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Lithium ion conducting argyrodites are among the most studied solid electrolytes due to their high ionic conductivities. A major concern in a solid-state battery is the solid electrolyte stability. Here we present a systematic study on the influence of cationic and anionic substitution on the electrochemical stability of Li6PS5X, using step-wise cyclic voltammetry, optical band gap measurements, hard X-ray photoelectron spectroscopy along with first-principles calculations. We observe that going from Li6PS5Cl to Li6+xP1-xMxS5I (M = Si4+, Ge4+), the oxidative degradation does not change. Considering the chemical bonding shows that the valence band edges are mostly populated by non-bonding orbitals of the PS43- units or unbound sulfide anions and that simple substitutions in these sulfide-based solid electrolytes cannot improve oxidative stabilities. This work provides insights on the role of chemical bonding on the stability of superionic conductors and shows that alternative strategies are needed for long-term stable solid-state batteries.

The lithium argyrodites Li6PS5X (X = Cl, Br, and I) have attracted interest as fast solid ionicconductors for solid-state battery. Within this class of materials, it has been previously suggestedthat more polarizable anions and larger substituents should influence the ionic conductivity (e.g.,substituting S by Se). Building upon this work, we explore the influence of Sn substitution inlithium argyrodites Li6+xSnxP1−xSe5I in direct comparison to the previously-reportedLi6+xSnxP1−xS5I series. The (P5+/Sn4+)Se43/4- polyhedral volume, unit cell volume, and lithiumcoordination tetrahedra Li(48h)-(S/Se)3-I increase with Sn substitution in this new selenideseries. Impedance spectroscopy reveals that increasing Sn4+ substitution results in a five-foldimprovement in the ionic conductivity when compared to Li6PSe5I. This work provides furtherunderstanding of compositional influences for optimizing the ionic conductivity of solidelectrolytes.

The recent interest in halide-based solid electrolytes Li3MX6 (M = Y, Er, and In; X = Cl, Br, and I) shows these materials to be promising candidates for solid-state battery application, due to the...

The development of high-performance inorganic solid electrolytes is central to achieving high-energy- density solid-state batteries. Whereas these solid-state materials are often prepared via classic solid-state syntheses, recent efforts in the community have shown that mechanochemical reactions, solution syntheses, microwave syntheses, and various post-synthetic heat treatment routines can drastically affect the structure and microstructure, and with it, the transport properties of the materials. On the one hand, these are important considerations for the upscaling of a materials processing route for industrial applications and industrial production. On the other hand, it shows that the influence of the different syntheses on the materials' properties is neither well understood fundamentally nor broadly internalized well. Here we aim to review the recent efforts on understanding the influence of the synthetic procedure on the synthesis – (micro)structure – transport correlations in superionic conductors. Our aim is to provide the field of solid-state research a direction for future efforts to better understand current materials properties based on synthetic routes, rather than having an overly simplistic idea of any given composition having an intrinsic conductivity. We hope this review will shed light on the underestimated influence of synthesis on the transport properties of solid electrolytes toward the design of syntheses of future solid electrolytes and help guide industrial efforts of known materials., Influence of synthesis and processing on the nature of ultimate product and the ionic transport properties of superionic conductors.

Sulfidic sodium ion conductors are currently investigated for the possible use in all-solid-state sodium ion batteries. The design of high performing electrolytes in terms of temperature-dependent ...

Owing to highly conductive solid ionic conductors, all-solid-state batteries attract significant attention as promising next-generation energy storage devices. A lot of research is invested in the search and optimization of solid electrolytes with higher ionic conductivity. However, a systematic study of an interlaboratory reproducibility of measured ionic conductivities and activation energies is missing, making the comparison of absolute values in literature challenging. In this study, we perform an uncertainty evaluation via a Round Robin approach using different Li-argyrodites exhibiting orders of magnitude different ionic conductivities as reference materials. Identical samples are distributed to different research laboratories and the conductivities and activation barriers are measured by impedance spectroscopy. The results show large ranges of up to 4.5 mScm-1 in the measured total ionic conductivity (1.3 – 5.8 mScm-1 for the highest conducting sample, relative standard deviation 35 – 50% across all samples) and up to 128 meV for the activation barriers (198 – 326 meV, relative standard deviation 5 – 15%, across all samples), presenting the necessity of a more rigorous methodology including further collaborations within the community and multiplicate measurements.

The lithium argyrodites Li

Halide-based solid electrolytes are currently growing in interest in solid-state batteries due to their high electrochemical stability window compared to sulfide electrolytes. However, often a bilayer separator of a sulfide and a halide is used and it is unclear why such setup is necessary, besides the instability of the halides against lithium metal. It is shown that an electrolyte bilayer improves the capacity retention as it suppresses interfacial resistance growth monitored by impedance spectroscopy. By using in-depth analytical characterization of buried interphases by time-of-flight secondary ion mass spectrometry and focused ion beam scanning electron microscopy analyses, an indium-sulfide rich region is detected at the halide and sulfide contact area, visualizing the chemical incompatibility of these two electrolytes. The results highlight the need to consider more than just the electrochemical stability of electrolyte materials, showing that chemical compatibility of all components may be paramount when using halide-based solid electrolytes in solid-state batteries.

The exploitation of high-capacity conversion-type materials such as sulfur in solid-state secondary batteries is a dream combination for achieving improved battery safety and high energy density in the push towards a sustainable future. Yet, the exact rate-limiting step, bottlenecking further development of solid-state lithium-sulfur batteries, has not been determined. Here, we directly visualize the spatial distribution of lithium via neutron imaging during operation and show that sluggish macroscopic ion transport within the composite cathode is rate-limiting. Observing a reaction front propagating from the separator side towards the current collector confirms detrimental influences of a low effective ionic conductivity. Furthermore, irreversibly concentrated lithium in the vicinity of the current collector, revealed via state-of-charge-dependent tomography, highlights a hitherto-overlooked loss mechanism triggered by sluggish effective ionic transport within a composite cathode. This discovery will be a cornerstone for future research on solid-state batteries, irrespective of the type of active material.

The exploitation of high-capacity conversion-type materials such as sulfur in solid-state secondary batteries is a dream combination for achieving improved battery safety and high energy density in the push towards a sustainable future. Yet, the exact rate-limiting step, bottlenecking further development of solid-state lithium-sulfur batteries, has not been determined. Here, we directly visualize the spatial distribution of lithium via neutron imaging during operation and show that sluggish macroscopic ion transport within the composite cathode is rate-limiting. Observing a reaction front propagating from the separator side towards the current collector confirms detrimental influences of a low effective ionic conductivity. Furthermore, irreversibly concentrated lithium in the vicinity of the current collector, revealed via state-of-charge-dependent tomography, highlights a hitherto-overlooked loss mechanism triggered by sluggish effective ionic transport within a composite cathode. This discovery will be a cornerstone for future research on solid-state batteries, irrespective of the type of active material.

Owing to their intrinsically low thermal conductivity and chemical diversity, materials within the I-V-VI2 family, and especially AgBiSe2, have recently attracted interest as promising thermoelectric materials. However, further investigations are needed in order to develop a more fundamental understanding of the origin of the low thermal conductivity in AgBiSe2, to evaluate possible stereochemical activity of the 6s2 lone pair of Bi3+, and to further elaborate on chemical design approaches for influencing the occurring phase transitions. In this work, a combination of temperature-dependent X-ray diffraction, Rietveld refinements of laboratory X-ray diffraction data, and pair distribution function analyses of synchrotron X-ray diffraction data is used to tackle the influence of Sb substitution within AgBi1-xSbxSe2 (0 ⩽ x ⩽ 0.15) on the phase transitions, local distortions, and off-centering of the structure. This work shows that, similar to other lone-pair-containing materials, local off-centering and distortions can be found in AgBiSe2. Furthermore, electronic and thermal transport measurements, in combination with the modeling of point-defect scattering, highlight the importance of structural characterizations toward understanding changes induced by elemental substitutions. This work provides new insights into the structure-transport correlations of the thermoelectric AgBiSe2.

All-solid-state batteries are promising candidates for next-generation energy-storage devices. Although the list of candidate materials for solid electrolytes has grown in the past decade, there ar...

All-solid-state lithium-ion batteries (ASSBs) are expected to represent a future alternative compared to conventional lithium-ion batteries with liquid electrolytes (LIBs). The excellent performanc...

Because of a remarkably high theoretical energy density, the lithium–sulfur (Li–S) battery has attracted significant attention as a candidate for next-generation batteries. While employing solid el...

Commercialization of solid-state batteries requires the upscaling of the material syntheses as well asthe mixing of electrode composites containing the solid electrolyte, cathode active materials, bindersand conductive additives. Inspired by recent literature about the tremendous influence of the employedmilling and dispersing procedure on the resulting ionic transport properties of solid ionic conductorsand the general performance of all solid-state batteries, in this review, we want to shed light on theunderlying physical and mechanochemical processes that influence this processing. By discussing andcombining the theoretical backgrounds of mechanical milling with regard to mechanochemicalsynthesis and dispersing of particles together with a wide range of examples, we aim to provide thefield with a better understanding of the critical parameters attached to mechanical milling of solidelectrolytes and solid-state battery components.

The energy density of the ubiquitous lithium-ion batteries is rapidly approaching its theoretical limit. To go beyond, a promising strategy is the replacement of conventional intercalation-type materials with conversion-type materials possessing substantially higher capacities. Among the conversion-type cathode materials, sulfur constitutes a cost-effective and earth-abundant element with a high theoretical capacity that has a potential to be game-changing, especially within an emerging solid-state battery configuration. Employment of nonflammable solid electrolytes that improves battery safety and boosts the energy density, as lithium metal anodes are also viable. The long-standing inherent problem of conventional lithium–sulfur batteries, arising from the reaction intermediates dissolved in liquid electrolytes, can be eliminated with inorganic solid ion conductors. In particular, the highly conducting and easily processable lithium-thiophosphates have successfully enabled the lab-scale solid-state lithium–sulfur cells to achieve close-to-theoretical capacities. For applications requiring safe, energy-dense, lightweight batteries, solid-state lithium–sulfur batteries are an ideal choice that could surpass conventional lithium-ion batteries.Nevertheless, there are challenges specific to practical solid-state lithium–sulfur batteries, beyond the typical challenges inherent to solid-state batteries in general. While the conversion reaction of sulfur realizes a large specific capacity, the associated significant total volume changes of the active material results in contact losses among the cathode components and, consequently, decreases reversible capacity. Additionally, the ionically and electronically insulating active material requires composite formation with solid electrolytes and electron-conductive additives to secure sufficient ion and electron supply at a triple-phase boundary. However, the compositing process itself makes the carrier transport pathways very tortuous and requires the balancing of carrier transport and optimization of the attainable energy density. Lastly, the requirement of a high interfacial area to establish sufficient triple-phase boundaries promotes the degradation of the solid electrolytes, and the formation of less-conductive interphases further deteriorates the transport in the composites.This Account focuses on the challenges associated with developing practical solid-state lithium–sulfur batteries and provides an overview over recently developed concepts to tackle these critical challenges: (1) Introduction of the conversion efficiency to enable quantitative assessments of the impact of chemo-mechanical failure. (2) For long-term cycling, the electrolyte degradation at the interface and the electrochemical activity of the formed interphases come into play. Practical stability tests with increased interfacial areas and subsequently altered reversal potentials can quantify the magnitude of the electrolyte degradation and confirm influences of reversible redox activity of the interphases. (3) Monitoring the effective conductivity in the composites clarifies correlations between transport and cyclability, further highlighting the need of quantitative measurements to address the composite carrier transport. (4) Impedance spectroscopy combined with transmission-line model analysis as a function of applied potentials can visualize the stability window of good effective ion transport to utilize both the capacity contributions from redox-active interphases and the high ionic conductivity. In the end, a roadmap toward the practical solid-state lithium–sulfur batteries will be presented.

Sulfidic sodium ion conductors are currently investigated for the possible use in all-solid-state sodium ion batteries. The design of high performing electrolytes in terms of temperature-dependent ionic transport is based upon the fundamental understanding of structure – transport relationships within the given structural phase boundaries inherent to the investigated materials class. In this work, the Na+ superionic structural family of Na11Sn2PS12 is explored by using the systematic antimony substitution with phosphorous in Na11+xSn2+x(Sb1-yPy)1-xS12. A combination of Rietveld refinements against X-ray synchrotron diffraction data with electrochemical impedance spectroscopy is used to monitor the changes in the anionic framework, the Na+ substructure and the ionic transport. A new simplified descriptor for the average Na+ diffusion pathways, the average Na+ polyhedral volume is introduced, which is used to correlate the contraction of the overall lattice and the found activation barriers in the system. This study exemplifies how substitution affects diffusion pathways in ionic conductors and widens the knowledge about the related structural motifs and their influence on the ionic transport in this novel class of ionic conductors.

Sulfidic sodium ion conductors are currently investigated for the possible use in all-solid-state sodium ion batteries. The design of high performing electrolytes in terms of temperature-dependent ionic transport is based upon the fundamental understanding of structure – transport relationships within the given structural phase boundaries inherent to the investigated materials class. In this work, the Na+ superionic structural family of Na11Sn2PS12 is explored by using the systematic antimony substitution with phosphorous in Na11+xSn2+x(Sb1-yPy)1-xS12. A combination of Rietveld refinements against X-ray synchrotron diffraction data with electrochemical impedance spectroscopy is used to monitor the changes in the anionic framework, the Na+ substructure and the ionic transport. A new simplified descriptor for the average Na+ diffusion pathways, the average Na+ polyhedral volume is introduced, which is used to correlate the contraction of the overall lattice and the found activation barriers in the system. This study exemplifies how substitution affects diffusion pathways in ionic conductors and widens the knowledge about the related structural motifs and their influence on the ionic transport in this novel class of ionic conductors.

Owing to highly conductive solid ionic conductors, all-solid-state batteries attract significant attention as promising next-generation energy storage devices. A lot of research is invested in the search and optimization of solid electrolytes with higher ionic conductivity. However, a systematic study of an interlaboratory reproducibility of measured ionic conductivities and activation energies is missing, making the comparison of absolute values in literature challenging. In this study, we perform an uncertainty evaluation via a Round Robin approach using different Li-argyrodites exhibiting orders of magnitude different ionic conductivities as reference materials. Identical samples are distributed to different research laboratories and the conductivities and activation barriers are measured by impedance spectroscopy. The results show large ranges of up to 4.5 mScm-1 in the measured total ionic conductivity (1.3 – 5.8 mScm-1 for the highest conducting sample, relative standard deviation 35 – 50% across all samples) and up to 128 meV for the activation barriers (198 – 326 meV, relative standard deviation 5 – 15%, across all samples), presenting the necessity of a more rigorous methodology including further collaborations within the community and multiplicate measurements.

Ternary compounds with a tetragonal chalcopyrite structure, such as CuGaTe2, are promising thermoelectric (TE) materials. It has been demonstrated in various chalcopyrite systems, including compounds with quaternary chalcopyrite-like structures, that the lattice parameter ratio, c/ a, being exactly 2.00 to have a pseudo-cubic structure is key to increase the degeneracy at the valence band edge and ultimately achieve high TE performance. Considering the fact that ZnSnSb2 with a chalcopyrite structure is reported to have c/ a close to 2.00, it is expected to have multiple valence bands leading to a high p-type zT. However, there are no complete investigations on the high temperature TE properties of ZnSnSb2 mainly because of the difficulty of obtaining a single-phase ZnSnSb2. In the present study, pure ZnSnSb2 samples with no impurities are synthesized successfully using a Sn flux-based method and TE properties are characterized up to 585 K. Transport properties and thermal analysis indicate that the structure of ZnSnSb2 remains chalcopyrite with no order-disorder transition and clearly show that ZnSnSb2 can be made to exhibit a high zT in the low-to-mid temperature range through further optimization.

Solid-state batteries with inorganic solid electrolytes are currently being discussed as a more reliable and safer future alternative to the current lithium-ion battery technology. To compete with state-of-the-art lithium-ion batteries, solid electrolytes with higher ionic conductivities are needed, especially if thick electrode configurations are to be used. In the search for optimized ionic conductors, the lithium argyrodites have attracted a lot of interest. Here, we systematically explore the influence of aliovalent substitution in Li6+xP1-xGexS5I using a combination of X-ray and neutron diffraction, as well as impedance spectroscopy and nuclear magnetic resonance. With increasing Ge content, an anion site disorder is induced and the activation barrier for ionic motion drops significantly, leading to the fastest lithium argyrodite so far with 5.4 ± 0.8 mS cm-1 in a cold-pressed state and 18.4 ± 2.7 mS cm-1 upon sintering. These high ionic conductivities allow for successful implementation within a thick-electrode solid-state battery that shows negligible capacity fade over 150 cycles. The observed changes in the activation barrier and changing site disorder provide an additional approach toward designing better performing solid electrolytes.

Recent work on superionic conductors has demonstrated the influence of lattice dynamics and the softness of the lattice on ionic transport. When examining either the changes in the acoustic phonon spectrum or the whole phonon density of states, both a decreasing activation barrier of migration and a decreasing entropy of migration have been observed, highlighting that the paradigm of “the softer the lattice, the better” does not always hold true. However, both approaches to monitor the changing lattice dynamics probe different frequency ranges of the phonon spectrum, and thus, it is unclear if they are complementary. In this work, we investigate the lattice dynamics of the superionic conductor Na3PS4–xSex by probing the optical phonon modes and the acoustic phonon modes, as well as the phonon density of states via inelastic neutron scattering. Notably, Raman spectroscopy shows the evolution of multiple local symmetry reduced polyhedral species, which likely affect the local diffusion pathways. Meanwhile, ...

Zintl compounds make excellent thermoelectrics with many opportunities for chemically tuning their electronic and thermal transport properties. However, the majority of Zintl compounds are persistently p-type even though computation predicts superior properties when n-type. Surprisingly, n-type Mg_3Sb_2-based thermoelectrics have been recently found with exceptionally high figure of merit. Excess Mg is required to make the material n-type, prompting the suspicion that interstitial Mg is responsible. Here we explore the defect chemistry of Mg_3Sb_2 both theoretically and experimentally to explain why there are two distinct thermodynamic states for Mg_3Sb_2 (Mg-excess and Sb-excess) and why only one can become n-type. This work emphasizes the importance of exploring all of the multiple thermodynamic states in a nominally single-phase semiconductor. This understanding of the existence of multiple inherently distinct different thermodynamic states of the same nominal compound will vastly multiply the number of new complex semiconductors to be discovered for high zT thermoelectrics or other applications.

Thermally activated mobility near room temperature is a signature of detrimental scattering that limits the efficiency and figure-of-merit zT in thermoelectric semiconductors. This effect has been observed dramatically in Mg3Sb2-based compounds, but also to a lesser extent in other thermoelectric compounds. Processing samples differently or adding impurities such that this effect is less noticeable produces materials with a higher zT. Experiments suggest that the behavior is related to grain boundaries, but impurity scattering has also been proposed. However, conventional models using Matthissen's rule are not able to explain the dramatic change in the temperature dependency of conductivity or drift mobility which is observed in Mg3Sb2-based compounds. We find that it is essential to consider the grain boundary region as an effectively separate phase rather than a scattering center, taking into account the weaker screening in semiconductors compared with classical metals. By modeling a grain boundary phase with a band offset, we successfully reproduce the experimentally observed conductivity versus temperature and thermopower versus conductivity relations, which indicate an improved description of transport. The model shows good agreement with measured grain size dependencies of conductivity, opening up avenues for quantitatively engineering materials with similar behavior. Model estimates predict room for >60% improvement in the room temperature zT of Mg3.2Sb1.5Bi0.49Te0.01 if the grain boundary resistance could be eliminated.

All-solid-state batteries are often expected to replace conventional lithium-ion batteries in the future. However, the practical electrochemical and cycling stability of the best-conducting solid electrolytes, i.e. lithium thiophosphates, are still critical issues that prevent long-term stable high-energy cells. In this study, we use stepwisecyclic voltammetry to obtain information on the practical oxidative stability limit of Li10GeP2S12, a Li2S‑P2S5glass, as well as the argyrodite Li6PS5Cl solid electrolytes. We employ indium metal and carbon black as the counter and working electrode, respectively, the latter to increase the interfacial contact area to the electrolyte as compared to the commonly used planar steel electrodes. Using a stepwise increase in the reversal potentials, the onset potential at 25 °C of oxidative decomposition at the electrode-electrolyte interface is identified. X‑ray photoelectron spectroscopy is used to investigate the oxidation of sulfur(-II) in the thiophosphate polyanions to sulfur(0) as the dominant redox process in all electrolytes tested. Our results suggest that after the formation of these decomposition products, significant redox behavior is observed. This explains previously reported redox activity of thiophosphate solid electrolytes, which contributes to the overall cell performance in solid-state batteries. The stepwise cyclic voltammetryapproach presented here shows that the practical oxidative stability at 25 °C of thiophosphate solid electrolytes against carbon is kinetically higher than predicted by thermodynamic calculations. The method serves as an efficient guideline for the determination of practical, kinetic stability limits of solid electrolytes.

Owing to their intrinsically low thermal conductivity and chemical diversity, materials within the I-V-VI

All-solid-state batteries are promising candidates for next-generation energy storage devices. Although the list of candidate materials for solid electrolytes has grown in the past decade, there are still many open questions concerning the mechanisms behind ionic migration in materials. In particular, the lithium thiophosphate family of materials has shown very promising properties for solid-state battery applications. Recently, the Ge-substituted Li6PS5I argyrodite was shown to be a very fast Li-ion conductor, despite the poor ionic conductivity of the unsubstituted Li6PS5I. Therein, the conductivity was enhanced by over three orders of magnitude due to the emergence of I−/S2−exchange, i.e.site-disorder, which led to a sudden decrease of the activation barrier with a concurrent flattening of the energy landscapes. Inspired by this work, two series of elemental substitutions in Li6+xP1−xMxS5I (M= Si and Sn) were investigated in this study and compared to the Ge-analogue. A sharp reduction in the activation energy was observed at the same M4+/P5+composition as previously found in the Ge-analogue, suggesting a more general mechanism at play. Furthermore, structural analyses with X-ray and neutron diffraction indicate that similar changes in the Li-sublattice occur despite a significant variation in the size of the substituents, suggesting that in the argyrodites, the lithium substructure is most likely influenced by the occurring Li+– Li+interactions. This work provides further evidence that the energy landscape of ionic conductors can be tailored by inducing local disorder.

Owing to a remarkably high theoretical energy density, the lithium-sulfur (Li-S) battery has attracted significant attention as a candidate for next-generation batteries. While employing solid electrolytes can provide a new avenue for high capacity Li-S cells, all-solid-state batteries have unique failure mechanisms such as chemo-mechanical failure due to the volume changes of active materials. In this study, we investigate all-solid-state Li-S model cells with differently processed cathode composites and elucidate a typical failure mechanism stemming from irreversible Li2S formation in the cathode composites. Reducing the particle size is key to minimizing the influence of volume changes and a capacity of over 1000 mAh gsulfur-1is achieved by ball-milling of the cathode composites. In addition, the long-term stability of the ball-milled cathode is investigated by varying upper and lower cut-off potentials for cycling, which results in unveiling the significantly detrimental role of the lower cut-off potential. Preventing a deep-discharge leads to a reversible capacity of 800 mAh gsulfur-1over 50 cycles in the optimized cell. This work highlights the importance of mitigating chemo-mechanical failure using microstructural engineering as well as the influence of the cut-off potentials in all-solid-state Li-S batteries.

Ternary compounds with a tetragonal chalcopyrite structure, such as CuGaTe

Solid-state batteries with inorganic solid electrolytes are currently being discussed as a more reliable and safer future alternative to the current lithium-ion battery technology. To compete with state-of-the-art lithium-ion batteries, solid electrolytes with higher ionic conductivities are needed, especially if thick electrode configurations are to be used. In the search for optimized ionic conductors, the lithium argyrodites have attracted a lot of interest. Here, we systematically explore the influence of aliovalent substitution in Li

Li

Recent work on superionic conductors has demonstrated the influence of lattice dynamics and the softness of the lattice on ionic transport. When examining either the changes in the acoustic phonon spectrum or the whole phonon density of states, both a decreasing activation barrier of migration and a decreasing entropy of migration have been observed, highlighting that the paradigm of "the softer the lattice, the better" does not always hold true. However, both approaches to monitor the changing lattice dynamics probe different frequency ranges of the phonon spectrum, and thus, it is unclear if they are complementary. In this work, we investigate the lattice dynamics of the superionic conductor Na

Intrinsically doped samples of YCuTe2 were prepared by solid state reaction of the elements. Based on the differential scanning calorimetry and the high temperature X-ray diffraction analyses, YCuTe2 exhibits a first order phase transition at ∼440 K from a low-temperature-phase crystallizing in the space group Pm1 to a high-temperature-phase in P. Above the phase transition temperature, partially ordered Cu atoms become completely disordered in the crystal structure. Small increases to the Cu content are observed to favour the formation of the high temperature phase. We find no indication of superionic Cu ions as for binary copper chalcogenides (e.g., Cu2Se or Cu2Te). All investigated samples exhibit very low thermal conductivities (as low as ∼0.5 W m−1 K−1 at 800 K) due to highly disordered Cu atoms. Electronic structure calculations are employed to better understand the high thermoelectric efficiency for YCuTe2. The maximum thermoelectric figure of merit, zT, is measured to be ∼0.75 at 780 K for Y0.96Cu1.08Te2, which is promising for mid-temperature thermoelectric applications.

The synthesis and transport properties of the family of coinage metal-stuffed Zintl compounds, Eu9Cd4–xCM2+x–y□ySb9 (CM = coinage metal, □ = vacancies), is presented as a function of coinage metal substitution. Eu9Cd4–xCM2+x–y□ySb9 compounds are shown to be rare examples of metallic Zintl phases with low thermal conductivities. While the lattice thermal conductivity is low, which is attributed to the complex structure and presence of interstitials, the electronic contribution to thermal conductivity is also low. In these p-type compounds, the carriers transmit less heat than expected, based on the Wiedemann–Franz law and metallic conduction, κe = L0T/ρ. Density functional theory (DFT) calculations indicate that the Fermi level resides in a pseudo-gap, which is consistent with the metallic description of the properties. While the contribution from the interstitial CM states to the Fermi level is small, the interstitial CMs are required to tune the position of the Fermi level. Analysis of the topology of el...

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...