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Chalcopyrite, CuFeS2 is considered one of the promising n-type thermoelectric materials with high natural abundance as a mineral. In this work, partial substitution of germanium in materials CuFe1−xGexS2, (0.0 ≤ x ≤ 0.10), leads to an almost six-fold enhancement of thermoelectric properties. X-Ray photoelectron spectroscopy (XPS) reveals that germanium is present in two oxidation states: Ge2+ and Ge4+. The stereochemically-active 4s2 lone-pair of electrons associated with Ge2+ induces a local structural distortion. Pair-distribution function (PDF) analysis reveal that Ge2+ ions are displaced from the centre of the GeS4 tetrahedron towards a triangular face, leading to pseudo-trigonal pyramidal coordination. This distortion is accompanied by lattice softening and an increase of the strain-fluctuation scattering parameter (ΓS), leading to a decrease in thermal conductivity. Phonon calculations demonstrate that germanium substitution leads to the appearance of resonant phonon modes. These modes lie close in energy to the acoustic and low-energy optical modes of the host matrix, with which they can interact, providing an additional mechanism for reducing the thermal conductivity. The weak chemical bonding of germanium with sulphur also leads to localized electronic states near the Fermi level which results in a high density-of-states effective mass, enabling a relatively high Seebeck coefficient to be maintained, despite the reduced electrical resistivity. This combination produces an almost three-fold improvement in the power factor, which when coupled with the substantial reduction in thermal conductivity, leads to a maximum figure-of-merit, zT ∼ 0.4 at 723 K for CuFe0.94Ge0.06S2.

Abstract Two new hybrid iodobismuthates, [C8H17N2][C10H22N2][BiI6] (1) and [C6H12N2]0.5[C10H22N2]3.5[Bi2I10][Bi2I9] (2), have been prepared by solvothermal synthesis in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO) and ethanol. Both compounds have been characterized by single-crystal and powder X-ray diffraction, infrared and UV–Vis spectroscopies and thermogravimetric analysis. Structure determination reveals that the crystal structure of 1 contains mononuclear [BiI6]3− anions, whilst 2 contains an unusual combination of dinuclear anions, [Bi2I9]3− and [Bi2I10]4−, consisting of two edge- and two face-sharing [BiI6]3− octahedra, respectively. Mono- and diethylated derivatives of DABCO, which are formed in situ under solvothermal conditions, act as countercations and are located between the discrete anions. The optical band gaps of 1 and 2, which are 2.29(1) and 2.03(2) eV respectively, are consistent with the red color of these compounds, and are comparable to the band gaps measured for other iodobismuthates containing discrete anions. Graphical Abstract Two new iodobismuthates, [C8H17N2][C10H22N2][BiI6] (1) and [C6H12N2]0.5[C10H22N2]3.5[Bi2I10][Bi2I9] (2), have been synthesized under solvothermal conditions, and their crystal structures determined by single-crystal X-ray diffraction.

The dearth of

The dearth of n-type sulfides with thermoelectric performance comparable to that of their p-type analogues presents a problem in the fabrication of all-sulfide devices. Chalcopyrite (CuFeS2) offers a rare example of an n-type sulfide. Chemical substitution has been used to enhance the thermoelectric performance of chalcopyrite through preparation of Cu1 xSnxFeS2 (0 ≤ x ≤ 0.1). Substitution induces a high level of mass and strain-field fluctuation, leading to lattice softening and enhanced point-defect scattering. Together with dislocations and twinning identified by transmission electron microscopy, this provides a mechanism for scattering phonons with a wide range of mean free paths. Substituted materials retain a large density-of-states effective mass, and hence a high Seebeck coefficient. Combined with a high charge-carrier mobility, and thus high electrical conductivity, a 3-fold improvement in power factor is achieved. Density functional theory (DFT) calculations reveal that substitution leads to the creation of small polarons, involving localised Fe2+ states, as confirmed by X-ray photoelectron spectroscopy. Small polaron formation limits the increase in carrier concentration to values that are lower than expected on electron-counting grounds. An improved power factor, coupled with substantial reductions (up to 40 %) in lattice thermal conductivity, increases the maximum figure-of-merit by 300%, to zT 0.3 at 673 K for Cu0.96Sn0.04FeS2.

Mechanochemical synthesis has recently emerged as a scalable “green” approach for the preparation of MOFs, but current understanding of the underlying reaction mechanisms is limited. In this work, an investigation of the reaction pathway of the mechanochemical synthesis of MOF-74 from ZnO and 2,5-dihydroxyterephthalic acid (H4HDTA), using DMF as a liquid additive, is presented. The complex reaction pathway involves the formation of four short-lived intermediate phases, prior to the crystallization of MOF-74. The crystal structures of three of these intermediates have been determined using a combination of single-crystal and powder X-ray diffraction methods and are described here. The initial stages of the reaction are very fast, with a DMF solvate of H4HDTA forming after only 2 min of milling. This is followed by crystallization, after only 4 min of milling, of a triclinic one-dimensional coordination polymer, Zn(H2DHTA)(DMF)2(H2O)2, which converts into a monoclinic polymorph on additional milling. Highly crystalline MOF-74 appears after prolonged milling, for at least 70 min., The mechanochemical synthesis of MOF-74 from ZnO and 2,5-dihydroxyterephthalic acid, using DMF as a liquid additive, has been investigated. The reaction pathway of involves the formation of four short-lived intermediate phases prior to the crystallization of the final product, MOF-74.

Kesterite-structured phase have attracted considerable interest as earth-abundant photovoltaic and thermoelectric materials. For the kesterite, Cu2ZnGeSe4, we have established a direct link between anomalies in the temperature dependence of transport properties and an order-disorder transition. Powder neutron diffraction as a function of temperature, reveals an order-disorder transition at 473 K, involving disordering of copper and zinc cations over three crystallographic positions. Vacancies are simultaneously created on the copper-ion sub-lattice, indicative of the concomitant onset of copper-ion mobility. Differential scanning calorimetry data show a weak thermal signature in this temperature region, typical of a second-order phase transition, which is consistent with the absence of anomalies in the temperature dependence of the unit cell volume. The partial melting of the copper-ion sub-lattice induces a transition in the electrical-transport properties. The changes in electrical resistivity and Seebeck coefficient suggest this involves a transition from a conventional, activated semiconductor, to a degenerate semiconductor. The entry of an increasing fraction of the copper-ion sub-lattice into a liquid-like state is reflected in a reduction in thermal conductivity above 473 K. The order-disorder phase transition identified here has consequences for the performance of photovoltaic and thermoelectric devices based on kesterites.

In4Se3 is an attractive n-type thermoelectric material for midrange waste heat recovery, owing to its low thermal conductivity (∼0.9 W·m– 1·K–1 at 300 K). Here, we explore the relationship between ...

Understanding the mechanism that connects heat transport with crystal structures and order/disorder phenomena is crucial to develop materials with ultralow thermal conductivity (κ), for thermoelectric and thermal barrier applications, and requires the study of highly pure materials. We synthesized the n-type sulfide CuPbBi

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)

Synthetic copper sulfides have emerged as promising nontoxic and low-cost materials for thermoelectric power generation in low-grade waste heat recovery systems. Similarly to tetrahedrite and colus...

Semiconducting quaternary chalcogenides with A2ZnBQ4 stoichiometry, where A and B are monovalent and tetravalent metal ions and Q is a chalcogen (e.g. Cu2ZnSnS4 or CZTS) have recently attracted attention as potential solar-cell absorbers made from abundant and non-toxic elements. Unfortunately, they exhibit relatively poor sunlight conversion efficiencies, which has been linked to site disorder within the tetrahedral cation sub-lattice. In order to gain a better understanding of the factors controlling cation disorder in these chalcogenides, we have used powder neutron diffraction, coupled with Density Functional Theory (DFT) simulations, to investigate the detailed structure of A2ZnBQ4 phases, with A = Cu, Ag; B = Sn, Ge; and Q = S, Se. Both DFT calculations and powder neutron diffraction data demonstrate that the kesterite structure (space group: I) is adopted in preference to the higher-energy stannite structure (space group: I2m). The contrast between the constituent cations afforded by neutron diffraction reveals that copper and zinc cations are only partially ordered in the kesterites Cu2ZnBQ4 (B = Sn, Ge), whereas the silver-containing phases are fully ordered. The degree of cation order in the copper-containing phases shows a greater sensitivity to the identity of the B-cation than to the chalcogenide anion. DFT indicates that cation ordering minimises inter-planar Zn2+⋯Zn2+ electrostatic interactions, while there is an additional intra-planar energy contribution associated with size mismatch. The complete Ag/Zn order in Ag2ZnBQ4 (B = Sn, Ge) phases can thus be related to the anisotropic expansion of the unit cell on replacing Cu with Ag.

Hybrid bismuth-containing halides are emerging as alternative candidates to lead-containing perovskites for light-harvesting applications, as Bi3+ is isoelectronic with Pb2+ and the presence of an active lone pair of electrons is expected to result in outstanding charge-carrier transport properties. Here, we report a family of one binary and three ternary iodobismuthates containing 1,4-diazabicyclo[2.2.2]octane (DABCO). These materials have been prepared solvothermally and their crystal structures, thermal stability, and optical properties determined. Reactions carried out in the presence of bismuth iodide and DABCO produced (C6H12N2)BiI3 (1), which consists of hybrid ribbons in which pairs of edge-sharing bismuth octahedra are linked by DABCO ligands. Short I···I contacts give rise to a three-dimensional network. Similar reactions in the presence of copper iodide produced (C8H17N2)2Bi2Cu2I10(2) and [(C6H13N2)2BiCu2I7](C2H5OH) (3) in which either ethylated DABCO cations (EtDABCO)+ or monoprotonated DABCO cations (DABCOH)+ are coordinated to copper in discrete tetranuclear and trinuclear clusters, respectively. In the presence of potassium iodide, a unique three-dimensional framework, (C6H14N2)[(C6H12N2)KBiI6] (4), was formed, which contains one-dimensional hexagonal channels approximately 6 Å in diameter. The optical band gaps of these materials, which are semiconductors, range between 1.82 and 2.27 eV, with the lowest values found for the copper-containing discrete clusters. Preliminary results on the preparation of thin films are presented., The solvothermal syntheses, crystal structures, and optical properties of four new iodobismuthates containing 1,4-diazabicyclo[2.2.2]octane (DABCO) are described. In the absence of hydriodic acid, DABCO can act as a ligand instead of acting as a countercation.

A series of copper-based p-type chalcogenide semiconductors, Cu2+x BGe1−x Se4 (B = Zn, Fe; 0 ⩽ x ⩽ 0.15), was prepared by high-temperature methods, to explore the impact of replacement of the closed-shell ion, Zn2+, with the magnetically active, Fe2+ cation. Powder x-ray diffraction in conjunction with Rietveld refinement reveals that zinc-containing materials are described in the kesterite-type structure (space group: I 4 ˉ ) and contain trace amounts of secondary phases, whereas in the iron analogues, described in the stannite-type structure (space group: I 4 ˉ 2 m ), single-phase behaviour persists to x = 0.1. Excess copper ions lead to the formation of holes and the electrical resistivity of both series is reduced from that of the stoichiometric end members. In the case of the iron-containing materials, this is shown to be due to an increase in the hole mobility, µ. This decrease in resistivity offsets the observed reduction in Seebeck coefficient and both series exhibit an improvement in thermoelectric performance. The lower electrical resistivity of the iron-containing materials, leads to higher figures-of-merit, compared to those of the zinc-containing materials at the same level of copper excess. The maximum figure-of-merit, ZT = 0.3 is attained for Cu2.075FeGe0.925Se4 at the comparatively low temperature of 575 K. This is an increase of ca. 62% from that of the end member phase and ca. 67% higher than that of the zinc analogue at the same level of substitution.

Thermoelectric performance of the p-type semiconductor bornite, Cu5FeS4, is greatly enhanced through chemical substitution. Nonstoichiometric materials in which the Cu:Fe ratio and overall cation-vacancy content were adjusted are reported, and a figure of merit, ZT = 0.79, is achieved at temperatures as low as 550 K in Cu4.972Fe0.968S4. All materials were synthesized mechanochemically and characterized by powder X-ray diffraction, differential scanning calorimetry (DSC), and thermal and electrical transport property measurements. Single-phase behavior is retained in copper deficient phases, Cu5–xFeS4, for vacancy levels up to x = 0.1, while in materials Cu5+yFe1–yS4, in which the Cu:Fe ratio is varied while maintaining full occupancy of cation sites, single-phase behavior persists for y ≤ 0.08. Adjusting the Cu:Fe ratio at a constant cation-vacancy level of 0.06 in Cu4.94+zFe1–zS4, leads to single phases for z ≤ 0.04. DSC measurements indicate the temperature of the intermediate- (2a) to high-temperature ...

Electron and hole doping in Co 3 Sn 2 S 2 , through chemical substitution of cobalt by the neighbouring elements, nickel and iron, affects both the structure and thermoelectric properties. Electron doping to form Co 3−x Ni x Sn 2 S 2 (0≤x≤3) results in an expansion of the kagome layer and materials become increasingly metallic as cobalt is substituted. Conversely, hole doping in Co 3−x Fe x Sn 2 S 2 (0≤x≤0.6) leads to a transition from metallic to n -type semiconducting behaviour at x=0.5. Iron substitution induces a small increase in the separation between the kagome layers and improves the thermoelectric performance. Neutron diffraction data reveal that substitution occurs at the Co 9( d ) site in a disordered fashion. Mossbauer spectroscopy reveals two iron environments with very different isomer shifts, which may be indicative of a mixed-valence state, while Sn exhibits an oxidation state close to zero in both series. Co 2.6 Fe 0.4 Sn 2 S 2 exhibits a maximum figure-of-merit, ZT=0.2 at 523 K while Co 2.4 Fe 0.6 Sn 2 S 2 reaches a power factor of 10.3 μW cm −1 K −2 close to room temperature.

Semiconducting quaternary chalcogenides with A

International audience; The crystal structure of the tetrahedrites Cu12 - xMnxSb4S13 (x = 0, 1) has been studied by powder neutron diffraction between room temperature and 773 K. At all temperatures investigated, manganese exclusively occupies tetrahedral sites, while the trigonal-planar sites contain only copper. In situ diffraction data confirm the stability of the tetrahedrite phase up to 773 K, with no evidence of copper mobility at elevated temperatures. Analysis of atomic displacement parameters indicates that there are low-energy vibrations associated with the trigonal-planar and the tetrahedral copper cations. The Einstein temperatures for the copper cations range between 79 and 91 K. Manganese substitution increases the electrical resistivity and the Seebeck coefficient, while the thermal conductivity is reduced. This results in a modest improvement in the thermoelectric figure of merit for Cu12MnSb4S13, which reaches ZT = 0.56 at 573 K. © 2019 Author(s).

International audience; The scale up of Spark Plasma Sintering (SPS) for the consolidation of large square monoliths (50 × 50 × 3 mm3) of thermoelectric materials is demonstrated and the properties of the fabricated samples compared with those from laboratory scale SPS. The SPS processing of n-type TiS2 and p-type Cu10.4Ni1.6Sb4S13 produces highly dense compacts of phase pure material. Electrical and thermal transport property measurements reveal that the thermoelectric performance of the consolidated n- and p-type materials is comparable with that of material processed using laboratory scale SPS, with ZT values that approach 0.75 and 0.35 at 700 K for Cu10.4Ni1.6Sb4S13 and TiS2, respectively. Mechanical properties of the consolidated materials show that large-scale SPS processing produces highly homogeneous materials with hardness and elastic moduli that deviate little from values obtained on materials processed on the laboratory scale. More generally, the process described in this paper is a promising way to produce high performance thermoelectric materials with square geometry, specifically required for thermoelectric device production. © The Royal Society of Chemistry 2016.

Bismuth-containing chalcohalides are currently under consideration for photovoltaic applications and easily-scalable synthetic routes for these materials are needed. The work presented here demonstrates the feasibility of synthesising large quantities of phase-pure chalcohalides by ball milling. This avoids deviations from stoichiometry due to the volatilisation of halides that can occur during high-temperature synthesis. Bulk polycrystalline samples of BiQI (Q ​= ​S, Se) were produced by ball milling and characterised by powder X-ray diffraction, UV–vis diffuse reflectance and thermogravimetric analysis. Rietveld refinements confirm that phase-pure BiSI and BiSeI were produced. These materials were found to be thermally stable up to 270–330 ​°C, and to exhibit optical band gaps of 1.56 and 1.26 ​eV for BiSI and BiSeI respectively. Preliminary results on the preparation of films by spin-coating slurries of the ball milled powders are also presented.

Tetrahedrite, Cu12Sb4S13, is an abundant mineral with excellent thermoelectric properties owing to its low thermal conductivity. The electronic and structural origin of the intriguing physical properties of tetrahedrite, including its metal-to-semiconductor transition, remains largely unknown. This work presents the first determination of the low-temperature structure of tetrahedrite that accounts for its unique properties. Contrary to prior conjectures, our results show that the trigonal-planar copper cations remain in planar coordination below the metal-to-semiconductor transition. The atomic displacement parameters of the trigonal-planar copper cations, which have been linked to low thermal conductivity, increase by 200% above the metal-to-semiconductor transition. The phase transition is consequence of the orbital degeneracy of the highest occupied 3d cluster orbitals of the copper clusters found inside the sodalite cages in the cubic phase. This study reveals that a Jahn-Teller electronic instability leads to the formation of “molecular-like” Cu57+ clusters and suppresses copper rattling vibrations due to the strengthening of direct copper-copper interactions. Our first-principles calculations demonstrate that the structural phase transition opens a small band gap in the electronic density of states and eliminates the unstable phonon modes. The present results provide insights on the interplay between phonon transport, electronic properties and crystal structure in mixed-valence compounds.

The construction and evaluation of wholly-skutterudite thermoelectric modules with a high volume-power-density is described. Such modules afford the maximum power output for the minimum use of material. Synthesis of the component n-type unfilled skutterudite CoSb2.75Sn0.05Te0.20 and p-type filled skutterudite Ce0.5Yb0.5Fe3.25Co0.75Sb12, was achieved using a scalable ball-milling route that provides sufficient material for the construction and assessment of performance of 12 modules. Impedance spectroscopy at room temperature is shown to provide a rapid means of evaluating the quality of module fabrication. The results show a high degree of reproducibility in module performance across the 12 modules, with an average internal resistance of 102(4) m. Electrical measurements on the component n- and p-type materials reveal power factors (S2σ) of 1.92 and 1.33 mW m-1 K-2, respectively, at room temperature and maximum figures of merit of ZT = 1.13 (n-type) and ZT = 0.91 (p-type) at 673 K and 823 K, respectively. The figure of merit of the module at room temperature (ZT = 0.12) is reduced by ca. 39% from the average of the n- and p-type component materials at the same temperature, as a result of thermal- and electrical-contact resistance losses associated with the architecture of the module. I-V curves for the module determined for T in the range 50 – 450 K show an almost linear dependence of the open-circuit voltage on T and allow calculation of the power output, which reaches a maximum value of 1.8 W (0.9 W cm-2) at T = 448 K.

Bi 2 O 2 Te was synthesised from a stoichiometric mixture of Bi, Bi 2 O 3 and Te by a solid state reaction. Analysis of powder X-ray diffraction data indicates that this material crystallises in the anti -ThCr 2 Si 2 structure type (space group I 4/ mmm ), with lattice parameters a =3.98025(4) and c =12.70391(16) A. The electrical and thermal transport properties of Bi 2 O 2 Te were investigated as a function of temperature over the temperature range 300≤ T (K)≤665. These measurements indicate that Bi 2 O 2 Te is an n-type semiconductor, with a band gap of 0.23 eV. The thermal conductivity of Bi 2 O 2 Te is remarkably low for a crystalline material, with a value of only 0.91 W m −1 K −1 at room temperature.

Understanding the underlying mechanisms that suppress thermal conduction in solids is of paramount importance for the targeted design of materials for thermal management and thermoelectric energy conversion applications. Bismuth copper oxychalcogenides, BiOCuQ (Q = Se, Te), are highly crystalline thermoelectric materials with an unusually low lattice thermal conductivity of ∼0.5 Wm(-1) K(-1), a value normally found in amorphous materials. Here we unveil the origin of the unusual thermal transport properties of these phases. First principles calculations of the vibrational properties combined with analysis of in-situ neutron diffraction data, demonstrate that weak bonding of copper atoms within the structure leads to an unexpected vibrational mode at low frequencies, which is likely to be a major contributor to the low thermal conductivity of these materials. In addition, we show that anharmonicity and the large Grüneisen parameter in these oxychalcogenides are mainly related to the low frequency copper vibrations, rather than to the Bi(3+) lone pairs.

The synthesis and characterization of five new indium selenides, [C9H17N2]3[In5Se(8+x)(Se2)(1-x)] (1-2), [C6H12N2]4[C6H14N2]3[In10Se15(Se2)3] (3), [C6H14N2][(C6H12N2)2NaIn5Se9] (4) and [enH2][NH4][In7Se12] (5), are described. These materials were prepared under solvothermal conditions, using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,4-diazabicyclo[2.2.2]octane (DABCO) as structure-directing agents. Compounds 1-4 represent the first examples of ribbons in indium selenides, and 4 is the first example of incorporation of an alkali metal complex. Compounds 1, 2 and 4 contain closely related [In5Se(8+x)(Se2)(1-x)](3-) ribbons which differ only in their content of (Se2)(2-) anions. These ribbons are interspaced by organic countercations in 1 and 2, while in 4 they are linked by highly unusual [Na(DABCO)2](+) units into a three-dimensional framework. Compound 3 contains complex ribbons, with a long repeating sequence of ca. 36 Å, and 4 is a non-centrosymmetric three-dimensional framework, formed as a consequence of the decomposition of DABCO into ethylenediamine (en) and ammonia.

Thermoelectric generator (TEG) has received more and more attention in its application in the harvesting of waste thermal energy in automotive engines. Even though the commercial Bismuth Telluride thermoelectric material only have 5% efficiency and 250°C hot side temperature limit, it is possible to generate peak 1kW electrical energy from a heavy-duty engine. If being equipped with 500W TEG, a passenger car has potential to save more than 2% fuel consumption and hence CO2 emission reduction. TEG has advantages of compact and motionless parts over other thermal harvest technologies such as Organic Rankine Cycle (ORC) and Turbo-Compound (TC). Intense research works are being carried on improving the thermal efficiency of the thermoelectric materials and increasing the hot side temperature limit. Future thermoelectric modules are expected to have 10% to 20% efficiency and over 500°C hot side temperature limit. This paper presents the experimental synthesis procedure of both p-type and n-type skutterudite thermoelectric materials and the fabrication procedure of the thermoelectric modules using this material. These skutterudite materials were manufactured in the chemical lab in the University of Reading and then was fabricated into modules in the lab in Cardiff University. These thermoelectric materials can work up to as high as 500°C temperature and the corresponding modules can work at maximum 400°C hot side temperature. The performance loss from materials to modules has been investigated and discussed in this paper. By using a validated TEG model, the performance improvement using these modules has been estimated compared to commercial Bisemous Telluride modules.

We describe an entirely new type of tetrahedral cluster, representing a new level of structural hierarchy: a hybrid tetrahedron of supertetrahedra, formed by five T3 supertetrahedral clusters connected by bipyridyl linkers. Covalent assembly of these 37 A super-supertetrahedra with smaller (10 A) T3 clusters results in the formation of a two-dimensional covalent network which contains pores in the mesoporous range.

The single and double filled p -type skutterudites Ce 0.8 Fe 3 CoSb 12 and Ce 0.5 Yb 0.5 Fe 3.25 Co 0.75 Sb 12 have been prepared by mechanical alloying. This offers a rapid method for the preparation of skutterudites that could be scaled up for adoption at industrial level. The large-scale samples prepared by ball-milling exhibit enhanced figures of merit ZT, compared with materials prepared by conventional solid-state reaction. At room temperature ZT is increased by ca . 19% for both single and double filled skutterudites. Maximum figures of merit, ZT = 0.68 and ZT = 0.93 are attained for Ce 0.8 Fe 3 CoSb 12 at 773 K and Ce 0.5 Yb 0.5 Fe 3.25 Co 0.75 Sb 12 at 823 K respectively. The improvement in thermoelectric values at room temperature may be traced to a reduction in thermal conductivity in the ball-milled samples arising from the reduced grain size. The influence of the microstructure on the thermoelectric properties, together with the stability in air and the performance of the materials after several heating and cooling cycles has been studied and are detailed in this work. The densified samples prepared by ball-milling also show a higher resistance to oxidation, which starts at 694 K for Ce 0.8 Fe 3 CoSb 12 and at 783 K for Ce 0.5 Yb 0.5 Fe 3.25 Co 0.75 Sb 12 .

International audience; Tetrahedrites are promising p-type thermoelectric materials for energy recovery. We present here the first investigation of the structure and thermoelectric properties of copper-rich tetrahedrites, Cu12+xSb4S13 (0 < x 0 consist of two tetrahedrite phases. In situ neutron diffraction data demonstrate that on heating, the two tetrahedrite phases coalesce into a single tetrahedrite phase at temperatures between 493 and 553 K and that this transition shows marked hysteresis on cooling. Our structural data indicate that copper ions become mobile above 393 K. Marked changes in the temperature dependence of the electrical and thermal transport properties of the copper-rich phases occur at the onset of copper mobility. Excess copper leads to a significant reduction in the total thermal conductivity, which for the nominal composition Cu14Sb4S13 reaches a value as low as 0.44 W m(-1) K-1 at room temperature, and to thermoelectric properties consistent with phonon liquid electron crystal (PLEC) behavior.

Bulk polycrystalline samples in the series Ti1+xS2 (x = 0 to x = 0.05) were prepared using high-temperature synthesis from the elements and spark plasma sintering. X-ray structure analysis shows that the lattice constant c expands as titanium intercalates between TiS2 slabs. A Seebeck coefficient close to −300 μV K−1 is observed for the first time in TiS2 compounds. The decrease in electrical resistivity and Seebeck coefficient that occurs upon Ti intercalation (Ti off stoichiometry) supports the view that charge carrier transfer to the Ti 3d band takes place and the carrier concentration increases. At the same time, the thermal conductivity is reduced by phonon scattering due to structural disorder induced by Ti intercalation. Optimum ZT values of 0.14 and 0.48 at 300 and 700 K, respectively, are obtained for x = 0.025.

The thermoelectric behavior of the transition-metal disulfides n-type NiCr2S4 and p-type CuCrS2 has been investigated. Materials prepared by high-temperature reaction were consolidated using cold-pressing and sintering, hot-pressing in graphite dies or spark-plasma sintering in tungsten carbide dies. The consolidation conditions have a marked influence on the electrical transport properties. In addition to the effect on sample density, altering the consolidation conditions results in changes to the sample composition, including the formation of impurity phases. Maximum room-temperature power factors were 0.18 mW m−1 K−2 and 0.09 mW m−1 K−2 for NiCr2S4 and CuCrS2, respectively. Thermal conductivities of ca. 1.4 W m−1 K−1 and 1.2 W m−1 K−1 lead to figures of merit of 0.024 and 0.023 for NiCr2S4 and CuCrS2, respectively.

The solvothermal synthesis and characterization of two indium selenides with stoichiometry [NH4][InSe2] is described. Yellow [NH4][InSe2] (1), which exhibits a layered structure, was initially prepared in an aqueous solution of trans-1,4-diaminocyclohexane, and subsequently using a concentrated ammonia solution. A red polymorph of one-dimensional character, [NH4][InSe2] (2), was obtained using 3,5-dimethylpyridine as solvent. [NH4][InSe2] (1) crystallizes in the non-centrosymmetric space group Cc (a=11.5147(6), b=11.3242(6), c=15.9969(9) A and β=100.354(3)°). The structural motif of the layers is the In4Se10 adamantane unit, composed of four corner-linked InSe4 tetrahedra. These units are linked by their corners, forming [InSe2]− layers which are stacked back to back along the c-direction, and interspaced by [NH4]+cations. The one-dimensional polymorph, (2), crystallizes in the tetragonal space group, I4/mcm (a=8.2519(16), c=6.9059 (14) A). This structure contains infinite chains of edge-sharing InSe4 tetrahedra separated by [NH4]+ cations.

The new thermoelectric material BiOCuTe exhibits an electrical conductivity of 224 S cm-1 and a Seebeck coefficient of +186 μV K-1 at 373 K, together with an extremely low lattice thermal conductivity of ∼ 0.5 W m-1 K-1. This results in a ZT of 0.42 at 373 K, which increases to 0.66 at the maximum temperature investigated, 673 K.

The shandite family of solids, with hexagonal structure and composition ${A}_{3}{M}_{2}{X}_{2}$ (A = Ni, Co, Rh, Pd; M = Pb, In, Sn, Tl; X = S, Se), has attracted recent research attention due to promising applications as thermoelectric materials. Herein we discuss the electron and phonon transport properties of shandite-structured ${\mathrm{Ni}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$, based on a combination of density functional theory, Boltzmann transport theory, and experimental measurements. ${\mathrm{Ni}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ exhibits a metallic and nonmagnetic ground state with ${\mathrm{Ni}}^{0}$ oxidation state and very low charge on Sn and S atoms. Seebeck coefficients obtained from theoretical calculations are in excellent agreement with those measured experimentally between 100 and 600 K. From the calculation of the ratio $\ensuremath{\sigma}/\ensuremath{\tau}$ between the electronic conductivity and relaxation time, and the experimental determination of electron conductivity, we extract the variation of the scattering rate ($1/\ensuremath{\tau}$) with temperature between 300 and 600 K, which turns out to be almost linear, thus implying that the dominant electron-scattering mechanism in this temperature range is via phonons. The electronic thermal conductivity, which deviates only slightly from the Wiedemann-Franz law, provides the main contribution to thermal transport. The small lattice contribution to the thermal conductivity is calculated from the phonon structure and third-order force constants, and is only $\ensuremath{\sim}2\phantom{\rule{4pt}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 300 K (less than 10% of the total thermal conductivity), which is confirmed by experimental measurements. Overall, ${\mathrm{Ni}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ is a poor thermoelectric material ($ZT\ensuremath{\sim}$ 0.01 at 300 K), principally due to the low absolute value of the Seebeck coefficient. However, the understanding of its transport properties will be useful for the rationalization of the thermoelectric behavior of other, more promising members of the shandite family.

The recent progress in metal chalcogenides for thermoelectric energy recovery has been striking. Significant advances in performance have arisen from strategies that address the considerable challenges presented by the relationship between the key physical properties of electrical conductivity (σ), Seebeck coefficient (S) and thermal conductivity (κ) that determine the energy conversion efficiency through the figure-of-merit, zT. In particular, optimisation of structure across a range of length scales achieves substantial reductions in thermal conductivity through interface scattering of heat carrying phonons. Similarly, manipulation of the electron-transport properties using techniques such as band engineering, Fermi-level tuning and resonant states provides a means of targeting improvements in the thermoelectric power factor (S2σ). The adoption of a combination of these approaches leads to materials with unprecedented thermoelectric performance. Figures of merit routinely exceed zT=1.0 at elevated temperatures and, in the best performing materials, zT>2.0 has been achieved. Moreover, many of the improvements in performance have been achieved in materials containing earth-abundant elements, suggesting that sustainable alternatives to the established Bi2Te3 are indeed viable.

We report structural, magnetic, and magnetotransport properties of the two-dimensional chromium sulfide Cr2S2.92, which contains both Cr3+ and Cr2+ ions. Below its magnetic-ordering temperature (TN=118 K), Cr2S2.92 exhibits a weak spontaneous magnetization with a maximum around 90 K and a value of 0.013(4) μB per Cr ion at 5 K. The resistivity and magnetoresistance exhibit pronounced local maxima around TN (MR=48% in a field of 14 T). The magnetotransport properties are discussed in terms of magnetic-polaron formation.

International audience; Bulk polycrystalline samples in the series Ti1-xNbxS2 (0 ≤ x ≤ 0.075) were prepared using mechanical alloying synthesis and spark plasma sintering. X-ray diffraction analysis coupled with high resolution transmission electron microscopy indicates the formation of trigonal TiS2 by high energy ball-milling. The as-synthesized particles consist of pseudo-ordered TiS2 domains of around 20-50 nm, joined by bent atomic planes. This bottom-up approach leads, after spark plasma sintering, to homogeneous solid solutions, with a niobium solubility limit of x = 0.075. Microstructural observations evidence the formation of small crystallites in the bulk compounds with a high density of stacking faults. The large grain boundary concentration coupled with the presence of planar defects, leads to a substantial decrease in the thermal conductivity to 1.8 W/mK at 700 K. This enables the figure of merit to reach ZT = 0.3 at 700 K for x = 0.05, despite the lower electron mobility in mechanically alloyed samples due to small crystallite/grain size and structural defects. © 2015 Elsevier Ltd.

International audience; A family of phases, CoxTiS2 (0 ≤ x ≤ 0.75) has been prepared and characterised by powder X-ray and neutron diffraction, electrical and thermal transport property measurements, thermal analysis and SQUID magnetometry. With increasing cobalt content, the structure evolves from a disordered arrangement of cobalt ions in octahedral sites located in the van der Waals' gap (x ≤ 0.2), through three different ordered vacancy phases, to a second disordered phase at x ≥ 0.67. Powder neutron diffraction reveals that both octahedral and tetrahedral inter-layer sites are occupied in Co0.67TiS2. Charge transfer from the cobalt guest to the TiS2 host affords a systematic tuning of the electrical and thermal transport properties. At low levels of cobalt intercalation (x andlt; 0.1), the charge transfer increases the electrical conductivity sufficiently to offset the concomitant reduction in |S|. This, together with a reduction in the overall thermal conductivity leads to thermoelectric figures of merit that are 25% higher than that of TiS2, ZT reaching 0.30 at 573 K for CoxTiS2 with 0.04 ≤ x ≤ 0.08. Whilst the electrical conductivity is further increased at higher cobalt contents, the reduction in |S| is more marked due to the higher charge carrier concentration. Furthermore both the charge carrier and lattice contributions to the thermal conductivity are increased in the electrically conductive ordered-vacancy phases, with the result that the thermoelectric performance is significantly degraded. These results illustrate the competition between the effects of charge transfer from guest to host and the disorder generated when cobalt cations are incorporated in the inter-layer space. © The Royal Society of Chemistry 2016.

The solvothermal synthesis and characterisation of [C6H16N2][GaS2]2 (1), [C6H16N2][Ga2Se3(Se2)] (2), and mixed-metal phases with composition [C6H16N2][Ga2–xInxSe3(Se2)] (0 < x < 2)(3–5), is described. These materials have been characterised by single-crystal and powder X-ray diffraction, thermogravimetric analysis and UV/Vis diffuse reflectance spectroscopy. The materials contain one-dimensional anionic chains. In 1, these chains consist of edge-linked GaS4 tetrahedra, whilst in 2–5, the chains contain perselenide (Se2)2– units and comprise alternating four-membered [M2Se2] and five-membered [M2Se3] rings (where M = Ga, In). Compounds 3–5 represent the first examples of ternary mixed-metal [M2Se3(Se2)]2– chains.

We report a straightforward methodology for the fabrication of high-temperature thermoelectric (TE) modules using commercially available solder alloys and metal barriers. This methodology employs standard and accessible facilities that are simple to implement in any laboratory. A TE module formed by nine n-type Yb x Co4Sb12 and p-type Ce x Fe3CoSb12 state-of-the-art skutterudite material couples was fabricated. The physical properties of the synthesized skutterudites were determined, and the module power output, internal resistance, and thermocycling stability were evaluated in air. At a temperature difference of 365 K, the module provides more than 1.5 W cm−3 volume power density. However, thermocycling showed an increase of the internal module resistance and degradation in performance with the number of cycles when the device is operated at a hot-side temperature higher than 573 K. This may be attributed to oxidation of the skutterudite thermoelements.

The skutterudites Yb{sub x}Fe{sub 2}Ni{sub 2}Sb{sub 12} (0{

A new organically templated indium selenide, [C{sub 6}H{sub 16}N{sub 2}][In{sub 2}Se{sub 3}(Se{sub 2})], has been prepared hydrothermally from the reaction of indium, selenium and trans-1,4-diaminocyclohexane in water at 170 deg. C. This material was characterised by single-crystal and powder X-ray diffraction, thermogravimetric analysis, UV-vis diffuse reflectance spectroscopy, FT-IR and elemental analysis. The compound crystallises in the monoclinic space group C2/c (a=12.0221(16) A, b=11.2498(15) A, c=12.8470(17) A, {beta}=110.514(6){sup o}). The crystal structure of [C{sub 6}H{sub 16}N{sub 2}][In{sub 2}Se{sub 3}(Se{sub 2})] contains anionic chains of stoichiometry [In{sub 2}Se{sub 3}(Se{sub 2})]{sup 2-}, which are aligned parallel to the [1 0 1] direction, and separated by diprotonated trans-1,4-diaminocyclohexane cations. The [In{sub 2}Se{sub 3}(Se{sub 2})]{sup 2-} chains, which consist of alternating four-membered [In{sub 2}Se{sub 2}] and five-membered [In{sub 2}Se{sub 3}] rings, contain perselenide (Se{sub 2}){sup 2-} units. UV-vis diffuse reflectance spectroscopy indicates that [C{sub 6}H{sub 16}N{sub 2}][In{sub 2}Se{sub 3}(Se{sub 2})] has a band gap of 2.23(1) eV. - Graphical abstract: [C{sub 6}H{sub 16}N{sub 2}][In{sub 2}Se{sub 3}(Se{sub 2})], prepared under hydrothermal conditions, contains one-dimensional chains of stoichiometry [In{sub 2}Se{sub 3}(Se{sub 2})]{sup 2-}, in which four-membered [In{sub 2}Se{sub 2}] and five-membered [In{sub 2}Se{sub 3}] rings alternate. Highlights: > New one-dimensional indium selenide prepared under hydrothermal conditions. >more » The [In{sub 2}Se{sub 3}(Se{sub 2})]{sup 2-} chains are a new structural motif for Group 13 chalcogenides. > Unusual presence of (Se{sub 2}){sup 2-} moieties. > Optical band gap of 2.23(1) eV.« less

The new thiogallate Na 5 (Ga 4 S)(GaS 4 ) 3 ·6H 2 O has been prepared solvothermally, using 3,5-dimethyl pyridine as a solvent, and characterised by powder and single crystal X-ray diffraction. This material, which exhibits a three-dimensional crystal structure, crystallises in the cubic space group F 4 ¯ 3 c ( a = 17.557(4) A). The crystal structure contains octahedral building blocks [Ga 4 S (GaS 4 ) 6 ] 20− , linked into a three-dimensional network with a perovskite-type topology, and sodium hydrate clusters, [Na 5 (H 2 O) 6 ] 5+ , filling the cavities in the [Ga 4 S(GaS 4 ) 6/2 ] 5− framework. UV–Vis diffuse reflectance measurements indicate that this material is a wide band gap semiconductor, with a band gap of ca . 4.4 eV.

Two semiconducting hybrid gallium selenides, [Ga6Se9(C6H14N2)4][H2O] (1) and [C6H14N2][Ga4Se6(C6H14N2)2] (2), were prepared using a solvothermal method in the presence of 1,2-diaminocyclohexane (1,2-DACH). Both materials consist of neutral inorganic layers, in which 1,2-DACH is covalently bonded to gallium. In 1, the organic amine acts as a monodentate and a bidentate ligand, while in 2, bidentate and uncoordinated 1,2-DACH molecules coexist.