Sb and Se Substitution in CsBi₄Te₆: The Semiconductors CsM₄Q₆ (M = Bi, Sb; Q = Te, Se), Cs₂Bi₁₀Q₁₅, and CsBi₅Q₈

The solid solutions of CsBi₄Te₆, a high ZT material at a low temperature region, with Sb and Se were synthesized with general formulas CsBi_4‑<i>x</i>Sb_<i>x</i>Te₆ and CsBi₄Te_6‑<i>y</i>Se_<i>y</i>. The introduction of Sb and Se in the lattice of CsBi₄Te₆ is possible but only to a limited extent. The Sb and Se atoms substituted are not uniformly distributed over all crystallographic sites but display particular site preferences. The structure of new Sb/Bi solid solutions retains the original framework of CsBi₄Te₆ composed of NaCl-type Bi/Te slabs interconnected by characteristic Bi–Bi bonds and Cs atoms located in the interlayer space. A structurally modified phase in Se/Te solid solutions was found from the reactions targeted for 0.2 < <i>y</i> < 2.4 with the formula of CsBi₅Te_{7.5‑<i>y</i>}Se_<i>y</i> (or Cs₂Bi₁₀Q₁₅, (Q = Se, Te)). The new structure is constructed by the same structural motif with an extended Bi/Te slab (29 Å) compared to that in CsBi₄Te₆ (23 Å). The CsBi₅Te_{7.5‑<i>y</i>}Se_<i>y</i> possesses Bi/Te slabs that extend by an additional “Bi₂Te₃” unit compared to the structure of CsBi₄Te₆, which implies the existence of a phase homology of compounds with the adjustable parameter being the width of the Bi/Q slab. In the reactions targeted for the compounds with higher <i>y</i>, a new phase CsBi₅Te_3.6Se_4.4 with a different type of framework was found. The electrical conductivity and thermopower for the selected samples show p-type conduction with metallic behavior. The room temperature values measured are in the range of 300–1100 S/cm and 100–150 μV/K for Sb-substituted samples and 20–500 S/cm and 70–140 μV/K for Se-substituted samples, respectively. Thermal conductivities of these samples are in the range of 0.9–1.2 W/m·K at room temperature. Tailoring the transport behavior of these materials for thermoelectric applications may be achieved by doping, as is possible for the parent compound CsBi₄Te₆

Carrier Mobility Modulation in Cu₂Se Composites Using Coherent Cu₄TiSe₄ Inclusions Leads to Enhanced Thermoelectric Performance

Carrier transport engineering in bulk semiconductors using inclusion phases often results in the deterioration of carrier mobility (μ) owing to enhanced carrier scattering at phase boundaries. Here, we show by leveraging the temperature-induced structural transition between the α-Cu2Se and β-Cu2Se polymorphs that the incorporation of Cu4TiSe4 inclusions within the Cu2Se matrix results in a gradual large drop in the carrier mobility at temperatures below 400 K (α-Cu2Se), whereas the carrier mobility remains unchanged at higher temperatures, where the β-Cu2Se polymorph dominates. The sharp discrepancy in the electronic transport within the α-Cu2Se and β-Cu2Se matrices is associated with the formation of incoherent α-Cu2Se/Cu4TiSe4 interfaces, owing to the difference in their atomic structures and lattice parameters, which results in enhanced carrier scattering. In contrast, the similarity of the Se sublattices between β-Cu2Se and Cu4TiSe4 gives rise to coherent phase boundaries and good band alignment, which promote carrier transport across the interfaces. Interestingly, the different cation arrangements in Cu4TiSe4 and β-Cu2Se contribute to enhanced phonon scattering at the interfaces, which leads to a reduction in the lattice thermal conductivity. The large reduction in the total thermal conductivity while preserving the high power factor of β-Cu2Se in the (1–x)Cu2Se/(x)Cu4TiSe4 composites results in an improved ZT of 1.2 at 850 K, with an average ZT of 0.84 (500–850 K) for the composite with x = 0.01. This work highlights the importance of structural similarity between the matrix and inclusions when designing thermoelectric materials with improved energy conversion efficiency

Fabrication and Thermoelectric Properties of n‑Type CoSb_2.85Te_0.15 Using Selective Laser Melting

We report a nonequilibrium fabrication method of n-type CoSb_2.85Te_0.15 skutterudites using selective laser melting (SLM) technology. A powder of CoSb_2.85Te_0.15 was prepared by self-propagating high-temperature synthesis (SHS) and served as the raw material for the SLM process. The effect of SLM processing parameters such as the laser power and scanning speed on the quality of the forming CoSb_2.85Te_0.15 thin layers was systematically analyzed, and the optimal processing window for SLM was determined. A brief postannealing at 450 °C for 4 h, following the SLM process, has resulted in a phase-pure CoSb_2.85Te_0.15 bulk material deposited on a Ti substrate. The Seebeck coefficient of the annealed SLM prepared bulk material is close to that of the sample prepared by the traditional sintering method, and its maximum <i>ZT</i> value reached 0.56 at 823 K. Moreover, a Ti–Co–Sb ternary compound transition layer of about 70 μm in thickness was found at a dense interface between CoSb_2.85Te_0.15 and the Ti substrate. The contact resistivity was measured as 37.1 μΩcm². The results demonstrate that SLM, coupled with postannealing, can be used for fabrication of incongruently melting skutterudite compounds on heterogeneous substrates. This lays an important foundation for the follow-up research utilizing energy efficient SHS and SLM processes in rapid printing of thermoelectric modules

High‑<i>T</i>_c Ferromagnetism and Electron Transport in p‑Type Fe_1–<i>x</i>Sn_<i>x</i>Sb₂Se₄ Semiconductors

Single-phase polycrystalline powders of Fe_1–<i>x</i>Sn_<i>x</i>Sb₂Se₄ (<i>x</i> = 0 and 0.13) were synthesized by a solid-state reaction of the elements at 773 K. X-ray diffraction on Fe_0.87Sn_0.13Sb₂Se₄ single-crystal and powder samples indicates that the compound is isostructural to FeSb₂Se₄ in the temperature range from 80 to 500 K, crystallizing in the monoclinic space group <i>C</i>2/<i>m</i> (No. 12). Electron-transport data reveal a marginal alteration in the resistivity, whereas the thermopower drops by ∼60%. This suggests a decrease in the activation energy upon isoelectronic substitution of 13% Fe by Sn. Magnetic susceptibility and magnetization measurements from 2 to 500 K reveal that the Fe_1–<i>x</i>Sb₂Sn_<i>x</i>Se₄ phases exhibit ferromagnetic behavior up to ∼450 K (<i>x</i> = 0) and 325 K (<i>x</i> = 0.13). Magnetotransport data for FeSb₂Se₄ reveal large negative magnetoresistance, suggesting spin polarization of free carriers in the sample. The high-<i>T</i>_c ferromagnetism in Fe_1–<i>x</i>Sn_<i>x</i>Sb₂Se₄ phases and the decrease in <i>T</i>_c of the Fe_0.87Sn_0.13Sb₂Se₄ sample are rationalized by taking into account (1) the separation between neighboring magnetic centers in the crystal structures and (2) the formation of bound magnetic polarons, which overlap to induce long-range ferromagnetic ordering

Band Ordering and Dynamics of Cu_2–<i>x</i>Te and Cu_1.98Ag_0.2Te

⁶³Cu, ⁶⁵Cu, and ¹²⁵Te NMR measurements are reported for Cu_2–<i>x</i>Te and Cu_1.98Ag_0.2Te. The results demonstrate an onset of Cu-ion hopping below room temperature, including an activation behavior consistent with high-temperature transport measurements but with a significant enhancement of the hopping barriers with Ag substitution. We also separated the Korringa behavior by combining NMR line shape and relaxation measurements, thereby identifying large negative chemical shifts for both nuclei, as well as large Cu and Te s-state contributions in the valence band. Further comparison was obtained through heat capacity measurements and chemical shifts computed by density functional methods. The large diamagnetic chemical shifts coincide with behavior previously identified for materials with topologically nontrivial band inversion, and in addition, the large metallic shifts point to analogous features in the valence band density of states, suggesting that Cu₂Te may have similar inverted features

Optimized Thermoelectric Properties of Sb-Doped Mg_{2(1+<i>z</i>)}Si_{0.5–<i>y</i>}Sn_0.5Sb_<i>y</i> through Adjustment of the Mg Content

Mg₂Si_1–<i>x</i>Sn_<i>x</i> compounds are low-cost and environmentally friendly thermoelectric materials expected to be applied as power generators in the intermediate temperature range. Optimization of the thermoelectric properties of Mg₂Si_1–<i>x</i>Sn_<i>x</i> compounds can be accomplished by the precise control and adjustment of the Mg content. A series of Mg_{2(1+<i>z</i>)}Si_{0.5–<i>y</i>}Sn_0.5Sb_<i>y</i> (0 ≤ <i>y</i> ≤ 0.015 and 0 ≤ <i>z</i> ≤ 0.15) compounds with controlled Mg content were synthesized by a two-step solid-state reaction method, followed by a spark plasma sintering technique. On the basis of optimized thermoelectric properties via doping with Sb, the effect of a variable content of Mg spanning from understoichiometry to overstoichiometry has been systematically explored. The results indicate that when the actual Mg content exceeds the stoichiometric amount, the dominant point defects in Mg_{2(1+<i>z</i>)}Si_0.49Sn_0.5Sb_0.01 compounds are interstitial Mg and Si/Sn vacancies. At the same time, the electron concentration is enhanced with increasing content of Mg. However, when the actual Mg content is substoichiometric, the point defects consist mainly of Mg vacancies that tend to counteract the doping effect of Sb. Thus, the electron concentration of the nominal Mg₂Si_0.49Sn_0.5Sb_0.01 compound (in reality a 2 mol % deficiency of Mg) is markedly lower compared with the Mg_2.10Si_0.49Sn_0.5Sb_0.01 compound, which actually had a 2 mol % excess of Mg. Furthermore, a modest overstoichiometry of Mg enhances the power factor and improves the dimensionless figure of merit. The highest value of <i>ZT</i> = 1.25 at 800 K among the compounds was obtained on Mg_2.20Si_0.49Sn_0.5Sb_0.01, which had an actual Mg excess of 5.5 mol %. The study suggests that point defects, such as interstitial Mg and Si/Sn vacancies, which are created by an overstoichiometric content of Mg, have a positive effect on the electron concentration and thermoelectric properties of n-type Mg₂Si_1–<i>x</i>Sn_<i>x</i>-based compounds. This research has also established an essential foundation for further optimization of the thermoelectric properties of Mg₂Si_1–<i>x</i>Sn_<i>x</i> compounds

Pb₇Bi₄Se₁₃: A Lillianite Homologue with Promising Thermoelectric Properties

Pb₇Bi₄Se₁₃ crystallizes in the monoclinic space group <i>C</i>2/<i>m</i> (No. 12) with <i>a</i> = 13.991(3) Å, <i>b</i> = 4.262(2) Å, <i>c</i> = 23.432(5) Å, and β = 98.3(3)° at 300 K. In its three-dimensional structure, two NaCl-type layers A and B with respective thicknesses <i>N</i>₁ = 5 and <i>N</i>₂ = 4 [<i>N</i> = number of edge-sharing (Pb/Bi)­Se₆ octahedra along the central diagonal] are arranged along the <i>c</i> axis in such a way that the bridging monocapped trigonal prisms, PbSe₇, are located on a pseudomirror plane parallel to (001). This complex atomic-scale structure results in a remarkably low thermal conductivity (∼0.33 W m^–1 K^–1 at 300 K). Electronic structure calculations and diffuse-reflectance measurements indicate that Pb₇Bi₄Se₁₃ is a narrow-gap semiconductor with an indirect band gap of 0.23 eV. Multiple peaks and valleys were observed near the band edges, suggesting that Pb₇Bi₄Se₁₃ is a promising compound for both n- and p-type doping. Electronic-transport data on the as-grown material reveal an n-type degenerate semiconducting behavior with a large thermopower (∼−160 μV K^–1 at 300 K) and a relatively low electrical resistivity. The inherently low thermal conductivity of Pb₇Bi₄Se₁₃ and its tunable electronic properties point to a high thermoelectric figure of merit for properly optimized samples

Pb₇Bi₄Se₁₃: A Lillianite Homologue with Promising Thermoelectric Properties

Pb₇Bi₄Se₁₃ crystallizes in the monoclinic space group <i>C</i>2/<i>m</i> (No. 12) with <i>a</i> = 13.991(3) Å, <i>b</i> = 4.262(2) Å, <i>c</i> = 23.432(5) Å, and β = 98.3(3)° at 300 K. In its three-dimensional structure, two NaCl-type layers A and B with respective thicknesses <i>N</i>₁ = 5 and <i>N</i>₂ = 4 [<i>N</i> = number of edge-sharing (Pb/Bi)­Se₆ octahedra along the central diagonal] are arranged along the <i>c</i> axis in such a way that the bridging monocapped trigonal prisms, PbSe₇, are located on a pseudomirror plane parallel to (001). This complex atomic-scale structure results in a remarkably low thermal conductivity (∼0.33 W m^–1 K^–1 at 300 K). Electronic structure calculations and diffuse-reflectance measurements indicate that Pb₇Bi₄Se₁₃ is a narrow-gap semiconductor with an indirect band gap of 0.23 eV. Multiple peaks and valleys were observed near the band edges, suggesting that Pb₇Bi₄Se₁₃ is a promising compound for both n- and p-type doping. Electronic-transport data on the as-grown material reveal an n-type degenerate semiconducting behavior with a large thermopower (∼−160 μV K^–1 at 300 K) and a relatively low electrical resistivity. The inherently low thermal conductivity of Pb₇Bi₄Se₁₃ and its tunable electronic properties point to a high thermoelectric figure of merit for properly optimized samples

Optimization of the Electronic Band Structure and the Lattice Thermal Conductivity of Solid Solutions According to Simple Calculations: A Canonical Example of the Mg₂Si_{1–<i>x</i>–<i>y</i>}Ge_<i>x</i>Sn_<i>y</i> Ternary Solid Solution

The dependence of the electronic band structure of Mg₂Si_{0.3–<i>x</i>}Ge_<i>x</i>Sn_0.7 and Mg₂Si_0.3Ge_<i>y</i>Sn_{0.7–<i>y</i>} (0 ≤ <i>x</i>, and <i>y</i> ≤ 0.05) ternary solid solutions on composition and temperature is explained by a simple linear model, and the lattice thermal conductivity of solid solutions with different Si/Ge/Sn ratios is predicted by the Adachi model. The experimental results show excellent consistency with the calculations, which suggests that the approach might be suitable for describing the electronic band structure and the lattice thermal conductivity of other solid solutions using these simple calculations. Beyond this, it is observed that the immiscible gap in the Mg₂Si_1–<i>x</i>Sn_<i>x</i> binary system is narrowed via the introduction of Mg₂Ge. Moreover, for the Sb-doped solid solutions Mg_2.16(Si_0.3Ge_<i>y</i>Sn_{0.7–<i>y</i>})_0.98Sb_0.02 (0 ≤ <i>y</i> ≤ 0.05), the energy offset between the light conduction band and the heavy conduction band at higher temperatures (500–800 K) will decrease with an increase in Ge content, thus making a contribution to the conduction band degeneracy and enhancing the power factor in turn. Meanwhile, mass fluctuation and strain field scattering processes are enhanced when Ge is substituted for Sn in Mg_2.16(Si_0.3Ge_<i>y</i>Sn_{0.7<i>–y</i>})_0.98Sb_0.02 (0 ≤ <i>y</i> ≤ 0.05) because of the large discrepancy between the mass and size of Ge and Sn, and the lattice thermal conductivity is decreased as a consequence. Thus, the thermoelectric performance is improved, with the figure of merit ZT being >1.45 at ∼750 K and the average ZT value being between 0.9 and 1.0 in the range of 300–800 K, which is one of the best results for Sb-doped Mg₂Si_{1–<i>x</i>–<i>y</i>}Ge_<i>x</i>Sn_<i>y</i> systems with a single phase

Manipulating the Combustion Wave during Self-Propagating Synthesis for High Thermoelectric Performance of Layered Oxychalcogenide Bi_1–<i>x</i>Pb_<i>x</i>CuSeO

Novel time- and energy-efficient synthesis methods, especially those adaptable to large-scale industrial processing, are of vital importance for broader applications of thermoelectrics. We herein reported a case study of layer-structured oxychalcogenides Bi_1–<i>x</i>Pb_<i>x</i>CuSeO (<i>x</i> = 0–10%) with emphases on the reaction mechanism of self-propagating high-temperature synthesis (SHS) and the impact of SHS conditions on the thermoelectric properties. The combined results of X-ray powder diffraction, differential scanning calorimetry, and quenching experiments corroborated that the SHS process of BiCuSeO consisted two fast binary SHS reactions (2 Bi+3 Se → Bi₂Se₃ and 2 Cu+Se → Cu₂Se) intimately coupled with two relatively slow solid-state diffusion reactions (2 Bi₂Se₃+B₂O₃ → 3 Bi₂SeO₂ and then Bi₂SeO₂+Cu₂Se → 2 BiCuSeO). The formation rate of the reaction intermediate Bi₂SeO₂ was the bottleneck in the SHS process of BiCuSeO. Importantly, we found that adding PbO in the starting materials has (i) facilitated the formation of Bi₂SeO₂ and thus significantly reduced the SHS reaction time; (ii) improved the phase purity and sample homogeneity; (iii) increased the power factor via increasing both carrier concentration and effective mass; and (iv) reduced the lattice thermal conductivity via more point defect phonon scattering. As a result, a state-of-the-art <i>ZT</i> value ∼1.2 has been attained at 923 K for Bi_0.94Pb_0.06CuSeO. These results not only open a new avenue for mass production of single phased multinary thermoelectric materials but also inspire more investigation into the SHS mechanisms of multinary materials in diverse fields of material science and engineering