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The elastic behavior of a material can be a powerful tool to decipher thermal transport. In thermoelectrics, measuring the elastic moduli - directly tied to sound velocity - is critical to understand trends in lattice thermal conductivity, as well as study bond anharmonicity and phase transitions, given the sensitivity of elastic moduli to the chemical bonding. In this review, we introduce the basics of elasticity and explain the origin of high-temperature lattice softening from a bonding perspective. We then review elasticity data throughout classes of thermoelectrics, and explore trends in sound velocity, anharmonicity, and thermal conductivity. We reveal how experimental sound velocities can improve the accuracy of common thermal conductivity models and present a critical discussion of Grüneisen parameter estimates from elastic moduli. Readers will be equipped with tools to leverage elasticity measurements or calculations to accurately interpret thermal transport trends.

A new family of Zintl antimonides with the approximate composition Ca10MSb9 (M = Ga, In, Mn, Zn) has been obtained via two synthetic approaches: (1) using Sb as a flux and (2) using the conventiona...

Alloys between Mg3Sb2 and Mg3Bi2 have recently been shown to be exceptional thermoelectric materials due in part to their anomalously low thermal conductivity. In the present study, in situ high-pr...

Accurate density functional theory calculations of the interrelated properties of thermoelectric materials entail high computational cost, especially as crystal structures increase in complexity and size. New methods involving ab initio scattering and transport (AMSET) and compressive sensing lattice dynamics are used to compute the transport properties of quaternary CaAl2Si2-type rare-earth phosphides RECuZnP2 (RE = Pr, Nd, Er), which were identified to be promising thermoelectrics from high-throughput screening of 20 000 disordered compounds. Experimental measurements of the transport properties agree well with the computed values. Compounds with stiff bulk moduli (>80 GPa) and high speeds of sound (>3500 m s−1) such as RECuZnP2 are typically dismissed as thermoelectric materials because they are expected to exhibit high lattice thermal conductivity. However, RECuZnP2 exhibits not only low electrical resistivity, but also low lattice thermal conductivity (∼1 W m−1 K−1). Contrary to prior assumptions, polar-optical phonon scattering was revealed by AMSET to be the primary mechanism limiting the electronic mobility of these compounds, raising questions about existing assumptions of scattering mechanisms in this class of thermoelectric materials. The resulting thermoelectric performance (zT of 0.5 for ErCuZnP2 at 800 K) is among the best observed in phosphides and can likely be improved with further optimization.

This paper presents tables of key thermoelectric properties, which define thermoelectric conversion efficiency, for a wide range of inorganic materials. The twelve families of materials included in these tables are primarily selected on the basis of well established, internationally-recognized performance and promise for current and future applications: tellurides, skutterudites, half Heuslers, Zintls, Mg-Sb antimonides, clathrates, FeGa3-type materials, actinides and lanthanides, oxides, sulfides, selenides, silicides, borides and carbides. As thermoelectric properties vary with temperature, data are presented at room temperature to enable ready comparison, and also at a higher temperature appropriate to peak performance. An individual table of data and commentary are provided for each family of materials plus source references for all the data., National Science Foundation; UK Engineering and Physical Sciences Research Council (EPSRC)

Crystals with anisotropic thermoelectric transport coefficients can yield a high figure-of-merit along the direction with the highest electronic mobility, provided the Seebeck coefficient and thermal conductivity are relatively isotropic. In this study, we combine experiment and theory to investigate the anisotropic properties of the quasi-1D Zintl phase Ca5In2Sb6. Ca5In2Sb6 is predicted by ab initio calculations to have extremely anisotropic p-type electrical conductivity and power factor, arising from light effective mass parallel to its ladder-like polyanionic chains. In contrast, the Seebeck coefficient and lattice thermal conductivity are predicted to be relatively isotropic. The latter is evidenced by the nearly isotropic computed speed of sound tensor and experimentally obtained thermal expansion coefficients. In order to characterize the anisotropic electrical conductivity, Ca5In2Sb6 single crystals were grown from an In-Sb rich molten flux and measured both parallel and perpendicular to the polyanionic chains. Due to the small crystal cross-sections, measurements perpendicular to the growth direction demanded a novel photolithography methodology whereby micro-ribbons were extracted using focused ion beam milling, and processed using laser photolithography to deposit contacts for electrical resistivity and Hall coefficient measurements. The conductivity parallel to the growth direction was found to be nearly 20x higher than the perpendicular direction, in agreement with our theoretical predictions. This study represents one of the first experimental confirmations of highly anisotropic electrical conductivity in Zintl thermoelectrics. (C) 2021 Elsevier Ltd. All rights reserved.

Accurate density functional theory calculations of the interrelated properties of thermoelectric materials entail high computational cost, especially as crystal structures increase in complexity and size. New methods involving ab initio scattering and transport (AMSET) and compressive sensing lattice dynamics are used to compute the transport properties of quaternary CaAl

The tetragonal fluoro-scheelite [Formula: see text] compounds ([Formula: see text]) have been shown to exhibit a variety of useful optical and magnetic properties. While [Formula: see text] has been widely studied, many of the fundamental thermodynamic properties of other members of this family remain unknown. Here, we report the complete elastic tensors ([Formula: see text]) of single-crystalline [Formula: see text], [Formula: see text], and [Formula: see text] using resonant ultrasound spectroscopy and density functional theory (DFT). We compare the results for [Formula: see text] with prior experimental results using time-of-flight ultrasound methods. This is the first report, however, of the experimental elastic tensors of [Formula: see text] and [Formula: see text]. The present results point to a softening of the elastic moduli of the [Formula: see text] system when Y is replaced by the larger ionic radius of Tb or Gd. Furthermore, we find that just 0.3% doping with Nd on the Y site also leads to a slight softening of the moduli. The variation of the elastic moduli as a function of temperature up to [Formula: see text] was also measured. A nearly linear softening of all seven independent elastic moduli was observed with increasing temperature. Phonon dispersions and phonon density of states obtained by DFT support the experimental finding of a significantly higher sound velocity due to lighter Y atoms in [Formula: see text], as compared to heavier [Formula: see text] and [Formula: see text].

In the study of thermoelectric materials, the elastic moduli are valuable for predicting and interpreting trends in lattice thermal conductivity and some aspects of electronic conductivity (e.g., electron-phonon interactions). Because elastic moduli are highly sensitive to the details of chemical bonding, they can also be used to detect subtle changes in crystal symmetry or site-disorder due to phase transitions. Consequently, measurements of elastic moduli are of fundamental value in the development of structure-property relationships in thermoelectric materials. However, experimental reports of the elastic constants of functional materials are relatively sparse, and temperature-dependent data is even less common. Further, unlike most types of functional materials, thermoelectric materials are used over an extremely wide temperature range (up to 1300 K in some cases). In our lab, we measure the elastic constants of polycrystalline and single crystalline thermoelectric materials across a wide temperature range (250–1300 K) using resonant ultrasound spectroscopy with Al2O3 buffer rods to separate the transducers from the hot zone. In this talk, I will discuss the high-temperature elastic properties of several classes of thermoelectric materials, including Mg3Sb2 and GeSe-AgBiSe2 alloys, emphasizing the role that such data can play in interpreting composition- and temperature-dependent trends in lattice thermal conductivity.

The atomistic origin of ultralow thermal conductivity in promising new thermoelectrics is revealed by experiments and simulations., The candidate thermoelectric compounds Mg3Sb2 and Mg3Bi2 show excellent performance near ambient temperature, enabled by an anomalously low lattice thermal conductivity (κl) comparable to those of much heavier PbTe or Bi2Te3. Contrary to common mass-trend expectations, replacing Mg with heavier Ca or Yb yields a threefold increase in κl in CaMg2Sb2 and YbMg2Bi2. Here, we report a comprehensive analysis of phonons in the series AMg2X2 (A = Mg, Ca, and Yb; X = Bi and Sb) based on inelastic neutron/x-ray scattering and first-principles simulations and show that the anomalously low κl of Mg3X2 has inherent phononic origins. We uncover a large phonon softening and flattening of low-energy transverse acoustic phonons in Mg3X2 compared to the ternary analogs and traced to a specific Mg-X bond, which markedly enlarges the scattering phase-space, enabling the threefold tuning in κl. These results provide key insights for manipulating phonon scattering without the traditional reliance on heavy elements.

In this work, we discover anomalously low lattice thermal conductivity (

Summary Refractory rare-earth tellurides with the Th 3 P 4 structure type have attracted considerable interest as high-performance thermoelectric materials since the 1980s due to their high dimensionless figure of merit ( ZT ). Extensive work has been conducted on La 3−x Te 4 with peak ZT values greater than 1.1 at 1,273 K. The high ZT of La 3-x Te 4 is in part due to a large peak in the density of states near the Fermi level from the La 5d states. Here, we revisit Pr 3−x Te 4 , for which our electronic structure calculations predict a favorable modification of the density of states by the introduction of praseodymium's 4f electrons. This was experimentally verified by preparing Pr 3−x Te 4 samples with varying Pr vacancy concentrations using a mechanochemical synthesis approach. The thermoelectric properties were measured and a ZT of 1.7 at 1,200 K was achieved with Pr 2.74 Te 4 . The 50% improvement in peak ZT compared with La 3−x Te 4 resulted from an increased effective mass, improved Seebeck coefficient, and lower thermal conductivity.

Compounds that crystallize in the layered CaAl2Si2 structural pattern have rapidly emerged as an exciting class of thermoelectric materials with attractive n- and p-type properties. More than 100 AM2X2 compounds that form this structure type – characterized by anionic M2X2 slabs sandwiched between layers of octahedrally coordinated A cations – provide numerous potential paths to chemically tune every aspect of thermoelectric transport. This review highlights the chemical diversity of this structure type, discusses the rules governing its formation and stability relative to competing AM2X2 structures (e.g., ThCr2Si2 and BaCu2S2), and attempts to bring some of the most recently discovered compounds into the spotlight. The discussion of thermoelectric transport properties in AM2X2 compounds focuses primarily on the intrinsic parameters that determine the potential for a high figure of merit: the band gap, effective mass, degeneracy, carrier relaxation time, and lattice thermal conductivity. We also discuss routes that have been used to successfully control the carrier concentration, including controlling the cation vacancy concentration, doping, and isoelectronic alloying (approaches that are highly interdependent). Finally, we discuss recent progress made towards n-type doping in this system, highlight opportunities for further improvements, as well as open questions that still remain.

Zintl Phases are a class of intermetallic compounds that combine ionic and covalent bonding to form charge-balanced structures, leading to semiconducting properties. The Ca5M2Sb6 family of Zintl compounds, (M =Al, Ga, In), have demonstrated promising thermoelectric efficiency, meaning they can be used to convert a temperature gradient into useful electrical energy. Ca5M2Sb6 compounds crystallize in a highly anisotropic structure comprised of chains of corner-linked MSb4 tetrahedra, in which each neighboring chain is joined via Sb-Sb covalent bonds, forming an infinite polyanionic “ladder”. In the present study, we used first principles calculations to study various surface terminations of the Ca5Ga2Sb6 structure as a mechanism to understand its crystal growth morphology. Our results indicate that the calcium stoichiometry of the crystal surface plays an important role in surface reconstruction, specifically leading to the formation or breaking of covalent bonds. Our results suggest that surface energy as a function of stoichiometry and crystallographic orientation is only one piece of the puzzle in understanding the growth habits of Ca5Ga2Sb6 crystals.

In the search for novel solid electrolytes for solid-state batteries, thiophosphate ionic conductors have been in recent focus owing to their high ionic conductivities, which are believed to stem from a softer, more polarizable anion framework. Inspired by the oft-cited connection between a soft anion lattice and ionic transport, this work aims to provide evidence on how changing the polarizability of the anion sublattice in one structure affects ionic transport. Here, we systematically alter the anion framework polarizability of the superionic argyrodites Li6PS5X by controlling the fractional occupancy of the halide anions (X = Cl, Br, I). Ultrasonic speed of sound measurements are used to quantify the variation in the lattice stiffness and Debye frequencies. In combination with electrochemical impedance spectroscopy and neutron diffraction, these results show that the lattice softness has a striking influence on the ionic transport: the softer bonds lower the activation barrier and simultaneously decrea...

Rare-earth (RE) tellurides have been studied extensively for use in high-temperature thermoelectric applications. Specifically, lanthanum and praseodymium-based compounds with the Th3P4 structure type have demonstrated dimensionless thermoelectric figures of merit (zT) up to 1.7 at 1200 K. Scandium, while not part of the lanthanide series, is considered a RE element due to its chemical similarity. However, little is known about the thermoelectric properties of the tellurides of scandium. Here, we synthesized scandium sesquitelluride (Sc2Te3) using a mechanochemical approach and formed sintered compacts through spark plasma sintering (SPS). Temperature-dependent thermoelectric properties were measured from 300&ndash, 1100 K. Sc2Te3 exhibited a peak zT = 0.3 over the broad range of 500&ndash, 750 K due to an appreciable power factor and low-lattice thermal conductivity in the mid-temperature range.

A M 2 X 2 compounds that crystallize in the CaAl 2 Si 2 structure type have emerged as a promising class of n- and p-type thermoelectric materials. Alloying on the cation (A) site is a frequently used approach to optimize the thermoelectric transport properties of A M 2 X 2 compounds, and complete solid solubility has been reported for many combinations of cations. In the present study, we investigate the phase stability of the AMg 2 Sb 2 system with mixed occupancy of Mg, Ca, Sr, or Ba on the cation (A) site. We show that the small ionic radius of Mg 2 + leads to limited solubility when alloyed with larger cations such as Sr or Ba. Phase separation observed in such cases indicates a eutectic-like phase diagram. By combining these results with prior alloying studies, we establish an upper limit for cation radius mismatch in A M 2 X 2 alloys to provide general guidance for future alloying and doping studies.

Thermoelectricity offers a sustainable path to recover and convert waste heat into readily available electric energy, and has been studied for more than two centuries. From the controversy between Galvani and Volta on the Animal Electricity, dating back to the end of the XVIII century and anticipating Seebeck’s observations, the understanding of the physical mechanisms evolved along with the development of the technology. In the XIX century Ørsted clarified some of the earliest observations of the thermoelectric phenomenon and proposed the first thermoelectric pile, while it was only after the studies on thermodynamics by Thomson, and Rayleigh’s suggestion to exploit the Seebeck effect for power generation, that a diverse set of thermoelectric generators was developed. From such pioneering endeavors, technology evolved from massive, and sometimes unreliable, thermopiles to very reliable devices for sophisticated niche applications in the XX century, when Radioisotope Thermoelectric Generators for space missions and nuclear batteries for cardiac pacemakers were introduced. While some of the materials adopted to realize the first thermoelectric generators are still investigated nowadays, novel concepts and improved understanding of materials growth, processing, and characterization developed during the last 30 years have provided new avenues for the enhancement of the thermoelectric conversion efficiency, for example through nanostructuration, and favored the development of new classes of thermoelectric materials. With increasing demand for sustainable energy conversion technologies, the latter aspect has become crucial for developing thermoelectrics based on abundant and non-toxic materials, which can be processed at economically viable scales, tailored for different ranges of temperature. This includes high temperature applications where a substantial amount of waste energy can be retrieved, as well as room temperature applications where small and local temperature differences offer the possibility of energy scavenging, as in micro harvesters meant for distributed electronics such as sensor networks. While large scale applications have yet to make it to the market, the richness of available and emerging thermoelectric technologies presents a scenario where thermoelectrics is poised to contribute to a future of sustainable future energy harvesting and management. This work reviews the broad field of thermoelectrics. Progress in thermoelectrics and milestones that led to the current state-of-the-art are presented by adopting an historical footprint. The review begins with an historical excursus on the major steps in the history of thermoelectrics, from the very early discovery to present technology. A panel on the theory of thermoelectric transport in the solid state reviews the transport theory in complex crystal structures and nanostructured materials. Then, the most promising thermoelectric material classes are discussed one by one in dedicated sections and subsections, carefully highlighting the technological solutions on materials growth that have represented a turning point in the research on thermoelectrics. Finally, perspectives and the future of the technology are discussed in the framework of sustainability and environmental compatibility.

KTb 3F 10 (KTF) has been developed in recent years as a candidate Faraday rotator material because of its cubic symmetry, high figures of merit, and low absorption coefficient. While considerable efforts have focused on crystal growth and optical properties, investigations of fundamental thermodynamic behavior of KTF have been limited. Here, we report elastic moduli C 11, C 12, and C 44 of single crystalline KTF measured by resonant ultrasound spectroscopy from 280 K up to 300 K and the obtained temperature derivatives of each modulus. We additionally report the single crystal elastic moduli of the rare-earth garnet Y 3Al 5O 12 (YAG), yielding results that agree well with the prior literature. We found C 11 = 135.62 GPa, C 12 = 58.11 GPa, and C 44 = 44.81 GPa for KTF and C 11 = 332.43 GPa, C 12 = 109.58 GPa, and C 44 = 114.81 GPa for YAG at room temperature. The present results have been compared with previous experimental and theoretical results and with common oxide and fluoride optical materials, revealing the relative softness of KTF. The low elastic moduli of KTF are, in turn, responsible for its low thermal conductivity and low Debye frequency compared to other laser host materials.

In the design of materials with low lattice thermal conductivity, compounds with high density, low speed of sound, and complexity at either the atomic, nano- or microstructural level are preferred. The layered compound Mg$_3$Sb$_2$ defies these prevailing paradigms, exhibiting lattice thermal conductivity comparable to PbTe and Bi$_2$Te$_3$, despite its low density and simple structure. The excellent thermoelectric performance ($zT$ $\sim$ 1.5) in $n$-type Mg$_3$Sb$_2$ has thus far been attributed to its multi-valley conduction band, while its anomalous thermal properties have been largely overlooked. To explain the origin of the low lattice thermal conductivity of Mg$_3$Sb$_2$, we have used both experimental methods and ab initio phonon calculations to investigate trends in the elasticity, thermal expansion and anharmonicity of $A$Mg$_2Pn_2$ Zintl compounds with $A$ = Mg, Ca, Yb, and $Pn$ = Sb and Bi. Phonon calculations within the quasi-harmonic approximation reveal large mode Gr\"uneisen parameters in Mg$_3$Sb$_2$ compared with isostructural compounds, in particular in transverse acoustic modes involving shearing of adjacent anionic layers. Measurements of the elastic moduli and sound velocity as a function of temperature using resonant ultrasound spectroscopy provide a window into the softening of the acoustic branches at high temperature, confirming their exceptionally high anharmonicity. We attribute the anomalous thermal behavior of Mg$_3$Sb$_2$ to the diminutive size of Mg, which may be too small for the octahedrally-coordinated site, leading to weak, unstable interlayer Mg-Sb bonding. This suggests more broadly that soft shear modes resulting from undersized cations provide a potential route to achieving low lattice thermal conductivity low-density, earth-abundant materials., Comment: 11 pages, 9 figures

Solid-solution Zintl compounds with the formula Eu11Cd6–xZnxSb12 have been synthesized from the elements as single crystals using a tin flux according to the stoichiometry Eu:Cd:Zn:Sb:Sn of 11:6–xp:xp:12:30 with xp = 0, 1, 2, 3, 4, 5, and 6, where xp is the preparative amount of Zn employed in the reaction. The crystal structures and the compositions were established by single-crystal as well as powder X-ray diffraction and wavelength-dispersive X-ray analysis measurements. The title solid-solution Zintl compounds crystallize isostructurally in the centrosymmetric monoclinic space group C 2/m (No. 12, Z = 2) as the Sr11Cd6Sb12 structure type (Pearson symbol mC58). There is a miscibility gap at 3 ≤ xp ≤ 4 where the major product crystallizes in a disordered structure related to the Ca9Mn4Bi9 structure type; otherwise, for all other compositions, the Sr11Cd6Sb12 structure is the majority phase. Eu11Cd6Sb12 shows lower lattice thermal conductivity relative to Eu11Zn6Sb12 consistent with its higher mean atomi...

BaZn2Sn2 (space group P4/nmm, a = 4.7459(5) A, c = 11.330(2) A, Z = 2) crystallizes in the CaBe2Ge2 structure type with a polyanionic framework comprising alternately stacked PbO-like {ZnSn4/4} and anti-PbO-like {SnZn4/4} layers along the c-axis. BaZn2Sn2 samples were obtained by either direct solid state reaction of the elements or from a Sn-flux method in very high yield with very small amount of β-Sn as the secondary phase. The samples were characterized by powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM). The chemical compositions were determined to be off-stoichiometric with Zn/Sn ratio lower than 1.0 and Sn2 atoms in the crystal structure were found to be either loosely bonded or not bonded which might lead to an incomplete charge balance. Electrical and thermal transport measurements have been performed in the temperature range 300–773 K. BaZn2Sn2 displays the electrical resistivity of a metal (or semimetal) along with very low Seebeck coefficients and relatively high thermal conductivity.

In the search for novel solid electrolytes for solid-state batteries, thiophosphate ionic conductors have been in recent focus owing to their high ionic conductivities, which are believed to stem from a softer, more polarizable anion framework. Inspired by the oft-cited connection between a soft anion lattice and ionic transport, this work aims to provide evidence on how changing the polarizability of the anion sublattice in one structure affects ionic transport. Here, we systematically alter the anion framework polarizability of the superionic argyrodites Li

Zintl phases are compounds that have shown promise for thermoelectric applications. The title solid–solution Zintl compounds were prepared from the elements as single crystals using a tin flux for compositions x = 0, 1, 2, and 3. Eu_(11)Cd_6Sb_(12–x)As_x (x < 3) crystallize isostructurally in the centrosymmetric monoclinic space group C2/m (no. 12, Z = 2) as the Sr_(11)Cd_6Sb_(12) structure type (Pearson symbol mC58). Efforts to make the As compositions for x exceeding ~3 resulted in structures other than the Sr_(11)Cd_6Sb_(12) structure type. Single-crystal X-ray diffraction indicates that As does not randomly substitute for Sb in the structure but is site specific for each composition. The amount of As determined by structural refinement was verified by electron microprobe analysis. Electronic structures and energies calculated for various model structures of Eu_(11)Cd_6Sb_(10)As_2 (x = 2) indicated that the preferred As substitution pattern involves a mixture of three of the six pnicogen sites in the asymmetric unit. In addition, As substitution at the Pn4 site opens an energy gap at the Fermi level, whereas substitution at the other five pnicogen sites remains semimetallic with a pseudo gap. Thermoelectric properties of these compounds were measured on hot-pressed, fully densified pellets. Samples show exceptionally low lattice thermal conductivities from room temperature to 775 K: 0.78–0.49 W/mK for x = 0; 0.72–0.53 W/mK for x = 1; and 0.70–0.56 W/mK for x = 2. Eu_(11)Cd_6Sb_(12) shows a high p-type Seebeck coefficient (from +118 to 153 μ V/K) but also high electrical resistivity (6.8 to 12.8 mΩ·cm). The value of zT reaches 0.23 at 774 K. The properties of Eu_(11)Cd_6Sb_(12–x)As_x are interpreted in discussion with the As site substitution.

Yb9Mn4.2Sb9 has been shown to have extremely low thermal conductivity and a high thermoelectric figure of merit attributed to its complex crystal structure and disordered interstitial sites. Motivated by previous work which shows that isoelectronic substitution of Mn by Zn leads to higher mobility by reducing spin disorder scattering, this study investigates the thermoelectric properties of the solid solution, Yb9Mn4.2−xZnxSb9 (x = 0, 1, 2, 3 and 4.2). Measurements of the Hall mobility at high temperatures (up to 1000 K) show that the mobility can be increased by more than a factor of 3 by substituting Zn into Mn sites. This increase is explained by the reduction of the valence band effective mass with increasing Zn, leading to a slightly improved thermoelectric quality factor relative to Yb9Mn4.2Sb9. However, increasing the Zn-content also increases the p-type carrier concentration, leading to metallic behavior with low Seebeck coefficients and high electrical conductivity. Varying the filling of the interstitial site in Yb9Zn4+ySb9 (y = 0.2, 0.3, 0.4 and 0.5) was attempted, but the carrier concentration (∼1021 cm−3 at 300 K) and Seebeck coefficients remained constant, suggesting that the phase width of Yb9Zn4+ySb9 is quite narrow.

GeTe-Sb 2Te 3 alloys have been widely studied for use in rewritable media, and in recent years, they have emerged as excellent thermoelectric materials, with reports of zT>2 for Ge-rich compositions. GeTe-Sb 2Te 3 alloys exhibit a solid-state phase transition from a layered structure with rhombohedral symmetry to a cubic rocksalt structure, which plays an important role in their thermoelectric behavior. Here, we investigate the impact of the phase transition on the thermal expansion and elastic moduli of (GeTe) 17Sb 2Te 3 using high-temperature X-ray diffraction and resonant ultrasound spectroscopy. The high-temperature elastic moduli of GeTe, Sb2Te3, and Bi 2Te 3 were also measured for comparison. While it is typical for materials to soften with increasing temperature due to thermal expansion, our study reveals anomalous hardening of the elastic moduli in (GeTe) 17Sb 2Te 3 at temperatures below the phase transition, followed by further hardening at the transition temperature. In contrast, the elastic moduli of GeTe, Sb 2Te 3, and Bi 2Te 3 soften with increasing temperature. We attribute the anomalous hardening of (GeTe) 17Sb 2Te 3 to the gradual vacancy diffusion accompanying the transition from a layered to a cubic structure. The stiffening elastic moduli lead to increasing speed of sound, which impacts the lattice thermal conductivity by flattening the temperature dependence.

Thermoelectric materials directly convert thermal energy into electrical energy, offering a promising solid-state solution for waste heat recovery. For thermoelectric devices to make a significant impact on energy and the environment the major impediments are the efficiency, availability and toxicity of current thermoelectric materials. Typically, efficient thermoelectric materials contain heavy elements such as lead and tellurium that are toxic and not earth abundant. Many materials with unusual structures containing abundant and benign elements are known, but remain unexplored for thermoelectric applications. In this paper we demonstrate, with the discovery of high thermoelectric efficiency in Ca3AlSb3, the use of elementary solid-state chemistry and physics to guide the search and optimization of such materials.

Understanding transport in Zintl compounds is important due to their unusual chemistry, structural complexity, and potential for good thermoelectric performance. Resistivity measurements indicate that undoped Ca_5Al_2Sb_6 is a charge-balanced semiconductor with a bandgap of 0.5 eV, consistent with Zintl–Klemm charge counting rules. Substituting divalent calcium with monovalent sodium leads to the formation of free holes, and a transition from insulating to metallic electronic behavior is observed. Seebeck measurements yield a hole mass of ∼2m_e, consistent with a structure containing both ionic and covalent bonding. The structural complexity of Zintl compounds is implicated in their unusually low thermal conductivity values. Indeed, Ca_5Al_2Sb_6 possesses an extremely low lattice thermal conductivity (0.6 W mK^(−1) at 850 K), which approaches the minimum thermal conductivity limit at high temperature. A single parabolic band model is developed and predicts that Ca_(4.75)Na_(0.25)Al_2Sb_6 possesses a near-optimal carrier concentration for thermoelectric power generation. A maximum zT > 0.6 is obtained at 1000 K.Beyond thermoelectric applications, the semiconductor Ca_5Al_2Sb_6 possesses a 1D covalent structure which should be amenable to interesting magnetic interactions when appropriately doped.

Single crystals of Eu11Cd6-xZnxSb12 (0 ≤ x ≤ 6) are prepared by reaction of the elements in a Sn flux (950 °C; cooling to 650 °C with a rate of 5 °C/h).

Intermetallic Zintl phases form complex composite structures characterized by covalently-bonded polyanions stabilized by the surrounding cations. The Zintl polyanions occur in an astounding variety of forms, including isolated 0-dimensional clusters, 1-D chains, and 2-D slabs, making this an ideal class of compounds to study structure-property relationships in complex semiconductors. AM 2 X 2 compounds in the CaAl2Si2 structure type are one of the most successful classes of Zintl thermoelectrics - particularly in light of recent reports of zT ~ 1.5 in n-type MgMg2Sb2. This talk will explore the role played by the defect chemistry of AM 2 X 2 compounds. We have shown both experimentally and using first principles calculations that the low formation energy for cation vacancies in this structure type enables tuning of the electronic properties via either alloying or by directly varying the cation content. In addition, by using high pressure, we have investigated the transport properties of AM 2 X 2 compounds synthesized using high pressure. While AM 2 X 2 compounds in the CaAl2Si2 structure are typically narrow gap semiconductors, those in the related tetragonal BaZn2P2 structure type are exclusively metals or semimetals and are known to exhibit superconductivity. In the first example of a phase transition between these two technologically important structures types, we show that SrAl2Si2 (trigonal CaAl2Si2-type structure at ambient pressure) can be obtained as a metastable phase in the tetragonal BaZn2P2 structure type by employing high-pressure, high-temperature synthesis. Our study shows that the changes in the electronic density of states lead to the emergence of phonon-driven superconductivity in the tetragonal polymorph. By accessing structural motifs that are not thermodynamically stable at ambient conditions, we are thus able to directly investigate the relationship between structure and electronic properties in AM 2 X 2 Zintls.

Single crystals of the title compounds are prepared from the elements in a Sn flux (alumina crucibles, 950 °C, 96 h) and characterized by powder and single crystal XRD, transport properties measurements, and TB-LMTO-ASA electronic structure calculations.

Motivated by excellent thermoelectric performance in the well-known Yb-based Zintl compounds Yb_(14)MnSb_(11) and YbZn_(2−x)Mn_xSb_2, this study investigates the thermoelectric properties of Yb_9Mn_(4.2)Sb_9. Unlike most transition metal containing Zintl phases, Yb_9Mn_(4.2)Sb)9 contains a partially occupied Mn site and thus does not have a valence-precise stoichiometry. Samples were synthesized by direct ball milling of the elements, followed by hot pressing. Consistent with previous reports, X-ray diffraction and wavelength dispersive spectroscopy confirmed a narrow composition range near Yb_9Mn_(4.2)Sb_9. High temperature measurements of the electronic properties of Yb_9Mn_(4.2)Sb_9 indicate that it is a degenerate p-type semiconductor with a band gap sufficiently large for high temperature thermoelectric applications. Hall measurements reveal that Yb_9Mn_(4.2)Sb_9 has a high extrinsic carrier concentration (~10^(20) h^+ cm^(−3)), which is due to the deviation from the theoretical “Zintl composition” of Yb_9Mn_(4.5)Sb_9. The measured carrier concentration coincides with the optimum concentration predicted using a single parabolic band model. Measurements of the thermal diffusivity and heat capacity reveal an extremely low, temperature-independent lattice thermal conductivity in this compound (κ_L < 0.4 W mK^(−1)), which is due to both the large unit cell size (44 atoms per primitive cell) and substantial disorder on the Mn site. This favorable combination of optimized electronic properties and low lattice thermal conductivity leads to a promising figure of merit at high temperature (zT = 0.7 at 950 K).

ScCeZrO2 ceramic powders produced by two different manufacturers have been used for the manufacturing of ScCeZrO2 electrolyte based solid oxide fuel cells. Thin, porous anode and cathode layers have been deposited on 150 μm thick, dense electrolyte tapes. Preliminary electrochemical testing of the cells has been performed.