Alan Olvera

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.

Enhanced ZT and attempts to chemically stabilize Cu2Se via Sn doping

Cu2Se is a p-type semiconducting compound that possesses excellent thermoelectric properties but degrades at elevated temperatures under large currents, precluding it from applications in harvesting waste heat. In this study, we make use of a doping approach to attempt to chemically stabilize Cu2Se while maintaining its superior thermoelectric properties. Specifically, we synthesized Cu2(1−x)SnxSe (x = 0, 0.01, 0.02 and 0.05) via melting, annealing and spark plasma sintering. We found that the ZT was enhanced the most in the x = 0.01 sample, averaging approximately a 15% increase over the pure Cu2Se throughout a broad temperature range of 473–823 K, and achieving a maximum ZT = 1 at T = 823 K. The enhancement is due to an increased power factor and a reduced thermal conductivity, which is a result of point defect scattering from Sn atoms in the Cu2Se matrix and grain boundary scattering from a micron-size secondary phase of SnSe. We further tested the ability of the Sn dopant to prevent material degradation at elevated temperatures under large currents. Increasing the Sn dopant content does indeed decrease the solid Cu precipitation but not enough to resolve the issue of material degradation. As a result, despite its improved ZT, Cu1.98Sn0.01Se is not yet ready for thermoelectric applications, and requires further effort to stabilize the structure.

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.

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.

Chemical manipulation of phase stability and electronic behavior in Cu4−xAgxSe2

Superionic chalcogenides have gained renewed research interest, within the last decade, as emerging thermoelectric materials due to attractive properties, such as glass-like phonon transport coupled with crystal-like carrier transport. Of particular interest has been p-type coinage metal-based materials (Cu2Se, CuAgSe), which have demonstrated figures-of-merit, ZT, exceeding unity through a broad temperature range. However, the lack of n-type counterparts within this class of compounds limits potential module deployment. Here we show that careful stoichiometry control of the Cu4−xAgxSe2 series enables the formation of stable n-type materials throughout the measured temperature range upon substitution of Cu by Ag (1 ≤ x ≤ 3). Thermopower data show that the sample with x = 1 (Cu3AgSe2) undergoes a transition from n- to p-type conducting behavior, whereas samples with x = 2 (CuAgSe) and x = 3 (CuAg3Se2) exhibit n-type character in the whole measured temperature range. The post superionic transition n-type conductivity of CuAgSe is quite surprising and is contrary to the n- to p-type transition previously reported for this composition. Room temperature X-ray diffraction studies indicate the formation of a two-phase mixture for samples with x = 1 (Cu3AgSe2 = α-Cu2Se + α-CuAgSe) and x = 3 (CuAg3Se2 = α-Ag2Se + α-CuAgSe), whereas a single-phase α-CuAgSe is observed for the sample with x = 2. At 523 K, X-ray diffraction patterns show that Cu3AgSe2 (x = 1) and α-CuAgSe (x = 2) transform into single phase structures with the space group Fmm, while the CuAg3Se2 (x = 3) sample remains a two-phase system (CuAg3Se2 = β-Ag2Se + β-CuAgSe) in contrast to previous studies. This structural study is consistent with the observed gradual evolution of the conduction type of Cu4−xAgxSe2 samples between the p-type character of Cu2Se (x = 0) and the n-type semiconducting behavior of Ag2Se (x = 4). This suggests that the conducting behavior in the Cu4−xAgxSe2 is modulated by the Cu : Ag ratio. All Cu4−xAgxSe2 samples exhibit extremely low thermal conductivity after their phase transitions (<0.5 W m−1 K−1), which result in modest ZT values (∼0.45 at 625 K).

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.

1H and 13C NMR assignments for a series of Diels–Alder adducts of anthracene and 9-substituted anthracenes

Keywords: 1H NMR; 13C NMR; HSQC; HMBC; Diels-Alder reaction; Anthracene derivatives; Crystal structures

Correction to Pb7Bi4Se13: A Lillianite Homologue with Promising Thermoelectric Properties

■ ACKNOWLEDGMENTS The synthesis and structural characterization portion of this work was supported by the National Science Foundation Career Award DMR-1237550. P.F.P.P. and C.U. also gratefully acknowledge partial financial support for electronic and thermal transport measurements from the Department of Energy, Office of Basic Energy Sciences, under Award DE-SC-0008574. G.S. was supported as part of the Center for Solar and Thermal Energy Conversion, an Energy Frontier Research Center, funded by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences, under Award DESC0000957 (computational studies). Computational resources were provided by the National Energy Research Scientific Computing Center, supported by the Office of Science of the U.S. Department of Energy under Contract DE-AC02-05CH11231.