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Oliver Oeckler - One of the best experts on this subject based on the ideXlab platform.
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lithium atom mobility in lithium germanium antimony Tellurides elucidated by neutron diffraction and quasielastic neutron scattering
Journal of Alloys and Compounds, 2020Co-Authors: Stefan Schwarzmuller, Markus Hoelzel, Katharina Fritsch, Zachary Evenson, Klaus Habicht, Oliver OecklerAbstract:Abstract Lithium germanium antimony Tellurides are known as promising thermoelectric materials. Earlier 7Li solid state NMR studies had revealed pronounced lithium atom mobility. Further insights into the diffusion mechanism are investigated by means of neutron powder diffraction in combination with quasielastic neutron scattering (QENS). In vacancy-containing LiGe3.5Sb2Te7, the isotropic displacement parameters of the cations are larger than in vacancy-free Li2Ge3Sb2Te7 and thus indicate more pronounced atom mobility in the investigated temperature range from room temperature to 600 °C. In contrast, temperature-dependent isotropic displacement parameters of the anions are similar for both solid solutions. Comparable to the situation in the important thermoelectric material PbTe, anharmonic displacement parameters for LiGe3.5Sb2Te7 – a lithium germanium antimony telluride with optimized thermoelectric properties – are significant already at room temperature. The extrapolation of displacement parameters toward low temperatures indicates that the displacements are predominantly dynamic. The temperature-dependent evaluation of lattice parameters and isotropic displacement parameters reveals three different regimes. From room temperature to 250 °C, a NaCl-type model with anharmonic displacement parameters for cation sites fits well. Between 250 °C and 400 °C, the unit cell volume increases less than in the other regimes and small lattice distortions may occur. At temperatures higher than 400 °C (investigated up to 600 °C), lithium atoms also occupy tetrahedral voids of the fcc Te substructure. Whereas QENS data probing the picosecond time scale show no significant lithium atom mobility in the Q range of 0.08–1.68 A−1, a temperature-dependent change in the elastic scattering term hints at a different time window or a combination of motions with different time scales.
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cobalt germanide precipitates indirectly improve the properties of thermoelectric germanium antimony Tellurides
Journal of Materials Chemistry C, 2019Co-Authors: Daniel Souchay, Stefan Schwarzmuller, Jeffrey G Snyder, Hanka Becker, Stefan Kante, Andreas Leineweber, Oliver OecklerAbstract:Different established synthesis methods such as melt-casting followed by annealing as well as melt-spinning or ball-milling followed by hot-pressing or spark plasma sintering may significantly influence the properties of thermoelectric materials. The comparison of microstructures obtained by different synthesis routes (water-quenching and melt-spinning followed by spark plasma sintering) reveals the indirect nature of the beneficial influence of cobalt germanide precipitates on the thermoelectric properties of germanium telluride and germanium antimony Tellurides (GST materials). Cobalt germanide precipitates significantly influence the thermoelectric properties of GST materials: the thermoelectric figure of merit zT of (GeTe)17Sb2Te3 obtained by quenching melts in water increases from 1.6 to 1.9 (at 450 °C) by introducing cobalt germanide precipitates. They drastically reduce the grain and sub-grain sizes of the GST matrix. Melt-spinning followed by spark plasma sintering leads to nanoscopic cobalt germanide precipitates, whose effect on the thermoelectric properties, especially the phononic thermal conductivity, surprisingly seems to be marginal. This is due to the already significantly reduced (sub-)grain sizes in such polycrystalline GST samples as revealed by orientation (“channeling”) contrast in backscattered-electron micrographs. Melt-spun and spark-plasma-sintered GST material exhibits similar thermoelectric figures of merit zT values as water-quenched samples with cobalt germanide precipitates. In addition to providing easier access to samples with small (sub-)grains by simple quenching, the precipitates may stabilize the GST or GeTe microstructure by Zener pinning as cobalt is insoluble in the matrix material. This is very favorable concerning cyclability during thermoelectric measurements and potential applications. The heterostructured materials and their thermoelectric properties are long-term stable. Hall effect measurements of heterostructured GST (melt-spun and spark-plasma-sintered) indicate that the charge carrier concentration is near optimum.
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high thermoelectric figure of merit values of germanium antimony Tellurides with kinetically stable cobalt germanide precipitates
Journal of the American Chemical Society, 2015Co-Authors: Felix Fahrnbauer, Daniel Souchay, G Wagner, Oliver OecklerAbstract:Heterostructures that consist of a germanium antimony telluride matrix and cobalt germanide precipitates can be obtained by straightforward solid-state synthesis including simple annealing and quenching procedures. The microscale precipitates are homogeneously distributed in a matrix with pronounced “herringbone-like” nanostructure associated with very low thermal conductivities. In comparison to the corresponding pure Tellurides, the figure of merit (ZT) values of heterostructured materials are remarkably higher. This is mostly due to an increase of the Seebeck coefficient with only little impact on the electrical conductivity. In addition, the phononic part of the thermal conductivity is significantly reduced in some of the materials. As a result, ZT values of ca. 1.9 at 450 °C are achieved. Temperature-dependent changes of the thermoelectric properties are well-understood and correlate with complex phase transitions of the telluride matrix. However, the high ZT values are retained in multiple measureme...
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gesb4te4 a new 9p type phase in the system ge sb te
Zeitschrift für anorganische und allgemeine Chemie, 2010Co-Authors: Matthias Schneider, Oliver OecklerAbstract:9P-GeSb4Te4 is a new germanium antimony telluride that can be obtained from the elements as a homogeneous phase by quenching a stoichiometric melt and subsequently annealing the sample at 500 °C. The crystal structure consists of alternating antimony layers similar to those in elemental antimony and rocksalt-type blocks similar to those in GeSb2Te4. Although not thermodynamically stable according to the phase diagram, GeSb4Te4 is remarkably stable up to 540 °C, where it starts to melt incongruently according to DSC and DTA measurements. The crystal structure has been refined from single-crystalX-ray data [Pm1, a = 4.2466(2) A, c = 17.483 A, R1 = 0.0355]. Its diffraction patterns are very similar to those of other 9P-type Tellurides like Ge2Sb2Te5 and Sb2Te, which tend to occur as very pronounced false minima in structure refinements. The electrical conductivity is low (33 S·cm–1) but exhibits metallic temperature dependence.
Paul G Spry - One of the best experts on this subject based on the ideXlab platform.
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stable te isotope fractionation in tellurium bearing minerals from precious metal hydrothermal ore deposits
Geochimica et Cosmochimica Acta, 2017Co-Authors: Andrew P Fornadel, Paul G Spry, M A Haghnegahdar, E A Schauble, Simon E Jackson, Stuart J MillsAbstract:The tellurium isotope compositions of naturally-occurring Tellurides, native tellurium, and tellurites were measured by multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) and compared to theoretical values for equilibrium mass-dependent isotopic fractionation of representative Te-bearing species estimated with first-principles thermodynamic calculations. Calculated fractionation models suggest that 130/125Te fractionations as large as 4‰ occur at 100 °C between coexisting tellurates (Te VI) and Tellurides (Te −II) or or native tellurium Te(0), and smaller, typically <1‰, fractionations occur between coexisting Te(−I) or Te(−II) (Au,Ag)Te2 minerals (i.e., calaverite, krennerite) and (Au,Ag)2Te minerals (i.e., petzite, hessite). In general, heavyTe/lightTe is predicted to be higher for more oxidized species, and lower for reduced species. Tellurides in the system Au–Ag–Te and native tellurium analyzed in this study have values of δ130/125Te = −1.54‰ to 0.44‰ and δ130/125Te = −0.74‰ to 0.16‰, respectively, whereas those for tellurites (tellurite, paratellurite, emmonsite and poughite) range from δ130/125Te = −1.58‰ to 0.59‰. Thus, the isotopic composition for both oxidized and reduced species are broadly coincident. Calculations of per mil isotopic variation per amu for each sample suggest that mass-dependent processes are responsible for fractionation. In one sample of coexisting primary native tellurium and secondary emmonsite, δ130/125Te compositions were identical. The coincidence of δ130/125Te between all oxidized and reduced species in this study and the apparent lack of isotopic fractionation between native tellurium and emmonsite in one sample suggest that oxidation processes cause little to no fractionation. Because Te is predominantly transported as an oxidized aqueous phase or as a reduced vapor phase under hydrothermal conditions, either a reduction of oxidized Te in hydrothermal liquids or deposition of Te from a reduced vapor to a solid is necessary to form the common Tellurides and native tellurium in ore-forming systems. Our data suggest that these sorts of reactions during mineralization may account for a ∼3‰ range of δ130/125Te values. Based on the data ranges for Te minerals from various ore deposits, the underpinning geologic processes responsible for mineralization seem to have primary control on the magnitude of fractionation, with Tellurides in epithermal gold deposits showing a narrower range of isotope values than those in orogenic gold and volcanogenic massive sulfide deposits.
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mineralogy and geochemical environment of formation of the perama hill high sulfidation epithermal au ag te se deposit petrota graben ne greece
Mineralogy and Petrology, 2011Co-Authors: Panagiotis Voudouris, Paul G Spry, Vasilios Melfos, Robert Moritz, C Papavassiliou, G FalalakisAbstract:The Perama Hill deposit is a high-sulfidation Au-Ag-Te-Se epithermal system hosted in silicic- and argillic altered andesitic rocks and overlying sandstones, which were emplaced on the eastern margin of the Petrota graben, northeastern Greece. The deposit evolved from an early stage silica-pyrite rock and argillic alteration followed by the deposition of sulfide-, sulfosalt- and telluride-bearing quartz-barite veins and stockworks. Early ore formation is characterized by a high-sulfidation-type enargite-galena-bearing ore assemblage (consisting of enargite, watanabeite, Fe-free sphalerite, covellite, kesterite, bismuthinite, selenian bismuthinite, lillianite homologues, kawazulite-tetradymite, goldfieldite, and native gold), followed by the formation of an intermediate-sulfidation-type tennantite-bearing assemblage characterized by ferrian/zincian tennantite, tellurobismuthite, tetradymite, melonite, native tellurium, Au-Ag-Tellurides (calaverite, krennerite, sylvanite, hessite, petzite, stutzite), altaite and electrum. Quartz, barite, kaolinite, sericite and minor aluminum-phosphate-sulfate minerals are gangue minerals. Fluid inclusion data demonstrate that the ore system evolved from an initial high temperature (up to 330°C) and low salinity (up to 4.9 wt.% NaCl equiv.) fluid towards a cooler (200°C) and very low salinity (0.7 wt.% NaCl equiv.) hydrothermal fluid suggesting progressive cooling and dilution of the ore fluid. The ore minerals at Perama Hill reflect variable fS2 and fTe2 conditions during base and precious metal deposition. Early ore deposition took place at ~300°C, at logfS2 values between ≈−8.2 and −5.5, and logfTe2 from −11.8 and −7.8. Late ore deposition occurred at logfS2 = −11.8 to −9.8 and logfTe2 of ≈−9.2 and −7.8. These data and paragenetic studies indicate a shift towards higher logfTe2 and lower logfS2 and logfSe2 values for the mineralizing fluids with time. The kawazulite/tetradymitess-gold association at Perama Hill suggests that it formed from a sulfide melt in the Bi-Au-Se-Te system as Au was scavenged from the hydrothermal ore-forming fluid at elevated temperatures. The presence of Tellurides, and Bi- and Sn-bearing minerals in the ore system is compatible with direct deposition of metals from the vapor phase of a degassing magmatic (porphyry) body.
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understanding gold silver telluride selenide mineral deposits
Episodes, 2009Co-Authors: Nigel J Cook, Paul G Spry, Cristiana L Ciobanu, Panagiotis VoudourisAbstract:Gold-(silver)-telluride (selenide) ores occur as epithermal orogenic and intrusion related deposits. Although Te and Se are chalcophile elements and share geochemical affinity with Au, formation of selenides and other elements Ag-Au require acidic or reducing environments. The thermodynamic stability conditions for Au and AgTellurides and native tellurium indicate an epithermal environment. Analysis of mineral paragenensis, textures and compositional variation in Tellurides/selenides suggest petrogenetic processes involving interaction with fluids leading to Au scavenging and entrapment in Tellurides, changes in chemistry/rates of fluid infiltration and attaining equilibrium in a given assemblage.
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rhenium rich molybdenite and rheniite in the pagoni rachi mo cu te ag au prospect northern greece implications for the re geochemistry of porphyry style cu mo and mo mineralization
Canadian Mineralogist, 2009Co-Authors: Panagiotis Voudouris, Paul G Spry, Vasilios Melfos, Robert Moritz, Luca Bindi, Tamara Kartal, Kyriakos Arikas, Melissa OrtelliAbstract:The porphyry-type Pagoni Rachi Mo–Cu–Te–Ag–Au prospect, in northern Greece, is a porphyry-epithermal system hosted by an Oligocene dacite porphyry and quartz–feldspar porphyry dikes. The rare mineral rheniite (ReS2) and molybdenite, with very high contents of Re (up to 4.7 wt% Re), occur in quartz veins along with Fe–Cu sulfides, Pb-, Sn-, and Cl-bearing oxides, hematite, ilmenite and Tellurides of Bi; these veins are spatially related to sericitic and transitional sericitic–sodic–potassic alteration. Earlier-formed gold-bearing quartz and magnetite veins with sodic–potassic–calcic alteration and late precious-metal telluride-rich carbonate–quartz veins with argillic alteration contain only minor amounts of molybdenite. The composition of rheniite ranges from almost pure, stoichiometric rheniite with low Mo content to rheniite with up to 5.99 wt% Mo. Petrographic, scanning electron microscope, and structural studies suggest that the high Re content of molybdenite from Pagoni Rachi is the result of the isovalent Re-for-Mo substitution in molybdenite. This fact is corroborated by the progressive shortening of the Mo–S mean bond-distance (from 2.414 A in pure molybdenite to 2.355 A), as well as by the isotropic decrease of the unit-cell values with the increase of the Re:Mo ratio. The structural analysis of four molybdenite crystals, with the highest Re content ever reported in nature, demonstrates that they crystallize as the 2 H polytype and not the 3 R polytype, as previously hypothesized, thus suggesting that Re concentration does not correlate with a specific polytype. The fluid inclusions in quartz in the Re-bearing molybdenite–rheniite veins at Pagoni Rachi show that they homogenize to either the liquid or vapor phases (354 to 428°C) or by halite dissolution at 317 to 585°C, which equates to salinities of 40 to 59 wt% NaCl equiv. Rheniite and molybdenite likely precipitated as temperatures fell below 400°C during phase separation under relatively oxidizing conditions, at elevated chlorine activity, and from relatively acid hydrothermal solutions. However, the presence of Pb oxides, Sn-bearing minerals and Tellurides is compelling evidence that rheniite and Re-rich molybdenite may have formed directly from the vapor as sublimates, in a manner similar to the way they are deposited at the Kudriavy volcano, Kurile Islands.
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telluride mineralogy of the low sulfidation epithermal emperor gold deposit vatukoula fiji
Mineralogy and Petrology, 2003Co-Authors: D W Pals, Paul G SpryAbstract:¶The epithermal, low sulfidation Emperor gold telluride deposit in Fiji, hosted by Late Miocene-Early Pliocene shoshonitic rocks, is spatially related to a low-grade porphyry Cu system on the western flank of the Tavua Caldera. Gold is largely in the form of “invisible” gold in arsenian pyrite but 10 to 50% of gold is in the form of precious metal Tellurides. Gold mineralization occurs in steeply dipping dikes and faults, flat-dipping structures (<45°), referred to locally as “flatmakes,” and at the intersection of two or more structures referred to as “shatter zones.” Petrographic, electron microprobe, and scanning electron microscope analyses of ores from some of the more recently discovered orebodies, Matanagata, Matanagata East, and R1 reveal that tellurium-bearing minerals, sylvanite, calaverite, krennerite, petzite, hessite, coloradoite, melonite, native tellurium, and rare benleonardite, formed during various hydrothermal stages, hosted in quartz, and to a lesser extent arsenian pyrite and tetrahedrite group minerals. Sylvanite followed by krennerite are the two most common Tellurides in these orebodies. These Tellurides show no systematic spatial distribution within flatmakes but there appears to be a higher concentration of Tellurides where the flatmake intersects steep structures. Gold-rich Tellurides preceded the formation of silver-rich Tellurides and were constrained at 250 °C in log f S2 and log f Te2 space at −12.7 to −10.1 and −9.4 to −7.8, respectively, based on sulfide and telluride stabilities, and the composition of sphalerite. Ore forming components, such as Au, Ag, Te, Cu, V, and S, were likely derived from Late Miocene-Early Pliocene monzonites in and adjacent to the Tavua caldera.
Markku Leskela - One of the best experts on this subject based on the ideXlab platform.
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atomic layer deposition of metal Tellurides and selenides using alkylsilyl compounds of tellurium and selenium
Journal of the American Chemical Society, 2009Co-Authors: Viljami Pore, Timo Hatanpaa, Mikko Ritala, Markku LeskelaAbstract:Atomic layer deposition (ALD) of metal selenide and telluride thin films has been limited because of a lack of precursors that would at the same time be safe and exhibit high reactivity as required in ALD. Yet there are many important metal selenide and telluride thin film materials whose deposition by ALD might be beneficial, for example, CuInSe2 for solar cells and Ge2Sb2Te5 for phase-change random-access memories. Especially in the latter case highly conformal deposition offered by ALD is essential for high storage density. By now, ALD of germanium antimony telluride (GST) has been attempted only using plasma-assisted processes owing to the lack of appropriate tellurium precursors. In this paper we make a breakthrough in the development of new ALD precursors for tellurium and selenium. Compounds with a general formula (R3Si)2Te and (R3Si)2Se react with various metal halides forming the corresponding metal Tellurides and selenides. As an example, we show that Sb2Te3, GeTe, and GST films can be deposited...
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atomic layer deposition of metal Tellurides and selenides using alkylsilyl compounds of tellurium and selenium
Journal of the American Chemical Society, 2009Co-Authors: Viljami Pore, Timo Hatanpaa, Mikko Ritala, Markku LeskelaAbstract:Atomic layer deposition (ALD) of metal selenide and telluride thin films has been limited because of a lack of precursors that would at the same time be safe and exhibit high reactivity as required in ALD. Yet there are many important metal selenide and telluride thin film materials whose deposition by ALD might be beneficial, for example, CuInSe2 for solar cells and Ge2Sb2Te5 for phase-change random-access memories. Especially in the latter case highly conformal deposition offered by ALD is essential for high storage density. By now, ALD of germanium antimony telluride (GST) has been attempted only using plasma-assisted processes owing to the lack of appropriate tellurium precursors. In this paper we make a breakthrough in the development of new ALD precursors for tellurium and selenium. Compounds with a general formula (R3Si)2Te and (R3Si)2Se react with various metal halides forming the corresponding metal Tellurides and selenides. As an example, we show that Sb2Te3, GeTe, and GST films can be deposited by ALD using (Et3Si)2Te, SbCl3, and GeCl2 x C4H8O2 compounds as precursors. All three precursors exhibit a typical saturative ALD growth behavior and GST films prepared at 90 degrees C show excellent conformality on a high aspect-ratio trench structure.
Panagiotis Voudouris - One of the best experts on this subject based on the ideXlab platform.
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mineralogy and geochemical environment of formation of the perama hill high sulfidation epithermal au ag te se deposit petrota graben ne greece
Mineralogy and Petrology, 2011Co-Authors: Panagiotis Voudouris, Paul G Spry, Vasilios Melfos, Robert Moritz, C Papavassiliou, G FalalakisAbstract:The Perama Hill deposit is a high-sulfidation Au-Ag-Te-Se epithermal system hosted in silicic- and argillic altered andesitic rocks and overlying sandstones, which were emplaced on the eastern margin of the Petrota graben, northeastern Greece. The deposit evolved from an early stage silica-pyrite rock and argillic alteration followed by the deposition of sulfide-, sulfosalt- and telluride-bearing quartz-barite veins and stockworks. Early ore formation is characterized by a high-sulfidation-type enargite-galena-bearing ore assemblage (consisting of enargite, watanabeite, Fe-free sphalerite, covellite, kesterite, bismuthinite, selenian bismuthinite, lillianite homologues, kawazulite-tetradymite, goldfieldite, and native gold), followed by the formation of an intermediate-sulfidation-type tennantite-bearing assemblage characterized by ferrian/zincian tennantite, tellurobismuthite, tetradymite, melonite, native tellurium, Au-Ag-Tellurides (calaverite, krennerite, sylvanite, hessite, petzite, stutzite), altaite and electrum. Quartz, barite, kaolinite, sericite and minor aluminum-phosphate-sulfate minerals are gangue minerals. Fluid inclusion data demonstrate that the ore system evolved from an initial high temperature (up to 330°C) and low salinity (up to 4.9 wt.% NaCl equiv.) fluid towards a cooler (200°C) and very low salinity (0.7 wt.% NaCl equiv.) hydrothermal fluid suggesting progressive cooling and dilution of the ore fluid. The ore minerals at Perama Hill reflect variable fS2 and fTe2 conditions during base and precious metal deposition. Early ore deposition took place at ~300°C, at logfS2 values between ≈−8.2 and −5.5, and logfTe2 from −11.8 and −7.8. Late ore deposition occurred at logfS2 = −11.8 to −9.8 and logfTe2 of ≈−9.2 and −7.8. These data and paragenetic studies indicate a shift towards higher logfTe2 and lower logfS2 and logfSe2 values for the mineralizing fluids with time. The kawazulite/tetradymitess-gold association at Perama Hill suggests that it formed from a sulfide melt in the Bi-Au-Se-Te system as Au was scavenged from the hydrothermal ore-forming fluid at elevated temperatures. The presence of Tellurides, and Bi- and Sn-bearing minerals in the ore system is compatible with direct deposition of metals from the vapor phase of a degassing magmatic (porphyry) body.
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understanding gold silver telluride selenide mineral deposits
Episodes, 2009Co-Authors: Nigel J Cook, Paul G Spry, Cristiana L Ciobanu, Panagiotis VoudourisAbstract:Gold-(silver)-telluride (selenide) ores occur as epithermal orogenic and intrusion related deposits. Although Te and Se are chalcophile elements and share geochemical affinity with Au, formation of selenides and other elements Ag-Au require acidic or reducing environments. The thermodynamic stability conditions for Au and AgTellurides and native tellurium indicate an epithermal environment. Analysis of mineral paragenensis, textures and compositional variation in Tellurides/selenides suggest petrogenetic processes involving interaction with fluids leading to Au scavenging and entrapment in Tellurides, changes in chemistry/rates of fluid infiltration and attaining equilibrium in a given assemblage.
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rhenium rich molybdenite and rheniite in the pagoni rachi mo cu te ag au prospect northern greece implications for the re geochemistry of porphyry style cu mo and mo mineralization
Canadian Mineralogist, 2009Co-Authors: Panagiotis Voudouris, Paul G Spry, Vasilios Melfos, Robert Moritz, Luca Bindi, Tamara Kartal, Kyriakos Arikas, Melissa OrtelliAbstract:The porphyry-type Pagoni Rachi Mo–Cu–Te–Ag–Au prospect, in northern Greece, is a porphyry-epithermal system hosted by an Oligocene dacite porphyry and quartz–feldspar porphyry dikes. The rare mineral rheniite (ReS2) and molybdenite, with very high contents of Re (up to 4.7 wt% Re), occur in quartz veins along with Fe–Cu sulfides, Pb-, Sn-, and Cl-bearing oxides, hematite, ilmenite and Tellurides of Bi; these veins are spatially related to sericitic and transitional sericitic–sodic–potassic alteration. Earlier-formed gold-bearing quartz and magnetite veins with sodic–potassic–calcic alteration and late precious-metal telluride-rich carbonate–quartz veins with argillic alteration contain only minor amounts of molybdenite. The composition of rheniite ranges from almost pure, stoichiometric rheniite with low Mo content to rheniite with up to 5.99 wt% Mo. Petrographic, scanning electron microscope, and structural studies suggest that the high Re content of molybdenite from Pagoni Rachi is the result of the isovalent Re-for-Mo substitution in molybdenite. This fact is corroborated by the progressive shortening of the Mo–S mean bond-distance (from 2.414 A in pure molybdenite to 2.355 A), as well as by the isotropic decrease of the unit-cell values with the increase of the Re:Mo ratio. The structural analysis of four molybdenite crystals, with the highest Re content ever reported in nature, demonstrates that they crystallize as the 2 H polytype and not the 3 R polytype, as previously hypothesized, thus suggesting that Re concentration does not correlate with a specific polytype. The fluid inclusions in quartz in the Re-bearing molybdenite–rheniite veins at Pagoni Rachi show that they homogenize to either the liquid or vapor phases (354 to 428°C) or by halite dissolution at 317 to 585°C, which equates to salinities of 40 to 59 wt% NaCl equiv. Rheniite and molybdenite likely precipitated as temperatures fell below 400°C during phase separation under relatively oxidizing conditions, at elevated chlorine activity, and from relatively acid hydrothermal solutions. However, the presence of Pb oxides, Sn-bearing minerals and Tellurides is compelling evidence that rheniite and Re-rich molybdenite may have formed directly from the vapor as sublimates, in a manner similar to the way they are deposited at the Kudriavy volcano, Kurile Islands.
Stefan Schwarzmuller - One of the best experts on this subject based on the ideXlab platform.
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lithium atom mobility in lithium germanium antimony Tellurides elucidated by neutron diffraction and quasielastic neutron scattering
Journal of Alloys and Compounds, 2020Co-Authors: Stefan Schwarzmuller, Markus Hoelzel, Katharina Fritsch, Zachary Evenson, Klaus Habicht, Oliver OecklerAbstract:Abstract Lithium germanium antimony Tellurides are known as promising thermoelectric materials. Earlier 7Li solid state NMR studies had revealed pronounced lithium atom mobility. Further insights into the diffusion mechanism are investigated by means of neutron powder diffraction in combination with quasielastic neutron scattering (QENS). In vacancy-containing LiGe3.5Sb2Te7, the isotropic displacement parameters of the cations are larger than in vacancy-free Li2Ge3Sb2Te7 and thus indicate more pronounced atom mobility in the investigated temperature range from room temperature to 600 °C. In contrast, temperature-dependent isotropic displacement parameters of the anions are similar for both solid solutions. Comparable to the situation in the important thermoelectric material PbTe, anharmonic displacement parameters for LiGe3.5Sb2Te7 – a lithium germanium antimony telluride with optimized thermoelectric properties – are significant already at room temperature. The extrapolation of displacement parameters toward low temperatures indicates that the displacements are predominantly dynamic. The temperature-dependent evaluation of lattice parameters and isotropic displacement parameters reveals three different regimes. From room temperature to 250 °C, a NaCl-type model with anharmonic displacement parameters for cation sites fits well. Between 250 °C and 400 °C, the unit cell volume increases less than in the other regimes and small lattice distortions may occur. At temperatures higher than 400 °C (investigated up to 600 °C), lithium atoms also occupy tetrahedral voids of the fcc Te substructure. Whereas QENS data probing the picosecond time scale show no significant lithium atom mobility in the Q range of 0.08–1.68 A−1, a temperature-dependent change in the elastic scattering term hints at a different time window or a combination of motions with different time scales.
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cobalt germanide precipitates indirectly improve the properties of thermoelectric germanium antimony Tellurides
Journal of Materials Chemistry C, 2019Co-Authors: Daniel Souchay, Stefan Schwarzmuller, Jeffrey G Snyder, Hanka Becker, Stefan Kante, Andreas Leineweber, Oliver OecklerAbstract:Different established synthesis methods such as melt-casting followed by annealing as well as melt-spinning or ball-milling followed by hot-pressing or spark plasma sintering may significantly influence the properties of thermoelectric materials. The comparison of microstructures obtained by different synthesis routes (water-quenching and melt-spinning followed by spark plasma sintering) reveals the indirect nature of the beneficial influence of cobalt germanide precipitates on the thermoelectric properties of germanium telluride and germanium antimony Tellurides (GST materials). Cobalt germanide precipitates significantly influence the thermoelectric properties of GST materials: the thermoelectric figure of merit zT of (GeTe)17Sb2Te3 obtained by quenching melts in water increases from 1.6 to 1.9 (at 450 °C) by introducing cobalt germanide precipitates. They drastically reduce the grain and sub-grain sizes of the GST matrix. Melt-spinning followed by spark plasma sintering leads to nanoscopic cobalt germanide precipitates, whose effect on the thermoelectric properties, especially the phononic thermal conductivity, surprisingly seems to be marginal. This is due to the already significantly reduced (sub-)grain sizes in such polycrystalline GST samples as revealed by orientation (“channeling”) contrast in backscattered-electron micrographs. Melt-spun and spark-plasma-sintered GST material exhibits similar thermoelectric figures of merit zT values as water-quenched samples with cobalt germanide precipitates. In addition to providing easier access to samples with small (sub-)grains by simple quenching, the precipitates may stabilize the GST or GeTe microstructure by Zener pinning as cobalt is insoluble in the matrix material. This is very favorable concerning cyclability during thermoelectric measurements and potential applications. The heterostructured materials and their thermoelectric properties are long-term stable. Hall effect measurements of heterostructured GST (melt-spun and spark-plasma-sintered) indicate that the charge carrier concentration is near optimum.
