Showing papers on "Lattice constant published in 1987"

Single-phase 60-K bulk superconductor in annealed Ba2YCu

Abstract: Employing a gettered annealing technique, we have prepared homogeneous polycrystalline samples of oxygen deficient Ba/sub 2/YCu/sub 3/O/sub 7-//sub delta/ for 0less than or equal todeltaless than or equal to0.7. Measurements of resistive T/sub c/, resistivity, magnetization, and lattice parameter indicate that a distinct bulk superconducting phase with a T/sub c/ of 60 K occurs in the range of 0.3

Structure of the single‐phase high‐temperature superconductor YBa2Cu3O7−δ

Abstract: We have determined the crystal structure of the single‐phase stoichiometric high‐temperature superconductor in the Y‐Ba‐Cu‐O system using high‐resolution neutron powder diffraction. This compound has an orthorhombic structure with space group Pmmm and lattice constants a=3.8231 A, b=3.8864 A, and c=11.6807 A. The structure consists of ‘‘dimpled’’ CuO2 layers in the a‐b planes loosely bonded by one‐dimensional fencelike square‐planar CuO3 chains in the b‐c planes.

Synthesis and Equation of State of (Mg,Fe) SiO3 Perovskite to Over 100 Gigapascals.

Abstract: Silicate perovskite of composition (Mg0.88Fe0.12) SiO3 has been synthesized in a laser-heated diamond-anvil cell to a pressure of 127 gigapascals at temperatures exceeding 2000 K. The perovskite phase was identified and its unit-cell dimensions measured by in situ x-ray diffraction at elevated pressure and room temperature. An analysis of these data yields the first high-precision equation of state for this mineral, with values of the zero-pressure isothermal bulk modulus and its pressure derivative being K0T = 266 ± 6 gigapascals and K90T = 3.9 ± 0.4. In addition, the orthorhombic distortion of the silicate-perovskite structure away from ideal cubic symmetry remains constant with pressure: the lattice parameter ratios are b/a = 1.032 ± 0.002 and c/a = 1.444 ± 0.006. These results, which prove that silicate perovskite is stable to ultrahigh pressures, demonstrate that perovskite can exist throughout the pressure range of the lower mantle and that it is therefore likely to be the most abundant mineral in Earth.

Elastic state and thermodynamical properties of inhomogeneous epitaxial layers: Application to immiscible III‐V alloys

Abstract: The elastic strain and stress fields and the elastic energy of the system composed of a crystalline epitaxial layer of finite thickness coherently grown on a bulk substrate are calculated, when the intrinsic stress‐free lattice parameter of the layer is modulated along directions parallel to the substrate surface. When the modulation has components with spatial periods of the same order as the thickness of the layer, the elastic energy is considerably reduced with respect to the same modulation occurring in a bulk sample. There exists an optimal period of elementary sinusoidal modulation, proportional to the layer thickness. Consequently, for immiscible alloys where changes of composition induce changes of intrinsic lattice parameter, the critical temperature (below which they become thermodynamically unstable with respect to composition modulations) is much higher (and the domain of instability larger) if the material is in the epitaxial layer form than in the bulk form. This extends Cahn’s theory of spinodal decomposition to epitaxial layers. It is also pointed out that if a modulation has started to occur in a growing layer, the elastic deformation induced near the free surface should have important consequences on the subsequent growth of this layer. These results are applied to III‐V semiconductors immiscible alloys, such as In x Ga1−x As y P1−y , where such composition modulations are known to exist. New values of the critical temperatures for these alloys are calculated and compared with lower former estimates. The mode of development of these modulations is discussed in light of previous experimental results and these new calculations.

Crystal Structure of the Tetragonal Form of Ba2YCu3O7-x

Abstract: The crystal structure of the high-temperature form of Ba2YCu3O7-x was refined by the Rietveld analysis of TOF neutron powder diffraction data taken at 49 K. This compound is tetragonal with lattice constants a = 3.8533 A and c = 11.7631 A. It has an oxygen-deficient triperovskite structure, in which oxygen atoms on a z = 1/2 plane are missing, an oxygen site at (0, 1/2, 0) is about 9% occupied, and Ba2+ and Y3+ ions are completely ordered in a [Ba-Y-Ba]∞, sequence along the c axis.

Ab initio pseudopotential study of structural and high-pressure properties of SiC

Abstract: An ab initio pseudopotential method is used to study the structural properties of SiC at low and high pressures. For cubic SiC, the calculated ground-state properties, such as the equilibrium lattice constant, bulk modulus, and cohesive energy, are in good agreement with experiment. The indirect band gap of cubic SiC is predicted to decrease with increasing pressure; this behavior is similar to Si but not to C. To study the structural phase stability at high pressures, the rocksalt and tetragonal \ensuremath{\beta}-Sn phases are considered. Our calculations suggest that the zinc-blende structure of SiC transforms into the rocksalt phase under a hydrostatic pressure of 660\ifmmode\pm\else\textpm\fi{}50 kbar. The calculated value should be regarded as an upper bound for the pressure of a structural phase transition from the zinc blende. The valence-charge distribution for cubic SiC resembles those for other zinc-blende semiconductors.

Elastic scattering from cubic lattice systems with paracrystalline distortion.

Abstract: The three-dimensional paracrystalline lattice factors Z(q)'s for a face-centered-cubic lattice and a body-centered-cubic lattice, in addition to a simple-cubic lattice, were calculated. The Z(q)'s thus calculated could reproduce the diffraction peaks at the characteristic positions for each cubic lattice system over a fairly wide range of g values (g is the degree of the paracrystalline distortion). In addition to the paracrystalline distortion, the thermal oscillation and crystal size effects were also considered. The numerically calculated profiles were compared with light and small-angle neutron scattering curves for polymer latex dispersions and with small-angle x-ray scattering curves for block copolymer films for quantitative determination of the ordered cubic systems.

Role of x-ray diffraction analysis in surface engineering: investigation of microstructure of nitrided iron and steels

Abstract: X-ray diffraction analysis can be very usefully applied in the field of surface engineering, since a depth of a few micrometres only is usually probed. This paper provides an overview of current X-ray diffraction methods for the analysis of surface layers. The treatment is illustrated by examples taken from recent work on nitrided iron and steels. Distinct ways to characterize the effective depth and layer thickness probed are presented. Composition-depth profiles can be measured accurately by tracing the lattice parameter (taking into account the possible presence of stresses) as a function of depth below the surface by successive sublayer removals. A correction for the penetration effect is required and methods for that are presented and demonstrated. The basis of the determination of (macro) stress (the sin2ψ method) is indicated. Attention is paid to the determination of the strain free lattice spacing and (diffraction) elastic constants, to the effect of a concentration-depth profile, and to ...

Preparation of Co-N films by rf-sputtering

Abstract: Co-N films in the wide compositional range can be prepared by reactive sputtering. Co-N sputtered films consist of one or two phases, such as CoN, Co2N, Co3N, Co4N and α-Co. Co4N phase with a cubic unit cell is observed, and its lattice constant isa = 0.3586 nm. The preferred orientation is observed on the Co-N films, CoN (200) plane, Co4N (1 1 1) plane and α-Co (002) plane parallel to the film surface, respectively. Saturation magnetization σs of Co-N sputtered film decreases from 160 to 1.7 e.m.u. g−1 with increasing content of N from 0 to 21.7 at%, and coercive forceI H c is the range of 43 to 5000e at room temperature.

A relativistic treatment of interacting spin-aligned electron systems: application to ferromagnetic iron, nickel and palladium metal

Abstract: The authors use a relativistically extended version of the one-particle formalism. The electron-electron interaction is assumed to be non-relativistic Coulombic as before. As a consequence, the one-particle Dirac-type equations which now stand in place of the formerly employed Kohn-Sham type equations, contain a diagonal 4*4 matrix whose elements represent the various potential contributions including spin-dependent local potentials describing exchange and correlation as in the non-relativistic case. If asphericity effects of the resulting potential are negligible, these Dirac equations can exactly be reduced to relativistic two-component Pauli-type equations with a diagonal exchange-correlation matrix and a spin-orbit coupling matrix. The latter equations are used to self-consistently calculate the electronic structure of nickel, iron and palladium metal. In addition, the calculation provides magnetic anisotropy energies. The band structure of iron is discussed in detail and compared with other theoretical studies based on different band theoretical methods. The calculations on Pd metal lead without spin-orbit coupling to ferromagnetic spin alignment when the metal is expanded by approximately 5% of the lattice constant. If spin-orbit coupling is included in the self-consistent calculations, the alignment disappears. This effect can only be counteracted by further expanding the lattice up to approximately 10%.

The Effect of Metallurgical Variables on the Electrocatalytic Properties of PtCr Alloys

Abstract: In this investigation, the effect of alloying on catalysis, especially with respect to oxygen and the ORR, was examined. A series of PtCr alloys allowed four major aspects of alloying to be investigated: (i) effect of decreasing lattice parameter with increasing Cr content in the Pt terminal solid solution, (ii) effect of an order/disorder transition, (iii) difference between intermetallic and solid solution structures, and (iv) effect of dendritic segregation. Results indicated that contrary to the findings of previous investigators, decreased lattice spacing did not correlate with an increase in ORR activity. Atomic ordering in low Cr content specimens, however, was found to generally increase catalysis. Results also indicated that disordered structures interact more strongly with impurities than the ordered structures. ORR Tafel slopes, however, were unaffected by atomic ordering although additional chromium, up to at least 35 atom percent (a/o), caused a decrease in Tafel slope apparently due to oxide reduction effects. At higher Cr concentrations, catalysis decreased due to the passive nature of chromium oxide. The change in crystal structure which occurs for the 80 a/o Cr sample and the dendritic structure, had little effect on most of the electrochemical parameters studied relative simply to the additional Cr content.more » The results of this work also indicated that, in general, no alloy in the PtCr system is more catalytic than pure Pt. This result is in contrast to gas diffusion cell studies which have shown significant increases in catalysis due to PtCr alloying.« less

The cell envelope of Thermoproteus tenax: three-dimensional structure of the surface layer and its role in shape maintenance.

Abstract: The sulphur-dependent archaebacterium Thermoproteus tenax has a cylindrical cell shape variable in length, but constant in diameter. Its whole surface is covered by a regular protein layer (S-layer). The lattice has p6 symmetry and a lattice constant of 32.8 nm. The three-dimensional reconstruction from a tilt series of isolated and negatively stained S-layer shows a complex mass distribution of the protein: a prominent, pillar-shaped protrusion is located at the 6-fold crystallographic axis with radiating arms connecting neighbouring hexamers in the vicinity of the 3-fold axis. The base vectors of the S-layer lattice have a preferred orientation with respect to the longitudinal axis of the cell. The layer can be seen as a helical structure consisting of a right-handed, two-stranded helix, with the individual chains running parallel. Supposing that new S-layer protein is inserted at lattice faults (wedge disclinations) near the poles, growing of the layer would then proceed by moving a disclination at the end of the helix. The constant shape of the cell, as well as the particular structure of the layer, strongly suggest that this S-layer has a shape-maintaining function.

Electronic structure and properties of NiSi2 and CoSi2 in the fluorite and adamantane structures.

Abstract: An electronic-band-structure study of ${\mathrm{NiSi}}_{2}$ and ${\mathrm{CoSi}}_{2}$ is performed by means of the linear-muffin-tin-orbital method for both the fluorite and the hypothetical adamantane structures, i.e., silicon with tetrahedral interstitial metal atoms. Energy bands along symmetry lines, densities of states, charge densities, and total energies are presented. In addition, the equilibrium lattice constant, bulk modulus, and cohesive energies are obtained from total-energy and pressure calculations as a function of volume. Special attention is given to the relative stability of both structures. The experimentally observed fluorite structure is found to be lower in energy by more than 1 eV in both cases. The total-energy difference is analyzed and discussed in terms of the electronic structure. The electrostatic energy due to the charge transfer is found to play a significant role, as well as the tendency to form Ni\char22{}Si covalent bonds in the fluorite case.

Elastic constants of crystals from linear-response theory.

Abstract: We propose a new scheme to calculate the elastic constants of solids which is based on linear-response theory. Elastic constants are given directly by a single self-consistent calculation (i.e., they are not obtained by numerical differentiation of total energies or stresses). As an illustration, we apply our procedure to the determination of the equilibrium lattice constant, bulk modulus, and pressure derivative of the bulk modulus of silicon.

Depth profiling of elastic strains in lattice-mismatched semiconductor heterostructures and strained-layer superlattices.

Abstract: The elastic strains in regions near the top surface of strained-layer structures can be quite different from those in deeper portions of the samples. This effect has been established in epitaxial layers of ZnSe grown on GaAs and ZnSe-$\mathrm{Zn}{\mathrm{S}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}$ strained-layer superlattices. The depth dependence of the strains was determined with Raman scattering measurements performed under laser excitation below and above the band gap. The strain values near the top surface are driven by a stronger relaxation of the in-plane lattice constants towards equilibrium. This relaxation appears to be independent of the generation of misfit dislocations.

Effect of energetic neutralized noble gas ions on the structure of ion beam sputtered thin metal films

Abstract: Energetic ion bombardment of a sputtering target often results in a significant fraction of incident ions being reflected as energetic neutrals in the direction of the growing film. Experiments are described which test the degree to which bombardment by these energetic backscattered neutrals can influence the microstructure of a growing film. As anticipated, microstructural changes of the film are influenced by the reflection coefficient and energy exchange of a given energetic gas ion at the sputtering target which in turn depends on the mass ratio of the sputtering gas to that of the target. Whereas the sputtered metal species leave the target with a near cosine distribution, evidence for a strong noncosine reflection preference in the forward direction for the energetic backscattered neutrals will be given. Very careful x‐ray diffraction studies show that films grown in a Ne/Pd system give rise to lattice dilation in the (111) lattice planes resulting in a 1% increase in the lattice parameter over Pd b...

Epitaxial growth of a metastable modification of copper with body-centered-cubic structure.

Abstract: Slow deposition of Cu onto a clean unheated Fe{001} surface produced films with large amounts of defects, but still detectable crystallinity. Low-energy electron diffraction showed that the films contained regions with a (somewhat distorted) body-centered-cubic structure and the same or nearly the same lattice parameter as the underlying substrate. Auger-electron spectroscopy suggested that the crystalline regions were essentially pure Cu, thus allowing the conclusion that a metastable bcc modification of Cu has been realized, consistent with the predictions of total-energy band calculations reported elsewhere.

Structural development during mechanical alloying of crystalline niobium and tin powders

Abstract: The structural development with milling time during mechanical alloying of niobium and tin powders, of average composition Nb3Sn, was followed by x‐ray diffraction. The elemental powders initially alloy mechanically to form an A15 structure phase. With continued milling, the A15 phase transforms to an amorphous structure. The kinetics of the structure changes are dependent on the milling media and atmosphere. The measured lattice parameter a0 for the A15 phase prepared with tungsten carbide milling media in an argon atmosphere agrees with the literature value of a0 for bulk Nb3Sn. However, milling with steel media introduced significant contamination by iron and an anamolous expansion of the A15 phase lattice. Amorphization of the A15 phase is believed to be due to the creation of a critical defect concentration by the mechanical deformation.

III-V Compound heteroepitaxial 3-D semiconductor structures utilizing superlattices

Abstract: A component of semiconductor material deposited by epitaxial growth on a substrate having a predetermined and different lattice parameter consists of an alternate succession of layers of a first type and layers of a second type deposited on the substrate. The lattice parameter of the first type of layers is substantially matched with the lattice parameter of the substrate. In the case of the second type of layers, the lattice parameter is matched and even equal to that of the first type of layers. A component having a lattice parameter equal to that of the second type of layers is formed on the last layer of the second type. Moreover, the energy gaps of the two types of layers are different.

Crystal Structure of the Orthorhombic Form of Ba2YCu3O7-x at 42 K

Abstract: The crystal structure of the low-temperature form of Ba2YCu3O7-x at 42 K was refined by the Rietveld analysis of TOF neutron powder diffraction data. This material is orthorhombic with space group Pmmm and lattice constants a=3.8800 A, b=3.8122 A, and c=11.6264 A. It has an oxygen-deficient triperovskite structure in which oxygen atomson a z=1/2 plane are lacking completely, two oxygen sites on a z=0 plane are partially occupied, and Ba2+ and Y3+ ions are ordered in a [Ba-Y-Ba]∞ sequence along the c axis. The occupation factors of oxygen at (1/2, 0, 0) and (0, 1/2, 0) are 0.84 and 0.05, respectively.

Structural features of the hexagonal phase of phasmidic liquid crystals

Abstract: Eighteen compounds having the same molecular architecture, a rod-like aromatic core with four to six long aliphatic terminal groups, which all possess a hexagonal mesophase, have been studied by X-ray diffraction. From the lattice constants of these mesophases, we have deduced some information about the molecular organization. We discuss the curvature and the density of the interface between the aromatic and the paraffinic moieties, the orientational disorder and the packing of the rod-like central parts of the molecules. The occurrence of smectic, oblique or cubic mesophases in the same series underlines the similarities with lyotropic polymorphism and justifies a description of the molecular organization in terms of interfaces.

Substitution Effect of Sr for Ba of High-Tc Superconducting YBa2Cu3O7-y Ceramics

Abstract: Single phase Y(Ba1-xSrx)2Cu3O7-y [x=0, 0.1, 0.2, 0.3, 0.4] ceramics were fabricated. In the fracture surface of the ceramics, many prism crystals were observed. Superconducting transition temperature measured by the magnetic susceptibility was linearly lowered from 94 to 84 K with increasing x from 0 to 0.4 and all the lattice constants of orthorhombic unit cell became short.