The impact of charge transfer and structural disorder on the thermoelectric properties of cobalt intercalated TiS2

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 < 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 impact of manganese substitution on the structure and properties of tetrahedrite

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 indicate 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

The influence of mobile copper ions on the glass-like thermal conductivity of copper-rich tetrahedrites

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 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-1K-1 at room temperature, and to thermoelectric properties consistent with phonon liquid electron crystal (PLEC) behaviour

Up-scaled synthesis process of sulphur-based thermoelectric materials

International audienceThe 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

The impact of charge transfer and structural disorder on the thermoelectric properties of cobalt intercalated TiS2

International audienceA 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

Thermoelectric properties of TiS2 mechanically alloyed compounds

International audienceBulk 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

Resolution of the Cationic Distribution in Synthetic Germanite Cu22Fe8Ge4S32 by an Experimental Combinatorial Approach Based on Synchrotron Resonant Powder Diffraction Data: A Case Study and Guidelines for Analogous Compounds

International audienceThe present work investigates the cationic distribution of a complex CuS compound belonging to a promising class of materials with significant prospects for thermoelectric generator and photovoltaic applications. We propose the use of powder resonant X-ray scattering in a combinatorial experimental approach with X-ray powder diffraction and single crystal, as a valid, general, and practical method to unravel the complex cation ordering encountered in the synthetic germanite Cu22Fe8Ge4S32. The generation and testing of all the possible structural models is rapid and rigorous as it is based on two modules written in python language: a module that takes care of model creation and ordering (permutation algorithm associated with some filtering and ordering algorithms) and a python parser for the refinement software (FullProf) input and output. Each possible cationic model is tested through combined Rietveld refinement of resonant X-ray data at selected edges and analyzed considering its global χ2 (Bragg contribution) agreement factor. This approach can easily integrate information coming from other techniques, as in our case EXAFS spectroscopy and single crystal X-ray diffraction. In spite of the complexity of the germanite, we were able to define the main characteristics of the cationic ordering (space group P4¯ 3n): (i) the &quot;interstitial&quot;site 2a is fully occupied by Fe, (ii) Ge is located only on the 6d site (or the symmetry equivalent one 6c), and (iii) the remaining Fe atoms are located preferentially on the 12f site and possibly on the 6d (or 6c) site along with Ge. Moreover, we single out a probable enrichment of Ge at the expense of Fe. The approach developed in this case study can be used as a guideline for the crystal structure resolution of analogous compounds. © 2022 American Chemical Society

Resolution of the Cationic Distribution in Synthetic Germanite Cu22Fe8Ge4S32 by an Experimental Combinatorial Approach Based on Synchrotron Resonant Powder Diffraction Data: A Case Study and Guidelines for Analogous Compounds

International audienceThe present work investigates the cationic distribution of a complex CuS compound belonging to a promising class of materials with significant prospects for thermoelectric generator and photovoltaic applications. We propose the use of powder resonant X-ray scattering in a combinatorial experimental approach with X-ray powder diffraction and single crystal, as a valid, general, and practical method to unravel the complex cation ordering encountered in the synthetic germanite Cu22Fe8Ge4S32. The generation and testing of all the possible structural models is rapid and rigorous as it is based on two modules written in python language: a module that takes care of model creation and ordering (permutation algorithm associated with some filtering and ordering algorithms) and a python parser for the refinement software (FullProf) input and output. Each possible cationic model is tested through combined Rietveld refinement of resonant X-ray data at selected edges and analyzed considering its global χ2 (Bragg contribution) agreement factor. This approach can easily integrate information coming from other techniques, as in our case EXAFS spectroscopy and single crystal X-ray diffraction. In spite of the complexity of the germanite, we were able to define the main characteristics of the cationic ordering (space group P4¯ 3n): (i) the &quot;interstitial&quot;site 2a is fully occupied by Fe, (ii) Ge is located only on the 6d site (or the symmetry equivalent one 6c), and (iii) the remaining Fe atoms are located preferentially on the 12f site and possibly on the 6d (or 6c) site along with Ge. Moreover, we single out a probable enrichment of Ge at the expense of Fe. The approach developed in this case study can be used as a guideline for the crystal structure resolution of analogous compounds. © 2022 American Chemical Society

Structure, microstructure and thermoelectric properties of germanite-type Cu22Fe8Ge4S32 compounds

International audienceThis paper describes the influence of the powder synthesis and densification techniques on the structure, microstructure and thermoelectric properties of Cu22Fe8Ge4S32, a synthetic derivative of the naturally occurring germanite mineral. Two powder synthesis approaches are compared, namely mechanical alloying and conventional sealed tube synthesis, combined with two densification methods spark plasma sintering and hot pressing. Structural analyses by Le Bail refinement of X-ray powder diffraction patterns and transmission electron microscopy confirmed the high crystallinity and the absence of structural defects in the samples. It is especially highlighted that mechanical alloying combined with low sintering temperature allows to reach high purity and to limit the formation of secondary phases due to sulfur volatilization in the bulk specimens. The changes in the electrical resistivity and Seebeck coefficient with the sample preparation methods evidence the high sensitivity of the material to slight stoichiometric deviations. Conversely, the thermal conductivity is less influenced by stoichiometric variations and microstructural changes. This investigation draws attention to the significant impact of powder synthesis and sintering methods on the electrical transport properties of complex quaternary Cu-based sulfides specifically designed to present intrinsically low thermal conductivity for potential thermoelectric applications