New Trick for an Old Dog: From Prediction to Properties of "Hidden Clathrates" Ba2Zn5As6 and Ba2Zn5Sb6

The zinc-antimony phase space has been heavily investigated due to the structural complexity and abundance of high-performing thermoelectric materials. Consequentially, the desire to use zinc and antimony as framework elements to encage rattling cations and achieve phonon-glass-electron-crystal-type properties has remained an enticing goal with only two alkali metal clathrates to date, Cs₈Zn₁₈Sb₂₈ and K₅₈Zn₁₂₂Sb₂₀₇. Guided by Zintl electron-counting predictions, we explored the Ba-Zn-Pn (Pn = As, Sb) phase space proximal to the expected composition of the type-I clathrate. In situ powder X-ray diffraction studies revealed two "hidden" compounds which can only be synthesized in a narrow temperature range. The ex situ synthesis and crystal growth unveiled that instead of type-I clathrates, compositionally close but structurally different new clathrate-like compounds formed, Ba₂Zn₅As₆ and Ba₂Zn₅Sb₆. These materials crystallize in a unique structure, in the orthorhombic space group Pmna with the Wyckoff sequence i²h⁶gfe. Single-phase synthesis enabled the exploration of their transport properties. Rattling of the Ba cations in oversized cages manifested low thermal conductivity, which, coupled with the high Seebeck coefficients observed, are prerequisites for a promising thermoelectric material. Potential for further optimization of the thermoelectric performance by aliovalent doping was computationally analyzed.

Aliovalent Dilute Doping and Nano-Moiré Fringe Advance the Structural Stability and Thermoelectric Performance in β-Zn4Sb3

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 n _H 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 κ _L = 0.2 W m^-1 K^-1 in a (Zn_0.992Ga_0.008)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.

Scattering lifetime and high figure of merit in CsAgO predicted by methods beyond relaxation time approximation

The electronic transport behaviour of CsAgO has been discussed using the theory beyond relaxation time approximation from room temperature to 800 K. Different scattering mechanisms such as acoustic deformation potential scattering, impurity phonon scattering, and polar optical phonon scattering are considered for calculating carrier scattering rates to predict the absolute values of thermoelectric coefficients. The scattering lifetime is of the order of 10^-14s. The lattice thermal transport properties like lattice thermal conductivity and phonon-lifetime have been evaluated. The calculated lattice thermal conductivity equals 0.12 and 0.18 W mK^-1along 'a' and 'c' axes, respectively, at room temperature, which is very low compared to state-of-the-art thermoelectric materials. The anisotropy in the electrical conductivity indicates that the holes are favourable for the out-of-plane thermoelectrics while the electrons for in-plane thermoelectrics. The thermoelectric figure of merit for holes and electrons is nearly same with a value higher than 1 at 800 K for different doping concentrations. The value of the thermoelectric figure of merit is significantly higher than the existing oxide materials, which might be appealing for future applications in CsAgO.

Structural evolution in thermoelectric zinc antimonide thin films studied by in situ X-ray scattering techniques

Zinc antimonides have been widely studied owing to their outstanding thermoelectric properties. Unlike in the bulk state, where various structurally unknown phases have been identified through their specific physical properties, a number of intermediate phases in the thin-film state remain largely unexplored. Here, in situ X-ray diffraction and X-ray total scattering are combined with in situ measurement of electrical resistivity to monitor the crystallization process of as-deposited amorphous Zn-Sb films during post-deposition annealing. The as-deposited Zn-Sb films undergo a structural evolution from an amorphous phase to an intermediate crystalline phase and finally the ZnSb phase during heat treatment up to 573 K. An intermediate phase (phase B) is identified to be a modified β-Zn8Sb7 phase by refinement of the X-ray diffraction data. Within a certain range of Sb content (∼42-55 at%) in the films, phase B is accompanied by an emerging Sb impurity phase. Lower Sb content leads to smaller amounts of Sb impurity and the formation of phase B at lower temperatures, and phase B is stable at room temperature if the annealing temperature is controlled. Pair distribution function analysis of the amorphous phase shows local ordered units of distorted ZnSb4 tetrahedra, and annealing leads to long-range ordering of these units to form the intermediate phase. A higher formation energy is required when the intermediate phase evolves into the ZnSb phase with a significantly more regular arrangement of ZnSb4 tetrahedra.

Nanoarchitectonics of p-type BiSbTe with improved figure of merit via introducing PbTe nanoparticles

Bi_0.4Sb_1.6Te₃ (BST) is known to be a unique p-type commercial thermoelectric (TE) alloy used at room temperatures, but its figure of merit (ZT) is relatively low for wide industrial applications. To improve its ZT value is vitally important. Here, we show that the incorporation of 0.5 wt% PbTe nanoparticles into BST concurrently causes a large enhancement of power factor (PF) and a significant reduction of lattice thermal conductivity κ_L. The increase in PF mainly benefits from the optimization of carrier concentration, maintenance of high carrier mobility and constant rise in Seebeck coefficient. The decrease in κ_L can be attributed to the enhanced phonon scattering by the dispersed PbTe nanoparticles and the interfaces between PbTe and the BST matrix by using the Callaway model. Specifically, an ultralow κ_L of 0.26 W m⁻¹ K⁻¹ at 429 K is achieved for the composites incorporating 0.5 wt% PbTe nanoinclusions. Consequently, an excellent ZT = 1.6 at 482 K and a high average ZT_ave = 1.38 at 300–500 K are achieved, indicating that incorporation of PbTe in BST is an effective approach to improve its thermoelectric performance.

We introduce 0.5 wt% PbTe nanoparticles into the Bi_0.4Sb_1.6Te₃ matrix and possess an ultralow lattice thermal conductivity of 0.26 W m⁻¹ K⁻¹ at 429 K and an excellent ZT value of 1.6 at 482 K as well as a high average ZT_ave of 1.38 at 300–500 K.

Compositional Fluctuations Locked by Athermal Transformation Yielding High Thermoelectric Performance in GeTe

Phase transition in thermoelectric (TE) material is a double-edged sword-it is undesired for device operation in applications, but the fluctuations near an electronic instability are favorable. Here, Sb doping is used to elicit a spontaneous composition fluctuation showing uphill diffusion in GeTe that is otherwise suspended by diffusionless athermal cubic-to-rhombohedral phase transition at around 700 K. The interplay between these two phase transitions yields exquisite composition fluctuations and a coexistence of cubic and rhombohedral phases in favor of exceptional figures-of-merit zT. Specifically, alloying GeTe by Sb₂ Te₃ significantly suppresses the thermal conductivity while retaining eligible carrier concentration over a wide composition range, resulting in high zT values of >2.6. These results not only attest to the efficacy of using phase transition in manipulating the microstructures of GeTe-based materials but also open up a new thermodynamic route to develop higher performance TE materials in general.

Achieving High Thermoelectric Performance in p-Type BST/PbSe Nanocomposites through the Scattering Engineering Strategy

To achieve high thermoelectric conversion efficiency in Bi_0.4Sb_1.6Te₃ (BST) alloy is vital for its applications in low-grade energy harvesting. Here, we show that 56% increase in the power factor (PF) (from 16 to 25 μW cm^-1 K^-2) and 32% reduction of lattice thermal conductivity κ_L (from 0.56 to 0.38 W m^-1 K^-1) as well as an approximately four-fold decrease in bipolar-effect contribution κ_b (from 0.48 to 0.12 W m^-1 K^-1) can be achieved at 512 K through the incorporation of 0.2 vol % PbSe nanoparticles in the BST matrix. Analyses indicate that the remarkable increase in PF for the composite samples can be mainly attributed to strong electron scattering at the large interface barriers, inhibiting effectively the electron contribution to the total thermopower at elevated temperatures, while the large drop of κ_L and κ_b originates from enhanced phonon scattering by PbSe nanoinclusions as well as phase boundaries (among BST and PbSe nanophase) and suppression of electron transport, respectively. As a result, a maximum figure of merit (ZT) of 1.56 (at 400 K) and an average ZT (ZT_ave) of 1.44 in the temperature range of 300-512 K are reached. Correspondingly, a record projected conversion efficiency η = 11% is achieved at the cold side 300 K and hot side 512 K in the BST-based composite incorporated with 0.2 vol % PbSe nanoinclusions.

Operando X-ray scattering study of thermoelectric β-Zn4Sb3

The application of thermoelectrics for energy harvesting depends strongly on operational reliability and it is therefore desirable to investigate the structural integrity of materials under operating conditions. We have developed an operando setup capable of simultaneously measuring X-ray scattering data and electrical resistance on pellets subjected to electrical current. Here, operando investigations of β-Zn₄Sb₃ are reported at current densities of 0.5, 1.14 and 2.3 A mm^-2. At 0.5 A mm^-2 no sample decomposition is observed, but Rietveld refinements reveal increased zinc occupancy from the anode to the cathode demonstrating zinc migration under applied current. At 1.14 A mm^-2 β-Zn₄Sb₃ decomposes into ZnSb, but pair distribution function analysis shows that Zn₂Sb₂ units are preserved during the decomposition. This identifies the mobile zinc in β-Zn₄Sb₃ as the linkers between the Zn₂Sb₂ units. At 2.3 A mm^-2 severe Joule heating triggers transition into the γ-Zn₄Sb₃ phase, which eventually decomposes into ZnSb, demonstrating Zn ion mobility also in γ-Zn₄Sb₃ under electrical current.

Thermoelectric power generation: from new materials to devices

Thermoelectric technology offers the opportunity of direct conversion between heat and electricity, and new and exciting materials that can enable this technology to deliver higher efficiencies have been developed in recent years. This mini-review covers the most promising advances in thermoelectric materials as they pertain to their potential in being implemented in devices and modules with an emphasis on thermoelectric power generation. Classified into three groups in terms of their operating temperature, the thermoelectric materials that are most likely to be used in future devices are briefly discussed. We summarize the state-of-the-art thermoelectric modules/devices, among which nanostructured PbTe modules are particularly highlighted. At the end, key issues and the possible strategies that can help thermoelectric power generation technology move forward are considered. This article is part of a discussion meeting issue 'Energy materials for a low carbon future'.

Design of Nano-Structured Micro-Thermoelectric Generator: Load Resistance and Inflections in the Efficiency

In recent years the interest for the harvest of energy with micro thermoelectric generators (μTEG) has increased, due to its advantages compared to technologies that use fossil fuels. There are three ways to improve the performance of the device, by modifying its structure, type of material and operation control. In this study, the role of the load resistance RL on the performance of a μTEG with nanostructured materials is investigated. The interaction of the load resistance with the thermoelements exhibits interesting features, arising from the coupling of the temperature-dependent electrical and thermal transport properties at different temperature ranges and the architecture of nanostructured thermoelectric materials. This coupling results in inflections on the efficiency, i.e., maximum and minimum values of the efficiency at higher temperatures, 600–900 K. We show the explicit dependence of the performance of the μTEG in terms of the load resistance and discuss the underlying physics. The unusual features of the efficiency of nanostructured thermoelectric materials are a result of the behavior of the power factor and the nonequilibrium properties of the system. We also analyze the effect of the geometric shape of the thermoelements on the device. We determine the performance of the μTEG, evaluating the generation power and its efficiency. The results show that the efficiency of the device can decrease or increase depending on the value of RL, while the power decreases with an increase of the load resistance.

The updated Zn-Sb phase diagram. How to make pure Zn13Sb10 ("Zn4Sb3")

The Zn-Sb system contains two well-known thermoelectric materials, Zn1-δSb and Zn13-δSb10 ("Zn4Sb3"), and two other phases, Zn9-δSb7 and Zn3-δSb2, stable only at high temperatures. The current work presents the updated phases diagram constructed using the high-temperature diffraction studies and elemental analysis. All phases are slightly Zn deficient with respect to their stoichiometric compositions, which is consistent with their p-type charge transport properties. Either at room or elevated temperatures, Zn1-δSb and Zn13-δSb10 display deficiencies of the main Zn sites and partial Zn occupancy of the other interstitial sites. A phase pure Zn13-δSb10 sample can be obtained from the Zn13Sb10 loading composition, and there is no need to use a Zn-richer composition such as Zn4Sb3. While the Zn13-δSb10 phase is stable till its decomposition temperature of 515 °C, it may incorporate some additional Zn around 412 °C, if elemental Zn is present.

Manipulation of Phonon Transport in Thermoelectrics

For several decades, thermoelectric advancements have largely relied on the reduction of lattice thermal conductivity (κ_L ). According to the Boltzmann transport theory of phonons, κ_L mainly depends on the specific heat, the velocity, and the scattering of phonons. Intensifying the scattering rate of phonons is the focus for reducing the lattice thermal conductivity. Effective scattering sources include 0D point defects, 1D dislocations, and 2D interfaces, each of which has a particular range of frequencies where phonon scattering is most effective. Because acoustic phonons are generally the main contributors to κ_L due to their much higher velocities compared to optical phonons, many low-κ_L thermoelectrics rely on crystal structure complexity leading to a small fraction of acoustic phonons and/or weak chemical bonds enabling an overall low phonon propagation velocity. While these thermal strategies are successful for advancing thermoelectrics, the principles used can be integrated with approaches such as band engineering to improve the electronic properties, which can promote this energy technology from niche applications into the mainstream.

In Situ TEM Studies of Nanostructured Thermoelectric Materials: An Application to Mg-Doped Zn4 Sb3 Alloy

We demonstrate an advanced approach using state of the art in situ transmission electron microscopy (TEM) to understand the interplay between nanostructures and thermoelectric (TE) properties of high-performance Mg-doped Zn₄ Sb₃ TE systems. By using the technique, microstructure and crystal evolutions of TE material have been dynamically captured as a function of temperature from 300 K to 573 K. On heating, we have clearly observed precipitation and growth of a Zn-rich secondary phase as nanoinclusions in the matrix of primary Zn₄ Sb₃ phase. Elemental mapping by STEM-EDX spectroscopy reveals enrichment of Zn in the secondary Zn₆ Sb₅ nanoinclusions during the thermal processing without decomposition. Such nanostructures strongly enhances phonon scattering, resulting in a decrease in the thermal conductivity leading to a zT value of 1.4 at 718 K.

Recent advances in inorganic material thermoelectrics

Time line of representative inorganic bulk thermoelectric materials from 1960s to the present.

Inhibition of minority transport for elevating the thermoelectric figure of merit of CuO/BiSbTe nanocomposites at high temperatures

In BiSbTe based composite with 0.2 vol% of CuO nanoinclusions, thermoelectric figure of merit increases monotonically and largest ZT = 1.37 was obtained at 496 K by inhibiting the transport of minorities at introduced asymmetric interface potentials.

High-pressure single crystal X-ray diffraction study of thermoelectric ZnSb and β-Zn4Sb3

The crystal structures of thermoelectric ZnSb and Zn₄Sb₃ have been studied by high pressure single crystal X-ray diffraction and the pressure behavior is different from thermal response.

Enhanced thermoelectric figure of merit in p-type β-Zn4Sb3/Bi0.4Sb1.6Te3 nanocomposites

In Bi_0.4Sb_1.6Te₃-based composites incorporated with 1.3 vol% β-Zn₄Sb₃ nanoparticles, figure of merit ZT = 1.43 is achieved at 443 K, which is ∼18% larger than that (=1.21) of Bi_0.4Sb_1.6Te₃ matrix.

Identification, structural characterization and transformations of the high-temperature Zn9-δSb7 phase in the Zn-Sb system

The Zn9-δSb7 phase has been identified via high-temperature powder diffraction studies. Zn9-δSb7 adopts two modifications: an α form stable between 514 °C and 539 °C and a Zn-poorer β form stable from 539 °C till its melting temperature of 581 °C. The Zn9-δSb7 structure was solved from the powder data using the simulated annealing approach. Both modifications adopt the same hexagonal structure (P6/mmm) but with slightly different lattice parameters. The α-to-β transformation is abrupt and first-order in nature. The Zn atoms occupy the tetrahedral holes created by Sb atoms. The ideal Zn9Sb7 composition can be explained by its tendency to adopt a charge balance configuration. Out of 7 Sb atoms, 3 Sb atoms form dimers (Sb(2-) ions) and 4 Sb atoms are isolated (Sb(3-) ions), which require 9 Zn(2+) cations for charge neutrality.

Elusive β-Zn8Sb7: A New Zinc Antimonide Thermoelectric

Zn8Sb7 was theoretically predicted to exhibit superior thermoelectric properties; however a crystalline phase with a similar composition was only stabilized in the form of nanoparticles. We report a new metastable compound, β-Zn8Sb7, which was synthesized in the form of bulk polycrystalline powder via high-temperature solid-state annealing followed by quenching. Single crystal X-ray diffraction indicates that β-Zn8Sb7 crystallizes in a new structure type (noncentrosymmetric orthorhombic space group Pmn21 (no. 31) with unit cell parameters of a = 15.029(1) Å, b = 7.7310(5) Å, c = 12.7431(9) Å, which is different from the nanoparticulate phase. According to differential scanning calorimetry, the β-Zn8Sb7 phase melts incongruently at 825(5) K. β-Zn8Sb7 is a p-type semiconductor with high Seebeck thermopower and low thermal conductivity stemming from the complex crystal structure. β-Zn8Sb7 exhibits a promising thermoelectric figure-of-merit, zT, of 0.33 at 400 K, which is comparable to the state-of-the-art thermoelectric materials based on binary zinc antimonides.

Superior intrinsic thermoelectric performance with zT of 1.8 in single-crystal and melt-quenched highly dense Cu(2-x)Se bulks

Practical applications of the high temperature thermoelectric materials developed so far are partially obstructed by the costly and complicated fabrication process. In this work, we put forward two additional important properties for thermoelectric materials, high crystal symmetry and congruent melting. We propose that the recently discovered thermoelectric material Cu2-xSe, with figure of merit, zT, over 1.5 at T of ~ 1000 K, should meet these requirements, based on our analysis of its crystal structure and the Cu-Se binary phase diagram. We found that its excellent thermoelectric performance is intrinsic, and less dependent on grain size, while highly dense samples can be easily fabricated by a melt-quenching approach. Our results reveal that the melt-quenched samples and single crystals exhibit almost the same superior thermoelectric performance, with zT as high as 1.7-1.8 at T of ~973 K. Our findings not only provide a cheap and fast fabrication method for highly dense Cu(2-x)Se bulks with superior thermoelectric performance, paving the way for possible commercialization of Cu2-xSe as an outstanding component in practical thermoelectric modules, but also provide guidance in searching for new classes of thermoelectric systems with high crystal symmetry or further improving the cost performance of other existing congruent-melting thermoelectric materials.

One pot green synthesis of graphene–iron oxide nanocomposite (GINC): an efficient material for enhancement of thermoelectric performance

We report for the first time, a green method for graphene–iron oxide nanocomposite (GINC) synthesis by dispersing graphene and nano iron oxide (Fe₂O₃) in ethanolviaultrasonication followed by micro-wave irradiation.

Fast direct synthesis and compaction of homogenous phase-pure thermoelectric Zn4Sb3

Zn4Sb3 is among the cheapest high performance thermoelectric materials, and it is made of relatively nontoxic elements. Strong activities are aimed at developing commercial power generation modules based on Zn4Sb3 making it vital to develop fast reliable synthesis processes for high-quality material. Here direct synthesis and compaction of homogeneous phase-pure thermoelectric Zn4Sb3 by spark plasma sintering (SPS) has been developed. Compared with the traditional quench and press method, the complexity and process time of the new method is very significantly reduced (order of magnitude), making large-scale production feasible. A composition gradient is observed in the pellet along the axis of applied pressure and current. The homogeneity of the pressed pellets is studied as a function of the SPS parameters: sintering time, applied current, sintering temperature and applied pressure, and the mechanism behind the formation of the gradient is discussed. The key finding is that pure and homogeneous Zn4Sb3 pellets can be produced by adding an extra layer of elemental Zn foil to compensate the Zn migration.

Low-cost high-performance zinc antimonide thin films for thermoelectric applications

Zinc antimonide thin films with high thermoelectric performance are produced by a simple sputtering method. The phase-pure Zn(4)Sb(3) and ZnSb thin films fulfill the key requirements for commercial TE power generation: cheap elements, cheap fabrication method, high performance and thermal stability. In addition, two completely new meta-stable crystalline phases of zinc antimonide have been discovered.

Electronic structure of thermoelectric Zn-Sb

The electronic structures of the two main compounds of the binary zinc antimonides that are stable at room temperature, Zn(1)Sb(1) and β-Zn(4)Sb(3), were probed with x-ray photoelectron spectroscopy. Additionally, electron energy loss measurements and density functional theory calculations are presented. The compounds are found to share a very similar electronic structure. They both feature only small charge transfers and differ moderately in their screening potentials. These results are in line with recent theoretical works on the Zn-Sb system and are discussed in light of the reported thermoelectric performance of the materials.

Predicted electronic and thermodynamic properties of a newly discovered Zn8Sb7 phase

A new binary compound, Zn(8)Sb(7), has recently been prepared in nanoparticulate form via solution synthesis. No such phase is known in the bulk phase diagram; instead, one would expect phase separation to the good thermoelectric semiconductors ZnSb and Zn(4)Sb(3). Here, density functional calculations are employed to determine the free energies of formation, including effects from vibrations and configurational disorder, of the relevant phases, yielding insight into the phase stability of Zn(8)Sb(7). Band structure calculations predict Zn(8)Sb(7), much like ZnSb and Zn(4)Sb(3), to be an intermetallic semiconductor with similar thermoelectric properties. If sufficient entropy or surface energy exists to stabilize the bulk material, it would be stable in a limited temperature window at high temperature.