Structure Low Dimensionality and Lone-Pair Stereochemical Activity: the Key to Low Thermal Conductivity in the Pb-Sn-S System.

Recently, metal sulfides have begun to receive attention as potential cost-effective materials for thermoelectric applications beyond optoelectronic and photovoltaic devices. Herein, based on a comparative analysis of the structural and transport properties of 2D PbSnS2 and 1D PbSnS3, we demonstrate that the intrinsic effects that govern the low lattice thermal conductivity (κL) of these sulfides originate from the combination of the low dimensionality of their crystal structures with the stereochemical activity of the lone-pair electrons of cations. The presence of weak bonds in these materials, responsible for phonon scattering, results in inherently low κL of 1.0 W/m K in 1D PbSnS3 and 0.6 W/m K in 2D PbSnS2 at room temperature. However, the nature of the thermal transport is quite distinct. 1D PbSnS3 exhibits a higher thermal conductivity with a crystalline-like peak at low temperatures, while 2D PbSnS2 demonstrates glassy thermal conductivity in the entire temperature range investigated. First-principles density functional theory calculations reveal that the presence of antibonding states below the Fermi level, especially in PbSnS2, contributes to the very low κL. In addition, the calculated phonon dispersions exhibit very soft acoustic phonon branches that give rise to soft lattices and very low speeds of sounds.

Metal Substitutions (M = Al, Ga, In; Sn, Ge; and Mn, Fe, Co) in ZnS: Sphalerite versus Wurtzite Formation.

A series of Zn1-xMxS polycrystalline samples were synthesized via a solid-state reaction in closed vessels to examine the solubility of foreign M cations within the wurtzite ZnS structure, employing quenching or slow cooling processes to favor specific polymorphs. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses revealed diverse structural behaviors across different cations. Group 13 elements (Al and Ga) formed solid solutions with a wurtzite structure, while In showed complex layer stacking defects. For 3d magnetic cations (Mn, Fe, and Co), a broad solubility range in the hexagonal structure was noted for Mn, whereas Fe and Co more readily formed cubic structures, with solubilities similar to Mn in the sphalerite form. Despite structural differences, magnetic susceptibilities and spin freezing temperatures for Fe and Co were comparable. Group 14 elements showed varied behaviors: Sn was insoluble in ZnS, as attested by unchanged unit cell parameters and surface crystallite Sn, whereas Ge only formed in the cubic phase with a solubility limit of x ≈ 0.2. The study discusses these variations in solubility and structure in terms of oxidation states, ionic-covalent radius, and coordination preferences in sulfides.

The Synthesis and Thermoelectric Properties of the n-Type Solid Solution Bi2-xSbxTe3 (x < 1).

Commercial Peltier cooling devices and thermoelectric generators mostly use bismuth telluride-based materials, specifically its alloys with Sb2Te3 for the p-type legs and its alloys with Bi2Se3 for the n-type legs. If the p-type materials perform with zT well above the unity around room temperature, the n-type counterpart is lacking efficiency in this temperature range, and has the disadvantage of containing selenium. Indeed, despite the fact that selenium is not environmentally benign and that its handling requires precautions, the use of selenium does not facilitate the optimization of thermoelectric performance at or around room temperature, as the presence of selenium results in a larger band gap. In this study, we investigate the feasibility of a selenium-free n-type (Bi, Sb)2Te3 using a simple two-step process: mechanical alloying synthesis followed by spark plasma sintering. All the members of the solid solution Bi2-xSbxTe3 with x < 1 are n-type materials, with zTs between 0.35 and 0.6. The zT is maximized at lower temperatures with an increasing Sb content, which is proof that the band gap is reduced accordingly. We also show here that an edge-free sintering process considerably improves thermoelectric performance.

CuFe2S3 as electrode material for Li-ion batteries.

Electrochemical performances of the isocubanite CuFe2S3 tested as electrode material for Li-ion batteries have been investigated. A first discharge capacity of 860 mA h g-1 shows a conversion process leading to Li2S, copper and iron nanoparticles. Interestingly, a reversible capacity of 560 mA h g-1 at 1.5 V is demonstrated with good cyclability up to 30 cycles.

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

A family of phases, Co x TiS2 (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 Co x TiS2 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.

Silver intercalation in SPS dense TiS2: staging and thermoelectric properties.

Polycrystalline samples in the series AgxTiS2 with x varying from 0 to 0.2 were prepared using solid-liquid-vapor reaction and spark plasma sintering. Depending on the x content, it is found that different stages can occur with intercalation, from the so-called 1T-TiS2 (stage 1) to ordered Ag1/6TiS2 (stage 2). Randomly intercalated Ag cations in the van der Waals gap of stage 1 and stage 2 based TiS2 structures induce a strong decrease of lattice thermal conductivity through structural disorder. A decrease in electrical resistivity and the absolute value of the Seebeck coefficient with increasing Ag content supports also the charge transfer to the Ti 3d conduction band, enhancing the power factor in the specific temperature range. Thus, the combined effects of Ag intercalation are beneficial to the improvement in ZT, reaching around 0.45 at 700 K in Ag intercalated compounds.