Phase Purity Of Samples Research Articles

Superconductivity in Nb21S8, a Phase with Metal Cluster Chains

Phase pure samples of Nb21S8 are obtained by chemical transport and molten flux reactions in sealed niobium containers at 850−950 °C. In the temperature range from 5 to 290 K the electrical conductivity is found to be moderate metallic with a specific resistivity of 3.90 mΩ cm at 273 K. Magnetic susceptibility measurements give weak, almost temperature independent paramagnetism above ∼40 K. These metallic properties are compared with the structure of Nb21S8, which contains linear single and double chains of fused body centered niobium cubes, separated by S and additional Nb atoms. Both physical measurements consistently show a transition into the superconducting state below 4.1(5) and 3.7(2) K, respectively. Recently ideas were developed that superconductivity is favored by the tendency of pairwise attraction of conducting electrons at the Fermi level. To see whether these ideas can be applied to Nb21S8, electronic band structure calculations (TB-LMTO-ASA-method) have been performed. In fact at the X symm...

The influence of metallic interfaces on the properties of superconductors

The influence of AgPd and Au interfaces on the formation of the (HgBi-1223) superconductor was investigated. Variations in the BaCaCuO precursor phase assemblage and the use of as the Bi source were examined in an attempt to lower the reaction temperatures suitable for the growth of HgBi-1223 in conjunction with AgPd and Au substrates. Phase pure samples of HgBi-1223 were synthesized at temperatures below . Use of as a Bi source combined with variations of the BaCaCuO precursor phase assemblage allowed the formation of some liquid phase during the reaction and resulted in dense HgBi-1223 samples with aligned colonies of grains. Magnetization measurements in fields up to 30 T indicated good intergrain coupling within the large grain colonies. Comparison of the microstructural features of bulk samples and the metal-superconductor interface showed that the interface has a positive influence on grain growth and alignment. The metallic interfaces, however, were found to have a detrimental effect on the purity of the superconducting phase due to the formation of amalgams.

Surface Enhanced Raman Spectroscopy: A Novel Physical Chemistry Experiment for the Undergraduate Laboratory

Raman spectra of molecules adsorbed on metal colloids exhibit large signal enhancements over those of the pure or solution phase samples. This paper presents background information on surface-enhanced Raman spectroscopy (SERS) and provides details for carrying out an SERS experiment in the undergraduate physical chemistry laboratory. A single system, pyridine on silver, has been adapted from the research literature for use with undergraduates. Instructions for sample preparation, experimental setup, and data analysis are provided in addition to sample spectra.

Mn 3O 4 and γ-MnOOH powders, preparation, phase composition and XPS characterisation

The paper reports data relative to the preparation of Mn 3O 4 and γ-MnOOH powders by following hydrothermal procedures. The mixed valence spinel was prepared starting either from manganous sulphate or chloride and oxidising in suspension (by either air or oxygen) the alkaline precipitate. Manganite was obtained by oxidising manganous sulphate solutions with H 2O 2, subsequent alkalisation and refluxing. The samples were characterised for phase composition by X-ray diffraction. Pure phase oxides were obtained in the case of both Mn 3O 4 and γ-MnOOH when Mn(II) sulphate was used as the precursor. The pure phase samples were submitted to X-ray photoelectron spectroscopy (XPS) investigation, in the case of manganese, for both 2p and 3s spectral regions. The binding energies of the un-fitted experimental Mn peaks compare well with the relative values reported in the literature for the compounds. With the aim of assessing the actual Mn speciation, in the external layers, the fitting procedure of the experimental XPS peaks has been performed by modulating the relative intensities of the spectral components on the grounds of distinct hypotheses consistent with the oxide formulas. The possibility of extending the outcome of this elaboration to the bulk composition of the samples is also discussed.

Hydrogen Sites In The α(TiCrSiO) 1/1 Approximant Phase

AbstractCrystal approximant phases are important because they are believed to have a similar local atomic structure to corresponding quasicrystals. Interstitial hydrogen is used as a probe of the local structures of the Ti-based quasicrystals and crystal approximants. Phase pure samples of ct(TiCrSiO), a Mackay type 1/1 crystal approximant, which were plasma etched and coated with a thin layer of Pd, were loaded with deuterium from the gas phase to a maximum deuterium atom to metal atom ratio (D/M) of 0.26. After deuteration, the sample becomes a fine powder and remains single-phase. A Rietveld structural refinement of powder x-ray and neutron diffraction data was made to determine the location of the interstitial sites. For the fully deuterated sample the deuterium atoms sit at both octahedral and tetrahedral sites. The octahedral sites are formed by six Ti atoms in the first and second shells of the icosahedra. Deuterium atoms are also located at the tetrahedral interstitial sites formed between the clusters along <100> directions. In the partially loaded samples with DiM = 0.11, the deuterium atoms occupy only octahedral sites, demonstrating a preference for these sites.

Effects of a Au interface on (HgBi)Ba2Ca2Cu3Ox superconductor

Phase pure samples of (HgBi0.2 )Ba2Ca2CU3.1Ox have been synthesized on gold foils. The gold superconductor interface was found to enhance grain growth and alignment as seen by XRD, SEM, and magnetization measurements.

Synthesis methods and unit-cell volume of end-member titanite (CaTiOSiO4)

Unit-cell parameters of synthetic, end-member titanite (CaTiOSiO 4 ) critically depend on the synthesis conditions, as is shown for studies reported in the literature and for new samples reported here. Our study suggests that phase-pure samples are likely to be obtained only if they are synthesized entirely below the solidus. In contrast, samples synthesized either directly from melt or by annealing of glass tend to have higher unit-cell volumes, contain Si-rich and Ca-Si-rich phase impurities, and may be nonstoichiometric. The observed variations in cell parameters among the samples strongly correlate with synthesis methods and can be explained by vacancies in the Ca or Si site or both. This result is particularly important because the thermodynamic properties currently in use for titanite are based on samples synthesized from melts of stoichiometric composition and thus are suspect even though they have been determined carefully. To establish a reference point for future studies concerned with the chemical and physical properties of this material we report our findings along with a redetermination of the unit-cell parameters [a = 7.062(1), b = 8.716(2), and c = 6.559(1) Aa; beta = 113.802(9) degrees , V = 369.4(3) Aa 3 ] from powder X-ray data of synthetic, stoichiometric titanite.

Synthesis, crystal structure and properties of In-containing 1212-type compounds

A new series of solid solutions (Bi 0.35In 0.65) 1− x Pb x Sr 2Gd 0.6Ca 0.4Cu 2O y ( x = 0−0.5) has been synthesised and characterised by means of powder X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectrometric analysis, and magnetic and resistivity measurements. High temperature preparations lead to the appearance of “bulk” superconducting properties which, however, can be attributed to the formation of a small amount of a Bi-2212 phase. The structure of a highly phase pure nonsuperconducting sample with composition Bi 0.21In 0.33Pb 0.25Sr 2Gd 0.89Ca 0.11Cu 2.21O 7+δ has been Rietveld refined using powder synchrotron X-Ray diffraction data.

Electrochemical Dissolution of Iron(III) Hydroxy-Oxides: More Information About the Particles

The process of reductive dissolution of α-Fe2O3, Ba-hexaferrite, α-FeOOH, γ-Fe2O3, γ-FeOOH, and ferrihydrite can be monitored continuously by abrasive stripping voltammetry and chronoamperometry to obtain information about homogeneity of reactivity and about the particle size of the pure phases and mixtures. Phase parameters (rate constant and charge transfer coefficient) and the geometrical factor γ, which is independent on the specific surface area, can be assigned to a phase-pure sample in the given medium. Differences in reactivity of the hydroxy-oxides are documented, and the dependence of reactivity of the α-Fe2O3 samples on the preparation procedure is characterized. The electrochemical techniques can also be employed to investigate changes in the particles during phase transformations.

Influence of fluorine in magnetic properties of Hg-1223 high-temperature superconductors

Bulk pure-phase samples of HgBa 2Ca 2Cu 3O y with high quality were prepared using a solid-state reaction with BaF 2 as a part of the starting material. X-ray diffraction data show that the crystal structure is nearly the same as for starting compounds without fluorine with a = 3.85 A ̊ and c = 15.85 A ̊ . The effect of fluorine in the starting material on the magnetic properties was studied by DC susceptibility and hysteresis-loop measurements, and our results revealed a remarkable enhancement of hysteretic magnetization in samples prepared using BaO/BaF 2. Fluorine in Hg-1223 also shows an effect similar to that of oxygenation, which increases T c onset of the as-synthesized samples to 133 K. The asymmetric magnetization-field data indicate the presence of a surface barrier, and the field dependence and time dependence of the magnetization data display an enhancement of hysteresis upon replacement of 10% of BaO by BaF 2 as a result of increased concentration of pinning centers without changing the nature of the individual pinning.

Superconductivity in the Sr-Ca-Cu-O system and the phase with infinite-layer structure.

Superconductivity and structure in samples of (Sr,Ca)${\mathrm{CuO}}_{2}$ with the infinite-layer structure, prepared by high-pressure synthesis, have been studied using magnetic susceptibility measurements, small angle x-ray diffraction, and neutron diffraction. It is found that the superconducting (${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}100 K) samples in this system are phase impure and contain, in addition to the infinite-layer phase, members of the two homologous series ${\mathrm{Sr}}_{\mathit{n}\mathrm{\ensuremath{-}}1}$${\mathrm{Cu}}_{\mathit{n}+1}$${\mathrm{O}}_{2\mathit{n}}$ (n=3,5,...; orthorhombic), and ${\mathrm{Sr}}_{\mathit{n}+1}$${\mathrm{Cu}}_{\mathit{n}}$${\mathrm{O}}_{2\mathit{n}+1+\mathrm{\ensuremath{\delta}}}$ (n=1,2,...; tetragonal), as minor phases. Samples with larger phase fractions of the ${\mathrm{Sr}}_{\mathit{n}+1}$${\mathrm{Cu}}_{\mathit{n}}$${\mathrm{O}}_{2\mathit{n}+1+\mathrm{\ensuremath{\delta}}}$ compounds showed higher superconducting fractions. Phase-pure infinite-layer samples are not superconducting. Based on these results, and results previously published in the literature, it is proposed that the superconductivity in these infinite-layer samples comes from the tetragonal ${\mathrm{Sr}}_{\mathit{n}+1}$${\mathrm{Cu}}_{\mathit{n}}$${\mathrm{O}}_{2\mathit{n}+1+\mathrm{\ensuremath{\delta}}}$ compounds, not from the phase with the infinite-layer structure.

Structural Chemistry of Some Phases in The YC-Ni-B System

ABSTRACTNiB, monoclinic Ni4B3, Ni2B and Ni3B were prepared by arc-melting and their roomtemperature crystal structures were refined by Rietveld analysis of neutron powder diffraction data. The NiB refinement is altogether new data. Although the B atoms in NiB form characteristic zigzag chains, the primary coordination of each atom by atoms of the other kind is similar and distinctively sevenfold, with one short (2.117 Å), two intermediate (2.152 Å), and four long (2.163 Å) bonds. Other samples with stoichiometries (YC)nNi2B2, n = 3, 4, did not yield single-phase material, but both x-ray and neutron powder diffraction suggest that the n = 4 structure is present in both of these samples. Phase-pure samples of these homologues may require non-stoichiometry and a more controlled thermal history than is attainable by arc melting.

AC Susceptibility and Upper Critical Fields in Rb3C60

Abstract The ac susceptibility of a phase-pure Rb3C60 sample has been investigated in dc magnetic fields up to 14 T. The upper critical fields show a significant positive curvature near Tc indicating a local pairing mechanism.

The process of forming 2223 phase from 2212 phase in Bi(Pb)-Sr-Ca-Cu-O System

The process of forming (2223) phase was studied by x-ray diffraction, thermal analysis, as well as electric and magnetic measurements. It was discovered that the formation of (2223) from (2212) is a phase transition-like process. Some possible mechanisms of (2212) to (2223) phase transition-like are discussed. Our results not only explain the difficulty for getting pure (2223) phase sample, but also give a way to prepare high quality (2223) phase sample.

Is mutual separation of rare earth elements crucial for preparation of high T c materials?

RE oxide/hydroxide mixtures can serve as starting materials for the synthesis of the 123 (REBa 2Cu 3O 7−δ) superconductor, and a process based on such materials is far more economic than those starting from pure RE oxide. However, in order to obtain phase pure samples, lanthanum in the RE oxide mixture must be treated as an element occupying the barium site.

Chronic Friedlander's Pneumonia

In this article, properties of laboratory and industrially prepared Mg–Al layered double hydroxides (LDH) with Mg:Al molar ratios varied in the range from 2 to 4 were compared. Physico-chemical properties of the studied materials were investigated with XRD, SEM, N2 physisorption, TGA and CO2-TPD methods. Catalytic behavior of the LDH samples was examined in the liquid phase aldol condensation of furfural and acetone at 20 and 60 °C. It was found that LDH from the small-scale preparation have different physico-chemical characteristics than the LDH materials originating from the large-scale (industrial) preparation which is also reflected in their catalytic behavior. As-synthesized laboratory LDH catalysts are phase-pure crystalline samples with hydrotalcite structure while the industrial materials also possess MgO as admixture. The laboratory samples possess larger BET surface area and mesopore volume in comparison with the industrially prepared ones. Despite the similarity in basic properties of the studied materials, the samples with molar ratio Mg:Al = 3 in both groups of LDH using different method of preparation possess the best activity in aldol condensation. Generally, the calcined laboratory samples are more active than the industrial materials in the reaction. This is explained by the better textural properties of the former ones and their susceptibility to in-situ reconstruction in the presence of water formed during the reaction.