Variation Of Lattice Constants Research Articles

Structural behaviour of ZnS xSe 1− x films deposited by close-spaced evaporation

ZnS x Se 1− x is considered to be one of the potential alternatives to CdS as a window and/or buffer layer in polycrystalline heterojunction solar cells. Thin films of ZnS x Se 1− x with compositions x = 0.0, 0.25, 0.5, 0.75 and 1.0 have been prepared using close-spaced evaporation technique. The films were deposited at different substrate temperatures in the range 200–400 °C. The grown films have been characterized using X-ray diffractometer and scanning electron microscope (SEM) in order to determine the crystalline phases present and the surface topography. The X–ray diffraction data showed that the films deposited at substrate temperatures in the range of 275–325 °C were polycrystalline and showed only ZnS x Se 1− x phase without any additional phases. These layers had the preferred orientation along the (1 1 1) direction and exhibited cubic structure. The variation of lattice constants with S/Se atomic ratio in the films followed the Vegard's law.

Tunable Photonic Bandgap Composites Based on Thermo- and pH-Responsive Poly-(N,N-Dimethylaminoethyl methacrylate) Hydrogels

A tunable polymeric photonic bandgap composite (TPBC) was created by encapsulating ordered, monodispersed polystyrene spheres within a hydrogel matrix based on poly(N,N-dimethylaminoethyl methacrylate). This polymer swells and contracts with changes in the temperature and pH of its surrounding solution. The volume phase transition property inherent to this poly(DMAEMA) hydrogel was used to reversibly shrink and swell the hydrogel, which causes a variation in the lattice constant and results in a mechanism to tune the stopband. A rejection band was reversibly tuned within the temperature range between 35 and 65° C and solution pH between 2 and 12. A 20 nm shift was observed for each 10° C change in temperature. The volume phase transitions of this poly(DMAEMA) hydrogel were characterized by differential scanning calorimetery (DSC), equilibrium swelling degree measurements and thermo-mechanical analysis (TMA).

Synthesis and structural studies on Ni0.5+xZn0.5CuxFe2-2xO4

The mixed ferrites of Ni–Zn–Cu are synthesized using ceramic double sintering technique. Cu and Ni are substituted in steps of x=0.1 at the interstitial sites of Fe. Powders of mixed ferrites of Ni–Zn–Cu are studied using XRD. The mixed ferrites show a single phase and face center cubic structure for all concentrations. The substitution of Cu and Ni are confirmed from the variation of lattice constant. The cation distribution in the A and B sites of the ferrites is estimated. Mixed ferrites of Ni–Zn–Cu are characterized using a.c. conductivity and magnetic susceptibility methods. The variation of activation energy, magnetic moment and Curie temperature with concentration of Fe ions explains the alterations of the energy levels of d bands. Hopping of charge carriers and the presence of different ionic states of Ni, Cu, and Fe ions are discussed using the FTIR and EPR spectra.

Growth and Characterization of Superconductive Nd2‐xCexCuO4‐y Single Crystals.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Growth and characterization of superconductive Nd2‐xCexCuO4‐y single crystals

AbstractNd2‐xCexCuO4‐y single crystals with different x values have been successfully grown by Traveling Solvent Floating Zone method (TSFZ). Electronic properties of Nd2‐xCexCuO4‐y single crystals with various x have been studied in detail. The results show that superconductivity can be found in crystals with x > 0.1 (0.13‐0.18) directly grown at oxygen‐reduced atmosphere without post‐annealed, while no superconductivity appears in crystals with x < 0.1 at the same atmosphere. It is also found that, the segregation coefficient of Ce is determined to be 0.946 and transition temperature Tc (onset) reaches maximum value of 23.5 K at nominal composition x = 0.165. With further increase of Ce concentration, transition temperature of single crystals declines due to the precipitation of secondary Phases. In addition, the variation of lattice constants of Nd2‐xCexCuO4‐y single crystals with different x is also given. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Rapid solidification of Cu–Fe–Ni alloys

Abstract Cu80−xNixFe20 (x=0, 5 and 20) alloys have been rapidly solidified by planar flow casting. X-ray diffraction (XRD) analysis of as-quenched ribbons shows bcc-Fe precipitates embedded in an fcc phase (x=0), two co-existing fcc solid solutions (x=5) and a complete solid solution of the parent elements (x=20). Thermal treatments in the temperature range between 400 and 600 °C give precipitation and spinodal decomposition reactions. These phase transformations have been evidenced from a variation of lattice constants, from a broadening of diffraction peaks and from TEM observations. The role of Ni content on competition between precipitation and decomposition reactions during rapid solidification and annealing is discussed in terms of thermodynamic arguments. Recent CALPHAD assessment of thermodynamic properties for Cu–Fe–Ni system has been used for an estimation of composition and volume fraction of equilibrium phases.

Structural Analysis of Ce1 − x M x O 2 − 0.5x − δ ( M = Gd , Sm , Y ) by High Temperature XRD under Various Oxygen Partial Pressures

Variation of lattice constants of Sm, Y; 0.1, 0.2) with temperature and oxygen ion vacancy concentration was investigated with X-ray diffraction (XRD) at 600-1000°C under various gas atmospheres. Linear thermal expansion and increase of expansion coefficient with trivalent cation content were observed in every specimen under and atmospheres. In gas mixtures, expansion due to generation of oxygen ion vacancies was observed at temperatures higher than 600°C for every specimen. Irrespective of the kinds of trivalent cations, the dependence of lattice constants on δ was almost linear for the specimen with whereas bending point was observed for those with and for pure ceria. The bending point was observed at around 0.1 of which was the total amount of oxygen ion vacancies. © 2004 The Electrochemical Society. All rights reserved.

Physical properties of layered homologous RE–B–C(N) compounds

Physical properties of a series of homologous RE–B–C(N) B 12 cluster compounds REB 17CN, REB 22C 2N, and RE B 28.5 C 4 (RE= Er, Ho) were investigated. The structures of the compounds are layer-like along the c-axis, with rare earth and B 6 octahedral layers separated by B 12 icosahedral and C–B–C chain layers whose number increases successively from two B 12 layers for the REB 17CN compound to four for the REB 28.5C 4 compound. The rare earth atoms are configured in two triangular flat layers which are stacked on top of one another in AB stacking where the nearest-neighbor rare earth directions are the three atoms forming a triangle in the adjacent layer. The series of homologous compounds exhibit a spin glass transition with T f shifting in correspondence with variations of the basal plane lattice constants, consistent with the magnetic interaction being effective in the basal planes. The isothermal remanent magnetization shows a stretched exponential decay I m (t)∝ exp[−Ct −(1−n)] . Exponents determined for the different homologous compounds were scaled as a function of T r= T/ T f and found to follow the empirical dependency determined for typical spin glasses. It is indicated that a mixture of disorder originating from the partial occupancy of the rare earth sites and frustration of interactions due to the unique configuration is responsible for the manifestation of spin glass transitions in these homologous systems.

Exchange bias in Co/Fe/FexMn1−x/Cu(100) ultrathin films

Stable and well-grown face-centered-cubic Fe films were prepared on buffer layers with varying lattice constants by depositing FexMn1−x alloy film on Cu(100) single crystal. No ferromagnetic ordering was observed at the stage of 30 ML Fe on the FexMn1−x/Cu(100) systems in the temperature range from 100 to 350 K. Furthermore, capping of Co on Fe/FexMn1−x/Cu(100) was employed as the probe of antiferromagnetic ordering by study of exchange bias coupling in these films. The exchange bias of the hysteresis loops can be observed after field cooling of the films. Further analyses by varying the measurement temperature and Fe coverage of the films were also carried out to clarify the origin of the exchange bias coupling observed. The exchange bias field found here is attributed to the interlayer coupling between the Co and Fe–Mn films through the spacing layer Fe.

Anomalous thermal expansion of MnN

The MnN compound is prepared as a single phase by d.c. reactive sputtering. The crystal structure of MnN is tetragonally distorted NaCl type (fct) one. The temperature variation of lattice constants for MnN is measured by X-ray diffraction experiments in the temperature range from 289 to 803 K. It is found that the MnN compound shows anomalous thermal expansion and the crystal structure of MnN has changed from fct structure to fcc one at about 650 K. On the other hand, we found formerly that MnN was an antiferromagnetic compound with a Néel temperature of 650 K. The tetragonal distortion below about 650 K is well explained by the strain dependence of the exchange interaction on the basis of molecular field theory.

Lattice constant dependence of elastic modulus for ultrafine grained mild steel

The microstructure of an ultrafine grained mild steel was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and nanoindentation techniques. The results indicate that with the decrease of grain size within the submicron range, the change of Young’s modulus was obvious, which has been attributed to the variation of lattice constants.

Structural and optical characterization of polycrystalline CdSe x Te 1-x thin films

Solid solutions of CdSexTe1-x (0.7≤x≤1) were synthesized by vacuum fusion of stoichiometric amounts of CdSe and CdTe constituents in a silica tube. X-ray and electron microscope diffractometry techniques revealed that the CdSexTe1-x thin films were polycrystalline with a hexagonal structure. The variation of lattice constants with composition was found to obey Vegard’s law. The compositional dependence of the optical constants, the refractive index n and the absorption index k, of the films was determined in the spectral range of 400–2000 nm. The dispersion of the refractive index of the films could be described using the Wemple–DiDomenico single oscillator model. Changes of the dispersion parameters were also studied as a function of the mole fraction x. A plot representing α2=f(hν) showed that the CdSexTe1-x thin films of different compositions have two direct transitions corresponding to the energy gaps Eg and Eg+Δ. The variation in either Eg or Eg+Δ with x indicates that this system belongs to the amalgamation type. The variation follows a quadratic dependence and the bowing parameters were found to be 0.4 and 0.5 eV, respectively.

Spin-density inversion symmetry driven first-order magnetic phase transition inGaCMn3

The first-order magnetic phase transtion of the metallic perovskite ${\mathrm{GaCMn}}_{3}$ has been investigated in terms of density-functional theory using the all-electron total-energy full-potential linearized augmented plane-wave method. We found that the antiferromagnetic (AFM) state is more stable energetically than the ferromagnetic (FM) state at the low-temperature lattice constant. Total-energy calculations yield a critical lattice constant of $7.3411\mathrm{a}.\mathrm{u}.,$ which is remarkably close to the experimental one $(7.34\mathrm{a}.\mathrm{u}.)$ The calculated magnetic moments of Mn in each magnetic state are almost constant with lattice-constant variation, $\ensuremath{\sim}1.58{\ensuremath{\mu}}_{\mathrm{B}}$ for the FM and $\ensuremath{\sim}1.80{\ensuremath{\mu}}_{\mathrm{B}}$ for the AFM states, which are very close to experiment. The FM spin density is found to have even inversion symmetry, while the AFM one has odd, which restricts the direction of the AFM ordering vector along the [111] direction via tilting of the orbital orientation axis. Hence, the spin-density parity about the spin-inversion symmetry may be a key for understanding the first-order phase transition in ${\mathrm{GaCMn}}_{3}.$ We also found that the oddness of the spin-density-inversion symmetry in the AFM state brings about the wave-function sign dependent spin polarization and changes the bonding character. From the calculated density of states, the hybridization between the $\mathrm{Ga}\ensuremath{-}4p$ and $\mathrm{Mn}\ensuremath{-}3d$ states is important for determining the magnetism of ${\mathrm{GaCMn}}_{3}.$

Structural Analysis of Ce1−XMXO2-0.5x−δ (M=Gd, Sm, Y) by High Temperature XRD Under Various Oxygen Partial Pressures

Variation of lattice constants of Ce 1-x M x O 2-0.5x-δ (M=Gd, Sm, Y; x=0, 0.1, 0.2) with temperature and oxide ion vacancy concentration was investigated with X-ray diffraction at 600°∼1000°C under various gas atmospheres. Linear thermal expansion and increase in expansion coefficient by trivalent cation content were observed in every specimen under N 2 and O 2 atmospheres. In N 2 /H 2 /H 2 O gas mixtures, expansion due to generation of oxide ion vacancies was observed at temperatures higher than 600°C in every specimen. Irrespective of the kind of trivalent cations, the dependence of lattice constants on δ was almost linear for the specimen with x = 0.2, whereas a bending point was observed for those with x = 0.1 and for pure ceria. The bending point was observed at around 0.1 of 0.5x+δ, which was the total amount of oxide ion vacancies.

Magnetic susceptibility study on Li-Mn spinel oxides

Li-Mn spinel oxide particles were prepared by the solid state reaction using Mn3O4 particles and Li-hydroxide monohydrate, and the effects of the reaction temperature on the produced phase were studied with the powder X-ray diffraction and the magnetic susceptibility measurement. From the lattice constant variation versus the reaction temperature, it was found that the spinel oxides prepared by low temperature reaction were cation deficient type, and that the cation deficient degree decreased with an increase in the reaction temperature. The asymptotic Curie temperature (Weiss constant), derived from the temperature dependent magnetic susceptibility, was always negative, and it implied that the antiferromagnetic coupling among Mn ions is dominant. The absolute value of the Weiss constant increased with the reaction temperature, took a maximum at the reaction temperature around 750°C, and then decreased with a further increase in the reaction temperature. These were attributed to the variation of the average valence of Mn ions. At the reaction temperature, where the absolute value of the Weiss constant takes a maximum, the non-stoichiometry of Li-Mn spinel oxide becomes minimum. In the specimen prepared above the temperature, it was likely that there existed oxygen vacancy, and the phase transformation between cubic and orthorhombic structures was found in the DSC curves.

S doping in ZnO film by supplying ZnS species with pulsed-laser-deposition method

S-doped ZnO (ZnO:S) film was fabricated by supplying ZnS species from laser ablation of a ZnS target during ZnO growth. Variations of lattice constants and band gaps with respect to S content did not follow Vegard’s law. The ZnO:S film showed semiconducting behavior with lower activation energy and resistivity than those of ZnO owing to higher carrier concentration. Despite the absence of magnetic elements, the large magnetoresistance amount of 26% was observed at 3 K from ZnO:S film.

Oscillation of in-plane lattices constant of Ge islands during molecular beam epitaxy growth on Si

Variations in the lattice constant of Ge film were determined by reflection high-energy electron diffraction in the course of the molecular beam epitaxy (MBE) film growth on the silicon surface. Oscillations of the in-plane atomic cell constant were observed for the Ge film growing according to the two-dimensional (2D) mechanism. Variations in the 2D lattice constant at the stage of 2D growth are caused by elastic deformation of edges of 2D islands.

Lattice constant variation in GaN:Si layers grown by HVPE

ABSTRACTThe structural, optical, and electrical properties of HVPE-grown GaN-on-sapphire templates were studied. The c and a lattice constants of the GaN layers were measured by x-ray diffraction. It was observed that the c and a lattice constants vary non-monotonically with Si-doping. The proper selection of Si-doping level and growth conditions resulted in controllable strain relaxation, and thus, influenced defect formation in GaN-on-sapphire templates. It was also observed that HVPE homoepitaxial GaN layers grown on the templates have better crystal quality and surface morphology than the initial templates.

Lattice constant variation and complex formation in zincblende gallium manganese arsenide

We perform high resolution x-ray diffraction on GaMnAs mixed crystals as well as on GaMnAs/GaAs and GaAs/MnAs superlattices for samples grown by low-temperature molecular-beam epitaxy under different growth conditions. Although all samples are of high crystalline quality and show narrow rocking curve widths and pronounced finite thickness fringes, the lattice constant variation with increasing manganese concentration depends strongly on the growth conditions: For samples grown at substrate temperatures of 220 and 270 °C, the extrapolated relaxed lattice constant of Zincblende MnAs is 0.590 nm and 0.598 nm, respectively. This is in contrast to low-temperature GaAs, for which the lattice constant decreases with increasing substrate temperature.

AFeO3 (A=La, Nd, Sm) and LaFe1−xMgxO3 perovskites: structural and redox properties

Abstract The synthesis and properties of polycrystalline AFeO3 (A=La, Nd, Sm) and LaFe1−xMgxO3 (x=0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) perovskites prepared at different temperatures (623, 773, 923 and 1073 K) from decomposition of citrate precursors are reported. Chemical analysis, X-ray diffraction (XRD) for phase analysis, lattice parameters and crystallite dimension, BET surface area determination, diffuse reflectance spectroscopy, and magnetic susceptibility measurements were employed for the characterization of the solids. The reducibility of the samples was followed by temperature programmed reduction (TPR) and by reduction with in situ XRD. The calcination at different temperatures revealed that samples having less disordered perovskite structure are more easily formed. The variation of lattice constants and the order of reducibility within the AFeO3 perovskites, LaFeO3>NdFeO3>SmFeO3, have been linked to different Fe–O bond strength. A certain fraction of Fe4+ was observed by redox titration and TPR to be present in AFeO3 perovskites, which, for charge balance, have small cation defectivity. The fraction of Fe4+ increases with the increase of x in LaFe1−xMgxO3 solid solutions. However, the Fe4+/Mg2+ ratio never reaches unity, so that these mixed perovskites, to preserve charge neutrality, contain oxygen defectivity. After reduction, all samples preserve the perovskite structure and the expansion of the unit cell volume in the reduced LaFe1−xMgxO3 perovskites is due to the higher content of larger Fe3+ species formed by reduction of Fe4+. The decreasing reducibility of LaFe1−xMgxO3 solid solutions at the increase of x has been correlated with oxygen atoms more strongly bonded. Optical and magnetic properties are also reported and discussed.