- Structures and transport properties of metastable solid solutions (NaSbTe2)1-x(GeTe)x
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In contrast to the solid solution series (LiSbTe2)1-x(GeTe)x, compounds with compositions (NaSbTe2)1-x(GeTe)x are not thermodynamically stable at room temperature, but can be obtained as metastable phases by quenching melts with x ≤ 0.5. For x > 0.5 and in slowly cooled melts, samples are inhomogeneous. Despite the metastability, the lattice parameters of (NaSbTe2)1-x(GeTe)x show a Vegard-like behavior. The limited tendency toward forming solid solutions is due to the fact that the ionic radius of Na+ is much larger than that of Ge2+ or Li+ and thus the lattice parameter of NaSbTe2 (a = 6.32 ?) being significantly larger than that of LiSbTe2 (a = 6.11 ?). Measurements of transport properties revealed an increasing Seebeck coefficient with increasing GeTe content. Electrical and thermal conductivities of all members of the series (NaSbTe2)1-x(GeTe)x with x ≤ 0.5 are low with ~15 mS cm?1 and 0.5 W m?1 K?1, respectively, which is consistent with the inherently low electrical conductivity of NaSbTe2. Nevertheless, this model system explains both the low thermoelectric performance of other chalcogenides with high Na contents and the widely recognized potential of Na as doping agent in thermoelectric materials.
- Schwarzmüller, Stefan,Yang, Fangshun,Oeckler, Oliver
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- Quaternary pnictides with complex, noncentrosymmetric structures. synthesis and structural characterization of the new zintl phases Na11Ca2Al3Sb8, Na4CaGaSb3, and Na15Ca3In5Sb12
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Three new Zintl phases, Na11Ca2Al3Sb8, Na4CaGaSb3, and Na15Ca3In5Sb12, have been synthesized by solid-state reactions, and their structures have been determined by single-crystal X-ray diffraction. Na11Ca2Al3Sb8 crystallizes with its own structure type (Pearson index oP48) with the primitive orthorhombic space group Pmn21 (No. 31). The structure is best viewed as [Al3Sb8]15- units of fused AlSb4 tetrahedra, a novel type of Zintl ion, with Na+ and Ca2+ cations that solvate them. Na4CaGaSb3 also crystallizes in its own type with the primitive monoclinic space group Pc (No. 7; Pearson index mP36), and its structure boasts one-dimensional [GaSb3]6- helical chains of corner-shared GaSb4 tetrahedra. The third new compound, Na15Ca3In5Sb12, crystallizes with the recently reported K2BaCdSb2 structure type (space group Pmc21; Pearson index oP12). The Na15Ca3In5Sb12 structure is based on polyanionic layers made of corner-shared InSb4 tetrahedra. Approximately one-sixth of the In sites are vacant in a statistical manner. All three structures exhibit similarities to the TiNiSi structure type, and the corresponding relationships are discussed. Electronic band structure calculations performed using the tight-binding linear muffin-tin orbital atomic sphere approximation method show small band gaps for all three compounds, which suggests intrinsic semiconducting behavior for these materials.
- Wang, Yi,Stoyko, Stanislav,Bobev, Svilen
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- Weak itinerant ferromagnetism and electronic and crystal structures of alkali-metal iron antimonides: NaFe4Sb12 and KFe 4Sb12
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The synthesis, chemical, structural, and magnetic properties of alkali-metal compounds with filled-skutterudite structure, NaFe 4Sb12 and KFe4Sb12, are described. X-ray and neutron diffraction and elemental analysis established the crystal structure without defects and disorder on the cation site. The temperature and pressure dependence of the cubic unit cell of NaFe4Sb12 and the displacement parameter of Na are investigated. The electronic structure is calculated by density functional methods (LMTO, FPLO). Quantum chemical calculations (electron localization function) reveal the covalent character of both Fe-Sb and Sb-Sb interactions. Electronic structure calculations within the local density approximation exhibit a band ferromagnetic ground state and predict a half-metallic behavior. In contrast to isostructural alkaline-earth compounds (CaFe4Sb12 and BaFe4Sb12), the alkali-metal skutterudites are itinerant electron ferromagnets with small magnetic moments (≈0.25 μB/Fe atom) and TC≈=85 K. Yet the paramagnetic moments of all four compounds are between 1.5 μB and 1.7 μB per Fe atom, indicating similar Stoner factors. Temperature-dependent 57Fe and 121Sb Moessbauer spectroscopies confirm the ferromagnetic state in the sodium compound with very small hyperfine fields at the iron and antimony sites.
- Leithe-Jasper,Schnelle,Rosner,Baenitz,Rabis,Gippius,Morozova,Borrmann,Burkhardt,Ramlau,Schwarz,Mydosh,Grin,Ksenofontov,Reiman
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- Reactions of sodium/antimony-clusters with molecular chlorine in a flow-tube-reactor coupled to a time-of-flight mass spectrometer
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A gas aggregation technique is used to produce NaxSby clusters. After passing a flow-tube reactor, the clusters are ionized with unfocused high fluence KrF-cxcimer laser radiation and then mass analyzed in a time-of-night mass spectrometer. Dramatic changes are observed in the cluster ion distribution upon adding high purity chlorine gas into the flow tube. As a result of reactions with Cl2, all Nax-Sby clusters disappear while pure antimony cluster ions arise. The distribution patterns of the produced Sbx+ clusters are the same as those observed in experiments with pure antimony. VCH Verlagsgesellschaft mbH, 1996.
- Hartmann,Castleman Jr.,Weil
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p. 1763 - 1765
(2007/10/03)
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