Structural, magnetic, and electronic properties of VxCr2−xS3(0

A new family of vanadium-substituted chromium sulfides (VxCr2-xS3, 0 < x < 2) has been prepared and characterized by powder X-ray and neutron diffraction, SQUID magnetometry, electrical resistivity, and Seebeck coefficient measurements. Vanadium substitution leads to a single-phase region with a rhombohedral Cr2S3 structure over the composition range 0.0 < x e 0.75, while at higher vanadium contents (1.6 e x < 2.0) a second single-phase region, in which materials adopt a cation-deficient Cr3S4 structure, is observed. Materials with the Cr2S3 structure all exhibit semiconducting behavior. However, both transport and magnetic properties indicate an increasing degree of electron delocalization with increasing vanadium content in this compositional region. Materials that adopt a Cr3S4-type structure exhibit metallic behavior. Magnetic susceptibility data reveal that all materials undergo a magnetic ordering transition at temperatures in the range 90–118 K. Low-temperature magnetization data suggest that this involves a transition to a ferrimagnetic state.

A copper-containing oxytelluride as a promising thermoelectric material for waste heat recovery

The new thermoelectric material BiOCuTe exhibits an electrical conductivity of 224 S cm-1 and a Seebeck coefficient of +186 μV K-1 at 373 K, together with an extremely low lattice thermal conductivity of ∼ 0.5 W m-1 K-1. This results in a ZT of 0.42 at 373 K, which increases to 0.66 at the maximum temperature investigated, 673 K.

Synthesis, structural characterisation and thermoelectric properties of Bi1−xPbxOCuSe

The effect of Pb2+ doping on the structure and thermoelectric properties of BiOCuSe (also known as BiCuSeO or BiCuOSe) is described. With increasing Pb2+ content, the expansion of the unit cell results in a weakening of the bonding between the [Bi2(1-x) Pb2xO2]2(1-x)+ and the [Cu2Se2]2(1-x)- layers. The electrical resistivity and Seebeck coefficient decrease in a systematic way with growing Pb2+ levels. The thermal conductivity rises due to the increase of the electronic contribution with doping. The power factor of materials with a 4-5% Pb2+ content takes values of ca. 8 W cm-1 K-2 over a wide temperature range. ZT at 673 K is enhanced by ca. 50% when compared to values found for other dopants, such as Sr2+ or Mg2+.

Thermoelectric properties of BiOCu1-xMxSe (M = Cd and Zn)

Doping of BiOCuSe at the copper site with divalent cadmium and zinc cations has been investigated. Analysis of the powder X-ray diffraction data indicates that the ZrCuSiAs structure of BiOCuSe is retained up to substitution levels of 10 and 5 at.% for Cd2+ and Zn2+, respectively. Substitution of monovalent Cu+ with divalent Cd2+ or Zn2+ leads to an increase in the magnitude of the electrical resistivity and the Seebeck coefficient. All synthesized materials behave as p-type semiconductors.

Electron doping and phonon scattering in Ti1+xS2 thermoelectric compounds

Bulk polycrystalline samples in the series Ti1+xS2 (x = 0 to 0.05) were prepared using high temperature synthesis from the elements and spark plasma sintering. X-ray structure analysis shows that the lattice constant c expands as titanium intercalates between TiS2 slabs. For x=0, a Seebeck coefficient close to -300 μV/K is observed for the first time in TiS2 compounds. The decrease in electrical resistivity and Seebeck coefficient that occurs upon Ti intercalation (Ti off stoichiometry) supports the view that charge carrier transfer to the Ti 3d band takes place and the carrier concentration increases. At the same time, the thermal conductivity is reduced by phonon scattering due to structural disorder induced by Ti intercalation. Optimum ZT values of 0.14 and 0.48 at 300K and 700K, respectively, are obtained for x=0.025.

The interplay of metal-atom ordering, Fermi leveltuning and thermoelectric properties in cobalt shandites Co3M2S2 (M = Sn, In)

A combination of structural, physical and computational techniques including powder X-ray and neutron diffraction, SQUID magnetometry, electrical and thermal transport measurements, DFT calculations and 119Sn Mössbauer and X-ray photoelec-tron spectroscopies has been applied to Co3Sn2-xInxS2 (0 ≤ x ≤ 2) in an effort to understand the relationship between metal-atom ordering and physical properties as the Fermi level is systematically varied. Whilst solid solution behavior is found throughout the composition region, powder neutron diffraction reveals that indium preferentially occupies an inter-layer site over an alternative kagome-like intra-layer site. DFT calculations indicate that this ordering, which leads to a lowering of energy, is related to the dif-fering bonding properties of tin and indium. Spectroscopic data suggest that throughout the composition range 0 ≤ x ≤ 2, all ele-ments adopt oxidation states that are significantly reduced from expectations based on formal charges. Chemical substitution ena-bles the electrical transport properties to be controlled through tuning of the Fermi level within a region of the density of states, which comprises narrow bands of predominantly Co d-character. This leads to a compositionally-induced double metal-to-semiconductor-to-metal transition. The marked increase in the Seebeck coefficient as the semiconducting region is approached leads to a substantial improvement in the thermoelectric figure of merit, ZT, which exhibits a maximum of ZT = 0.32 at 673 K. At 425 K, the figure of merit for phases in the region 0.8 ≤ x ≤ 0.85 is amongst the highest reported for sulphide phases, suggesting these materials may have applications in low-grade waste heat recovery.

Synthesis, characterization and physical properties of the skutterudites YbxFe2Ni2Sb12 (0≤x≤0.4)

The skutterudites YbxFe2Ni2Sb12 (0≤x≤0.4) have been prepared by solid-state reaction and characterised by powder X-ray diffraction. The compounds crystallise in the cubic space group Im View the MathML source3¯ (a≈9.1 Å) with Yb atoms partially filling the voids in the skutterudite framework. A neutron time-of-flight diffraction experiment for Fe2Ni2Sb12 confirms the disorder of Fe and Ni atoms on the transition-metal site. Electrical resistivity, Seebeck coefficient and thermal conductivity measurements indicate that the thermoelectric performance of the skutterudites shows a marked dependence on the Yb content. Magnetic measurements over the temperature range 2≤T/K≤300 show paramagnetic behaviour for all compounds. Decomposition studies under an oxidising atmosphere at elevated temperatures have also been carried out by thermogravimetric analysis.

Synthesis and thermoelectric properties of the new skutterudites Yb x Fe2Ni2Sb12 (0 ≤ x ≤ 0.4)

The family of materials Yb x Fe2 Ni 2Sb12 (0 ≤ x ≤ 0.4) has been prepared by solid-state synthesis from the pure elements and characterized by powder X-ray diffraction. These materials crystallize in the skutterudite structure, with the framework voids partially filled with Yb atoms. Electrical resistivity, Seebeck coefficient and thermal conductivity measurements have been performed on hot-pressed samples, and indicate that the thermoelectric performance is significantly improved by increasing the Yb content. The decomposition of the compounds under oxidizing atmosphere at elevated temperatures has also been studied by thermogravimetric analysis. The physical properties and thermal stability of the new compounds are further discussed in comparison with those of the reported isostructural and isoelectronic Yb x Co4Sb12 (0 ≤ x ≤ 0.19).

Synthesis and characterisation of the anion-ordered tellurides MGeTe (M = Co, Rh)

The synthesis and structural characterisation, carried out using a combination of single-crystal and powder X-ray diffraction, of the materials MGeTe (M = Co, Rh) are described. These phases adopt an ordered α-NiAs2 structure, which can be considered intermediate between those of pyrite and marcasite. Electrical resistivity and Seebeck coefficient measurements, carried out over the temperature range 77 ≤ T/K ≤ 325, indicate that these materials are n-type semiconductors.

A powder neutron diffraction study of the metallic ferromagnet Co3Sn2S2

Variable-temperature powder neutron diffraction data reveal that Co3Sn2S2 crystallizes in the shandite structure (space group R (3) over barm, a = 5.36855(3)angstrom, c = 13.1903(1) angstrom at 300 K). The structural relationship between Co3Sn2S2 and the intermetallic compound CoSn, both of which contain Kagome nets of cobalt atoms, is discussed. Resistivity and Seebeck coefficient measurements for Co3Sn2S2 are consistent with metallic behaviour. Magnetic susceptibility measurements indicate that Co3Sn2S2 orders ferromagnetically at 180(10) K, with a saturation moment of 0.29 mu(B) per cobalt atom at 5 K. The onset of magnetic ordering is accompanied by marked anomalies in the electrical transport properties. (c) 2008 Elsevier Masson SAS. All rights reserve

Structure and electrical transport properties of the ordered skutterudites MGe1.5S1.5 (M = Co, Rh, Ir)

High-resolution powder neutron diffraction data collected for the skutterudites MGe1.5S1.5 (M=Co, Rh, Ir) reveal that these materials adopt an ordered skutterudite structure (space group R3¯), in which the anions are ordered in layers perpendicular to the [111] direction. In this ordered structure, the anions form two-crystallographically distinct four-membered rings, with stoichiometry Ge2S2, in which the Ge and S atoms are trans to each other. The transport properties of these materials, which are p-type semiconductors, are discussed in the light of the structural results. The effect of iron substitution in CoGe1.5S1.5 has been investigated. While doping of CoGe1.5S1.5 has a marked effect on both the electrical resistivity and the Seebeck coefficient, these ternary skutterudites exhibit significantly higher electrical resistivities than their binary counterparts.

A comparison of diatom phosphorus transfer functions and export coefficient models as tools for reconstructing lake nutrient histories

1. We compared the baseline phosphorus (P) concentrations inferred by diatom-P transfer functions and export coefficient models at 62 lakes in Great Britain to assess whether the techniques produce similar estimates of historical nutrient status. 2. There was a strong linear relationship between the two sets of values over the whole total P (TP) gradient (2-200 mu g TP L-1). However, a systematic bias was observed with the diatom model producing the higher values in 46 lakes (of which values differed by more than 10 mu g TP L-1 in 21). The export coefficient model gave the higher values in 10 lakes (of which the values differed by more than 10 mu g TP L-1 in only 4). 3. The difference between baseline and present-day TP concentrations was calculated to compare the extent of eutrophication inferred by the two sets of model output. There was generally poor agreement between the amounts of change estimated by the two approaches. The discrepancy in both the baseline values and the degree of change inferred by the models was greatest in the shallow and more productive sites. 4. Both approaches were applied to two lakes in the English Lake District where long-term P data exist, to assess how well the models track measured P concentrations since approximately 1850. There was good agreement between the pre-enrichment TP concentrations generated by the models. The diatom model paralleled the steeper rise in maximum soluble reactive P (SRP) more closely than the gradual increase in annual mean TP in both lakes. The export coefficient model produced a closer fit to observed annual mean TP concentrations for both sites, tracking the changes in total external nutrient loading. 5. A combined approach is recommended, with the diatom model employed to reflect the nature and timing of the in-lake response to changes in nutrient loading, and the export coefficient model used to establish the origins and extent of changes in the external load and to assess potential reduction in loading under different management scenarios. 6. However, caution must be exercised when applying these models to shallow lakes where the export coefficient model TP estimate will not include internal P loading from lake sediments and where the diatom TP inferences may over-estimate TP concentrations because of the high abundance of benthic taxa, many of which are poor indicators of trophic state.

Preparation of nanomagnetic absorbent for partition coefficient measurement

In this paper, we report a new method based on supercritical carbon dioxide (scCO(2)) to fill and distribute the porous magnetic nanoparticles with n-octanol in a homogeneous manner. The high solubility of n-octanol in scCO(2) and high diffusivity and permeability of the fluid allow efficient delivery of n-octanol into the porous magnetic nanoparticles. Thus, the n-octanol-loaded magnetic nanoparticles can be readily dispersed into aqueous buffer (pH 7.40) to form a homogenous suspension consisting of nano-sized n-octanol droplets. We refer this suspension as the n-octanol stock solution. The n-octanol stock solution is then mixed with bulk aqueous phase (pH 7.40) containing an organic compound prior to magnetic separation. The small-size of the particles and the efficient mixing enable a rapid establishment of the partition equilibrium of the organic compound between the solid supported n-octanol nano-droplets and the bulk aqueous phase. UV-vis spectrophotometry is then applied to determine the concentration of the organic compound in the aqueous phase both before and after partitioning (after magnetic separation). As a result, log D values of organic compounds of pharmaceutical interest determined by this modified method are found to be in excellent agreement with the literature data. (c) 2006 Elsevier B.V. All rights reserved.