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Thermoelectric materials can convert heat into electricity. They are used to generate electricity when other power sources are not available or to increase energy efficiency by recycling waste heat. The Yb21Mn4Sb18 phase was previously shown to have good thermoelectric performance due to its large Seebeck coefficient (∼290 μV K−1) and low thermal conductivity (0.4 W m−1 K−1). These characteristics stem respectively from the unique [Mn4Sb10]22− subunit and the large unit cell/site disorder inherent in this phase. The solid solutions, Yb21Mn4−xCdxSb18 (x = 0, 0.5, 1.0, 1.5) and Yb21−yCayMn4Sb18 (y = 3, 6, 9, 10.5) have been prepared, their structures characterized and thermoelectric properties from room temperature to 800 K measured. A detailed look into the structural disorder for the Cd and Ca solid solutions was performed using synchrotron powder X-ray diffraction and pair distribution function methods and shows that these are highly disordered structures. The substitution of Cd gives rise to more metallic behavior whereas Ca substitution results in high resistivity. As both Cd and Ca are isoelectronic substitutions, the changes in properties are attributed to changes in the electronic structure. Both solid solutions show that the thermal conductivities remain extremely low (∼0.4 W m−1 K−1) and that the Seebeck coefficients remain high (>200 μV K−1). The temperature dependence of the carrier mobility with increased Ca substitution, changing from approximately T−1 to T−0.5, suggests that another scattering mechanism is being introduced. As the bonding changes from polar covalent with Yb to ionic for Ca, polar optical phonon scattering becomes the dominant mechanism. Experimental studies of the Cd solid solutions result in a max zT of ∼1 at 800 K and, more importantly for application purposes, a ZTavg ∼ 0.6 from 300 K to 800 K.

Here we report the synthesis and characterization of three quaternary complex chalcogenides, Ag0.72Bi5.48Cu0.88S9 (I), Ag0.70Bi5.30Cu1.3S9 (II), Ag0.34Bi4.54Cu1.98PbS9 (III). All the compounds in this homologous series crystallize in the C2/m space group and can be described as Pavonite structures. This structure type consists of alternating NaCl-like layers with varied thickness (nP), separated by a pair of square pyramids. All the compounds reported here are synthetic analogues of the n = 4 pavonite family and are known as makovickyite minerals. Compounds I–III possess complex crystal structures, consisting of mixed occupancies of Bi/Ag/Pb sites, as well as partially occupied Cu sites. These intrinsic assets lead to ultra-low lattice thermal conductivities, in the range of 0.75–0.42 Wm−1 K−1 from 300–500 K, and make these materials promising candidates for thermoelectric applications. All three structures exhibit very narrow indirect band gaps of less than 0.5 eV as confirmed by DRS measurements. Charge transport properties are consistent with n-type semiconducting behavior with moderate electrical conductivities and large Seebeck coefficients. Low temperature electrical resistivity and Seebeck coefficient measurements are also performed on II. A promising figure of merit, zT, of 0.09 for I and II, 0.11 for III can be achieved at 475 K.

FeAs2−xSex (x = 0.30–1.0) samples were synthesized as phase pure powders by conventional solid-state techniques and as single crystals (x = 0.50) from chemical vapor transport. The composition of the crystals was determined to be Fe1.025(3)As1.55(3)Se0.42(3), crystallizing in the marcasite structure type, Pnnm space group. FeAs2−xSex (0 < x < 1) was found to undergo a marcasite-to-arsenopyrite (P21/c space group) structural phase transition at x ∼ 0.65. The structures are similar, with the marcasite structure best described as a solid solution of As/Se, whereas the arsenopyrite has ordered anion sites. Magnetic susceptibility and thermoelectric property measurements from 300–2 K were performed on single crystals, FeAs1.50Se0.50. Paramagnetic behavior is observed from 300 to 17 K and a Seebeck coefficient of −33 μV K−1, an electrical resistivity of 4.07 mΩ cm, and a very low κl of 0.22 W m−1 K−1 at 300 K are observed. In order to determine the impact of the structural transition on the high-temperature thermoelectric properties, polycrystalline FeAs2−xSex (x = 0.30, 0.75, 0.85, 1.0) samples were consolidated into dense pellets for measurements of thermoelectric properties. The x = 0.85 sample shows the best thermoelectric performance. The electronic structure of FeAsSe was calculated with DFT and transport properties were approximately modeled above 500 K.

Correction for ‘Measured and simulated thermoelectric properties of FeAs2−xSex (x = 0.30–1.0): from marcasite to arsenopyrite structure’ by Christopher J. Perez et al., Mater. Adv., 2020, 1, 1390–1398, DOI: 10.1039/D0MA00371A.

Large crystals of Yb14MgSb11 were grown through a Sn flux method. Magnetic susceptibility measurements yield an effective magnetic moment of 3.4(1) μB, revealing the presence of both divalent and trivalent Yb in Yb14MgSb11. Previously assumed to only contain Yb2+ as in Yb14MnSb11, the mixed valency demonstrates that Yb14MgSb11 is a Zintl phase.

Zintl phases Yb14MnSb11 and Yb14MgSb11, which share the same complex structure type, have been demonstrated as the best p-type thermoelectric materials for the high temperature region (800-1200 K). A new iso-structural compound, Yb14MgBi11, was synthesized in order to investigate the structure and thermoelectric properties of the Bi analogs. Yb14MgBi11 crystallizes in the Ca14AlSb11 structure-type with the space group I41/acd [a = 16.974(2) A, c = 22.399(4) A, V = 6454(2) A3, R1/wR2 = 0.0238/0.0475]. The structure follows the previous description of this structure type and the trend observed in previous analogs. Thermoelectric properties of Yb14MgBi11 are measured together with Yb14MnBi11 and both compounds are metallic. Compared to Yb14MgSb11, Yb14MgBi11 has a higher carrier concentration with a similar mobility and effective mass. The lattice thermal conductivity of Yb14MgBi11 is extremely low, which is as low as 0.16-0.36 W(mK)-1. The zT values of Yb14MgBi11 and Yb14MnBi11 reach 0.2 at 875 K.

Two series of crystals were prepared via Sn flux synthesis with the compositional fluxes of Yb14−xLnxMnSb11, where x = 0.1–0.9 and x = 0.4. By X-ray structural measurements and microprobe analysis, the maximum amount of Ln incorporated in the unit cell matrix were determined to be 0.37 ± 0.04 for the La–Nd and 0.45 ± 0.04 for Sm–Tm elements with solid substitution solution formation. The Ln incorporation did not change the unit cell significantly but the cell volume decreased going from the largest La–Nd to the smallest Tm–Lu cations. The Ln0.30–0.40 samples demonstrated congruent melting and their melting points increased by ∼30–50 °C compared to the pristine matrix. The temperatures were attributed to the ordered structural state due to the Ln distribution in the unit cell only through the one regular system site. Based on geometrical fitting between crystal radii of Yb2+ and Ln3+ in six-coordination, the Yb(2) sites were found to be more preferable for substitution by La–Nd, Yb(1) by Sm–Ho and Yb(3) by Tm and Lu atoms. Thermal losses as a temperature function of the alloyed by La and Lu samples were determined by a step-by-step heating procedure with analysis of the vapor condensate deposited on the viewing window of the chamber. This experiment demonstrated a high mobility of the tetrahedral Mn and Sb along with Yb ions in the Yb14MnSb11 matrix with incongruent sublimation beyond 1300 °C and a decrease of the thermal weight losses by half if the matrix was alloyed by La. Occupation of the Yb sites by Ln atoms varied the geometry of the MnSb4 tetrahedron as well as electron properties and bonding in this structural fragment, and these changes are considered in the context of the coupling between chemical structure and thermal stability of the compounds. The improved thermal stability due to increasing the total ionic state of the alloyed samples was found to possibly be a useful factor for the long-time use of these materials for space applications.

After the discovery of Yb14MnSb11 as an outstanding p-type thermoelectric material for high temperatures (≥900 K), site substitution of other elements has been proven to be an effective method to further optimize the thermoelectric properties. Yb14−xRExMnSb11 (RE = Pr and Sm, 0 < x < 0.55) compounds were prepared by powder metallurgy to study their thermoelectric properties. According to powder X-ray diffraction, these samples are iso-structural with Yb14MnSb11 and when more than 5% RE is used in the synthesis the presence of (Yb,RE)4Sb3 is apparent after synthesis. After consolidation and measurement, (Yb,RE)Sb and (Yb,RE)11Sb10 appear in the powder X-ray diffraction patterns. Electron microprobe analysis results show that consolidated pellets have small (Yb,RE)Sb domains and that the maximum amount of RE in Yb14−xRExMnSb11 is x = 0.55, however, (Yb,RE)11Sb10 cannot be distinguished by electron microprobe analysis. By replacing Yb2+ with RE3+, one extra electron is introduced into Yb14MnSb11 and the carrier concentration is adjusted. Thermoelectric performance from room temperature to 1275 K was evaluated through transport and thermal conductivity measurements. The measurement shows that Seebeck coefficients initially increase and then remain stable and that electrical resistivity increases with substitutions. Thermal conductivity is slightly reduced. Substitution of Pr and Sm leads to enhanced zT. Yb13.82Pr0.18Mn1.01Sb10.99 has the best maximum zT value of ∼1.2 at 1275 K, while Yb13.80Sm0.19Mn1.00Sb11.02 has its maximum zT of ∼1.0 at 1275 K, respectively, ∼45% and ∼30% higher than Yb14MnSb11 prepared in the same manner.

High resolution single crystal synchrotron X-ray diffraction data measured at 15(2) K were used to solve the structure of the complex intermetallic Zintl phase, Yb(11)AlSb(9) (space group Iba2), made up of Yb cations and polyanions along with isolated Sb anions. The 15(2) K cell parameters are a = 11.7383(4) Å, b = 12.3600(4) Å, c = 16.6796(6) Å. The temperature dependence of the structure was investigated through high resolution synchrotron powder X-ray diffraction (PXRD) data measured from 90 K to 1000 K. Rietveld refinements of the crystal structure revealed near linear thermal expansion of Yb(11)AlSb(9) with expansion coefficients of 1.49(2) × 10(-5) K(-1), 1.71(3) × 10(-5) K(-1), 1.13(1) × 10(-5) K(-1) for a, b and c, respectively. The chemical bonding in Yb(11)AlSb(9) was analyzed using atomic Hirshfeld surfaces, and the analysis supports the presence of the structural elements of Yb cations, [AlSb(4)](9-) tetrahedra, [Sb(2)](4-) dimers and isolated Sb(3-) anions. However, indications of interatomic interactions between the Zintl anions and the Yb cations were also observed.

A series of single crystal samples, Yb14MnSb11−xTex (x ≤ 0.2), were investigated via Sn flux. The element distributions were mapped using an electron microprobe, which demonstrated inhomogeneity and limited solubility of Te in the crystals. The magnetic properties of two crystals of compositions Yb13.80(2)Mn1.03(1)Sb11.11(4)Te0.06(3) and Yb13.82(3)Mn1.02(1)Sb11.02(8)Te0.14(6) were investigated. Increasing amounts of Te increase the saturation moment of Yb14MnSb11−xTex, and slightly lower the ordering temperature TC by about 1 K. This is attributed to the filling of the hole in Yb14MnSb11 and the reduction of the screening of the Mn2+ d5 electrons, resulting in a higher effective moment. Hot-pressed, fully dense pellets of Yb13.72(3)Mn1.08(1)Sb11.13(6)Te0.07(5), Yb13.76(5)Mn1.11(1)Sb10.96(8)Te0.16(8), and Yb13.76(4)Mn1.10(2)Sb10.95(9)Te0.19(7) were prepared for thermoelectric property measurements. Both Seebeck coefficient and electrical resistivity increase with increasing amount of Te as a result of decreasing carrier concentration. Increasing amounts of YbTe with increasing x could be identified in the electron microprobe. The total thermal conductivity was approximately the same for all compositions, similar to that of Yb14MnSb11. The figure of merit zT reaches a maximum of 1.11 for x = 0.07 at 1240 K, which is approximately 12% higher than the parent compound, Yb14MnSb11.

This paper describes the preparation of air and moisture stable octanol derivatized crystalline silicon nanoparticles by room temperature sodium naphthalenide reduction of silicon halides.