Alexander Page

Pb7Bi4Se13: A Lillianite Homologue with Promising Thermoelectric Properties

Pb(7)Bi(4)Se(13) crystallizes in the monoclinic space group C2/m (No. 12) with a = 13.991(3) Å, b = 4.262(2) Å, c = 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 N(1) = 5 and N(2) = 4 [N = number of edge-sharing (Pb/Bi)Se6 octahedra along the central diagonal] are arranged along the c 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 Pb(7)Bi(4)Se(13) 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 Pb(7)Bi(4)Se(13) 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 Pb(7)Bi(4)Se(13) and its tunable electronic properties point to a high thermoelectric figure of merit for properly optimized samples.

Partial indium solubility induces chemical stability and colossal thermoelectric figure of merit in Cu2Se

High thermoelectric figure of merit, ZT ∼ 2.1 at 1000 K, have been reported in Cu2−xSe-based materials. However, their deployments in operational devices have been hampered by chemical instability and low average ZT (ZTave) values. Here, we demonstrate improved chemical stability and a record high ZTave ∼ 1.5 over a broad temperature range (T ≤ 850 K) in Cu2Se/CuInSe2 nanocomposites, with ZT values ranging from 0.6 at 450 K to an unprecedentedly large value of 2.6 at 850 K for the sample with 1 mol% In. This remarkable performance is attributed to the localization of Cu+ ions induced by the incorporation of In into the Cu2Se lattice, which simultaneously boost the electrical conductivity and reduce the thermal conductivity of the nanocomposites. These findings pave the way for large-scale utilization of Cu2Se-based materials in thermoelectric generators.

Coexistence of high-Tc ferromagnetism and n-type electrical conductivity in FeBi2Se4

The discovery of n-type ferromagnetic semiconductors (n-FMSs) exhibiting high electrical conductivity and Curie temperature (Tc) above 300 K would dramatically improve semiconductor spintronics and pave the way for the fabrication of spin-based semiconducting devices. However, the realization of high-Tc n-FMSs and p-FMSs in conventional high-symmetry semiconductors has proven extremely difficult due to the strongly coupled and interacting magnetic and semiconducting sublattices. Here we show that decoupling the two functional sublattices in the low-symmetry semiconductor FeBi2Se4 enables unprecedented coexistence of high n-type electrical conduction and ferromagnetism with Tc ≈ 450 K. The structure of FeBi2Se4 consists of well-ordered magnetic sublattices built of [FenSe4n+2]∞ single-chain edge-sharing octahedra, coherently embedded within the three-dimensional Bi-rich semiconducting framework. Magnetotransport data reveal a negative magnetoresistance, indicating spin-polarization of itinerant conducting electrons. These findings demonstrate that decoupling magnetic and semiconducting sublattices allows access to high-Tc n- and p-FMSs as well as helps unveil the mechanism of carrier-mediated ferromagnetism in spintronic materials.

Electronic and phonon transport in Sb-doped Ti0.1Zr0.9Ni1+xSn0.975Sb0.025 nanocomposites

The thermoelectric behavior of n-type Sb-doped half-Heusler (HH)-full-Heusler (FH) nanocomposites with general composition Ti(0.1)Zr(0.9)Ni(1+x)Sn(0.975)Sb(0.025) (x = 0, 0.02, 0.04, 0.1) was investigated in the temperature range from 300 to 775 K. Samples used for structural characterization and transport measurements were obtained through the solid-state reaction of high purity elements at 950 °C and densification of the resulting polycrystalline powders using a uniaxial hot press. X-ray diffraction study of the powder samples suggested the formation of single-phase HH alloys regardless of the Ni concentration (x value). However, high resolution transmission electron microscopy investigation revealed the presence of spherical nanoprecipitates with a broad size distribution coherently embedded in the HH matrix. The size range and dispersion of the precipitates depend on the concentration of Ni in the starting mixture. Well dispersed nanoprecipitates with size ranging from 5 nm to 50 nm are observed in the nanocomposite with x = 0.04, while severe agglomeration of large precipitates (>50 nm) is observed in samples with x = 0.1. Hall effect measurements of various samples indicate that the carrier concentration within the Sb-doped HH matrix remains nearly constant (~7 × 10(20) cm(-3)) for samples with x = 0.02 and x = 0.04, whereas a significant increase of the carrier concentration to ~9 × 10(20) cm(-3) is observed for the sample with x = 0.1. Interestingly, only a marginal change in thermopower value is observed for various samples despite the large difference in the carrier density. In addition, the carrier mobility remains constant up to x = 0.04 suggesting that the small nanoprecipitates in these samples do not disrupt electronic transport within the matrix. Remarkably, a large reduction in the total thermal conductivity is observed for all nanocomposites, indicating the effectiveness of the embedded nanoprecipitates in scattering phonons while enabling efficient electron transfer across the matrix/inclusion interfaces.

Origins of phase separation in thermoelectric (Ti, Zr, Hf)NiSn half-Heusler alloys from first principles

A high thermoelectric performance has been achieved in half-Heusler alloys of composition MNiSn (M = Ti, Zr, Hf). The enhanced properties are attributed to the formation of microscale Ti-rich and Ti-poor grains. The mechanism of phase separation remains unclear, as the composition and microstructure of the grains are highly dependent upon synthesis conditions. This work uses first principles density functional theory, combined with the cluster expansion method, to calculate a thermodynamic phase diagram of the full pseudo-ternary (Hf1−x−yZrxTiy)NiSn system. The results show that ZrNiSn and HfNiSn are fully miscible at all temperatures, and that a miscibility gap between (Zr, Hf)NiSn and TiNiSn exists at temperatures below 850 K. For temperatures above 850 K, a solid solution is found to minimize the free energy. The low critical temperature of 850 K challenges the prevailing interpretation that the microstructure is created by an intrinsic phase separation mechanism, such as spinodal decomposition. Calculated migration barriers for Ti, Zr, and Hf atoms show that Ti-rich and Ti-poor grains may be non-equilibrium states that are kinetically trapped after solidification.

Crystal Structure and Thermoelectric Properties of the 7,7L Lillianite Homologue Pb6Bi2Se9

Pb6Bi2Se9, the selenium analogue of heyrovsyite, crystallizes in the orthorhombic space group Cmcm (#63) with a = 4.257(1) Å, b = 14.105(3) Å, and c = 32.412(7) Å at 300 K. Its crystal structure consists of two NaCl-type layers, A and B, with equal thickness, N1 = N2 = 7, where N is the number of edge-sharing [Pb/Bi]Se6 octahedra along the central diagonal. In the crystal structure, adjacent layers are arranged along the c-axis such that bridging bicapped trigonal prisms, PbSe8, are located on a pseudomirror plane parallel to (001). Therefore, Pb6Bi2Se9 corresponds to a 7,7L member of the lillianite homologous series. Electronic transport measurements indicate that the compound is a heavily doped narrow band gap n-type semiconductor, with electrical conductivity and thermopower values of 350 S/cm and -53 μV/K at 300 K. Interestingly, the compound exhibits a moderately low thermal conductivity, ∼1.1 W/mK, in the whole temperature range, owing to its complex crystal structure, which enables strong phonon scattering at the twin boundaries between adjacent NaCl-type layers A and B. The dimensionless figure of merit, ZT, increases with temperature to 0.25 at 673 K.

Increasing the thermoelectric power factor of Ge17Sb2Te20 by adjusting the Ge/Sb ratio

We have investigated the thermoelectric properties of Ge17Sb2Te20. This compound is a known phase change material with electronic properties that depend strongly on temperature. The thermoelectric properties of this compound can be tuned by altering the stoichiometry of Ge and Sb without the use of additional foreign elements during synthesis. This tuning results in a 26% increase in the thermoelectric power factor at 723 K. Based on a single parabolic band model we show that the pristine material is optimally doped, and thus, a reduction in the lattice thermal conductivity of pure Ge17Sb2Te20 should result in an enhanced thermoelectric figure of merit.

High-Tc Ferromagnetism and Electron Transport in p-Type Fe1–xSnxSb2Se4 Semiconductors

Single-phase polycrystalline powders of Fe(1-x)Sn(x)Sb2Se4 (x = 0 and 0.13) were synthesized by a solid-state reaction of the elements at 773 K. X-ray diffraction on Fe0.87Sn0.13Sb2Se4 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 C2/m (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-x)Sb2Sn(x)Se4 phases exhibit ferromagnetic behavior up to ∼450 K (x = 0) and 325 K (x = 0.13). Magnetotransport data for FeSb2Se4 reveal large negative magnetoresistance, suggesting spin polarization of free carriers in the sample. The high-Tc ferromagnetism in Fe(1-x)Sn(x)Sb2Se4 phases and the decrease in Tc of the Fe0.87Sn0.13Sb2Se4 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.

Insights on the Synthesis, Crystal and Electronic Structures, and Optical and Thermoelectric Properties of Sr1–xSbxHfSe3 Orthorhombic Perovskite

Single-phase polycrystalline powders of Sr1- xSb xHfSe3 ( x = 0, 0.005, 0.01), a new member of the chalcogenide perovskites, were synthesized using a combination of high temperature solid-state reaction and mechanical alloying approaches. Structural analysis using single-crystal as well as powder X-ray diffraction revealed that the synthesized materials are isostructural with SrZrSe3, crystallizing in the orthorhombic space group Pnma (#62) with lattice parameters a = 8.901(2) Å; b = 3.943(1) Å; c = 14.480(3) Å; and Z = 4 for the x = 0 composition. Thermal conductivity data of SrHfSe3 revealed low values ranging from 0.9 to 1.3 W m-1 K-1 from 300 to 700 K, which is further lowered to 0.77 W m-1 K-1 by doping with 1 mol % Sb for Sr. Electronic property measurements indicate that the compound is quite insulating with an electrical conductivity of 2.9 S/cm at 873 K, which was improved to 6.7 S/cm by 0.5 mol % Sb doping. Thermopower data revealed that SrHfSe3 is a p-type semiconductor with thermopower values reaching a maximum of 287 μV/K at 873 K for the 1.0 mol % Sb sample. The optical band gap of Sr1- xSb xHfSe3 samples, as determined by density functional theory calculations and the diffuse reflectance method, is ∼1.00 eV and increases with Sb concentration to 1.15 eV. Careful analysis of the partial densities of states (PDOS) indicates that the band gap in SrHfSe3 is essentially determined by the Se-4p and Hf-5d orbitals with little to no contribution from Sr atoms. Typically, band edges of p- and d-character are a good indication of potentially strong absorption coefficient due to the high density of states of the localized p and d orbitals. This points to potential application of SrHfSe3 as absorbing layer in photovoltaic devices.

Unconventional large linear magnetoresistance in Cu2−xTe

We report a large linear magnetoresistance in Cu2−xTe, reaching Δρ/ρ(0) = 250% at 2 K in a 9 T field, for samples with x = 0.13 to 0.22. These results are comparable to those for Ag2X materials, though for Cu2−xTe the carrier densities are considerably larger. Examining the magnitudes and the crossover from quadratic to high-field linear behavior, we show that models based on classical transport behavior best explain the observed results. The effects are traced to the misdirection of currents in high mobility transport channels, likely due to behavior at grain boundaries such as topological surface states or a high mobility interface phase. The resistivity also exhibits a T2 dependence in the temperature range where the large linear MR appears, an indicator of electron-electron interaction effects within the high mobility states. Thus this is an example of a system in which electron-electron interactions dominate the low-temperature linear magnetoresistance.We report a large linear magnetoresistance in Cu2−xTe, reaching Δρ/ρ(0) = 250% at 2 K in a 9 T field, for samples with x = 0.13 to 0.22. These results are comparable to those for Ag2X materials, though for Cu2−xTe the carrier densities are considerably larger. Examining the magnitudes and the crossover from quadratic to high-field linear behavior, we show that models based on classical transport behavior best explain the observed results. The effects are traced to the misdirection of currents in high mobility transport channels, likely due to behavior at grain boundaries such as topological surface states or a high mobility interface phase. The resistivity also exhibits a T2 dependence in the temperature range where the large linear MR appears, an indicator of electron-electron interaction effects within the high mobility states. Thus this is an example of a system in which electron-electron interactions dominate the low-temperature linear magnetoresistance.