A new increasing delocalization of M=3d-elements (Ti, Fe, Co) in the channels network of the ternary MyMo6Se8 Chevrel phases

A structural investigation on single crystals of M_yMo₆Se₈ Chevrel phases (M=3d: Ti, Fe, and Co) has been carried out. These latter compounds as well as others with Cr, Mn, and Ni atoms make part of a new original family of Chevrel phases. The M ions occupy new interstices in the tridimensional channels network never observed in the other classical Chevrel phases as Li_y, Cu_y Mo₆X₈. They occupy only cavity 2 centered on and with a progressive delocalization of the M cation versus the nature of the cation from the center of the cavity to the outside . The position in cavity 2 is different from the position of the Cu (2) atoms, the second position of the copper atoms in the channels of the Cu_yMo₆X₈ compounds. This new position reinforces the interaction with the Mo₆ cluster, and may be able to involve new remarkable physical properties as thermoelectric properties.     

Thermoelectric and structural properties of a new Chevrel phase: Ti0.3Mo5RuSe8

The new Chevrel phase Ti_0.3Mo₅RuSe₈ has been synthesized and characterized by quantitative microprobe analysis, powder X-ray diffraction, and high-temperature thermoelectric properties measurements. The thermoelectric properties of this compound are compared to the previously reported data for other related Chevrel phases. We report also the results of Rietveld analysis of powder X-ray diffraction data for Ti_0.3Mo₅RuSe₈. This compound adopts the rhombohedral Chevrel phase structure (space group , Z=3) with hexagonal lattice constants a=9.75430(25) Å and c=10.79064(40) Å. The low level of incorporation and low scattering power of Ti precluded the identification of the Ti positions, and Rietveld refinement was carried out only for the Mo₅RuSe₈ framework of Ti_0.3Mo₅RuSe₈ (R_p=10.5%, R_wp=14.6%). Rietveld analysis was also used to refine the structure of the unfilled phase Mo₅RuSe₈ (, Z=3, a=9.63994(8) Å, c=10.97191(11) Å, R_p=8.0%, R_wp=10.5%). Comparisons between the two structures are made.     

Thermoelectric properties of Cu-doped n-type (Bi2Te3)0.9–(Bi2−xCuxSe3)0.1(x=0–0.2) alloys

n-Type (Bi₂Te₃)_0.9–(Bi_2−xCu_xSe₃)_0.1 (x=0–0.2) alloys with Cu substitution for Bi were prepared by spark plasma-sintering technique and their structural and thermoelectric properties were evaluated. Rietveld analysis reveals that approximate 9.0% of Bi atomic sites are occupied by Cu atoms and less than 4.0 wt% second phase Cu_2.86Te₂ precipitated in the Cu-doped parent alloys. Measurements show that an introduction of a small amount of Cu (x0.1) can reduce the lattice thermal conductivity (κ_L), and improve the electrical conductivity and Seebeck coefficient. An optimal dimensionless figure of merit (ZT) value of 0.98 is obtained for x=0.1 at 417 K, which is obviously higher than those of Cu-free Bi₂Se_0.3Te_2.7 (ZT=0.66) and Ag-doped alloys (ZT=0.86) prepared by the same technologies.     

Sur de nouvelles phases séléniées ternaires du molybdène

The synthesis and the crystal properties of new selenides of formula M_xMo₃Se₄ are described. If M = Zn, Ag, Cd, Sn and Pb, they are stoichiometric with x = 0.6; if M = Fe, Mn, Cr, V, Ti, triclinic solid solutions are observed with 0.5 < x < 0.7; if M = Cu, Co, Ni, rhombohedral solid solutions are obtained with 0 < x < 1.4 for M = Cu, 0 < x < 0.7 for M = Co and 0 < x < 0.8 for M = Ni All these phases can be deduced from the Mo₃Se₄ structure by introducing metal atoms into the tunnels between the “Mo₆Se₈” metal atom cluster configuration.     

Structure–reactivity relationships in Chevrel phase electrocatalysts for small-molecule reduction reactions

Chevrel phases, M_xMo₆X₈ (M = metal intercalant, X = chalcogen), constitute a family of materials with composition-dependent physicochemical properties that have shown promising electrocatalytic activity for various small-molecule reduction reactions. The wide range of possible compositions among the Chevrel phase family offers the opportunity to tune the local and electronic structure of discrete Mo₆X₈ cluster units within the extended M_xMo₆X₈ framework. Thus, making them an ideal platform for studying structure–function relationships and generating design principles for improved electrocatalytic reactivity. This review summarizes the state of the art in experimental and computational evaluations of Chevrel phases as electrocatalysts for hydrogen evolution, CO₂ reduction, and nitrogen reduction reactions. We aim to elucidate the uncharted small-molecule electrochemical reactivity of Chevrel phases as a function of composition and consequently guide the design of promising multinary chalcogenides for energy conversion reactions.     

Influence of Ni nanoparticle addition and spark plasma sintering on the TiNiSn–Ni system: Structure,microstructure, and thermoelectric properties

The electronic and thermal properties of thermoelectric materials are highly dependent on their microstructure and therefore on the preparation conditions, including the initial synthesis and, if applicable, densification of the obtained powders. Introduction of secondary phases on the nano- and/or microscale is widely used to improve the thermoelectric figure of merit by reduction of the thermal conductivity. In order to understand the effect of the preparation technique on structure and properties, we have studied the thermoelectric properties of the well-known half-Heusler TiNiSn with addition of a small amount of nickel nanoparticles. The different parameters are the initial synthesis (levitation melting and microwave heating), the amount of nickel nanoparticles added and the exact pressing profile using spark plasma sintering. The resulting materials have been characterized by synchrotron X-ray diffraction and microprobe measurements and their thermoelectric properties are investigated. We found the lowest (lattice) thermal conductivity in samples with full-Heusler TiNi₂Sn and Ni₃Sn₄ as secondary phases.     

Multiband Transport in CoSb3 Prepared by Rapid Solidification

Nano‐grained CoSb₃ was prepared by melt‐spinning and subsequent spark plasma sintering. The phonon thermal conductivity of skutterudites is known to be sensitive to the kind and the amount of guest atoms. Thus, unfilled CoSb₃ can serve as model compound to study the impact of a nanostructure on the thermoelectric properties, especially the phonon thermal conductivity. Therefore, a series of materials was prepared differing only by the cooling speed during the quenching procedure. In contrast to clathrates, the microstructure of meltspun CoSb₃ was found to be sensitive to the cooling speed. Although the phonon thermal conductivity, studied by means of Flash and 3ω measurements, was found to be correlated with the grain size, the bulk density of the sintered materials had an even stronger impact. Interestingly, the reduced bulk density did not result in an increased electrical resistivity. The influence of Sb and CoSb₂ as foreign phase on the electronic properties of CoSb₃ was revealed by a multi‐band Hall effect analysis. While CoSb₂ increases the charge carrier density, the influence of the highly mobile charge carriers introduced by elemental Sb on the thermoelectric properties of the composite offer an interesting perspective for the preparation of efficient thermoelectric composite materials.     

Bournonite PbCuSbS3: Stereochemically Active Lone‐Pair Electrons that Induce Low Thermal Conductivity

An understanding of the structural features and bonding of a particular material, and the properties these features impart on its physical characteristics, is essential in the search for new systems that are of technological interest. For several relevant applications, the design or discovery of low thermal conductivity materials is of great importance. We report on the synthesis, crystal structure, thermal conductivity, and electronic‐structure calculations of one such material, PbCuSbS₃. Our analysis is presented in terms of a comparative study with Sb₂S₃, from which PbCuSbS₃ can be derived through cation substitution. The measured low thermal conductivity of PbCuSbS₃ is explained by the distortive environment of the Pb and Sb atoms from the stereochemically active lone‐pair s² electrons and their pronounced repulsive interaction. Our investigation suggests a general approach for the design of materials for phase‐change‐memory, thermal‐barrier, thermal‐rectification and thermoelectric applications, as well as other functions for which low thermal conductivity is purposefully sought.     

Excellent Room-Temperature Thermoelectricity of 2D GeP3: Mexican-Hat-Shaped Band Dispersion and Ultralow Lattice Thermal Conductivity

Although some atomically thin 2D semiconductors have been found to possess good thermoelectric performance due to the quantum confinement effect, most of their behaviors occur at a higher temperature. Searching for promising thermoelectric materials at room temperature is meaningful and challenging. Inspired by the finding of moderate band gap and high carrier mobility in monolayer GeP_3, we investigated the thermoelectric properties by using semi-classical Boltzmann transport theory and first-principles calculations. The results show that the room-temperature lattice thermal conductivity of monolayer GeP₃ is only 0.43 Wm⁻¹K⁻¹ because of the low group velocity and the strong anharmonic phonon scattering resulting from the disordered phonon vibrations with out-of-plane and in-plane directions. Simultaneously, the Mexican-hat-shaped dispersion and the orbital degeneracy of the valence bands result in a large p-type power factor. Combining this superior power factor with the ultralow lattice thermal conductivity, a high p-type thermoelectric figure of merit of 3.33 is achieved with a moderate carrier concentration at 300 K. The present work highlights the potential applications of 2D GeP₃ as an excellent room-temperature thermoelectric material.     

Cr1.45Tl1.87Mo15Se19, a monoclinic variant of the hexagonal In3Mo15Se19 type

The monoclinic compound Cr_1.45Tl_1.87Mo₁₅Se₁₉ (chromium thallium pentadecamolybdenum nonadecaselenide) represents a variant of the hexagonal In₃Mo₁₅Se₁₉ structure type. Its crystal structure consists of an equal mixture of Mo₆Se₈Se₆ and Mo₉Se₁₁Se₆ cluster units. The Mo and Se atoms of the median plane of the Mo₉Se₁₁Se₆ unit, as well as three Cr ions, lie on sites with m symmetry (Wyckoff site 2e). The fourth Cr ion is in a 2b Wyckoff position with site symmetry.     

Synthesis, electrical properties, and structural features of copper-containing chalcogenide films produced by the chemical deposition method

Multicomponent copper-containing CuI-AsI₃-As₂Se₃ and CuI-Sb₃I-As₂Se₃ chalcogenide films were produced by chemical deposition from solutions of chalcogenide glasses in n-butylamine and their electrical conductivity was studied. It was shown that the electrical properties of chalcogenide glasses and films based on these glasses have the same values within experimental error. It was found sing Mossbauer spectroscopy that antimony atoms are in the Sb(III) state in the environment of three selenium atoms, and copper ions in the Cu(I) state and are surrounded by iodine atoms. The chalcogenide films can be used to fabricate ion-selective electrodes sensitive to copper cations.     

Single Crystal Studies of Ternary Transition-Metal Molybdenum Selenides MMo6Se8 (M = Ti, Fe, Mn, Cr and Co)

Magnetic properties have been measured in single crystals of selenium-based Chevrel-type M _xMo₆Se₈, with M = Ti, Fe, Mn, Cr and Co, in which x(M) is equal to unity, except for Ti (x ~ 0.9) and Co (x ~ 0.5). Presence of impurities, such as the binary Mo₃Se₄ sub-product, is absolutely excluded and reported results are representative of the intrinsic properties of the reported compounds. For polycrystalline ScMo₆Se₈ and single crystals of Ti_0.88Mo₆Se₈, the Pauli-like temperature-independent magnetic susceptibility allows having a good estimation of the non-magnetic contribution of the Mo₆Se₈ sublattice. Crystal field effects were seen for polycrystalline VMo₆Se₈. Spectacular results were obtained for FeMo₆Se₈ (canted antiferromagnetic compound with high magnetic anisotropy), CrMo₆Se₈ (antiferromagnetism, T_N = 12 K), MnMo₆Se₈ (metamagnetic compound) while the properties of Co_0.54Mo₆Se₈ can be interpreted by magnetic exchange interactions J_AF ~ 100 K. These results are discussed in connection with structural properties and transport measurements obtained in single crystal specimens.     

Highly Porous Thermoelectric Nanocomposites with Low Thermal Conductivity and High Figure of Merit from Large‐Scale Solution‐Synthesized Bi2Te2.5Se0.5 Hollow Nanostructures

To enhance the performance of thermoelectric materials and enable access to their widespread applications, it is beneficial yet challenging to synthesize hollow nanostructures in large quantities, with high porosity, low thermal conductivity (κ ) and excellent figure of merit (z T ). Herein we report a scalable (ca. 11.0 g per batch) and low‐temperature colloidal processing route for Bi₂Te_2.5Se_0.5 hollow nanostructures. They are sintered into porous, bulk nanocomposites (phi 10 mm×h 10 mm) with low κ (0.48 W m⁻¹ K⁻¹) and the highest z T (1.18) among state‐of‐the‐art Bi₂Te_3−x Se_x materilas. Additional benefits of the unprecedented low relative density (68–77 %) are the large demand reduction of raw materials and the improved portability. This method can be adopted to fabricate other porous phase‐transition and thermoelectric chalcogenide materials and will pave the way for the implementation of hollow nanostructures in other fields.     

Etude structurale de combinaisons sulfurées et séleniées du molybdène. III. Structure cristalline de NiMo3S4

The crystals of NiMo₃S₄, are rhombohedral, space group $\text{R}\text{3}$, with two formula units in a cell: a = 6.462 Å, α = 94.68°.The structure was solved by the heavy atom method and refined by a full-matrix least-squares program to R = 0.077 for 1026 independent reflexions. The arrangement of Mo and S atoms in the cell is approximately the same as described before for Mo and Se atoms in Mo₃Se₄ and Ni_0.33Mo₃Se₄; the Ni atoms are situated along the channel based on x = 0, y = 0. However, in these two compounds the lattice of Se presents a covalent character when the lattice of S in NiMo₃S₄ has an essentially ionic character.     

Conduction band splitting and transport properties of Bi2Se3

Detailed transport studies of single crystals of Bi₂Se₃ were made in the temperature range of 2–300 K, and the data were analyzed in terms of a model consisting of two groups of electrons—a centrosymmetrical lower conduction band and an upper conduction band located away from the Γ-point. Very good agreement with the experimental data is obtained assuming the electrons are scattered on acoustic phonons and ionized impurities. A rather strong influence of the latter mechanism is attributed to a large number of charged selenium vacancies in Bi₂Se₃. The fitted transport parameters were used to calculate the electronic portion of the thermal conductivity that, in turn, allowed for the determination of the lattice thermal conductivity. The Debye model provides a good approximation to the temperature dependence of the lattice thermal conductivity.     

Electronic Structure of EuMo6Se8 Studied by X-Ray Absorption Spectroscopy

The rare-earth based molybdenum chalcogenides, REMo₆Se₈ (RE = rare-earth metals) have been extensively studied because of their unique crystal structure based on Mo₆Se₈ clusters and their outstanding properties involving coexistence of superconductivity and magnetism. Among all these compounds, Ce and Eu based chalcogenides are magnetic and non-superconductors and possess many novel properties. Understanding their electronic structure is likely to provide valuable information about these materials. We employ X-ray absorption near-edge structure (XANES) spectroscopy at Mo and Se K-edges of EuMo₆Se₈ to identify the local environment respectively around Mo and Se ions and XANES spectra at L₃-edge of Eu ion to identify their valence state. Results from this study demonstrate that Se ions in EuMo₆Se₈ are in two inequivalent sites and the valency of Eu is divalent.     

Anisotropic thermoelectric properties of Bi1.9Lu0.1Te2.7Se0.3 textured via spark plasma sintering

Spark plasma sintering method was applied to prepare bulk n-type Bi_1.9Lu_0.1Te_2.7Se_0.3 samples highly textured along the 001 direction parallel to the pressing direction. The texture development is confirmed by X-ray diffraction analysis and scanning electron microscopy. The grains in the textured samples form ordered lamellar structure and lamellar sheets lie in plane perpendicular to the pressing direction. The average grain size measured along the pressing direction is much less as compared to the average grain size measured in the perpendicular direction (∼50 nm against ∼400 nm). A strong anisotropy in the transport properties measured along directions parallel and perpendicular to the pressing direction within the 290 ÷ 650 K interval was found. The specific electrical resistivity increases and the thermal conductivity decreases for the parallel orientation as compared to these properties for the perpendicular orientation. The Seebeck coefficient for both orientations is almost equal. Increase of the electrical resistivity is stronger than decrease of the thermal conductivity resulting in almost three-fold enhancement of the thermoelectric figure-of-merit coefficient for the perpendicular orientation (∼0.68 against ∼0.24 at ∼420 K). The texturing effect can be attributed to (i) recovery of crystal structure anisotropy typical for the single crystal Bi₂Te₃-based alloys and (ii) grain boundary scattering of electrons and phonons. An onset of intrinsic conductivity observed above T_d ≈ 410 K results in appearance of maxima in the temperature dependences of the specific electrical resistivity, the Seebeck coefficient and the thermoelectric figure-of-merit coefficient and minimum in the temperature dependence of the total thermal conductivity. The intrinsic conductivity is harmful for the thermoelectric efficiency enhancement since thermal excitation of the electron-hole pairs reduces the Seebeck coefficient and increases the thermal conductivity.     

Creating Zipper‐Like van der Waals Gap Discontinuity in Low‐Temperature‐Processed Nanostructured PbBi2nTe1+3n: Enhanced Phonon Scattering and Improved Thermoelectric Performance

Nanoengineered materials can embody distinct atomic structures which deviate from that of the bulk‐grain counterpart and induce significantly modified electronic structures and physical/chemical properties. The phonon structure and thermal properties, which can also be potentially modulated by the modified atomic structure in nanostructured materials, however, are seldom investigated. Employed here is a mild approach to fabricate nanostructured PbBi_2nTe_1+3n using a solution‐synthesized PbTe‐Bi₂Te₃ nano‐heterostructure as a precursor. The as‐obtained monoliths have unprecedented atomic structure, differing from that of the bulk counterpart, especially the zipper‐like van der Waals gap discontinuity and the random arrangement of septuple‐quintuple layers. These structural motifs break the lattice periodicity and coherence of phonon transport, leading to ultralow thermal conductivity and excellent thermoelectric z T.     

Modulating the thermoelectric transport properties of the novel material Er2Te3 via strain

The rare-earth chalcogenide Er₂Te₃, characterized by its low lattice thermal conductivity, represents a highly promising and innovative thermoelectric material. However, there have been limited studies exploring its thermoelectric properties in depth. Additionally, it has been discovered that strain engineering is an effective method for enhancing thermoelectric properties, a technique successfully applied to relevant materials. In this study, we employed a first-principles approach in conjunction with the semi-classical Boltzmann transport theory to investigate the thermoelectric properties of Er₂Te₃ materials under −4% to 4% strain. The results indicate that applying compressive strain modulates thermoelectric properties more effectively than tensile strain for Er₂Te₃. Under strain modulation, the maximum power factor for both p-type and n-type Er₂Te₃ increases significantly, from 0.9 to 2.5 mW m⁻¹ K⁻² and from 14 to 18 mW m⁻¹ K⁻² at 300 K, respectively. Moreover, the figure of merit (ZT) for p-type and n-type Er₂Te₃ improves notably, from 0.15 to 0.25 and from 1.15 to 1.35, respectively, under −4% strain. Consequently, the thermoelectric properties of Er₂Te₃ materials can be significantly enhanced through strain application, with n-type Er₂Te₃ demonstrating substantial potential as a thermoelectric material.     

The effect of Cu substitution on microstructure and thermoelectric properties of LaCoO(3) ceramics

La(Co, Cu)O(3-δ) ceramics were prepared by pressureless sintering of citrate precursor powders, and their thermoelectric properties were investigated with an emphasis on the influence of Cu doping and phase structure as well as microstructure. It was found that a secondary phase first appeared in the form of a network along the grain boundaries and then changed to dispersion with increasing Cu content, which effectively reduced the lattice thermal conductivity of the materials. The thermal conductivity was only 1.21 W m(-1) K(-1) for the sample LaCo(0.75)Cu(0.25)O(3-δ), being much lower as for the thermoelectric oxide materials. In addition, a small amount of Cu substitution for Co increased the electrical conductivity greatly and the absolute Seebeck coefficient, whose sign was also reversed from negative to positive. The dimensionless figure of merit, ZT, of LaCoO(3-δ) oxides at low and middle temperatures can be remarkably enhanced by substituting Co with Cu.