Paul Brazis

Flux synthesis, structure and physical properties of new pseudo-binary REAl3−xGex compounds

Abstract New compounds in the system RE-Al-Ge (RE=Tb, Gd, Ho) were prepared in molten aluminum. Large, (up to 5 mm) single crystals of new pseudo-binary phases GdAl3−xGex, (x=0.1), TbAl3−xGex, (x=0.3), and HoAl3−xGex, (x=0.2) were recovered in high yield. The crystal structures were refined by single crystal X-ray diffraction techniques, and found to vary as a function of rare-earth element. Thus, GdAl3−xGex crystallizes in the Ni3Sn structure type (P63/mmc) of GdAl3. For the TbAl3−xGex the BaPb3 structure type (R-3m) is adopted, as it is for TbAl3. In the case of HoAl3−xGex, the structure type of HoAl3 is not stabilized, and the compound crystallizes in the BaPb3 structure (R-3m). Crystal data: GdAl3−xGex a=6.3115(4) A, c=4.6052(4) A, V=158.87(2) A3, P63/mmc (No. 194, Z=2); TbAl3−xGex a=6.1956(9) A, c=21.025(4) A, V=698.9(2) A3, R-3m (No. 166, Z=9); HoAl3−xGex a=6.1579(10) A, c=21.062(5) A, V=691.7(2) A3, R-3m (No. 166, Z=9). Charge transport properties indicate that these materials are good metallic conductors. At low temperatures they order antiferromagnetically, whereas above ∼50 K they are Curie–Weiss paramagnets. The temperature of maximum magnetic susceptibility (Tmax) is 5.93 K, 18.8 K, and 24.0 K for HoAl2.8Ge0.2, TbAl2.7Ge0.3, and GdAl2.9Ge0.1 respectively.

Observed properties and electronic structure of RNiSb compounds (R = Ho, Er, Tm, Yb and Y). Potential thermoelectric materials

The RNiSb compounds (R=Ho, Er, Tm, Yb and Y) and some selected solid solution members such as (Zr 1-x Er x )Ni(Sn 1-x Sb x ) and ErNiSb 1-x Pn x (Pn=As, Sb, Bi) have been studied. They all crystallize in the MgAgAs structure type, which can be considered as a NaCI structure type in which half of the interstitial tetrahedral sites are occupied by Ni atoms. The measured values of the Seebeck coefficients, at room temperature, are positive for RNiSb (R=Ho, Er, Yb and Y) compounds and ErNiSb 1-x Pn x (Pn=As, Sb, Bi) solid solutions, but for (Zr 1-x Er x )Ni(Sn 1-x Sb x ) members vary from negative to positive values when 0

Complex bismuth chalcogenides as thermoelectrics

A solid state chemistry synthetic approach towards identifying new materials with potentially superior thermoelectric properties is presented. Materials with complex compositions and structures also have complex electronic structures which may give rise to high thermoelectric powers and at the same time possess low thermal conductivities. The structures and thermoelectric properties of several new promising compounds of K-Bi-S, K-Bi-Se, and Cs-Bi-Te are discussed.

Ln2Al3Si2 (Ln = Ho, Er, Tm): New silicides from molten aluminum - Determination of the Al/Si distribution with neutron crystallography and metamagnetic transitions

Highly metallic compounds with a quasi-one-dimensional structure, the new ternary compounds Ln2 Al3 Si2 (Ln=Ho, Er, Tm) are synthesized in molten aluminum from lanthanoid and silicon as reagents. Their structures show a formally [Al3 Si2 ]6- framework that contains infinite Al zigzag chains and Si-Si dimers and accommodates rows of Ln3+ ions in parallel tunnels. The compounds exhibit metamagnetic transitions at high magnetic fields.

Solid State Chemistry Approach to Advanced Thermoelectrics. Ternary and Quaternary Alkali Metal Bismuth Chalcogenides as Thermoelectric Materials

Our exploratory research to identify new promising candidates for next generation thermoelectric applications has produced several interesting new materials which are briefly described here. We present their compositions, solid state structures, properties and charge transport behavior. The compounds CsBi 4 Te 6 , β-K 2 Bi 8 Se 13 , Ba 4 Bi 6 Se 13 , Eu 2 Pb 2 Bi 6 Se 13 , KBi 6.33 S 10 , Eu 2 Pb 2 Bi 4 Se 10 , Ba 2 Pb 2 Bi 6 S 13 and K 1.25 Pb 3.5 Bi 7.25 Se 15 are particularly noteworthy.

Electrical Properties and Figures of Merit for New Chalcogenide-Based Thermoelectric Materials

New Bi-based chalcogenide compounds have been prepared using the polychalcogenide flux technique for crystal growth. These materials exhibit characteristics of good thermoelectric materials. Single crystals of the compound CsBi{sub 4}Te{sub 6} have shown conductivity as high as 2440 S/cm with a p-type thermoelectric power of {approx}+110 {micro}V/K at room temperature. A second compound, {beta}-K{sub 2}Bi{sub 8}Se{sub 13} shows lower conductivity {approx}240 S/cm, but a larger n-type thermopower {approx}{minus}200 {micro}V/K. Thermal transport measurements have been performed on hot-pressed pellets of these materials and the results show comparable or lower thermal conductivities than Bi{sub 2}Te{sub 3}. This improvement may reflect the reduced lattice symmetry of the new chalcogenide thermoelectrics. The thermoelectric figure of merit for CsBi{sub 4}Te{sub 6} reaches ZT {approx} 0.32 at 260 K and for {beta}-K{sub 2}Bi{sub 8}Se{sub 13} ZT {approx} 0.32 at room temperature, indicating that these compounds are viable candidates for thermoelectric refrigeration applications.

Ln2Al3Si2 (Ln = Ho, Er, Tm): neue Silicide aus Aluminiumschmelzen – Bestimmung der Al/Si-Verteilung mit Neutronenkristallographie und metamagnetische Übergänge

Ausgepragt metallische Verbindungen mit einer quasi-eindimensionalen Struktur sind die ternaren Verbindungen Ln2Al3Si2 (Ln = Ho, Er, Tm), die in Aluminiumschmelzen ausgehend von metallischem Lanthanoid und Silicium synthetisiert wurden. In ihnen liegt ein formales [Al3Si2]6−-Gerust aus unendlichen Al-Zickzackketten und Si-Si-Dimeren vor, das Reihen von Ln3 +-Ionen in parallelen Tunneln beherbergt. Die Verbindungen weisen bei hohen Magnetfeldstarken metamagnetische Ubergange auf.

α-RuCl 3 : A New Host for Polymer Intercalation. Lamellar Polymer/α-RuCI 3 Nanocomposites.

α-RuCl 3 was identified as a new host for polymer intercalation using different intercalative methods. It is found that Li 0.2 RuCl 3 exfoliates in water into single [RuCl 3 ] x− layers which can be used to encapsulate macromolecules. RuC1 3 /polymer nanocomposites such as Li 0.2 (PEO) x RuCl 3 (PEO=poly[ethylene oxide]), Li 0.2 (PVP) x RuCl 3 (PVP=polyvinylpyrrolidone) and (PPY) x RuCl 3 (PPY=polypyrrole) were successfully prepared. (PANI) x RuCl 3 (PANI=polyaniline) was prepared by in situ intercalative polymerization directly from α-RuCl 3 . These nanocomposites were characterized with Thermogravimetric Analysis (TGA), Infrared spectroscopy (IR), Powder X-ray Diffraction (XRD), magnetic measurements, conductivity measurements and thermopower measurements.

Transport properties of doped CsBi4Te6 thermoelectric materials

In previous investigations we have introduced a variety of new chalcogenide-based materials with promising properties for thermoelectric applications. The chalcogenide CsBi 4 Te 6 was previously reported to have a high ZT product with a maximum value at 260K. In order to improve this value, a series of doped CsBi 4 Te 6 samples has been synthesized. Current doping studies have been very encouraging, with one sample found to have a maximum power factor of 51.5 μW/cm·K 2 at 184 K. This paper reports on material characterization studies through the usual transport measurements to determine optimum doping concentration for various dopants.

Thermoelectric properties of the cubic family of compounds AgPbBiQ3 (Q = S, Se, Te). Very low thermal conductivity materials

The AgPbBiQ 3 class of compounds and their solid solution members are related to the NaCl structure type, where Ag, Pb and Bi atoms are statistically disordered on the Na site and Q atoms occupy the Cl site. These compounds were synthesized by combining the elements in the appropriate ratio and heating under static vacuum at 900° C for 3 days. They are narrow gap semiconductors with band gaps in the range of 0.6 to 0.28 eV. The charge-transport properties were measured on ingots as a function of temperature. The compounds AgPbBiTe 3 , AgPbBiSe 3 , AgPbBiTe 2.75 Se 0.25 and AgPbBiTe 2 Se, undoped, possess an electrical conductivity in the range of 70 S/cm to 400 S/cm. These materials exhibit negative thermopower ranging from -40 μV/K to -160 μV/K at room temperature and thermal conductivity less than 1.30 W/mK.