Phase Equilibria and Thermoelectric Properties in the Pb-Ga-Te System in the Vicinity of the PbGa6Te10 Phase.

PbGa6Te10 is a promising thermoelectric (TE) material due to its ultralow thermal conductivity and moderated values of the Seebeck coefficient. However, the reproducible synthesis of the PbGa6Te10-based materials for the investigation and tailoring of physical properties requires detailed knowledge of the phase diagram of the system. With this aim, a combined thermal, structural, and microstructural study of the Pb-Ga-Te ternary system near the PbGa6Te10 composition is presented here, in which polycrystalline samples with the compositions (PbTe)1-x(Ga2Te3)x (0.67 ≤ x ≤ 0.87) and PbyGa6Te10 (0.85 ≤ y ≤ 1.5) were synthesized and characterized. Differential scanning calorimetry measurements revealed that PbGa6Te10 melts incongruently at 1007 ± 2 K and has a polymorphic phase transition at 658-693 K depending on composition. Powder X-ray diffraction of annealed samples confirmed that below 658 K, the trigonal modification of PbGa6Te10 exists (space groups P3121 or P3221) and above 693 K, the rhombohedral one (space group R32). A homogeneity range was found for PbyGa6Te10, y = 0.9-1.1, based on refined lattice parameters of PbyGa6Te10 in samples annealed at 873 K. The revised version of the PbTe-Ga2Te3 phase diagram in the vicinity of the PbGa6Te10 phase is proposed. Based on the new results of the phase equilibria, the TE properties of the PbyGa6Te10 samples were studied in detail. The deviation from the stoichiometric composition leads to a tuning of the charge transport in PbyGa6Te10, and as a result, the Seebeck coefficient and electrical conductivity were significantly modified over the homogeneity range. The Pb-deficient Pb0.9Ga6Te10 sample shows an improved power factor up to 9.5 µW m-1 K-2 and a reduced thermal conductivity as low as 0.17 W m-1 K-1 due to attuned chemical potential and additional scattering of phonons on point defects. Thus, the ZT parameter for this composition was improved up to ∼0.043 at 773 K, which is almost 4 times higher than that of the stoichiometric specimen. This work shows that the knowledge of phase equilibria and crystal chemistry plays a key role in improving the energy conversion efficiency for new functional TE materials.

Structure Evolution and Bonding Inhomogeneity toward High Thermoelectric Performance in Cu2CoSnS4-xSex Materials.

Lightweight diamond-like structure (DLS) materials are excellent candidates for thermoelectric (TE) applications due to their low costs, eco-friendly nature, and property stability. The main obstacles restricting the energy-conversion performance by the lightweight DLS materials are high lattice thermal conductivity and relatively low carrier mobility. By investigating the anion substitution effect on the structural, microstructural, electronic, and thermal properties of Cu2CoSnS4-xSex, we show that the simultaneous enhancement of the crystal symmetry and bonding inhomogeneity engineering are effective approaches to enhance the TE performance in lightweight DLS materials. Particularly, the increase of x in Cu2CoSnS4-xSex makes the DLS structure with the ideal tetrahedral bond angles of 109.5° favorable, leading to better crystal symmetry and higher carrier mobility in samples with higher selenium content. In turn, the phonon transport in the investigated DLS materials is strongly disturbed due to the bonding inhomogeneity between anions and three sorts of cations inducing large lattice anharmonicity. The increase of Se content in Cu2CoSnS4-xSex only intensified this effect resulting in a lower lattice component of the thermal conductivity (κL) for Se-rich samples. As a result of the enhanced power factor S2ρ-1 and the low κL, the dimensionless thermoelectric figure of merit ZT achieves a high value of 0.75 for Cu2CoSnSe4 DLS material. This work demonstrates that crystal symmetry and bonding inhomogeneity play an important role in the transport properties of DLS materials and provide a path for the development of new perspective materials for TE energy conversion.

Ultralow Lattice Thermal Conductivity and Improved Thermoelectric Performance in Cl-Doped Bi2Te3-xSex Alloys.

Bi2Te3-based alloys are the main materials for the construction of low- and medium-temperature thermoelectric modules. In this work, the microstructure and thermoelectric properties of Cl-doped Bi2Te3-xSex alloys were systematically investigated considering the high anisotropy inherent in these materials. The prepared samples have a highly oriented microstructure morphology, which results in very different thermal transport properties in two pressing directions. To accurately separate the lattice, electronic, and bipolar components of the thermal conductivity over the entire temperature range, we employed a two-band Kane model to the Cl-doped Bi2Te3-xSex alloys. It was established that Cl atoms act as electron donors, which tune the carrier concentration and effectively suppress the minority carrier transport in Bi2Te3-xSex alloys. The estimated value of the lattice thermal conductivity was found to be as low as 0.15 Wm-1 K-1 for Bi2Te3-x-ySexCly with x = 0.6 and y = 0.015 at 673 K in parallel to the pressing direction, which is among the lowest values reported for crystalline materials. The large reduction of the lattice thermal conductivity in both pressing directions for the investigated Bi2Te3-xSex alloys is connected with the different polarities of the Bi-(Te/Se)1 and Bi-(Te/Se)2 bonds, while the lone-pair (Te/Se) interactions are mainly responsible for the extremely low lattice thermal conductivity in the parallel direction. As a result of the enhanced power factor, suppressed bipolar conduction, and ultralow lattice thermal conductivity, a maximum ZT of 1.0 at 473 K has been received in the Bi2Te2.385Se0.6Cl0.015 sample.

Phase Analysis and Thermoelectric Properties of Cu-Rich Tetrahedrite Prepared by Solvothermal Synthesis.

Because of the large Seebeck coefficient, low thermal conductivity, and earth-abundant nature of components, tetrahedrites are promising thermoelectric materials. DFT calculations reveal that the additional copper atoms in Cu-rich Cu14Sb4S13 tetrahedrite can effectively engineer the chemical potential towards high thermoelectric performance. Here, the Cu-rich tetrahedrite phase was prepared using a novel approach, which is based on the solvothermal method and piperazine serving both as solvent and reagent. As only pure elements were used for the synthesis, the offered method allows us to avoid the typically observed inorganic salt contaminations in products. Prepared in such a way, Cu14Sb4S13 tetrahedrite materials possess a very high Seebeck coefficient (above 400 µVK-1) and low thermal conductivity (below 0.3 Wm-1K-1), yielding to an excellent dimensionless thermoelectric figure of merit ZT ≈ 0.65 at 723 K. The further enhancement of the thermoelectric performance is expected after attuning the carrier concentration to the optimal value for achieving the highest possible power factor in this system.

Lone-Pair-Like Interaction and Bonding Inhomogeneity Induce Ultralow Lattice Thermal Conductivity in Filled ß-Manganese-Type Phases.

Finding a way to interlink heat transport with the crystal structure and order/disorder phenomena is crucial for designing materials with ultralow lattice thermal conductivity. Here, we revisit the crystal structure and explore the thermoelectric properties of several compounds from the family of the filled ß-Mn-type phases M 2/n n+Ga6Te10 (M = Pb, Sn, Ca, Na, Na + Ag). The strongly disturbed thermal transport observed in the investigated materials originates from a three-dimensional Te-Ga network with lone-pair-like interactions, which results in large variations of the Ga-Te and M-Te interatomic distances and substantial anharmonic effects. In the particular case of NaAgGa6Te10, the additional presence of different cations leads to bonding inhomogeneity and strong structural disorder, resulting in a dramatically low lattice thermal conductivity (∼0.25 Wm-1 K-1 at 298 K), being the lowest among the reported ß-Mn-type phases. This study offers a way to develop materials with ultralow lattice thermal conductivity by considering bonding inhomogeneity and lone-pair-like interactions.

Crystal Structure and Thermoelectric Properties of Novel Quaternary Cu2MHf3S8 (M-Mn, Fe, Co, and Ni) Thiospinels with Low Thermal Conductivity.

Uncovering of the origin of intrinsically low thermal conductivity in novel crystalline solids is among the main streams in modern thermoelectricity. Because of their earth-abundant nature and environmentally friendly content, Cu-based thiospinels are attractive functional semiconductors, including thermoelectric (TE) materials. Herein, we report the crystal structure, as well as electronic and TE properties of four new Cu2MHf3S8 (M-Mn, Fe, Co, and Ni) thiospinels. The performed density functional theory calculations predicted the decrease of the band gap and transition from p- to n-type conductivity in the Mn-Fe-Co-Ni series, which was confirmed experimentally. The best TE performance in this work was observed for the Cu2NiHf3S8 thiospinel due to its highest power factor and low thermal conductivity. Moreover, all the discovered compounds possess very low lattice thermal conductivity κlat over the investigated temperature range. The κlat for Cu2CoHf3S8 has been found to be as low as 0.8 W m-1 K-1 at 298 K and 0.5 W m-1 K-1 at 673 K, which are significantly lower values compared to the other Cu-based thiospinels reported up to date. The strongly disturbed phonon transport of the investigated alloys mainly comes from the peculiar crystal structure where the large cubic unit cells contain many vacant octahedral voids. As it was evaluated from the Callaway approach and confirmed by the speed of sound measurements, such a crystal structure promotes the increase in lattice anharmonicity, which is the main reason for the low κlat. This work provides a guideline for the engineering of thermal transport in thiospinels and offers the discovered Cu2MHf3S8 (M-Mn, Fe, Co, and Ni) compounds, as new promising functional materials with low lattice thermal conductivity.

High Thermoelectric Performance of p-Type PbTe Enabled by the Synergy of Resonance Scattering and Lattice Softening.

In this work, we show the simultaneous enhancement of electrical transport and reduction of phonon propagation in p-type PbTe codoped with Tl and Na. The effective use of advanced electronic structure engineering improves the thermoelectric power factor S2σ over the temperature range from 300 to 825 K. A rise in the Seebeck coefficient S was obtained due to the enhanced effective mass m*, coming from the Tl resonance state in PbTe. Due to the presence of additional carriers brought by Na codoping, electrical conductivity became significantly improved. Furthermore, Tl and Na impurities induced crystal lattice softening, remarkably reducing lattice thermal conductivity, which was confirmed by a measured low speed of sound vm and high internal strain CεXRD. Eventually, the combination of both the attuned electronic structure and the lattice softening effects led to a very high ZT value of up to ∼2.1 for the Pb1-x-yTlxNayTe samples. The estimated energy conversion efficiency shows the extraordinary value of 15.4% (Tc = 300 K, Th = 825 K), due to the significantly improved average thermoelectric figure of merit ZTave = 1.05. This work demonstrates that the combination of impurity resonance scattering and crystal lattice softening can be a breakthrough concept for advancing thermoelectrics.

Entropy-Induced Multivalley Band Structures Improve Thermoelectric Performance in p-Cu7P(SxSe1-x)6 Argyrodites.

Searching for novel low-cost and eco-friendly materials for energy conversion is a good way to provide widespread utilization of thermoelectric technologies. Herein, we report the thermal behavior, phase equilibria data, and thermoelectric properties for the promising argyrodite-based Cu7P(SxSe1-x)6 thermoelectrics. Alloying of Cu7PSe6 with Cu7PS6 provides a continuous solid solution over the whole compositional range, as shown in the proposed phase diagram for the Cu7PS6-Cu7PSe6 system. As a member of liquid-like materials, the investigated Cu7P(SxSe1-x)6 solid solutions possess a dramatically low lattice thermal conductivity, as low as ∼0.2-0.3 W m-1 K-1, over the entire temperature range. Engineering the configurational entropy of the material by introducing more elements stabilizes the thermoelectrically beneficial high-symmetry γ-phase and promotes the multivalley electronic structure of the valence band. As a result, a remarkable improvement of the Seebeck coefficient and a reduction of electrical resistivity were observed for the investigated alloys. The combined effect of the extremely low lattice thermal conductivity and enhanced power factor leads to the significant enhancement of the thermoelectric figure of merit ZT up to ∼0.75 at 673 K for the Cu7P(SxSe1-x)6 (x = 0.5) sample with the highest configurational entropy, which is around twice higher compared with the pure selenide and almost four times higher than sulfide. This work not only demonstrates the large potential of Cu7P(SxSe1-x)6 materials for energy conversion but also promotes sulfide argyrodites as earth-abundant and environmentally friendly materials for energy conversion.