Rhombohedral Bi Research Articles

Experimental magnetization evidence for two superconducting phases in Bi bicrystals with large crystallite disorientation angles

Magnetization measurements prove that the magnetic properties of large-angle $(\ensuremath{\theta}>30\ifmmode^\circ\else\textdegree\fi{})$ bismuth bicrystals with a crystallite interface (CI) of twisting types essentially differ from well-known results on single-crystalline specimens. Two superconducting phases with ${T}_{c}\ensuremath{\sim}8.4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $\ensuremath{\sim}4.3\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ were observed at the CI of bicrystals while ordinary rhombohedral Bi is not a superconductor. We conclude that these phases have to do with the central part and the adjacent layers of the CI of bicrystals.

Effect of the surface configuration on the oxidation of bismuth nanowire

Incorporating nanoprocessing into the metal oxidation, it was a facile way to synthesize functional oxide with desired nanostructure. In this work, δ-Bi 2O 3 nanowires were successfully fabricated by the oxidation of electroplated Bi nanowires at 350 °C. δ-Bi 2O 3 is the high-temperature phase of Bi 2O 3 and only stable at 723–823 °C. Partially oxidized nanowires showed core-shell structure composed of metallic Bi and δ-Bi 2O 3. To investigate the mechanism of oxidation reaction, the Bi/Bi 2O 3 interface was characterized by high resolution transmission electron microscopy (HRTEM). HRTEM images showed rapid growth of oxide layer on ( 2 1 ¯ 0 ) plane of rhombohedral Bi metal. The coherency between ( 10 2 ¯ ) of metallic Bi and (1 0 0) of cubic Bi 2O 3 was observed. A schematic model was also used to describe the oxidation process. The coherency Bi and Bi 2O 3 and the stabilization of high-temperature (fluorite structure) Bi 2O 3 were also discussed based on this model.

Effect of deposition temperature on the structural and thermoelectric properties of bismuth telluride thin films grown by co-sputtering

Thermoelectric bismuth telluride thin films were prepared on SiO 2/Si substrates by radio-frequency (RF) magnetron sputtering. Co-sputtering method with Bi and Te targets was adopted to control films' composition. Bi x Te y thin films were elaborated at various deposition temperatures with fixed RF powers, which yielded the stoichiometric Bi 2Te 3 film deposition without intentional substrate heating. The effects of deposition temperature on surface morphology, crystallinity and electrical transport properties were investigated. Hexagonal crystallites were clearly visible at the surface of films deposited above 290 °C. Change of dominant phase from rhombohedral Bi 2Te 3 to hexagonal BiTe was confirmed with X-ray diffraction analyses. Seebeck coefficients of all samples have negative value, indicating the prepared Bi x Te y films are n-type conduction. Optimum of Seebeck coefficient and power factor were obtained at the deposition temperature of 225 °C (about − 55 μV/K and 3 × 10 − 4 W/K 2·m, respectively). Deterioration of thermoelectric properties at higher temperature could be explained with Te deficiency and resultant BiTe phase evolution due to the evaporation of Te elements from the film surface.

On the origin and stability of the metastable phase in rapidly solidified Sn–Bi alloy particles embedded in Al matrix

The paper reports the synthesis of nanoscaled tin–bismuth alloy particles embedded in an aluminum matrix by rapid solidification. The observed microstructure and composition analyses suggest the co-existence of tetragonal β-Sn and rhombohedral Bi phase within the nanoparticles. Additionally, another phase, primarily a tin-based solid solution, gets stabilized and co-exists with the equilibrium bismuth phase in some of the smaller particles. In order to understand the kinetic stability of this phase we have carried out detailed thermal analyses using differential scanning calorimetry, high temperature X-ray diffraction and transmission electron microscopy. The results suggest that at small length scales this phase can be more stable than the eutectic phase. We have tried to explain the formation and stability of this new phase in terms of favorable nucleation kinetics, and the pressure developed in the melt within the cavity during solidification.

Fabrication of Bi2O3 Thin Film by Thermal-Enhanced Electroplating

High-temperature fluorite structure Bi 2 O 3 is a well-known solid electrolyte due to its high oxygen ion conductivity. The stabilization of cubic Bi 2 O 3 may be enhanced by adding rare earth oxide. It was thought that phase transformation was caused by the presence of oxygen vacancies. In this study, Bi 2 O 3 thin film was prepared by oxidation on the electroplated Bi. The effect of applied voltage on crystallization of metallic Bi thin film was investigated. The oxidation reaction of Bi with various crystalline orientations is discussed. Two preferred orientations, (102) and (202), appeared in XRD. After oxidation, β-Bi 2 O 2 and α-Bi 2 O 3 appeared due to the specific atomic configurations of (102) and (202). The microstructure observed by SEM showed microcracks due to lattice expansion from phase transformation. The volume changes from rhombohedral Bi to β-Bi 2 O 3 and α-Bi 2 O 3 were estimated to be 16.73 and 19.14%.

Powder Neutron Diffraction and TEM Investigations of Bi 0.775Ln0.225O 1.5 Oxide Conductors ( Ln=La, Pr, Nd, Sm, Tb, Dy) with Rhombohedral Bi–Sr–O type: Structural Relationships with Monoclinic ε-Bi 4.86La 1.14O 9 Form

Structural investigations of Bi 0.775 Ln 0.225O 1.5 phases with rhombohedral Bi–Sr–O type, have been carried out using complementary techniques. Average crystal structures were refined from neutron diffraction powder data using the Rietveld method for Ln=La, Pr, Nd and Tb. The comparison with the monoclinic Bi 4.86La 1.14O 9 structure previously determined using the same technique has been done. For both types of structure, a piling of slabs constituted of a (Bi (1), Ln) oxide layer, inserted between two Bi (2) oxide layers, is observed. In the rhombohedral phases there are moreover oxide ions in the inter-slab spaces responsible for the well-known conductivity of these phases, while in the monoclinic Bi 4.86La 1.14O 9 structure, the absence of oxygen in the inter-slab spaces, is compensated with the presence of the Bi (2) lone pair electrons localized at a significant distance (≅1.20 Å), which participate in the cohesion of the crystal structure. TEM study of Bi 0.775 Ln 0.225O 1.5 samples, has confirmed the existence of superstructures for these phases and has allowed to propose two types (respectively for La and for Nd, Sm, Tb or Dy) of q 1 * and q 2 * modulation wave vectors.

Structure and melting of Bi nanocrystals embedded in aB2O3−Na2Oglass

A composite material consisting of spherical Bi nanoclusters (nanocrystals and/or liquid nanodroplets) embedded in a $28{\mathrm{Na}}_{2}\mathrm{O}\ensuremath{-}72{\mathrm{B}}_{2}{\mathrm{O}}_{3}$ glass was studied by the wide-angle x-ray scattering (WAXS) and small-angle x-ray scattering (SAXS) techniques over the temperature range in which the Bi crystal-liquid transition occurs. Because of the wide radius distribution of Bi clusters and due to the dependence of the melting temperature on crystal radius, the overall transition occurs over a wide range, from 365 up to 464 K. In this transition range, large Bi nanocrystals coexist with small liquid droplets. A weak contraction in a and c lattice parameters of rhombohedral Bi nanocrystals with respect to the bulk crystal was detected. As expected, the average radius of crystalline Bi clusters, deduced from WAXS data, increases for increasing temperatures over the whole solid-to-liquid transition range. The SAXS spectrum recorded at different temperatures within the transition range is essentially invariant, indicating that the radius distribution of Bi nanoclusters (nanocrystals and nanodroplets) is temperature independent. The volume distribution of Bi nanoclusters is a single-mode function with the radius ranging from about 15 up to 41 \AA{} with a maximum at 28 \AA{}. The integral of Bragg peaks of Bi nanocrystals decreases for increasing temperatures as a consequence of the progressive melting of nanocrystals of increasing size. By combining the results of WAXS and SAXS experiments, we determined the melting temperature of the nanocrystals as a function their radius suppressing unwanted size dispersion effects. Our results clearly indicate a linear dependence of the melting temperature on nanocrystal reciprocal radius, thus confirming previous theoretical predictions.

Evidence for a New Monoclinic Bi–La Oxide Solid Solution: Identification and Structural Relationship to the Rhombohedral Bi–Sr–O Type

As a term of the Bi–Sr–O rhombohedral type Bi1−yLayO1.5 solid solution, the Bi17La4O31.5 mixed oxide (y no. 0.19) has been investigated. Air-quenching from the high temperature domain of the β1 disordered form allows one to preserve at room temperature a metastable form of β1. This form transforms exothermally on heating near 200°C to a new monoclinic phase. This transformation and the transition monoclinic phase→stable ordered rhombohedral β2, which occurs at 445°C, are both irreversible on cooling for this composition. The β2→β1 transition occurs reversibly near 660°C, under usual dynamic conditions (300°h−1). The monoclinic cell lattice constants, a=9.492(2), b=7.951(2), c=7.028(2) Å, β=104.74(1)°, lead to Z=12 for the unit formula Bi0.81La0.19O1.5 and imply a disordered cationic sublattice. This mixed oxide appears as a term of a new variety of a wide solid solution (that will name ε hereafter).

Bi–La-Based Oxide Conductors with Rhombohedral Bi–Sr–O Type: Structural and Conductivity Properties Optimization by Polycationic Substitutions for La

In Bi0.775La0.225O1.5, the best oxide conductor of the hexagonal-layered materials family known as the Bi–Sr–O type, partial polycationic substitutions for La3+, such as (Ce4++M2+II) for 2 La3+ (MII=Ba, Pb) or (M6+VI+3M2+II) for 4 La3+ (MVI=Mo, W; MII=Ba, Pb), have been realized; these substitutions have induced an enlargement of the cationic slab thickness and thus allowed improvement of the conductivity. The resulting modifications of a and c lattice constants have been interpreted on the basis of the characteristics of the substituting elements (cationic radii and symmetries) and were correlated to the observed conductivity modifications.

Growth habit of rhombohedral Bi thin films on zinc-blende CdTe substrates with various orientations

Rhombohedral Bi thin films have been grown on zinc-blende (111), (211) and (100) CdTe substrates by molecular beam epitaxy. Reflection high-energy electron diffraction patterns showed that Bi grew layer-by-layer on the CdTe substrates without any reconstruction. X-ray diffraction θ−2θ scans revealed that the Bi films grown on (111) CdTe are (00.l) oriented and that the Bi films on (100) CdTe are (10.4) oriented. X-ray pole figures from the Bi films and the CdTe substrates indicated that Bi (00.l) on (211) CdTe grew along the CdTe (111) direction, like in the case of the Bi films on (111) CdTe. Most Bi grains on (100) CdTe select one particular direction out of the four possible directions in the presence of an off-axis tilt relative to the (100) substrate surface normal.

Structural and Conductivity Properties of Bi0.775Ln0.225O1.5Oxide Conductors (Ln=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy) with Rhombohedral Bi–Sr–O Type

This paper deals with structural and conductivity properties of the dimorphic Bi0.775Ln0.225O1.5(Ln=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy) hexagonal structure layer phases with rhombohedral Bi–Sr–O type. The evolutions, versusLn3+VIradius, of cell parameters, thermal expansion coefficients, electrical properties (conductivity and activation energy), and structural characteristics determined from Rietveld powder structure refinements, suggest two different domains (La–Sm) and (Gd–Dy), with the intermediate Eu sample, thus possibly characterizing two kinds of ordering in the mixed cationic layer. The best conductivity properties obtained for Bi0.775La0.225O1.5(σ400°C=10−3S cm−1withEa=0.8 eV) are discussed and justified.

MICROSTRUCTURAL FEATURE OF NANOPHASE Sn-Bi PARTICLES PRODUCED BY AN ELECTROHYDRODYNAMIC TECHNIQUE

Sn-Bi nanometer powders were produced by an electrohydrodynamic technique. The microstructural feature of Sn-Bi nanometer powders was investigated by means of XRD, TEM and HREM. The results show that Sn-Bi nanometer powders produced are composed of tetragonal β-Sn monophase supersaturated solid solution and rhombohedral Bi monophase supersaturated solid solution. Most of the Sn-Bi nanometer powders are single crytals, only a few Sn-Bi nanometer powders are twin crystals. The particles of﹤10nm in diameter are perfect single crystals. There exist dislocations in the particles larger than 10nm in diameter. The formation mechanism of microstructure for Sn-Bi nanometer powders is analyzed.

Phase stability and oxygen ion conduction in Bi 2O 3−Pr 6O 11

Oxygen ion conductivity was measured on a number of (Bi 2O 3) 1− x (Pr 6O 11) x systems from room temperature to 550°C. At x=0.05, the rhombohedral Bi 1.35Pr 0.65O 3 structure was found to be stable up to 550°C. At higher concentrations, a mixture of rhombohedral and possibly monoclinic type structures were determined, which at x=0.2 became fully “monoclinic”. High temperature X-ray powder diffraction confirmed the stability of the observed phases over the temperature range. The optimum ionic conductivity was found for the mixed phase and a value of 1.23×10 −2s/cm was ascertained for x=0.15 at 450°C.

Anionic conduction properties of BiCaPb mixed oxides

This study of the Bi 2O 3CaOPbO system has determined the equilibrium domains of existence of three solid solutions which belong respectively to the fcc (δ-Bi 2O 3), rhombohedral (BiSrO) and bcc (anti-α-AgI) structural types. The first two solid solutions can be preserved at room temperature in a metastable or/and a stable form within a significant range of composition. They have been investigated by impedance spectroscopy and transport number measurements. In all cases, the substitution of Bi 3+ by the divalent cations leads to a decrease of the conductivity. However, for a constant M 2+ concentration, the conductivity increases slightly with the Pb 2+/Ca 2+ ratio without significant modification of the activation energy. Transport number measurements, performed particularly on the δ-phase, have shown, at low temperature (≤400 °C), a very attractive quick response to any P O2 variation. The corresponding Nernst potentials were shown to be significantly higher than expected from the classical O 2/O 2- electrode equilibrium.

Evidence for magnetic-field-dependent electric-field gradients in Bi from muon-spin-relaxation measurements.

The field dependence of the anisotropic second moment of the nuclear dipolar field distribution in rhombohedral Bi at 11 K observed by muon-spin relaxation in a single-crystal sample is in complete contrast to the results of well established calculations. A phenomenological model of an electric field gradient rotated by the external magnetic field allows description of the data; its physical justification, however, is not clear.

Multilayered structure and anisotropic electrical properties of (SiO2)100−x(Pb0.6Bi0.4)x alloys prepared by a combined technique of mechanical alloying and hot pressing

Compacted (SiO2)100−x(Pb0.6Bi0.4)x (x=20–80 at. %) materials, prepared by mechanical alloying (MA) of amorphous SiO2, fcc Pb, and rhombohedral hexagonal Bi followed by hot pressing of the MA powder at 473 K for 1.8 ks under an applied stress of 250 MPa, were found to have a unique layered structure consisting of amorphous SiO2 and hexagonal close-packed (hcp) ε(Pb-Bi) phases which developed along the direction perpendicular to the uniaxial applied load. The thickness and interlayer distance of the layered ε (Pb-Bi) phase are about 1.5 and 7 μm, respectively, for (SiO2)50Pb30Bi20. Furthermore, the multilayered compacts exhibited a strong anisotropy of electrical resistivity and superconducting properties when their properties are measured along the directions parallel or perpendicular to the layer. Electrical resistivity at 300 K, superconducting critical temperature (Tc), and upper critical magnetic field (Hc2) at 4.2 K are 10.5 μΩ m, 8.5 K, and 2.29 T, respectively, for the parallel direction and 950 μΩ m, 6.9 K, and 0.24 T, respectively, for the perpendicular direction. The strong anisotropy of electrical resistivity and superconducting properties is presumably due to a significant difference in the linkage of electrical path between the parallel and perpendicular directions.

Higher-order angular dependence of the positive-muon Knight shift in bismuth.

The ${\ensuremath{\mu}}^{+}$ Knight shift in rhombohedral Bi below 10 K is found to exhibit an anisotropy of solely fourth order in the direction cosines, or explicitly a ${sin}^{2}(2\ensuremath{\theta})$ dependence, where $\ensuremath{\theta}$ is the angle between the $c$ axis and the applied magnetic field. The implications of this dependence, allowed by symmetry arguments but never seen before in NMR studies, are discussed. In addition, the lattice site of the ${\ensuremath{\mu}}^{+}$ below 10 K has been determined and a local lattice contraction of approximately 11% was observed.

Structural investigations on bismuth-based mixed oxides

The structures of the low-temperature modification of the rhombohedral solid solutions Bi 1−x M x O 1.5 x 2 with M=Ca, Sr or Ba ( x=0.130, 0.235 and 0.156 respectively) have been solved by means of Fourier syntheses and least-squares refinement. Layers of cations are stacked along the c axis, with the following sequence … BiBi(M, Bi)BiBi(M, Bi)…. Two kinds of oxide ions were detected inside the blocks consisting of a mixed layer surrounded by two Bi layers. The conduction properties can be ascribed to the migration of the remaining anions in the space between two successive Bi layers.

Elastic energy, stability, and interaction of slip dislocations and the choice of slip systems in rhombohedral crystals, Bi, Sb, As, and Hg

Elastic energy, dislocation parameters, reaction energy, and stability of dislocations are analyzed by the theory of anisotropic elasticity, and the relation between the choice of slip systems and the dislocation energy is considered on the basis of the criterion of minimum dislocation energy (C.M.D.E.). The choice of slip planes for Bi, Sb, and As depends on the number of covalent bonds (density of covalent bonds ϱB) contained in a slip-interplanar spacing, and the easiest slip plane is determined as a plane with the smallest value of ϱB. On the other hand, the slip directions for these crystals can be determined by C.M.D.E. The slip systems of Hg are fairly well explained by C.M.D.E. Stability of dislocations is examined by the inverse Wulff plot method, and it is found that the following dislocations have unstable regions in certain ranges of the dislocation line: the {110} 〈110〉, {111} 〈110〉, {111} 〈110〉, {001} 〈110〉, and {001} 〈110〉 systems in As, the {111} 〈110〉 and {001} 〈110〉 systems in Hg. Es werden die Elastizitätsenergie, die Versetzungsparameter, die Reaktionsenergie und die Stabilität von verschiedenen Versetzungen mit der anisotropen Elastizitätstheorie analysiert, und der Zusammenhang zwischen der Gleitsystemwahl und der Versetzungsenergie wird aufgrund der Annahme minimaler Versetzungsenergie diskutiert. Die Wahl der Gleitebene im Bi-, Sb- sowie As-Kristall ist von der in der jeweiligen Gleitebene enthaltenen Kovalenzbindungszahl (Kovalenzbindungs-Dichte ϱB) abhängig und die Hauptgleitebene ist diejenige minimaler ϱB. Die Wahl der Gleitrichtung wird durch die minimale Versetzungsenergie bestimmt. Das Gleitsystem von Hg wird mit der Annahme minimaler Versetzungsenergie zufriedenstellend erklärt. Die Untersuchung der Versetzungsstabilität mit Hilfe der inversen Wulff-Darstellung zeigt, daß für folgende Gleitsystemversetzungen ein instabiles Orientierungsgebiet vorliegt: {110} 〈110〉-, {111} 〈110〉-, {111} 〈110〉-, {001} 〈110〉- und {001} 〈110〉-Systeme in As, {111} 〈110〉- und {001} 〈110〉-Systeme in Hg.

Reflectivity Spectra of the Rhombohedral Crystals Bi2Te3, Bi2Se3, and Sb2Te3 over the Range from 0.7 to 12.5 eV

AbstractThe reflectivity spectra (polarisation E ⟂ c) of the homologous group of rhombohedral crystals Bi2Te3, Bi2Se3, and Sb2Te3 have been investigated in the range 0.7 to 12.5 eV. The complex structure of the two reflectivity main maxima, lying in the range 1 to 5 and 6 to 11 eV, respectively, and its similarity in all three crystals have been established. The energy variation of the reflectivity peaks in dependence of the atomic composition shows the anomalous decrease of the interband gaps in Sb2Te3 respective to Bi2Te3. The results obtained are discussed in terms of published bismuth telluride band structure.