Variation Of Lattice Constants Research Articles

Photoluminescent and electrical properties of InGaPAs mixed crystals liquid phase epitaxially grown on (100) GaAs

The liquid phase epitaxial growth of InGaPAs layers on (100) GaAs is described for growth temperatures of 785 and 815 °C. Photoluminescence (PL) spectra and electron Hall mobility of the layers are investigated as a function of crystal composition. The width of PL spectra measured at 300 K is 40 meV, approximately the same value as that of GaAs, when the energy gap Eg of the layers is 1.9 eV or lower than 1.6 eV. The spectral width is around 80 meV when the energy gap of the layers is around 1.75 eV. The increase in the spectral width is attributed to variation of the local energy gap from analysis of dependence of the PL spectra on temperature and on excitation intensity. The indication that local energy gap varies in layers with Eg around 1.75 eV is consistent with the composition dependence of electron mobility. The mobility shows a sharp dip down to 1000 cm2/Vs around this composition, while it depends on composition only slightly for layers with Eg lower than 1.6 eV. The dip in composition dependence is attributed to space-charge scattering from analysis of temperature dependence of mobility. X-ray diffraction spectrum and surface morphology are also reported. The width of x-ray diffraction spectra is large for layers with Eg around 1.75 eV. This indicates that the local lattice constant varies significantly. Thick and uniform layers are particularly difficult to grow for such composition. These composition dependences of PL spectra, x-ray diffraction, and surface morphology are more significant in layers grown at 785 °C than in layers grown at 815 °C. The local variation of energy gap and lattice constant is attributed to variation of local composition caused by immiscibility of InGaPAs.

Local force variations caused by isoelectric impurities: Method of determination from first principles

It is shown how the variation of lattice dynamical force constants caused by substitutional isoelectronic impurities can be evaluated ab initio. The approach, illustrated on the example of Al in GaAs, is based on local density functional and uses ionic pseudopotentials of Al, Ga, As as the only input; Hellmann-Feynman theorem is applied in order to extract from self-consistent electronic charge densities the forces acting on atoms in periodic patterns in which entire planes of impurities are displaced. The defect-induced variations of interplanar force constants are converted into the interatomic ones, which can be compared with those determined by phenomenological models from the measured local mode frequencies. A method is presented which allows to account for the effect of relaxation without requiring an explicit determination of the latter. Particular problems resulting from dealing with entire plane of defects are discussed and an estimate for relaxation is given.

Chemical shift study of spinel ferrites

Copper-zinc and Copper-magnesium spinels, with four different stoichometric compositions, are studied using X-ray diffraction and absorption techniques. The variation of lattice constant, with copper content and with Fe K-edge shift, is attributed with the change in the iono-covalent behaviour of the spinel lattice.

Effect of additives on the properties of zinc-manganese ferrite

The dielectric properties and the variation in lattice constant, Curie temperature, saturation magnetization, and initial permeability have been investigated for various dopant concentrations of Cr, Al, and In in zinc-manganese ferrite. Possible mechanisms contributing to conduction process and dielectric properties are discussed. From the temperature dependence of dielectric relaxation, activation energies for various samples have been obtained and compared with those obtained from the observed temperature dependence of dc resistivity.

On the prediction of order in the σ phases on the basis of a sphere-packing model

A theory for the order in σ phases based on the sphere-packing model of Wilson & Spooner [Acta Cryst. (1973), A29, 342-352] has been examined using a mathematical analysis. The analysis suggests that the prediction of order in these phases based on lattice-constant variations associated with variations in atomic diameter should be treated with caution, particularly where the results are at variance with experimentally determined results.

Relating structural, magnetic-moment, and hyperfine-field behavior to a local-environment model inFe3−xCoxSi

A detailed NMR, magnetization, x-ray, and neutron-diffraction study is reported for the ${\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Si}$ system over the entire range of Co concentration ($0\ensuremath{\le}x\ensuremath{\le}3.00$). As the Co concentration $x$ is increased, the x-ray measurements indicate that a single phase having the fcc $D{O}_{3}$-type structure is maintained (with some variation in lattice constant) up to $x=2.15$. It is found that Co selectively enters the ($A$,$C$) sites for the higher concentrations studied in this work ($x\ensuremath{\le}2.00$) with a high degree of order for $xl1.50$. The variation in lattice constant with Co concentration correlates well with the bulk magnetization and can be described by a simple empirical relation which is an extension of Vegard's law. Neutron-diffraction and magnetization measurements have enabled a determination of the $\mathrm{Fe}(B)$, $\mathrm{Fe}(A,C)$, and $\mathrm{Co}(A,C)$ magnetic-moment dependence on Co concentration for these alloys. In particular, we note that the moment on the substituted Co atoms appears well localized and remains essentially constant (+1.7${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$) throughout the range $0\ensuremath{\le}x\ensuremath{\le}2.15$. In addition, the variations of the internal hyperfine fields with Co concentration at all sites [$\mathrm{Fe}(B)$, $\mathrm{Fe}(A,C)$, $\mathrm{Co}(A,C)$, and $\mathrm{Si}(D)$] have been studied by spin-echo NMR. In order to explain the magnetic-moment and internal field behavior, a model emphasizing the short range interaction approach is presented. Since Co enters the ($A$,$C$) sites, the short range interaction model involves only the $1nn$ configurations for the $\mathrm{Fe}(B)$ atoms. The model successfully describes the detailed behavior of the magnetic moments and internal fields at all sites and enables a subdivision of the observed internal field into contributions due to the $4s$ spin polarization transferred from neighboring moments and the polarization resulting from the on-site moment. Such an approach has already proved successful for transition metal substitutions into the $B$ sites.

VARIATION OF LATTICE CONSTANTS IN THE α-LiIO<sub>3</sub> SINGLE CRYSTAL UNDER THE ACTION OF AN ELECTROSTATIC FIELD

The irregular variation of lattice constants of the α-LiIO3 single crystal originated in the growth process and the fluctuation phenomena of lattice constants under the action of an electrostatic field were investigated by an X-ray double-crystal spectro-meter in the (n, -n) arrangement.The results obtained support the view point that the enhancement of neutron diffraction intensity of the l≠0 reflection under the action of an electrostatic field along z-axis is due to the existence of a gradient of lattice constant c.

FeCo合金の規則化に伴う格子定数変化

In FeCo alloy, the variation of lattice constant in isothermal annealing was observed by means of X-ray diffraction, and the experimental results were analyzed by the use of an approximate rate equation. This rate equation has been derived from the two conditions, namely, the symmetry between two sublattices in the B2 type structure, and the stability of the equilibrium state. The equation has an advantage over the others in that it is simpler and more general, but the physical meaning of the parameters used in this analysis is not clear. For this reason, the rate equation was compared with the one derived by Takagi and Oguchi the analogy to the chemical rate theory, and the activation energy for ordering in FeCo alloy was estimated to be 52.0 kcal/mol.

ChemInform Abstract: KINETICS AND MECHANISM OF THE TRANSFORMATION OF VATERITE TO CALCITE

AbstractDie Vaterit‐Calcit‐Umwandlung wird systematisch untersucht.

Thermal expansion of NaF, KBr and RbBr and temperature variation of the frequency spectrum of NaF

Coefficients of thermal expansion of NaF, KBr and RbBr are determined at different temperatures using a diffractometer, Geiger counter, chart recorder and a specially designed furnace. Equations are given for the variation of lattice constants with temperature. The temperature dependence of the thermal expansion at high temperatures is shown to be related to the concentration of thermally generated Schottky defects. The energies of formation of Schottky pairs in the three halides are estimated and are found to be consistent with those deduced from ionic-conductivity studies. The halides are found to obey the 'law of corresponding states' established by Pathak & Vasavada [Acta Cryst. (1970). A26, 655-658]. The geometric mean of the frequency spectrum of NaF is found to decrease by about 14% between room temperature and 1000°K.

Kinetics and Mechanism of the Transformation of Vaterite to Calcite

Abstract The transformation of vaterite to calcite was investigated systematically. The transition temperature and the energetics of the transformation were determined from differential thermal curves. The variations of lattice constants and crystallite size, accompanying the transformation were studied by X-ray diffractometry. The kinetics of transformation were investigated in the temperature range 460–490°C. The kinetic data were analysed with the help of three separate solid-state models.

Thermal expansion of LiF by X-ray diffraction and the temperature variation of its frequency spectrum

The coefficients of thermal expansion of LiF are determined at different temperatures using a diffractometer, Geiger counter, chart recorder and a specially designed furnace. Equations are obtained for the variation of lattice constants with temperature. The temperature dependence of the thermal expansion at high temperatures is shown to be related to the concentration of thermally generated Schottky defects. The energy of formation of Schottky pairs is found to be 2.42 eV. The Grüneisen constant γ is found to decrease with temperature. The mean frequency of the vibrational spectrum is found to decrease with temperature and the variation between 300 and 1000°K is about 13%.

Variations of lattice constants in clinoamphiboles

Variations of lattice constants in clinoamphiboles

The intermediate transformation in steels ShKh15 and ShKh15SG

1. For each of the steels investigated a relationship was found between the variation of the austenite lattice constant during the intermediate transformation and the carbon content of the original austenite before transformation of the γ-solid solution similar to that for a number of steels with approximately the same alloying. 2. Heats containing larger amounts of carbon in the original austenite have an elevated hardness in the intermediate transformation zone due to the more intensive formation of carbide.

Variations of lattice constants in clinoamphiboles*

Variations of lattice constants in clinoamphiboles*

Coordination compound formed by iron trichloride dissolved in liquid hydrogen cyanide

After dissolving iron(III) chloride in anhydrous hydrogen cyanide at room temperature single crystals are obtained. When studied by infrared spectroscopy these crystals show that the HCN molecule is bound to the iron atom by nitrogen atom. The crystals are trigonal with a = 10.29 ± 0.01 Å and c = 6.280 ± 0.006 Å. A three-dimensional X-ray crystal structure analysis has shown that the unit cell contains two FeCl 4 tetrahedrons and one Fe(NCH) 6 octahedron. Inverting the full matrix and using anisotropic temperature factors, R factor based on 871 reflexions is equal to 0.07 excluding zero values. Interatomic distances computed from X-ray results show that nitrogen atom is bound to the iron atom. The color of crystals changing from yellow to green is related to the presence of iron(II) the variable amount of which leads to a linear variation of lattice constants.

硫酸バリウム―硫酸鉛系の格子定数と屈折率

硫酸バリウム―硫酸鉛系の格子定数と屈折率

Preparation and study of some properties of NbC - WC and TaC - WC alloys

1. The conditions of preparation of homogeneous solid solutions and alloys of the carbide systems NbC - WC and TaC - WC were investigated. 2. Studies were made of the effects of composition on structure, lattice-constant variation, electrical resistivity, microhardness, the coefficient of thermal expansion, and the angle of contact of nickel for the systems TaC - WC and NbC - WC. 3. The findings are entirely consistent with the type of composition vs property relationship characteristic of metallic binary alloys with limited solid-state solubility. 4. It is demonstrated that, for solid solutions of WC in the cubic carbides TaC and NbC, raising the concentration of tungsten carbide is accompanied by an increase in hardness and electrical resistivity, and a decrease in the coefficient of thermal expansion and the angle of contact of nickel on solid solutions of the carbides.

Optical properties and cell parameters in the glaucophane-riebeckite series

A complete set of new optical and x-ray data is given for eleven analyzed alkali amphiboles [Na2(Mg, Fe″)3(Al, Fe‴)2Si8O22(OH)2]. Nine new wet chemical analyses are reported. Using additional selected data from the literature, variation in refractive indices, extinction angles (γ-α), optic angles, density, lattice constants and cell volume are expressed graphically as a function of composition in the glaucophane-riebeckite and magnesiorie-beckite-ferroglaucophane series. Four orientations (G, C, O, and R) of the optical indicatrix within the structure are described and shown to be characteristic of the chemical species glaucophane (G), crossite (C), magnesioriebeckite (O), riebeckite (O), and riebeckite-arfvedsonite (R and O).

A Study of Zinc Oxide-Chromium Oxide Catalysts. VI. The Changes Brought about by Heat Treatment in the Activity for Methanol Synthesis and in the Structure of the Raw Catalysts

Abstract The activity for methanol synthesis has been determined for two series of catalysts prepared from zinc oxide and chromic acid with and without subsequent heat treatment in the air. Also, the correlations between catalytic activity and other catalyst properties have been studied. Each catalyst series has consisted of four catalysts, three of them prepared from slurries of zinc oxide powder in aqueous chromic acid, and the other prepared directly from zinc oxide powder. The chromium/zinc ratios in the catalysts made from the slurries are 0.1, 0.5, and 1.0. The catalysts of one series have been heated in the air at 400°C after their preparation (the “heated series”). In both series, the activity and the specific activity have been found to be influenced by the chromium/zinc ratio. The catalysts not heated in the air after their preparation (the “unheated series”) show higher activities than the corresponding catalysts of the heated series, though the former catalysts have much smaller surface areas. The results of electron microscopic examination show that the two series of catalysts, after having been used for methanol synthesis, are structually different. The structural difference has been ascribed to the difference in the way of the decomposition of the layer-structure of zinc hydroxychromate. In the case of the unheated series, the layer-structure is decomposed into crystallites of zinc oxide and zinc chromite by reduction in the synthesis gas, whereas in the case of the heated series the layer-structure is decomposed by heat treatment in the air. X-Ray diffraction data show that, in all cases, the lattice constant for the zinc oxide in catalysts is normal, whereas the lattice constant for the zinc chromite is larger than the normal value. The average particle diameter as calculated from the surface area and density of catalysts does not agree with the size as estimated by electron microscopic observations. The difference in specific activity between the two series may be due to the difference in the degree of the mutual penetration of zinc oxide and zinc chromite crystallites rather than to the variation in the zinc chromite lattice constant. In addition, the effects upon the activity and upon the catalyst structure of catalyst treatment inhydrogen at 500°C after methanol synthesis have been discussed.