Showing papers on "Lattice constant published in 1974"

Structure of vapor‐deposited GaxIn1−xAs crystals

Abstract: Deformation of the crystal lattice of layers of GaxIn1−xAs (0.5 < x < 1.0) grown onto GaAs substrates by a chemical vapor deposition process was investigated. For this study, Ga and In were transported as chlorides with HCl and AsH3 was the source of As. It was observed that the lattice constant along the direction of the film thickness was larger than that along the surface. This fact indicates that the crystal lattice of GaxIn1−xAs is compressed along the substrate surface as a result of lattice mismatch. The influence of both the composition of the epitaxial layers and the crystal plane of the substrate surface on the degree of this deformation was investigated. It was also found that the epitaxial orientation grew inclined to the substrate crystal when the crystal surface of the substrate was {100} or {110}.

Bandgap and lattice constant of GaInAsP as a function of alloy composition

Abstract: Published data for the composition dependence of the room-temperature bandgap (Eg) and lattice constant (ao) in the pseudobinary GayIn1-yAs, GayIn1-yP, GaAsxPl-x, and InAsxPl-x systems have been used to derive the following equations for the quaternary GayInl-yAsx Pl-x, alloys: $$\begin{gathered} a_o ({\AA}) = 5.87 + 0.18x - 0.42y + 0.02xy \hfill \\ E_g (eV) = 1.35 - x + 1.4y - 0.33xy - (0.758 - 0.28x)y(1 - y) \hfill \\ - (0.101 + 0.109y) x(1 - x). \hfill \\ \end{gathered} $$

Stress Corrosion and Hydrogen Embrittlement in an Aluminum Alloy

Abstract: A strain aging type of reversible hydrogen embrittlement has been demonstrated for a high strength aluminum alloy in a 3% NaCl environment. Permeation of hydrogen was related to SCC under cathodic polarization as well as anodic conditions. Internal friction and lattice parameter measurements relate directly to lattice dissolved hydrogen arising from the environment. It is concluded that the evidence is quite consistent with a mechanism of stress corrosion cracking (SCC) involving absorption of hydrogen leading to lattice dissolved hydrogen and embrittlement.

Isothermal compression of magnetite to 320 KB

Abstract: The effect of pressure on the lattice parameter of magnetite has been determined at room temperature up to 320 kbar by means of X ray diffraction employing a diamond anvil high-pressure cell. By using the Birch-Murnaghan equation with a (∂KT/∂P)T∣P=0 value of 4 ± 0.4 the isothermal bulk modulus at zero pressure was calculated to be 1.83 ± 0.10 Mbar. The X ray diffraction study also revealed that at pressures greater than 250 kbar, magnetite transforms to a high-pressure phase, which reverts to magnetite at pressures below 50 kbar. It is possible to index this high-pressure phase as being monoclinic. If it is assumed that the unit cell contains two molecules, the density is in agreement with the density predicted for a phase of Fe3O4 having all of the iron atoms in sixfold coordination.

Volume and surface photoemission from tungsten. I. Calculation of band structure and emission spectra

Abstract: The electronic energy-band structure of tungsten has been calculated by means of the relativistic-augmented-plane-wave method. A series of mutually related potentials are constructed by varying the electronic configuration and the amount of Slater exchange included. The best band structure is obtained from an ad hoc potential based on a Dirac-Slater atomic calculation for the ground-state configuration and with full Slater exchange in the atomic as well as in the crystal potential. The selection of this best potential is justified by comparing the calculated band structure to Fermi-surface experiments and to optical-reflectance measurements up to 5-eV photon energy. The temperature and strain responses in the band structure are estimated from band calculations with four different lattice constants. The band structure was determined in the entire Brillouin zone and is applied to a calculation of photoemission spectra from W single crystals. The nondirect as well as the direct models for bulk photoemission processes are investigated. The emission from the three low-index surfaces (100), (110), and (111) exhibits strong dependence on direction and acceptance cone. According to the present band model there should essentially be no emission normal to the (110) face for photon energies between 9.4 and 10.6 eV. Experimental observation of emission in this gap, however, implies effects not included in the simple bulk models. In particular, effects arising from surface emission have been considered, i.e., emission of those electrons which are excited in a single-step process from initial states near the surface to final states outside the crystal. The electrons that are emitted from the surface in directions perpendicular to the crystal planes carry information on the one-dimensional surface density of states. The present work includes a crude estimate of this surface density of states, which is derived from the bulk band structure by narrowing the $d$ bands according to an effective number of neighbors per surface atom. Estimates of surface relaxation effects are also included.

Lattice thermal conductivity of minerals at high temperatures

Abstract: Simple perturbation theory predicts that the lattice thermal conductivity should be inversely proportional to the absolute temperature. It is shown that the lattice thermal conductivity of minerals should not depart markedly from that variation, even when the average phonon mean free path is comparable to the lattice spacing.

Band narrowing and charge transfer in six intermetallic compounds

Abstract: Self-consistent band calculations on six intermetallic compounds of the CsCl structure and on constituent bcc solids with the same lattice spacing show $3d$-band narrowing in the compounds and charge transfer between the compound constituents. The $d$-band narrowing in the compounds results from a lack of $d$ states on neighboring atoms into which electron tunneling can occur. In the case of compounds with two $d$-band constituents, we find an additional $d$-band narrowing due to mutual $d$-band repulsion. Charge transfers, found by a direct comparison of the charge densities in the compound and in the elemental solid, are always from the element with the higher Fermi energy to the element with the lower Fermi energy.

Piezoelectric nonlinear optic CuGaSe2 and CdGeAs2: Crystal structure, chalcopyrite microhardness, and sublattice distortion

Abstract: CuGaSe2 and CdGeAs2 crystallize in the chalcopyrite‐type structure. The lattice constants of tetragonal CuGaSe2 are a = 5.5963 ± 0.0001 and c = 11.0036 ± 0.0002 A at 298 °K, those of CdGeAs2 are a = 5.9432 ± 0.0001 and c = 11.2163 ± 0.0003 A at 298°K. The space group is I42d with four formulas in a unit cell. 1747 reflections were measured with PEXRAD for CuGaSe2 and 1892 for CdGeAs2 using MoKα x radiation. Least squares refinement of the structural parameters using 149 symmetry‐independent CuGaSe2 and 300 CdGeAs2 structure factors led to final agreement factors of 0.035 and 0.038, respectively. CuGaSe2 is slightly Cu deficient: the value of the x coordinate in the final model is 0.2431 ± 0.0002, with considerable vibrational anisotropy. CdGeAs2 is stoichiometric: the final model has x = 0.2785 ± 0.0002, also with significant vibrational anisotropy, but less than that in CuGaSe2. The two compounds confirm the pattern of sublattice distortion in AIBIIIC2VI and AIIBIVC2V chalcopyrites previously found. The...

Lattice parameter study of silicon uniformly doped with boron and phosphorus

Abstract: Powder and single-crystal X-ray techniques have been employed to obtain precise lattice parameters of silicon uniformly doped with boron or phosphorus. Good agreement is found between the two methods. Previous accurate determination of the CuKα1, effective wavelength has yielded λ=1.540621±0.000006 A. Particular care has been devoted to the chemical and electrical characterization of the alloys, whose maximum dopant concentrations were 8×1019 atoms cm−3 for P and 4.4×1020 atoms cm−3 for B.

X-Ray investigations of the crystal structure and phase transitions of YMnO3

Abstract: Lattice constants and atomic parameters of YMnO3 were investigated from room temperature up to 1010°C. Lattice constant c decreases slowly with temperature, contrary to earlier publication. With rising temperature small changes of the crystal structure were observed. The paraelectric phase is probably disordered and the phase transition is rather diffuse. At 1000°C we found a new phase transition resulting in the crystal structure with a small hexagonal lattice cell with a = 3.61 A, c = 11.39 A, Z = 2, space group P63/mmc D4 6hAtomic parameters of this high temperature crystal structure are presented.

An x‐ray, spectroscopic, and magnetic study of the structure of nickel squarate dihydrate, NiC4O4 · 2H2O

Abstract: The structure of nickel squarate dihydrate, NiC4O4 · 2H2O, has been investigated by initial susceptibility measurements, infrared spectroscopy, and single crystal x‐ray diffraction techniques The results indicate that the structure consists of Ni ions situated at the centers of the edges of a cube, with squarate ions in the cube faces Each nickel is roughly octahedrally coordinated by four squarate oxygens, and by two water oxygens The lattice constants were found to be a = b = c =80685A, with α = β = γ = 90° within experimental accuracy The structure was found to be noncubic A spin‐orbit coupling parameter of −280 (30) cm−1, which is 86% of the free ion value, is obtained from the magnetic and spectroscopic results The structure of this compound, which is isostructural with the Mn(II), Fe(II), and CO(II) squarates, is different than that proposed by two other sets of workers All crystals investigated were found to be macroscopically twinned, and a reason for such twinning is proposed

New compound in the iron-zinc system

Abstract: A new compd. (G1) consisting of 18.5-23.5 at. % Fe and the rest Zn was found in the Fe-Zn system at 380 Deg by the diffusion-couple technique. The lattice of this compd. is related to that of the G phase and its lattice parameter (17.963 .ANG.) is obtained by doubling that of the bcc. unit cell of the G phase. But the new cell is fcc. The homogeneity range of G1 decreases with increasing temp. whereas the reverse was obsd. for the G phase. Based on these facts, a modified phase diagram is proposed. [on SciFinder (R)]

The lattice parameters of La, Ce, Pr, Nd, Sm, Eu and Yb

Abstract: Lattice parameters of La, Ce (β and γ), Pr, Nd, Sm, Eu and Yb were determined on well-characterized, high-purity metals in the form of small-grained wires. The effect of sample preparation on the lattice parameters of these metals is discussed. A method to obtain crystallographically-pure polycrystalline d.h.c.p. lanthanum is presented.

Dislocations in vapor phase epitaxial GaP

Abstract: Dislocations in VPE GaP grown on (100) oriented LEC GaP substrates have been characterized, and their origins and effects on LED performance have been investigated. In non-nitrogen doped epilayers, the dislocations are found to originate in the substrate and propagate through the epilayers in straight lines in [100] and directions. The dislocation density of the epilayer is found to be nearly equal to that of the substrate. Introduction of nitrogen during growth of the epilayer has been observed to bend these so-called “inclined≓ dislocations propagating through the layer into [0−1 1] directions in the (100) plane and thus produces segments of [0 −1 1] dislocations to relieve the lattice parameter mismatch due to N. The mismatch dislocation density is observed to be proportional to the N doping level. At very high N doping levels, > 1019 cm-3, a large number of new inclined dislocations are observed, which may be in part due to GaN precipitation. The effects of dislocations on LED properties were investigated by measuring dislocation densities in the individual diodes using the electron beam induced current mode of the SEM and comparing this with the spot brightness and luminous flux. The dislocations were observed to produce dark spots in the EL emission in many cases. For a series of runs where all growth and processing parameters were fixed, a good correlation between B/J and dislocation density was observed with B/J decreasing with increasing dislocation density in the range < 1 × 104 cm−2 to 1 × 106 cm−2.

Superconductivity in new A-15 niobium alloys

Abstract: A number of previously unreported A-15 Nb alloys have been studied, using samples prepared by dual target sputtering.T c was measured resistively and structure determined by x-ray diffraction. Samples were “chemically” analyzed by anodization. New alloys include A-15 Nb3Si and pseudobinary derivatives, as well as some derivatives of “Nb3P.”T c and the A-15 lattice parameter were surveyed for these, and for previously studied alloys related to Nb3Al, Nb3Ga, and Nb3Ge. The “chemical” variation ofT c was slight, except near pure Nb3Si and for alloys based upon “Nb3P,” where the samples show considerable disorder. Maxima inT c occur for the alloys Nb3Al0.7Ge0.3 and Nb3Al0.5Ga0.5. The first is well known; trends measured here indicate that the second may be equally high. It is proposed that the dominant factor forT c in this group of alloys is the state of order achieved in the samples. No evidence was seen to associate enhancedT c with A-15 phase boundaries.

Equilibrium vacancy concentration measurements on aluminum

Abstract: Thermal expansion on high purity aluminum has been measured to study defects in thermal equilibrium by the (ΔL/L 0−Δa/a 0) method. Measurements of the changes in macroscopic length ΔL and lattice parameter Δa were made from room temperature to 637° C. Length changes were measured by a laser interferometer and lattice parameter changes by a X-ray diffractometer technique on the same crystal at identical temperatures. At temperatures above 400° C ΔL/L 0 becomes greater than Δa/a 0, indicating the generation of a noticeable amount of new lattice sites due to vacancy formation. Extrapolation gives a vacancy concentration ΔN/N 0=9.8·10−4 at the melting point (660° C). The experimental findings can be explained by assuming formation of monovacancies and divacancies. Values for the enthalpy and entropy of formation for mono- and divacancies are given.

The crystal structure and lattice constants of RE3Pd4, Y3Pd4 and Th3Pd4 compounds

Abstract: Alloys of the rare earths (R.E.), yttrium, thorium, hafnium and uranium with palladium of the M3Pd4 composition have been studied. For this composition, with the exception of Eu, U and Hf, single phases are formed, crystallizing with the Pu3Pd4-type of structure. The stability of these compounds appears to be restricted to the radius ratio ( R M R Pd ) range between 1.20 and 1.37.

The rare-earth arsenides: Non-stoichiometry in the rocksalt phases

Abstract: The rare-earth arsenides with the rocksalt structure are non-stoichiometric compounds. Both anion and cation lattice sites are incompletely occupied. A range of arsenic solubility is observed in all cases, and in samarium arsenide at 700 °C, it extends from Sm 0.98 As 0.81 to Sm 0.98 As 0.98 . Over part of the solubility range at the arsenic-rich end, variations in arsenic content can occur with no detectable change in lattice parameter. Further decrease in arsenic content can occur at 700 °C in all but Y, Ho, Er and Yb arsenides, and is accompanied by a reduction in the lattice parameter. A range of metal solubility exists at high temperatures for all phases and results in considerable reduction of lattice parameter at the lower metal concentrations. There is no evidence for vacancy ordering.

Organisation des cations assurant la conduction dans l'alumine β

Abstract: The organization of the conducting ions is studied by means of X-ray diffuse scattering as a function of temperature. While at high temperature (750°K) the conducting ions are organized in a two-dimensional 'quasi-liquid' structure perpendicular to the c axis, at low temperature (77°K) they form micro-domains corresponding to a two-dimensional superstructure with a lattice constant of a√3 = 9.68 A (a = lattice constant of β alumina). The size of these micro-domains, which is about 50 A at 77 °K, decreases progressively with increasing temperature.

Epitaxial indium-gallium-arsenide phosphide layer on lattice-matched indium-phosphide substrate and devices

Abstract: An epitaxial layer of a quaternary III-V alloy of Ga, In, As, and P has its constituents proportioned for lattice matching to a substrate having a lattice constant falling within the range of 5.45 to 6.05 A. In addition, the constituents of the alloy are proportioned to provide a selected bandgap energy falling within the range of 2.23 to 0.35 electron volts; this corresponds to wavelengths of 0.55 to 3.5 microns. Near perfect lattice matched heterojunctions are provided between the epitaxial layer and the lattice matched substrate; these are useful for providing improved photocathodes and lasers, particularly in the infrared range of wavelengths between 0.8 and 2.0 microns.

Self-radiation effects on lattice parameter of $sup 238$PuO$sub 2$

Abstract: A study was made, as a function of time, of the self-radiationinduced expansion of the lattice parameter of freshly prepared onential equation. The observations were compared with previous results reported for PuO/sub 2/ and with recent observations on the lattice expansion of /sup 244/CmO/sub 2/. The isochronal annealing of the damaged lattice revealed a two-stage process. Isothermal treatments showed a rapid contraction during the first few minutes of annealing, the equilibrium of recovery being reached within one hour. (auth)

Method of forming light emitting diode array with dome geometry

Abstract: A structure for providing arrays or individual hemispherical diodes and methods of producing the diodes. When the diode array is to be part of a configuration utilizing a substrate, the substrate is selected to have radiation transparency, a lower refractive index, and lattice constant and lattice structure similar to that of a crystal layer grown in hemispheres formed in the substrate. When the diode array is to be removed from the substrate, a material that can be preferentially etched is grown between the hemispheres formed in the substrate and the grown crystal layer that is to have light emitting areas.