallic CaSi3, the chemical bonding in Si2 units is investigated by computational quantum chemical methods considering the dual nature of the wave function. This concerted wave-vector and real space study substantiates that the Si2 dumbbells in Ca5Si3 can be referred to as molecular building units Si26- with additional metallic and ionic contributions in the solid. In the covalent metal CaSi3, however, the bonding within the dumbbells falls short of fulfilling the octet rule. As a result, antibonding states of the Si2 building units are depopulated and attend metallic interactions, simultaneously giving rise to stronger covalent Si-Si bonds.

Fragment Orbitals Extracted from First-Principles Plane-Wave Calculations

Decomposing extended structures into smaller, molecular, even functional groups or simple fragments has a long tradition in chemistry because it allows for understanding certain electronic peculiarities in truly chemical terms. By doing so, invaluable property information is chemically accessible, for example, needed to rationalize catalytic or magnetic or optical nature. In order to also follow that train of thought for periodic materials, we have developed a tool which in a straightforward manner derives fragment molecular orbitals from plane-wave electronic-structure data of whatever kind of solid-state material. We here report on the mathematical apparatus of the method dubbed linear combination of fragment orbitals (LCFO) used for that purpose, implemented within the LOBSTER code. The method is illustrated from various sorts of molecular entities contained in such crystalline materials, together with an assessment of both accuracy and robustness of the new tool.

β-Phase Yb5Sb3Hx: Magnetic and Thermoelectric Properties Traversing from an Electride to a Semiconductor

An electride is a compound that contains a localized electron in an empty crystallographic site. This class of materials has a wide range of applications, including superconductivity, batteries, photonics, and catalysis. Both polymorphs of Yb5Sb3 (the orthorhombic Ca5Sb3F structure type (β phase) and hexagonal Mn5Si3 structure type (α phase)) are known to be electrides with electrons localized in 0D tetrahedral cavities and 1D octahedral chains, respectively. In the case of the orthorhombic β phase, an interstitial H can occupy the 0D tetrahedral cavity, accepting the anionic electron that would otherwise occupy the site, providing the formula of Yb5Sb3Hx. DFT computations show that the hexagonal structure is energetically favored without hydrogen and that the orthorhombic structure is more stable with hydrogen. Polycrystalline samples of orthorhombic β phase Yb5Sb3Hx (x = 0.25, 0.50, 0.75, 1.0) were synthesized, and both PXRD lattice parameters and 1H MAS NMR were used to characterize H composition. Magnetic and electronic transport properties were measured to characterize the transition from the electride (semimetal) to the semiconductor. Magnetic susceptibility measurements indicate a magnetic moment that can be interpreted as resulting from either the localized antiferromagnetically coupled electride or the presence of a small amount of Yb3+. At lower H content (x = 0.25, 0.50), a low charge carrier mobility consistent with localized electride states is observed. In contrast, at higher H content (x = 0.75, 1.0), a high charge carrier mobility is consistent with free electrons in a semiconductor. All compositions show low thermal conductivity, suggesting a potentially promising thermoelectric material if charge carrier concentration can be fine-tuned. This work provides an understanding of the structure and electronic properties of the electride and semiconductor, Yb5Sb3Hx, and opens the door to the interstitial design of electrides to tune thermoelectric properties.

Vacancies tailoring lattice anharmonicity of Zintl-type thermoelectrics

While phonon anharmonicity affects lattice thermal conductivity intrinsically and is difficult to be modified, controllable lattice defects routinely function only by scattering phonons extrinsically. Here, through a comprehensive study of crystal structure and lattice dynamics of Zintl-type Sr(Cu,Ag,Zn)Sb thermoelectric compounds using neutron scattering techniques and theoretical simulations, we show that the role of vacancies in suppressing lattice thermal conductivity could extend beyond defect scattering. The vacancies in Sr2ZnSb2 significantly enhance lattice anharmonicity, causing a giant softening and broadening of the entire phonon spectrum and, together with defect scattering, leading to a ~ 86% decrease in the maximum lattice thermal conductivity compared to SrCuSb. We show that this huge lattice change arises from charge density reconstruction, which undermines both interlayer and intralayer atomic bonding strength in the hierarchical structure. These microscopic insights demonstrate a promise of artificially tailoring phonon anharmonicity through lattice defect engineering to manipulate lattice thermal conductivity in the design of energy conversion materials.

Alloying-Induced Structural Transition in the Promising Thermoelectric Compound CaAgSb

AMX Zintl compounds, crystallizing in several closely related layered structures, have recently garnered attention due to their exciting thermoelectric properties. In this study, we show that orthorhombic CaAgSb can be alloyed with hexagonal CaAgBi to achieve a solid solution with a structural transformation at x ∼ 0.8. This transition can be seen as a switch from three-dimensional (3D) to two-dimensional (2D) covalent bonding in which the interlayer M-X bond distances expand while the in-plane M-X distances contract. Measurements of the elastic moduli reveal that CaAgSb1-xBix becomes softer with increasing Bi content, with the exception of a steplike 10% stiffening observed at the 3D-to-2D phase transition. Thermoelectric transport measurements reveal promising Hall mobility and a peak zT of 0.47 at 620 K for intrinsic CaAgSb, which is higher than those in previous reports for unmodified CaAgSb. However, alloying with Bi was found to increase the hole concentration beyond the optimal value, effectively lowering the zT. Interestingly, analysis of the thermal conductivity and electrical conductivity suggests that the Bi-rich alloys are low Lorenz-number (L) materials, with estimated values of L well below the nondegenerate limit of L = 1.5 × 10-8 W Ω K-2, in spite of the metallic-like transport properties. A low Lorenz number decouples lattice and electronic thermal conductivities, providing greater flexibility for enhancing thermoelectric properties.

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.

Material descriptors for thermoelectric performance of narrow-gap semiconductors and semimetals

Thermoelectric (TE) cooling is an environment-friendly alternative to vapor compression cooling. New TE materials with high coefficients of performance are needed to further advance this technology. Narrow-gap semiconductors and semimetals have garnered interest for Peltier cooling, yet large-scale computational searches often rely on material descriptors that do not account for bipolar conduction effects. In this work, we derive three material descriptors to assess the TE performances of narrow-gap semiconductors and semimetals - band gap, n- and p-type TE quality factors, and the asymmetry in transport between the majority and minority carriers. We show that a large asymmetry is critical to achieving high TE performance through minimization of bipolar conduction effects. We validate the predictive power of the descriptors by correctly identifying Mg3Bi2 and Bi2Te3 as high-performing room-temperature TE materials. By applying these descriptors to a broad set of 650 Zintl phases, we identify three candidate room-temperature TE materials, namely SrSb2, Zn3As2, and NaCdSb. The proposed material descriptors will enable fast, targeted searches of narrow-gap semiconductors and semimetals for low-temperature TEs. We further propose a refined TE quality factor, Bbp, which is a composite descriptor of the peak zT in materials exhibiting significant bipolar conduction; Bbp can be used to compare the TE performances of narrow-gap semiconductors.

Zintl Phases: From Curiosities to Impactful Materials

The synthesis of new compounds and crystal structures remains an important research endeavor in pursuing technologically relevant materials. The Zintl concept is a guidepost for the design of new functional solid-state compounds. Zintl phases are named in recognition of Eduard Zintl, a German chemist who first studied a subgroup of intermetallics prepared with electropositive metals combined with main-group metalloids from groups 13-15 in the 1930s. Unlike intermetallic compounds, where metallic bonding is the norm, Zintl phases exhibit a combination of ionic and covalent bonding and are typically semiconductors. Zintl phases provide a palette for iso- and aliovalent substitutions that can each contribute uniquely to the properties. Zintl electron-counting rules can be employed to interrogate a structure type and develop a foundation of structure-property relationships. Employing substitutional chemistry allows for the rational design of new Zintl compounds with technological properties, such as magnetoelectronics, thermoelectricity, and other energy storage and conversion capabilities. Discovering new structure types and compositions through this approach is also possible. The background on the strength and innovation of the Zintl concept and a few highlights of Zintl phases with promising thermoelectric properties in the context of structural and electronic design will be provided.

A Quantum-Chemical Bonding Database for Solid-State Materials

An in-depth insight into the chemistry and nature of the individual chemical bonds is essential for understanding materials. Bonding analysis is thus expected to provide important features for large-scale data analysis and machine learning of material properties. Such chemical bonding information can be computed using the LOBSTER software package, which post-processes modern density functional theory data by projecting the plane wave-based wave functions onto an atomic orbital basis. With the help of a fully automatic workflow, the VASP and LOBSTER software packages are used to generate the data. We then perform bonding analyses on 1520 compounds (insulators and semiconductors) and provide the results as a database. The projected densities of states and bonding indicators are benchmarked on standard density-functional theory computations and available heuristics, respectively. Lastly, we illustrate the predictive power of bonding descriptors by constructing a machine learning model for phononic properties, which shows an increase in prediction accuracies by 27% (mean absolute errors) compared to a benchmark model differing only by not relying on any quantum-chemical bonding features.

Cation Substitution and Size Effects in Ca2ZnSb2 and Yb2MnSb2: Crystal and Electronic Structures and Thermoelectric Properties

Zintl compounds often feature complex structural fragments and small band gaps, favoring promising thermoelectric properties. In this work, a new phase Ca₂ZnSb₂ is synthesized and characterized to be a LiGaGe-type structure. It is isotypic to Yb₂MnSb₂ with half vacancies at transition metal sites and undergoes a phase transition to Ca₉Zn_4+xSb₉ after annealing. Interestingly, Ca₂ZnSb₂ and Yb₂MnSb₂ are amenable to diverse doping mechanisms at different sites. Here, by substituting smaller Li on cation sites, two novel layered compounds Ca_1.84(1)Li_0.16(1)Zn_0.84(1)Sb₂ and Yb_1.82(1)Li_0.18(1)Mn_0.96(1)Sb₂ with the P6₃/mmc space group are discovered, which can be viewed as derivatives of LiGaGe type. Despite having lower occupancy, the structural stability is improved compared with the prototype compounds owing to the reduced interlayered distances. Besides, the band structure analyses demonstrate that the bands near the Fermi level are mainly governed by the interlayered interaction. Due to the highly disordered structure, Yb_1.82Li_0.18Mn_0.96Sb₂ features ultralow thermal conductivity from 0.79 to 0.47 W·m^-1·K^-1 among the testing range; in addition, a remarkable Seebeck coefficient of 270.77 μV·K^-1 at 723 K is observed. The discovery of the Ca₂ZnSb₂ phase enriches the 2-1-2 map, and the size effect induced by cations provides new ideas for material designing.

Synthesis and Crystal Structure of the Zintl Phases NaSrSb, NaBaSb and NaEuSb

This work details the synthesis and the crystal structures of the ternary compounds NaSrSb, NaBaSb and NaEuSb. They are isostructural and adopt the hexagonal ZrNiAl-type structure (space group P6¯2m; Pearson code hP9). The structure determination in all three cases was performed using single-crystal X-ray diffraction methods. The structure features isolated Sb3- anions arranged in layers stacked along the crystallographic c-axis. In the interstices, alkali and alkaline-earth metal cations are found in tetrahedral and square pyramidal coordination environments, respectively. The formal partitioning of the valence electrons adheres to the valence rules, i.e., Na+Sr2+Sb3-, Na+Ba2+Sb3- and Na+Eu2+Sb3- can be considered as Zintl phases with intrinsic semiconductor behavior. Electronic band structure calculations conducted for NaBaSb are consistent with this notion and show a direct gap of approx. 0.9 eV. Additionally, the calculations hint at possible inverted Dirac cones, a feature that is reminiscent of topological quantum materials.

NaCdSb: An Orthorhombic Zintl Phase with Exceptional Intrinsic Thermoelectric Performance

Many Zintl phases are promising thermoelectric materials owning to their features like narrow band gaps, multiband behaviors, ideal charge transport tunnels, and loosely bound cations. Herein we show a new Zintl phase NaCdSb with exceptional intrinsic thermoelectric performance. Pristine NaCdSb exhibits semiconductor behaviors with an experimental hole concentration of 2.9×10¹⁸  cm^-3 and a calculated band gap of 0.5 eV. As the temperature increases, the hole concentration rises gradually and approaches its optimal one, leading to a high power factor of 11.56 μW cm^-1  K^-2 at 673 K. The ultralow thermal conductivity is derived from the small phonon group velocity and short phonon lifetime, ascribed to the structural anharmonicity of Cd-Sb bonds. As a consequence, a maximum zT of 1.3 at 673 K has been achieved without any doping optimization or structural modification, demonstrating that NaCdSb is a remarkable thermoelectric compound with great potential for performance improvement.

Ba6Ge2Se12 and Ba7Ge2Se17: Two Centrosymmetric Barium Seleno-Germanates with Polyatomic Anion Disorder

Herein, the crystal structures and physical properties of two previously unreported barium seleno-germanates, Ba₆Ge₂Se₁₂ and Ba₇Ge₂Se₁₇, are presented. Ba₆Ge₂Se₁₂ adopts the P2₁/c space group with a = 10.0903(2) Å, b = 9.3640(2) Å, c = 25.7643(5) Å, and β = 90.303(1)°, whereas Ba₇Ge₂Se₁₇ crystallizes in the Pnma space group with a = 12.652(1) Å, b = 20.069(2) Å, c = 12.3067(9) Å. Both structures feature polyatomic anion disorder: [Se₂]^2- in the case of Ba₆Ge₂Se₁₂ and [GeSe₅]^4- in the case of Ba₇Ge₂Se₁₇. The anion disorder is verified by comparing pair distribution functions of ordered and disordered models of the structures. These anions are split unevenly across two possible sets of atomic coordinates. The optical band gaps obtained from the powdered samples are found to be 1.75 and 1.51 eV for Ba₆Ge₂Se₁₂ and Ba₇Ge₂Se₁₇, respectively. Differential scanning calorimetry experiments indicate that the compounds are stable under the exclusion of air up to at least 673 K. The thermal diffusivity measurements revealed thermal conductivities reaching values as low as 0.33 W m^-1 K^-1 in both compounds at 573 K.

Temperature-Driven Twin Structure Formation and Electronic Structure of Epitaxially Grown Mg3Sb2 Films on Mismatched Substrates

Mg₃Sb₂-based compounds are one type of important room-temperature thermoelectric materials and the appropriate candidate of type-II nodal line semimetals. In Mg₃Sb₂-based films, compelling research topics such as dimensionality reduction and topological states rely on the controllable preparation of films with high crystallinity, which remains a big challenge. In this work, high quality Mg₃Sb₂ films are successfully grown on mismatched substrates of sapphire (000l), while the temperature-driven twin structure evolution and characteristics of the electronic structure are revealed in the as-grown Mg₃Sb₂ films by in situ and ex situ measurements. The transition of layer-to-island growth of Mg₃Sb₂ films is kinetically controlled by increasing the substrate temperature (T_sub), which is accompanied with the rational manipulation of twin structure and epitaxial strains. Twin-free structure could be acquired in the Mg₃Sb₂ film grown at a low T_sub of 573 K, while the formation of twin structure is significantly promoted by elevating the T_sub and annealing, in close relation to the processes of strain relaxation and enhanced mass transfer. Measurements of scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES) elucidate the intrinsic p-type conduction of Mg₃Sb₂ films and a bulk band gap of ~0.89 eV, and a prominent Fermi level downshift of ~0.2 eV could be achieved by controlling the film growth parameters. As elucidated in this work, the effective manipulation of the epitaxial strains, twin structure and Fermi level is instructive and beneficial for the further exploration and optimization of thermoelectric and topological properties of Mg₃Sb₂-based films.

Unlocking the thermoelectric potential of the Ca14AlSb11 structure type

Yb₁₄MnSb₁₁ and Yb₁₄MgSb₁₁ are among the best p-type high-temperature (>1200 K) thermoelectric materials, yet other compounds of this Ca₁₄AlSb₁₁ 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 Yb₁₄ZnSb₁₁, which has been presumed to be a poor thermoelectric material. We show that the previously reported low power factor of Yb₁₄ZnSb₁₁ is not intrinsic and is due to the presence of a Yb₉Zn_4+xSb₉ impurity uniquely present in the Zn system. Phase-pure Yb₁₄ZnSb₁₁ 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 Yb₁₄ZnSb₁₁, the favorable band structure features for thermoelectric performance are universal among the Ca₁₄AlSb₁₁ structure type, opening the possibility for high-performance thermoelectric materials.

Synthesis and structural characterization of the new Zintl phases Eu10Mn6Bi12 and Yb10Zn6Sb12

Two new ternary compounds, Eu₁₀Mn₆Bi₁₂ and Yb₁₀Zn₆Sb₁₂, were synthesized and structurally characterized. The synthesis was achieved either through reactions in sealed niobium tubes or in alumina crucibles by combining the elements in excess molten Sb. Their structures were elucidated using single-crystal X-ray diffraction, and they were determined to crystallize in the orthorhombic space group Cmmm (no. 65) with the Eu₁₀Cd₆Bi₁₂ structure type. Akin to the archetype phase, both Mn and Zn sites contain about 25% of vacancies. The anionic substructure of the title phases can be described as [M₆Pn₁₂] (M = Zn, Mn; Pn = Sb, Bi) double layers composed of the corner and edge-sharing [MPn₄] tetrahedra, linked by [Pn₂]^4- dumbbells. Eu²⁺/Yb²⁺ cations fill the space between the layers, with the valence electron counts adhering closely to the Zintl-Klemm rules, i.e., both Eu₁₀Mn₆Bi₁₂ and Yb₁₀Zn₆Sb₁₂ are expected to be valence-precise compounds. Analysis of the electronic structure and transport properties of Yb₁₀Zn₆Sb₁₂ indicate semimetallic behavior with relatively low Seebeck coefficient and resistivity that slightly decreases as a function of temperature.

Suppression of Interfacial Diffusion in Mg3Sb2 Thermoelectric Materials through an Mg4.3Sb3Ni/Mg3.2Sb2Y0.05/Mg4.3Sb3Ni-Graded Structure

The Zintl compound, n-type Mg3Sb2, has been extensively investigated as a promising thermoelectric material. However, performance degradation caused by the loss of Mg element during device preparation and service is a main disadvantage in its utilization in thermoelectric devices. To suppress volatilization, diffusion, or reaction of Mg, we designed a graded concentration junction to control the interfacial elemental diffusion and improve the stability of the thermoelectric joint. We utilized the reaction product at the Ni/Mg3.2Sb2Y0.05 interface, the phase Mg4.3Sb3Ni, as a barrier layer material, and prepared Mg4.3Sb3Ni/Mg3.2Sb2Y0.05/Mg4.3Sb3Ni junctions. The results show that the interface behavior of the thermoelectric junction is optimized by the gradation of elemental concentration, thermal expansion coefficient, and work function. The Mg4.3Sb3Ni/Mg3.2Sb2Y0.05/Mg4.3Sb3Ni single-leg device showed high thermal stability at 673 K for 20 days, the contact resistance was stable at around 10 μΩ cm2, and the shear strength was maintained at about 20 MPa. The conversion efficiency of its single-leg device maintains nearly 90% of the best performance after aging at 673 K for 20 days.

Materials design, synthesis, and transport properties of disordered rare-earth Zintl bismuthides with the anti-Th3P4 structure type

The synthesis, structural elucidation, and transport properties of the extended series Ca_4-xRE_xBi₃ (RE = Y, La-Nd, Sm, Gd-Tm, and Lu; x ≈ 1) and Ca_4-xRE_xBi_3-δSb_δ (RE = La, Ho, Er, and Lu; x ≈ 1, δ ≈ 1.5) are presented. Structural elucidation is based on single-crystal X-ray diffraction data and confirms the chemical drive of Ca₄Bi₃ with the cubic anti-Th₃P₄ structure type (space group I4̄3d, no. 220, Z = 4) into a Zintl phase by the introduction of trivalent rare-earth atoms. The structure features complex bonding, heavy elements, and electron count akin to that of valence-precise semiconductors, making it an ideal target for thermoelectrics development. Introducing crystallographic site disorders at the cation site for the Ca_4-xRE_xBi₃ phase and both the cation and anion sites for the Ca_4-xRE_xBi_3-δSb_δ phase brings about additional desirable characteristics for thermoelectric materials in the context of tuning knobs for lowering thermal conductivity. Electronic structure calculations of idealized Ca₃YBi₃ and Ca₃LaBi₃ compounds indicate the opening of indirect bandgaps at the Fermi level with magnitudes E_g = 0.38 eV and 0.57 eV, respectively. The electrical resistivity ρ(T) of some of the investigated phases measured on single crystals evolve in a metallic manner with magnitudes of order 1.4 mΩ cm near 500 K, thus supporting the notion of a degenerate semiconducting state, with the temperature dependence of the Seebeck coefficient α(T) suggesting the p-type behavior. The low electrical resistivity and the realization of a degenerate semiconducting state in the title phases present a window of opportunity for optimizing their carrier concentrations for enhanced thermoelectric performance.

Improved Thermal Stability and Enhanced Thermoelectric Properties of p-Type BaCu2Te2 by Doping of Cl

Doping in semiconductors is a widely implemented strategy for manipulation of carrier concentration, which is a critical parameter to regulate the thermoelectric performance. Stoichiometric BaCu₂Te₂ shows high hole concentration and unstable transport properties owing to the inherent Cu vacancy and dynamic precipitation behavior. In this work, Te has been partially substituted by Cl in BaCu₂Te₂ to suppress the overhigh hole concentration. Due to the high electronegativity of Cl, strong Cl-Cu bonds can significantly inhibit the Cu migration and the consequent dynamic precipitation. Meanwhile, nano-precipitate BaCl₂ distributes in the grain boundary, acting as ionic blocking layers. Therefore, the thermal stability of the samples can be essentially improved via chemical bonding strengthening and grain boundary engineering. In terms of thermal transport, the introduced point defects and second phase strengthen the short-wavelength and medium-wavelength phonon scattering, leading to further reduced thermal conductivity. Eventually, the repeatable ZT value of BaCu₂Te_1.98Cl_0.02 reached 1.22 at 823 K, which is higher by 19.6% compared with 1.02 of pristine BaCu₂Te₂. The average ZTs of BaCu₂Te_2-xCl_x (x = 0, 0.02, 0.04, and 0.06) in the temperature range of 323-823 K are 0.737 for x = 0.02, 0.689 for x = 0.04, and 0.667 for x = 0.06, which are 24.6, 17.2, and 13.4% higher than the average ZT of 0.588 corresponding to the undoped sample, respectively. The study shows that synergetic enhancements of thermal stability and thermoelectric properties can be achieved by strengthening chemical bonding and constructing ionic blocking layers in the grain boundary, which can be applied to other fast-ionic conductor thermoelectric materials.

Enhancement of the thermoelectric properties of Zintl phase SrMg2Bi2 by Na-doping

Due to their low lattice thermal conductivity and manipulable electronic properties, AB₂X₂ Zintl phases have been widely studied for thermoelectric applications. This has motivated numerous efforts to focus on the exploration of novel AB₂X₂ Zintl thermoelectrics. In this study, SrMg₂Bi₂ was systematically investigated to reveal its potential for thermoelectric application. Pristine SrMg₂Bi₂ exhibits an intrinsic p-type semiconducting behavior. The Hall carrier concentration (n_H) was efficiently increased to ∼9 × 10¹⁹ cm^-3 by Na-substitution at the Sr site. The increased n_H leads to enhanced electrical conductivity, but reduced Seebeck coefficient. Moreover, Na doping effectively decreased the thermal conductivity because of the intensified scattering from defects and lattice distortion. Thus, the zT of the Na-doped SrMg₂Bi₂ can reach 0.44 and be extremely higher than that of the pristine one. The well-known single parabolic band (SPB) model estimated that the increase in N_v and m* through doping boosts the electrical conductivity. This work sheds light on the discovery of new prospective thermoelectric materials and demonstrates that Bi-based p-type AB₂X₂ Zintl phases can achieve high thermoelectric performance.

From Three-Dimensional Clathrates to Two-Dimensional Zintl Phases AMSb2 (A = Rb, Cs; M = Ga, In) Composed of Pentagonal M-Sb Rings

Three new antimonide Zintl phases, RbGaSb₂, CsGaSb₂, and CsInSb₂, were discovered during exploration of corresponding A-M-Sb (A = Rb, Cs; M = Ga, In) ternary systems while searching for new clathrates. The AGaSb₂ phases crystallize in the tetragonal space group P4₂/nmc (No. 137) in the LiBS₂ structure type, while CsInSb₂ crystallizes in lower symmetry in the orthorhombic space group Cmce (No. 64) in the KGaSb₂ structure type with additional disorder of one of the Cs sites. The crystal structures of all three reported AMSb₂ compounds are composed of two-dimensional [MSb₂]^- tetrahedral layers separated by Rb⁺ or Cs⁺ cations. [MSb₂]^- layers are built from fused M-Sb pentagons and hexagons, which are also the main structural units for A₈M₂₇Sb₁₉ clathrate cages. The semiconductor nature of AMSb₂ was suggested by band structure calculations and confirmed by transport property characterization. CsGaSb₂ is a rare example of an n-type pnictide Zintl phase. All reported compounds exhibit low thermal conductivity typical for complex antimonides of heavy elements.

A novel ternary bismuthide, NaMgBi: crystal and electronic structure and electrical properties

A new ternary sodium magnesium bismuthide, NaMgBi, has been synthesized from the constituent metals, and its crystal structure was determined by single-crystal X-ray diffraction. NaMgBi crystallizes in a tetragonal PbFCl-type structure corresponding to the space group P4/nmm, where Z = 2, a = 4.7123(4) and c = 7.8158(7) Å. The structure is composed of layers formed by edge-sharing Bi tetrahedra centered with Mg stacked in the c-axis direction, and these layers sandwich the Na atoms. First-principles computations based on density functional theory calculations have verified that the most stable atomic configuration is the one in which the Na and Mg atoms occupy the 2a and 2c sites, respectively. The electrical resistivity measured for a sintered polycrystalline sample of NaMgBi with a relative density of 70% was found to gradually decrease from 868 to 26.4 mΩ cm upon increasing the temperature from 297 to 506 K, and the Seebeck coefficient decreased from 273 to 180 μV K−1 upon increasing the temperature from 298 to 496 K. Electronic structure calculations have revealed that NaMgBi must be a semiconductor with a small band gap of ∼0.1 eV.

Demonstration of valley anisotropy utilized to enhance the thermoelectric power factor

Valley anisotropy is a favorable electronic structure feature that could be utilized for good thermoelectric performance. Here, taking advantage of the single anisotropic Fermi pocket in p-type Mg3Sb2, a feasible strategy utilizing the valley anisotropy to enhance the thermoelectric power factor is demonstrated by synergistic studies on both single crystals and textured polycrystalline samples. Compared to the heavy-band direction, a higher carrier mobility by a factor of 3 is observed along the light-band direction, while the Seebeck coefficient remains similar. Together with lower lattice thermal conductivity, an increased room-temperature zT by a factor of 3.6 is found. Moreover, the first-principles calculations of 66 isostructural Zintl phase compounds are conducted and 9 of them are screened out displaying a pz-orbital-dominated valence band, similar to Mg3Sb2. In this work, we experimentally demonstrate that valley anisotropy is an effective strategy for the enhancement of thermoelectric performance in materials with anisotropic Fermi pockets.

Ternary ACd4P3 (A = Na, K) Nanostructures via a Hydride Solution-Phase Route

Complex pnictides such as I-II₄-V₃ compounds (I = alkali metal; II = divalent transition metal; V = pnictide element) display rich structural chemistry and interesting optoelectronic properties, but can be challenging to synthesize using traditional high-temperature solid-state synthesis. Soft chemistry methods can offer control over particle size, morphology, and properties. However, the synthesis of multinary pnictides from solution remains underdeveloped. Here, we report the colloidal hot-injection synthesis of ACd₄P₃ (A = Na, K) nanostructures from their alkali metal hydrides (AH). Control studies indicate that NaCd₄P₃ forms from monometallic Cd⁰ seeds and not from binary Cd₃P₂ nanocrystals. IR and ssNMR spectroscopy reveal tri-n-octylphosphine oxide (TOPO) and related ligands are coordinated to the ternary surface. Computational studies show that competing phases with space group symmetries R3̅m and Cm differ by only 30 meV/formula unit, indicating that synthetic access to either of these polymorphs is possible. Our synthesis unlocks a new family of nanoscale multinary pnictide materials that could find use in optoelectronic and energy conversion devices.

Theoretical Study on Thermoelectric Properties and Doping Regulation of Mg3X2 (X = As, Sb, Bi)

For searching both high-performances and better fits for near-room temperature thermoelectric materials, we here carried out a theoretical study on thermoelectric properties and doping regulation of Mg3X2 (X = As, Sb, Bi) by the combined method of first principle calculations and semi-classical Boltzmann theory. The thermoelectric properties of n-type Mg3As2, Mg3Sb2, and Mg3Bi2 were studied, and it was found that the dimensionless figures of merit, zT, are 2.58, 1.38, 0.34, and the p-type ones are 1.39, 0.64, 0.32, respectively. Furthermore, we calculated the lattice thermal conductivity of doped structures and screened out the structures with a relatively low formation energy to study the phonon dispersion and thermal conductivity in Mg3X2 (X = As, Sb, Bi). Finally, high thermoelectric zT and ultralow thermal conductivity of these doped structures was discussed.

Crystal structure characterization and electronic structure of a rare-earth-containing Zintl phase in the Yb-Al-Sb family: Yb3AlSb3

A rare-earth-containing compound, ytterbium aluminium antimonide, Yb₃AlSb₃ (Ca₃AlAs₃-type structure), has been successfully synthesized within the Yb-Al-Sb system through flux methods. According to the Zintl formalism, this structure is nominally made up of (Yb²⁺)₃[(Al^1-)(1b - Sb^2-)₂(2b - Sb^1-)], where 1b and 2b indicate 1-bonded and 2-bonded, respectively, and Al is treated as part of the covalent anionic network. The crystal structure features infinite corner-sharing AlSb₄ tetrahedra, [AlSb₂Sb_2/2]^6-, with Yb²⁺ cations residing between the tetrahedra to provide charge balance. Herein, the synthetic conditions, the crystal structure determined from single-crystal X-ray diffraction data, and electronic structure calculations are reported.

Superior Conversion Efficiency Achieved in GeP3/h-BN Heterostructures as Novel Flexible and Ultralight Thermoelectrics

GeP₃ materials are attracting broad research interest due to their typical puckered layer structure, high carrier mobility, and chemical stability. This peculiarity expedites the independent control of anisotropic electrical and thermal conductance, which is thus expected to possess great thermoelectric potential. Nevertheless, the metal characteristics of GeP₃ in the bulk and thick films are adverse to real application because of the low Seebeck coefficient. Thus, it is highly desirable to explore effective solutions to broaden the band gap and also maintain its excellent electrical conductance. Herein, we designed the interlaced GeP₃/hexagonal boron nitride (h-BN) bulk heterostructure using various component thicknesses. By using ab initio calculations based on the Boltzmann transport theory, we found that capping h-BN layer can obviously increase the band gap of the GeP₃ layer by 0.24 eV, and more interestingly, the anisotropic electronic structure in the GeP₃/h-BN heterostructure was accordingly modulated toward a favorable direction for high thermoelectricity. An ultrahigh ZT value of around 5 was predicted at 300 K in p-type GeP₃/h-BN, attributed to the adjusted multivalley band structure. Overall, our work provided an effective route to design novel high-performance thermoelectrics through the appropriate construction of heterostructures.

Enhanced Thermoelectric Performance of LiZnSb-Alloyed CaZn0.4Ag0.2Sb by Band Engineering

LiZnSb is a Zintl phase that has been predicted to be a good material in thermoelectric applications for a long time. However, experimental work indicated that the synthesized LiZnSb materials were p type, and their maximum zT value is only 0.08 at 525 K. CaZn_0.4Ag_0.2Sb, which belongs to the LiGaGe structure type and is also closely associated with the LiZnSb structure, did show high zT plateaus in a wide range of temperature, with the mixed transition metal Zn/Ag sites regulated. By comparing their crystallographic and electronic band structures, it is evident that the interlayered distances in both compounds have a great effect on the regulation of the corresponding electrical transport properties. When alloying CaZn_0.4Ag_0.2Sb with LiZnSb, solid solutions form within a specific range, which led to a marked enhancement in the Seebeck coefficient through the orbital alignment and carrier concentration optimization. In addition, a low thermal conductivity was obtained owing to the reduced electronic component. With the above optimization, a maximum zT value of ∼1.3 can be realized for (CaZn_0.4Ag_0.2Sb)_0.87(LiZnSb)_0.13 at 873 K, more than twice that of the pristine CaZn_0.4Ag_0.2Sb and about 10-fold compared to that of LiZnSb. This work may shed new light on the optimization of thermoelectric properties based on Zintl phases, for which the crystal structures are usually very complicated and a direct correlation between the structures and properties is difficult to make.

Complex Structural Disorder in the Zintl Phases Yb10MnSb9 and Yb21Mn4Sb18

A systematic investigation of the ternary system Yb-Mn-Sb led to the discovery of the novel phase Yb₁₀MnSb₉. Its crystal structure was characterized by single-crystal X-ray diffraction and found to be complex and highly disordered. The average Yb₁₀MnSb₉ structure can be considered to represent a defect modification of the Ca₁₀LiMgSb₉ type and to crystallize in the tetragonal P4₂/mnm space group (No. 136) with four formula units per cell. The structural disorder can be associated with both occupational and positional effects on several Yb and Mn sites. Similar traits were observed for the structure of the recently reported Yb₂₁Mn₄Sb₁₈ phase (monoclinic space group C2/c, No. 15), which was reevaluated as part of this study as well. In both structures, distorted Sb₆ octahedra centered by Yb atoms and Sb₄ tetrahedra centered by Mn atoms form disordered fragments, which appear as the hallmark of the structural chemistry in this system. Discussion along the lines of how difficult, and important, it is to distinguish Yb₁₀MnSb₉ from the compositionally similar binary Yb₁₁Sb₁₀ and ternary Yb₁₄MnSb₁₁ compounds is also presented. Preliminary transport measurements for polycrystalline Yb₁₀MnSb₉ indicate high values of the Seebeck coefficient, approaching 210 μV K^-1 at 600 K, and a semiconducting behavior with a room-temperature resistivity of 114 mΩ cm.

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.

Synthesis and Characterization of NaCd0.92Sn1.08, Na(Cd0.28Sn0.72)2 and Na2CdSn5 with Three-Dimensional Cd-Sn Frameworks

The crystal structures of three new ternary compounds, NaCd0.92Sn1.08 (I), Na(Cd0.28Sn0.72)2 (II), and Na2CdSn5 (III) synthesized in a sodium-cadmium-tin system were determined by single-crystal X-ray analysis to be the following: (I) LiGeZn-type structure (hexagonal, a = 4.9326(1) Å, c = 10.8508(3) Å, space group P-6m2); (II) CaIn2-type structure (hexagonal, a = 4.8458(2) Å, c = 7.7569(3) Å, P63/mmc); and (III) isotype with tI-Na2ZnSn5 (tetragonal, a = 6.4248(1) Å, c = 22.7993(5) Å, I-42d). Each compound has a three-dimensional framework structure mainly composed of four-fold coordinated Cd and Sn atoms with Na atoms located in the framework space. Elucidation of the electrical properties of the polycrystalline samples indicated that compounds (I) and (II) are polar intermetallics with metallic conductivity, and compound (III) is a semiconducting Zintl compound. These properties were consistent with the electronic structures calculated using the ordered structure models of the compounds.

Synthesis and structural characterization of orthorhombic Cu3–δ Sb (δ ≈ 0.1) and hexagonal Cu3Sb1–xInx (x ≈ 0.2) phases

Cu3Sb is a known copper-rich phase in the Cu–Sb binary phase diagram. It is reported to be dimorphic, with a low-temperature form adopting the orthorhombic Cu3Ti structure type (space group Pmmn, No. 59). The high-temperature form crystallizes in the cubic space group F m 3 ‾ m $Fm‾{3}m$ (No. 225), and is isostructural with BiF3. Neither polymorph has been carefully characterized to date, with both structures being assigned to the respective structure type, but never refined. With this study, we provide structural evidence, based on single-crystal and powder X-ray diffraction data that the low-temperature orthorhombic phase exists with a significant amount of defects on one of the Cu-sites. As a result, its composition is not Cu3Sb, but rather Cu3–δ Sb (δ = 0.13(1)). The cubic form could not be accessed as a part of this study, but another Cu-rich phase, Cu3Sb≈0.8In≈0.2, was also identified. It adopts the hexagonal Ni3Sn structure type (space group P63/mmc, No. 194) and represents an In-substituted variant of a hitherto unknown structural modification of Cu3Sb. Whether the latter can exist as a binary phase, or what is the minimum amount of In inclusions needed to stabilize it remains to be determined. Measurements of the thermopower of Cu3–δ Sb (δ = 0.13(1)) were conducted in the range of 300–600 K and demonstrated a maximum value of ca. 50 μV/K at 600 K, indicative of a p-type transport mechanism. Electrical resistivity measurements for the same sample confirmed that it exhibits metallic-like behavior, with a room temperature value of 0.43 mΩ cm. Electronic structure calculations show the absence of a band gap. Thermal analysis was utilized to ascertain the congruent melting of both phases.

Vacancy ordering induced topological electronic transition in bulk Eu2ZnSb2

Metal-semiconductor transitions from changes in edge chirality from zigzag to armchair were observed in many nanoribbon materials, especially those based on honeycomb lattices. Here, this is generalized to bulk complex Zintl semiconductors, exemplified by Eu₂ZnSb₂ where the Zn vacancy ordering plays an essential role. Five Eu₂ZnSb₂ structural models are proposed to guide transmission electron microscopy imaging. Zigzag vacancy ordering models show clear metallicity, while the armchair models are semiconducting with indirect bandgaps that monotonously increase with the relative distances between neighboring ZnSb₂ chains. Topological electronic structure changes based on cation ordering in a Zintl compound point toward tunable and possibly switchable topological behavior, since cations in these are often mobile. Thus, their orderings can often be adjusted by temperature, minor alloying, and other approaches. We explain the electronic structure of an interesting thermoelectric and point the way to previously unidentified types of topological electronic transitions in Zintl compounds.

First-principles prediction of structural stability and thermoelectric properties of SrGaSnH

Thermoelectric (TE) materials based on earth-abundant and non-toxic elements are very useful in cost-effective and eco-friendly waste heat management systems. The constituents of SrGaSnH are earth-abundant and non-toxic, thus we have chosen SrGaSnH to study its structural stability and thermoelectric properties by using density functional theory (DFT), density functional perturbation theory (DFPT), and semi-classical Boltzmann transport theory. Our elastic and phonons calculations show that the compound has good structural stability. The electronic structure calculation discloses that it is an indirect bandgap (0.63 eV by mBJ potential including spin-orbit coupling (SOC) effect) semiconductor. Light band hole effective mass leads to higher electrical conductivity along the x-axis than that of along the z-axis. On the other side, the weak phonon scattering leads to high lattice thermal conductivity ∼ 6.7 W m^-1 K^-1 at 300 K. Although the power factor (PF) is very high along the x-axis (above 10 mW m^-1 K^-2 at 300 K), such large κ _l dramatically reduces ZT. The maximum values of in-plane and cross-plane ZT are ∼1 (n-type), 0.8 (p-type) and 0.6 (n-type), (0.2 p-type) at 700 K, respectively. The present study has revealed that this compound has strong potential in eco-friendly TE applications.

Uncovering new transition metal Zintl phases by cation substitution: the crystal chemistry of Ca3CuGe3 and Ca2+nMnxAg2−x+zGe2+n−z (n = 3, 4)

Crystal engineering in transition metal Zintl phases.

Enhanced Average Thermoelectric Figure of Merit of p-Type Zintl Phase Mg2ZnSb2 via Zn Vacancy Tuning and Hole Doping

Isoelectronic Zn substitution at the Mg site has been proved to be effective in regulating the carrier concentration of p-type Mg₃Sb₂ Zintl phase. However, the reported thermoelectric performance is still unsatisfactory compared with that of n-type Mg₃Sb₂ due to the poor electrical transport properties. Here, we report an enhanced average ZT through improving low-temperature ZTs by introducing Zn vacancy followed suppressing the bipolar effect by doping. First, the Zn vacancy simultaneously increases the power factor and decreases the thermal conductivity, leading to a peak ZT value of ∼0.52 at 773 K in Mg₂Zn_0.98Sb₂. Additionally, doping Li or Ag at the Mg site is identified as a high-efficiency strategy for further increasing the carrier concentration and hence suppressing the bipolar effect. Finally, a peak ZT of ∼0.73 at 773 K and an average ZT of ∼0.46 between 300 and 773 K were obtained in Mg_1.98Li_0.02Zn_0.98Sb₂.

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.

Advanced Thermoelectric Design: From Materials and Structures to Devices

The long-standing popularity of thermoelectric materials has contributed to the creation of various thermoelectric devices and stimulated the development of strategies to improve their thermoelectric performance. In this review, we aim to comprehensively summarize the state-of-the-art strategies for the realization of high-performance thermoelectric materials and devices by establishing the links between synthesis, structural characteristics, properties, underlying chemistry and physics, including structural design (point defects, dislocations, interfaces, inclusions, and pores), multidimensional design (quantum dots/wires, nanoparticles, nanowires, nano- or microbelts, few-layered nanosheets, nano- or microplates, thin films, single crystals, and polycrystalline bulks), and advanced device design (thermoelectric modules, miniature generators and coolers, and flexible thermoelectric generators). The outline of each strategy starts with a concise presentation of their fundamentals and carefully selected examples. In the end, we point out the controversies, challenges, and outlooks toward the future development of thermoelectric materials and devices. Overall, this review will serve to help materials scientists, chemists, and physicists, particularly students and young researchers, in selecting suitable strategies for the improvement of thermoelectrics and potentially other relevant energy conversion technologies.

Coordination polymers for n-type thermoelectric applications

Coordination polymers (CPs) are potential thermoelectric (TE) materials to replace the sometimes costly, brittle and toxic heavy metal inorganic TEs for near-ambient-temperature applications. Air-stable and highly conductive p-type thermoelectric CPs are relatively well known, but the their n-type counterparts are only now emerging and both are needed for most practical applications. This perspective reviews recent advances in the development of n-type thermoelectric CPs, particularly the 1D and 2D metal bisdithiolenes, and introduces a relatively new class of guest@metal-organic framework(MOF)-based composites. Low dimensional CPs with reasonable n-type thermoelectric performance are emerging with good charge mobility and air-stability but still relatively low electrical conductivity.

On the New Oxyarsenides Eu5Zn2As5O and Eu5Cd2As5O

The new quaternary phases Eu5Zn2As5O and Eu5Cd2As5O have been synthesized by metal flux reactions and their structures have been established through single-crystal X-ray diffraction. Both compounds crystallize in the centrosymmetric space group Cmcm (No. 63, Z = 4; Pearson symbol oC52), with unit cell parameters a = 4.3457(11) Å, b = 20.897(5) Å, c = 13.571(3) Å; and a = 4.4597(9) Å, b = 21.112(4) Å, c = 13.848(3) Å, for Eu5Zn2As5O and Eu5Cd2As5O, respectively. The crystal structures include one-dimensional double-strands of corner-shared MAs4 tetrahedra (M = Zn, Cd) and As–As bonds that connect the tetrahedra to form pentagonal channels. Four of the five Eu atoms fill the space between the pentagonal channels and one Eu atom is contained within the channels. An isolated oxide anion O2– is located in a tetrahedral hole formed by four Eu cations. Applying the valence rules and the Zintl concept to rationalize the chemical bonding in Eu5M2As5O (M = Zn, Cd) reveals that the valence electrons can be counted as follows: 5 × [Eu2+] + 2 × [M2+] + 3 × [As3–] + 2 × [As2–] + O2–, which suggests an electron-deficient configuration. The presumed h+ hole is confirmed by electronic band structure calculations, where a fully optimized bonding will be attained if an additional valence electron is added to move the Fermi level up to a narrow band gap (Eu5Zn2As5O) or pseudo-gap (Eu5Cd2As5O). In order to achieve such a formal charge balance, and hence, narrow-gap semiconducting behavior in Eu5M2As5O (M = Zn, Cd), europium is theorized to be in a mixed-valent Eu2+/ Eu3+ state.

Realizing a High ZT of 1.6 in N-Type Mg3Sb2-Based Zintl Compounds through Mn and Se Codoping

Mg₃Sb₂-based compounds by virtue of nontoxicity and low-cost have become a promising class of candidates for midtemperature thermoelectric power generation. Here, we successfully fabricated n-type Mg₃Sb₂-based materials using an inexpensive and efficient approach of one-step ball milling and spark plasma sintering, and demonstrate that a complementary and favorable effect of multiple elements coalloying/-doping leads to an excellent thermoelectric performance. The intrinsic p-type conducting behavior for Mg₃Sb₂ could be changed to n-type through Bi and Se coalloying on Sb sublattices with excess Mg, resulting from the suppression of Mg vacancies and the formation of Mg interstitial. Furthermore, Mn doping on Mg sublattices could soften the chemical bonds, leading to the increase of carrier mobility and concentration simultaneously. Additionally, multielement coalloying/-doping could significantly increase the lattice disorder, which undoubtedly strengthens the phonon scattering and readily results in a suppressed lattice thermal conductivity. As a result, a highest ZT value of 1.6 at 723 K and an average ZT value up to 1.1 were obtained in the temperature range of 323-723 K in the Mg_3.18Mn_0.02Sb_1.5Bi_0.49Se_0.01 sample, which is one of the highest values among the Te free Mg₃Sb₂. This work could give guidance for improving the thermoelectric performance of Zintl phase materials or even others using the multielement codoping/-alloying strategy.