Absence of Nanostructuring in NaPbmSbTem+2: Solid Solutions with High Thermoelectric Performance in the Intermediate Temperature Regime

Implications and Optimization of Domain Structures in IV-VI High-Entropy Thermoelectric Materials

High-entropy semiconductors are now an important class of materials widely investigated for thermoelectric applications. Understanding the impact of chemical and structural heterogeneity on transport properties in these compositionally complex systems is essential for thermoelectric design. In this work, we uncover the polar domain structures in the high-entropy PbGeSnSe1.5Te1.5 system and assess their impact on thermoelectric properties. We found that polar domains induced by crystal symmetry breaking give rise to well-structured alternating strain fields. These fields effectively disrupt phonon propagation and suppress the thermal conductivity. We demonstrate that the polar domain structures can be modulated by tuning crystal symmetry through entropy engineering in PbGeSnAgxSbxSe1.5+xTe1.5+x. Incremental increases in the entropy enhance the crystal symmetry of the system, which suppresses domain formation and loses its efficacy in suppressing phonon propagation. As a result, the room-temperature lattice thermal conductivity increases from κL = 0.63 Wm-1 K-1 (x = 0) to 0.79 Wm-1 K-1 (x = 0.10). In the meantime, the increase in crystal symmetry, however, leads to enhanced valley degeneracy and improves the weighted mobility from μw = 29.6 cm2 V-1 s-1 (x = 0) to 35.8 cm2 V-1 s-1 (x = 0.10). As such, optimal thermoelectric performance can be achieved through entropy engineering by balancing weighted mobility and lattice thermal conductivity. This work, for the first time, studies the impact of polar domain structures on thermoelectric properties, and the developed understanding of the intricate interplay between crystal symmetry, polar domains, and transport properties, along with the impact of entropy control, provides valuable insights into designing GeTe-based high-entropy thermoelectrics.

Low thermal conductivity in a new mixed metal telluride Mn1.8(1)In0.8(1)Si2Te6

The design of new complex mixed metal tellurides (containing low toxicity cations) with intrinsic ultralow thermal conductivity is of paramount importance in the field of thermoelectrics. Herein, we report the synthesis and characterization of polycrystalline and single crystals of a new mixed-metal quaternary telluride Mn1.8(1)In0.8(1)Si2Te6. The structural aspects and chemical formula of this phase at room temperature have been established using single crystal X-ray diffraction and EDX studies. The trigonal centrosymmetric (space group: P3̄1m) structure of the title phase has cell constants of a = b = 7.0483(7) Å and c = 7.1277(8) Å. The structure has three independent cationic sites, one mixed (In1/Mn1), one partially filled Mn2, and one Si1, which are bonded with Te1 atoms. Each metal atom (In and Mn) in the structure is octahedrally coordinated with six neighboring Te1 atoms. The structure also features dimers of Si atoms, and each Si atom is bonded to three Te1 atoms to form ethane-like Si2Te6 units. The optical absorption study of a polycrystalline Mn1.8In0.8Si2Te6 sample shows a narrow optical bandgap of 0.6(2) eV. Temperature-dependent resistivity and Seebeck coefficient studies confirmed the p-type semiconducting nature of the sample with high values of S (301 μV K-1 to 444 μV K-1). The total thermal conductivity (ktot) study of the polycrystalline sample shows a decreasing trend on heating with an extremely low value of 0.28 W m-1 K-1 at 773 K. Magnetic measurements indicate a glassy magnetic behavior for the sample below 8 K.

Multiple Effects Promoting the Thermoelectric Performance of SnTe by Alloying with CuSbTe2 and CuBiTe2

In a SnTe-based thermoelectric material, the naturally high hole concentration caused by cation vacancies and high total thermal conductivity seriously hinder its thermoelectric performance. A recent work shows that alloying SnTe with other compounds from the I-V-VI2 family (I = Ag, Na; V = Sb, Bi; VI = Te) can be considered an effective strategy to boost the figure of merit efficiently via the synergy of manipulating hole concentration and lowering lattice thermal conductivity. Herein, we present a markedly enhanced thermoelectric performance in p-type SnTe through CuPnTe2 (Pn = Sb, Bi) alloying. Moreover, we found that the alloying with both CuSbTe2 and CuBiTe2 can facilitate the valence band convergence of SnTe, but their relative influence is different. Interestingly, compared to CuBiTe2, alloying with CuSbTe2 increases the carrier concentration of SnTe, which suppresses the bipolar effect. Ultimately, under the positive effect of valence band convergence, increased vacancy concentration, and decreased lattice thermal conductivity, compounds with a nominal composition of (SnTe)0.90(CuSbTe2)0.10 attains a peak zT of ∼1.26 at 823 K. In contrast, the thermoelectric performance of (SnTe)1-x(CuBiTe2)x is restricted by the reduced carrier concentration and diminished band gap, showing only a humble maximum zT value of ∼0.91 at 823 K in the sample with a nominal composition of (SnTe)0.96(CuBiTe2)0.04. These results demonstrate the multiple effects on thermoelectric transport during the formation of complex solid solutions.

Defect engineering in thermoelectric materials: what have we learned?

Thermoelectric energy conversion is an all solid-state technology that relies on exceptional semiconductor materials that are generally optimized through sophisticated strategies involving the engineering of defects in their structure. In this review, we summarize the recent advances of defect engineering to improve the thermoelectric (TE) performance and mechanical properties of inorganic materials. First, we introduce the various types of defects categorized by dimensionality, i.e. point defects (vacancies, interstitials, and antisites), dislocations, planar defects (twin boundaries, stacking faults and grain boundaries), and volume defects (precipitation and voids). Next, we discuss the advanced methods for characterizing defects in TE materials. Subsequently, we elaborate on the influences of defect engineering on the electrical and thermal transport properties as well as mechanical performance of TE materials. In the end, we discuss the outlook for the future development of defect engineering to further advance the TE field.

Mechanistic insight of KBiQ2 (Q = S, Se) using panoramic synthesis towards synthesis-by-design

Solid-state synthesis has historically focused on reactants and end products; however, knowledge of reaction pathways, intermediate phases and their formation may provide mechanistic insight of solid-state reactions. With an increased understanding of reaction progressions, design principles can be deduced, affording more predictive power in materials synthesis. In pursuit of this goal, in situ powder X-ray diffraction is employed to observe crystalline phase evolution over the course of the reaction, thereby constructing a “panoramic” view of the reaction from beginning to end. We conducted in situ diffraction studies in the K–Bi–Q (Q = S, Se) system to understand the formation of phases occurring in this system in the course of their reactions. Powder mixtures of K₂Q to Bi₂Q₃ in 1 : 1 and 1.5 : 1 ratios were heated to 800 °C or 650 °C, while simultaneously collecting diffraction data. Three new phases, K₃BiS₃, β-KBiS₂, and β-KBiSe₂, were discovered. Panoramic synthesis showed that K₃BiQ₃ serves an important mechanistic role as a structural intermediate in both chalcogen systems (Q = S, Se) in the path to form the KBiQ₂ structure. Thermal analysis and calculations at the density functional theory (DFT) level show that the cation-ordered β-KBiQ₂ polymorphs are the thermodynamically stable phase in this compositional space, while Pair Distribution Function (PDF) analysis shows that all α-KBiQ₂ structures have local disorder due to stereochemically active lone pair expression of the bismuth atoms. The formation of the β-KBiQ₂ structures, both of which crystallize in the α-NaFeO₂ structure type, show a boundary where the structure can be disordered or ordered with regards to the alkali metal and bismuth. A cation radius tolerance for six-coordinate cation site sharing of ∼ 1.3 is proposed. The mechanistic insight the panoramic synthesis technique provides in the K–Bi–Q system is progress towards the overarching goal of synthesis-by-design.

This work uses in situ powder X-ray diffraction studies to observe crystalline phase evolution over the course of multiple K-Bi-Q (Q = S, Se) reactions, thereby constructing a “panoramic” view of each reaction from beginning to end.

Thermoelectric Material SnPb2Bi2S6: The 4,4L Member of Lillianite Homologous Series with Low Lattice Thermal Conductivity

Although the binary sulfides Bi₂S₃, PbS, and SnS have attracted extensive interest as thermoelectric materials, no quaternary sulfides containing Sn/Pb/Bi/S elements have been reported. Herein, we report the synthesis of a new quaternary sulfide, SnPb₂Bi₂S₆, which crystallizes in the orthorhombic space group Pnma with unit cell parameters of a = 20.5458(12) Å, b = 4.0925(4) Å, c = 13.3219(10) Å. SnPb₂Bi₂S₆ has a lillianite-type crystal structure consisting of two alternately aligned NaCl-type structural motifs separated by a mirror plane of PbS₇ monocapped trigonal prisms. In the lillianite homologous series, SnPb₂Bi₂S₆ can be classified as ^4,4L, where the superscripted numbers indicate the maximum numbers of edge-sharing octahedra in the two adjacent NaCl-shaped slabs along the diagonal direction. The obtained SnPb₂Bi₂S₆ phase exhibited good thermal stability up to 1000 K and n-type degenerate semiconducting behavior, with a power factor of 3.7 μW cm^-1 K^-2 at 773 K. Notably, this compound exhibited a very low lattice thermal conductivity of 0.69-0.92 W m^-1 K^-1 at 300-1000 K. Theoretical calculations revealed that the low thermal conductivity is caused by the complex crystal structure and the related elastic properties of a low Debye temperature, low phonon velocity, and large Grüneisen parameters. A reasonable figure of merit (ZT) of ∼0.3 was obtained at 770 K.