Variation Of Lattice Constants Research Articles

Recent developments of high quality synthetic diamond single crystals for synchrotron X-ray monochromators

For several years now, the ESRF, the University of the Witwatersrand and Messrs. De Beers Industrial Diamonds (Pty) Ltd. through their Diamond Research Laboratory, have pursued a development programme to assess and improve the quality of synthetic diamonds. Recently, in an effort to study the influence of nitrogen impurities on the defect structure, X-ray excited optical luminescence and spatially resolved double-crystal diffractometry were employed as new techniques. The correlation between nitrogen impurities and the raw defect structure was clearly visible. It was confirmed that concentration variations are related to lattice imperfections, where tilts are much more important than lattice constant variations. High reflectivity was observed and quite large zones of a sample bigger than 1 cm 2 showed to be perfect to within better than 0.5 arcsec.

Optical properties of BN in cubic and layered hexagonal phases

Linear optical functions of cubic and hexagonal BN have been studied within first principles DFT-LDA theory. Calculated energy-loss functions compare well with experiments and previous theoretical results both for h-BN and for c-BN. Discrepancies arise between theoretical results and experiments in the imaginary part of the dielectric function for c-BN. Possible explanation to this mismatch are proposed and evaluated; lattice constant variations, h-BN contamination in c-BN samples and self-energy effects.

Crystallization of Ba xSr 1−x(NO 3) 2 solid solutions from aqueous solutions and the influence of Ba 2+ ion on Sr(NO 3) 2 crystals

A series of Ba xSr 1−x(NO 3) 2 crystals have been grown from aqueous solutions with different initial Ba 2+/Sr 2+ ratios. These crystals were characterized by powder X-ray diffraction. All diffraction data were well indexed according to the simple cubic structure, and the variation of lattice constants with the concentration of Ba 2+ in the crystals accorded quite well to Vegard’s Law. The composition of the Sr(NO 3) 2 crystal doped with Ba 2+ was studied with an electron microprobe. It was found that Ba 2+ was enriched in the 100 sectors. The quality of the crystals with different composition estimated by the high-resolution X-ray diffraction indicated that the second cation added to the crystalline phase degrades the crystalline quality of the crystals. Equilibrium behavior in the Ba(NO 3) 2–Sr(NO 3) 2–H 2O system was analyzed using Lippmann’s phase diagram, and the equilibrium distribution coefficient D Ba = 769.2. This large value of D indicates that the Ba 2+ ion is drastically preferentially partitioned towards the solid phase.

Crystallization of PbxSr1-x(NO3)2 Solid Solutions from Aqueous Solutions

A series of PbxPr1-x(NO3)(2) crystals have been grown from aqueous solutions and characterized by X-ray powder diffraction. All diffraction data were well indexed according to the simple cubic structure. The variation of lattice constants with the concentrations of Pb2+ in the crystals accorded quite well to the Vegard's Law. The composition of the Sr(NO,), crystal doped with Pb2+ was studied by electron microprobe and it was found that Pb2+ was enriched in the 111 sectors. Equilibrium behavior in the Pb(NO3)(2)-Sr(NO3)(2)-H2O system was analyzed by Lippmann's phase diagram and the equilibrium distribution coefficient D-ph = 133.6. This large value of D indicates that Pb2+ ion is preferentially distributed to the solid phase.

Role of Ba2+ Ion in the Crystal Growth of Sr(NO3)2

A series of crystals of BaxSr1-x(NO3)2 (x = 0 - 0.98) were grown from aqueous solutions by the evaporation method and characterized by x-ray powder diffraction and high resolution x-ray diffraction. All diffraction data are well indexed according to simple cubic structure. The variation of lattice constants with the concentrations of Ba2+ in the crystals accorded quite well to the Vegard's Law. The results of high-resolution x-ray diffraction show that the crystalline quality of BaxSr1-x(NO3)2 decreases with x increasing. The distribution of Ba2+ between the solution and solid phase indicated that Ba2+ is enriched in the solid phase. It could be attributed to the solubility of Ba(NO3)2 in water being less than that of Sr(NO3)2.

ZrO 2–CeO 2 and CeO 2–La 2O 3 film growth on oxide substrates and their applications in oxide heterostructures

The pulsed laser deposition of several cerium-based oxides has been studied in order to prepare buffer layers tailored to the epitaxial growth of oxides. It has been shown that ceria-based oxide films with a sufficiently wide variation of lattice constant (0.541–0.551 nm) could be prepared. CeO 2, Zr 1− X Ce X O 2 and Ce 1− X La X O 2− X/2 epitaxial films with low roughness were obtained. The results obtained by RHEED, XRD, RBS and AFM methods have confirmed the high quality of heteroepitaxial CeO 2, Zr 1− X Ce X O 2 and Ce 1− X La X O 2− X/2 layers, and we have evaluated the difference in crystallinities for the films grown from metallic alloy targets and sintered oxide targets. These films can be used in YBa 2Cu 3O 7-based heterostructures as insulating barriers. In the case of Ce 1− X La X O 2− X/2 (0⩽ x⩽0.4) high-quality epitaxial films are obtained on LaAlO 3 and SrTiO 3, the buffer layers are epitaxially grown with a 45° rotation of the in-plane axes with respect to those of the substrate and the surface roughness is low ( R rms=0.24 nm/1 μm 2). The properties of YBa 2Cu 3O 7 films deposited on these buffered substrates are as good as those for the film grown without any buffer layer, and furthermore the field dependence of the R's at 77 K and 10 GHz is better, being about 3.8×10 −3 mΩ/Oe which is smaller than the 9×10 −3 mΩ/Oe recorded in the case of unbuffered substrates.

Study of cation distribution in lithium doped nickel ferrite

The structural and magnetic properties of ferrites are sensitive to the nature, the valence state and distribution of metal ions over tetrahedral (A) and octahedral (B) sites in the spinel lattice. Therefore, the knowledge of cation distribution and spin alignment is essential to understand the magnetic properties of spinel ferrites. The outstanding fact about the ferrites is that they combine extremely high electrical resistivity with good magnetic properties. This means that the ferrites are more suitable for high frequency and low loss applications as compared to ferromagnetic metals. For more than a decade, lithium ferrite materials have dominated in the field of microwave applications. Several series of lithium ferrite covering wide range of properties have become commercially available. Suitable substituents in lithium ferrites have made it possible to tailor the materials properties for a variety of diverse requirements of microwave devices. Lithium ferrites also have the inherent properties of high Neel temperatures, rectangular hysterisis loop and high dielectric constant combined with low cost. There are many research reports in the literature on the studies of Li-Zn, Li-Mg, Li-Ti, Li-Cd ferrites [1– 4]. However, no information is available in the literature regarding the Li-Ni ferrite and relative site preference of Li+1, Fe+3 and Ni+2 in the spinel lattice. The spinel system Li0.5Fe2.5−x Mx O4 (where M = V, Cr and Rh) has been studied by Blasse [5]. It has been observed that substitution of V, Cr and Rh causes migration of Li ions for B-sites to A-sites. It has also been observed that Li ions occupy B-sites in pure Li-ferrite. The above results indicate that the Li ions have B-site preference and therefore it is interesting to study the sites occupancy of Li ions in the presence of the cations having strong B-site preference. Kapitonove [6] studied the distribution of cations in Li0.5Fex Ga2.5−x O4 with varying composition and preparation conditions. Nickel ferrite (NiFe2O4, x = 0.0) is an inverse spinel with all the Ni ions on B-sites. In order to study the relative site preferences for Li1+ and Ni2+ ion for octahedral site in spinel lattice, the spinel series Li0.5x Ni1−x Fe2+0.5x O4 with x = 0.0–0.8 has been prepared. The cation distributions have been determined through X-ray diffraction and confirmed by magnetization measurements. It is found that a greater percentage of Li1+ ions occupy the A-sites in the Ni-rich samples. This is reflected in the variation of lattice constant, magnetization and Neel temperature with Li-substitution supported by IR spectroscopy. The thermal variation of low field a.c. susceptibility exhibits normal ferrimagnetic behavior. Eight compositions of Li-Ni system were prepared by standard ceramic technique where stoichiometric proportions of Li2Co3, Fe2O3 and NiO powder were ground in acetone. The homogeneous mass was pelletized using 2% solution of polyvinyl acetate as binder medium. The pellets were presintered at 1000 ◦C for 12 h. In the final sintering process the materials were held at 1200 ◦C for 12 h and furnace cooled at the rate of 2 ◦C per min to room temperature. The X-ray diffraction patterns were recorded for all the samples on a Philips diffractometer model PW 1710. The magnetization measurements were made at 300 K using the high field hysterisis loop tracer. The low field (39.8 A/m) a.c. susceptibility measurements of powdered samples were taken in the temperature range 300–900 K using the double coil set-up, operating at a frequency of 263 Hz. The infrared spectra were recorded at 300 K on Perkin-Elemer IR spectrometer in KBr medium, in the frequency range 1000 cm−1–400 cm−1. The X-ray diffraction (XRD) patterns showed that the samples were single phase spinels. No reflections other than those belonging to a spinel structure were observed in the patterns. The variation of lattice constant a (nm) with content x is shown in Fig. 1. From Fig. 1 it is seen that the lattice constant value is not much influenced by Li1+ substitution (within the error) in NiFe2O4 (x = 0.0) up to x = 0.3, thereafter, it gradually decreases with the average rate of 0.0008 nm per 10% of Li1+-substitution. This happens for the Ni-rich samples (x 0.3, a greater fraction of Li1+ migrates to octahedral (B) sites with decreasing Ni concentrations. The X-ray density (dx) decreases with Li1+ substitutions because of the fact that the rate of decrease of molecular weight is faster than that of the volume of the unit cell per 10% of lithium concentration. The XRD intensities were calculated by using the formula suggested by Burger [7].

Synthesis of Li–Mn spinel oxide using Mn 2O 3 particles

Li–Mn spinel oxide cathode materials were prepared using Mn 2O 3 particles, which were derived from the thermal decomposition of Mn–acetate or Mn–oxalate particles. The effects of the molar ratio of Li/Mn and the reaction temperature on the produced phase were studied. From the lattice constant variation, it was found that the spinel oxides prepared by low-temperature reaction were cation deficient type. The cation deficiency decreased with an increase in the reaction temperature, due to the lowering in the oxidation potential. The temperature, in which the cation deficient degree attained was almost zero, was dependent on the molar ratio of Li/Mn and the kind of the Mn 2O 3 starting material. The higher temperature reaction produced spinel oxides with oxygen deficiency. The effect of the preparation conditions on the reversible Li-intercalation/deintercalation reaction was also examined from the view points of the structural deficiency. These experimental results implied that the defect structure in the Li–Mn spinel oxide is not always in the thermal equilibrium state, but depends on the other factors.

Molecular Dynamics Studies of the Phase Transitions of Homopolymers of p-Hydroxybenzoic Acid

Studies of liquid crystals composed of the homopolymer of p-hydroxybenzoic acid are presented based upon force-field (FF) and molecular dynamics (MD) methods. Structural data has been extracted from MD simulations to characterize the nature of the high-temperature phase transitions which have been experimentally identified for these polymers. Discontinuities in the isobaric heat capacity calculated as a function of temperature are taken to be signatures for the various phase transitions that HBA undergoes. Typically, the structural transitions are predicted by the simulations at temperatures agreeing within 10 to 15 °C of the transitions observed in differential scanning calorimetry measurements. Variations in crystallographic lattice constants determined from X-ray diffraction patterns are reproduced well by the simulations. The transition from an orthorhombic unit cell in a low-temperature phase (below 340 °C) to a pseudohexagonal unit cell at higher temperatures is experimentally observed for HBA homopolymers and has also been observed in the course of these simulations. In general, the simulations and methods applied are suitable for a description of the occurrence of phase transitions within these novel liquid crystal materials, are useful for characterization of the packing and relative orientations between chains, as well as providing a detailed description of the reorganization of the phenyl rings and ester linkages along the chain axes.

Bandgap Variation and Miscibility Gaps of Thallium-Based Pseudo-Binary Alloys

AbstractTo stimulate further research on the TI-based semiconductor alloys for long-wave infrared (LWIR) application, we carried out detailed calculations for the variation of band gaps versus lattice constants and the alloy miscibility gaps of cation-substituted pseudo-binary alloys. These results are used to identify promising TI-based LWIR alloys and the lattice-matched non-TI bearing alloys for possible substrates and passivation caps. Although all the six Tl-bearing pseudo-binary alloys studied can have a 0.1 eV band gap, none of these systems are lattice matched to a pure III-V compound. Five of these six alloys can have a non-TI-bearing lattice-matched ternary alloy as the substrate, but only two do not have a serious problem with the miscibility gap. When further considering the difficulty of incorporating TI with a large concentration, the only promising LWIR pseudo-binary alloy left is In1−xTlxAs with x≍0. 16.

Crystallite Sizes and Lattice Distortions of Gel-Spun Ultra-High Molecular Weight Polyethylene Fibers

A detailed method is reported for deriving the lattice distortions and crystallite sizes of ultra-high molecular weight polyethylene (UHMWPE) fibers employing a full wide angle X-ray diffraction (WAXD) pattern. The predominant crystals are orthorhombic form and the refined unit cell dimensions for a and b are 7.40(±0.03) and 4.94(±0.03) A, respectively. The average crystallite size normal to 110 planes was found to be in the range of 163–182 A. The microstrain lattice distortion was predominant in the gel-spun UHMWPE fibers, with the lattice constant variation of 0.5%.

Nonlinear magneto-optics of Fe monolayers from first principles: Structural dependence and spin-orbit coupling strength

We calculate the nonlinear magneto-optical response of free-standing fcc (001), (110), and (111) oriented Fe monolayers. The band structures are determined from first principles using a full-potential linear augmented plane wave method with the additional implementation of spin-orbit coupling. The variation of the spin-orbit coupling strength and the nonlinear magneto-optical spectra upon layer orientation are investigated. We find characteristic differences that indicate an enhanced sensitivity of nonlinear magneto-optics to surface orientation and variation of the in-plane lattice constants. In particular, the crossover from one-dimensional stripe structures to two-dimensional films of (111) layers exhibits a clean signature in the nonlinear Kerr spectra and demonstrates the versatility of nonlinear magneto-optics as a tool for in situ thin-film analysis.

Phase characterisation of simulated high burn-up UO 2 fuel

To determine the lattice constant variation with burn-up and the oxidation behaviour at high temperatures, simulated UO 2-spent fuels with burn-ups up to 200 GWd tM −1 were prepared by cold pressing and sintering and characterised by X-ray diffraction analysis and thermogravimetry. The fcc-lattice parameter of the sintered samples determined at room temperature, showed a continuous decrease with the simulated burn-up. During oxidation the conversion of the simulated fuels to the (U,FP) 4O 9 and (U,FP) 3O 8-type phases was strongly delayed when the simulated burn-up exceeded 80 GWd tM −1. Also, at high burn-ups (>100 GWd tM −1), large fractions of the (U,FP) 4O 9-type phase remained untransformed to (U,FP) 3O 8 after prolonged exposure to air at 950–1000°C. This suggests a marked widening of the (U,FP) 4O 9-phase field when the amount of substituted uranium atoms exceeds values of about 8%.

Metal-insulator transition in the Pr substituted Bi-2212 bulk textured crystals

Bulk textured crystals of Bi 2.2Sr 1.9Ca 1− x Pr xCu 2O 8 (0.0 ≤ x ≤ 0.6) were grown by the platinum strip heater floating zone technique. The effect of Pr on the growth and superconducting properties was studied. From the X-ray diffraction studies, variation of lattice constants, phase purity and texturing quantification with respect to Pr substitution were calculated. Variation in the Sr/Ca ratio upon Pr doping was observed. Metal insulator transition was observed in the higher order Pr substituted crystals.

Structural transition of La.Ba.Cu.O doped with iron

Crystal structures of LaBa 2Cu 3−xFe xO y with 0.0 ≤ rmx ≤ 0.25 shows an anomalous structural phase transition depending on Fe contents. With Fe content increasing the structure varies in the form of orthorhombic-to-tetragonal phases and then regeneration of orthorhombicity from tetragonal phase, which is observed from the separation or overlap of (0 2 0) and (2 0 0) lines in the X-ray diffractograms. Variation of lattice constants a, b and c with Fe content also supports by the anomalous phase transition.

Effects of P Doping in Nd2Fe17 and Phase Diagram of Fe Rich Nd-Fe-P

It was found that a P-doping process occurs in Nd2Fe17 by gas phase reaction around 673 K. A decrease of the lattice constants a and c by up to 0.6 and 0.8%, respectively, and an increase of the Curie temperature by up to 30 K were observed. The P-doped 2:17 phase dissolves around 763 K. Four phase regions in the phase diagram of Fe rich Nd-Fe-P alloys below 1223 K were determined. The phases in each region are: A: Nd 2 Fe 17 +α-Fe+NdP; B: α-Fe+NdP+ Nd 2 Fe 12 P 7 ; C: α-Fe + Nd 2 Fe 12 P 7 + Fe 3 P; D: Nd 2 Fe 12 P 7 + Fe 2 P + Fe 3 P. The variation of the lattice constants with composition of the alloys is as large as (Δa/a) max 0.8% for NdP and (Δa/a) max 1.2% and (Δc/c) max 0.8% for Nd 2 Fe 12 P 7 suggesting a certain soluble range for the compounds.

Atomic Scale Aluminum and Strain Distribution in a Gan/AlxGa1−XN Heterostructure

AbstractA GaN/AlxGalxN multi-quantum well test structure with Al concentrations 0 ≤ xAl ≤ 1 was utilized to investigate the growth of AlxGal–xN barrier layers deposited by metal organic chemical vapor deposition (MOCVD). A transition from a two dimensional (2D) to a three dimensional (3D) growth mode was observed in AlxGa1–xN barriers with XAl ≥ 0.75. It is argued that the transition occurs because of growth at temperatures that are low compared with the materials melting points Tmelt. The resulting rough AlxGa1–xN surfaces can be planarized by overgrowth with GaN. Quantitative high resolution electron microscopy (HREM) was applied to measure composition and strain profiles across the GaN/AlxGa1−xN stacks at an atomic level. The measurements reveal a substantial variation of lattice constants at the AlxGa1−xN/GaN interfaces that is attributed to an Al accumulation.

Elastic energy anisotropy effects on antiphase boundaries

The effect of elastic energy on the anisotropy of antiphase boundary (APB) interfacial energy has been studied theoretically. The model used, a generalization of one developed by Kikuchi and Cahn, treats composition and atomic ordering as local variables defined on each of the atomic planes parallel to the APB. The equilibrium APB structure is found as the set of planar variables that minimizes the free energy. A strain energy term involving an orientation-dependent stiffness parameter and a phenomenological law giving lattice constant as a function of local composition and order parameter is included in the definition of the free energy. Elastic effects in APBs in B2 ordered FeAl alloys with 0.2 – 0.3 atomic fraction Al were investigated. It was found that, despite the high elastic anisotropy of FeAl alloys, elastic effects would not result in any substantial anisotropy in APB interfacial energy. This is because in FeAl alloys the variation in lattice constant due to order parameter variation at the APB is almost completely cancelled out by the variation in lattice constant due to compositional variation. In a hypothetical model with physically reasonable parameters, APB energies were found to vary by up to 9% with orientation, purely as a result of elastic effects. It was concluded that elastically induced APB anisotropy would be observable in alloys such as FeSi in which lattice constant variations due to order parameter variation are cooperative with those due to compositional variation.

Refractive index and lattice constant variations in flux grown KTP crystals

The refractive indices and lattice constants of large KTP single crystals grown from a flux of K 6P 4O 13 have been investigated. Changes in their refractive index and lattice constant along the growth direction result from compositional deviation of the solution. Strong correlation is implied in the similar behavior of both the lattice constant and the refractive index. The lattice constant dependence on the compositional deviation seems to be due to variations in the solid solution range among the KTP components.

Anomalies in the pressure dependence of the effective charge in cubic semiconductors.

We have investigated the effect of hydrostatic pressure on the Born effective charge of tetrahedral semiconductors by means of first-principles pseudopotential calculations. In agreement with recent experiments, we find that the effective charge of IV-IV compounds, such as SiC, increases in magnitude with pressure, whereas that of III-V compounds decreases. We explain this behavior in terms of conflicting effects associated with the lattice-constant variation of the long- and short-range components of the crystal potential. We show that an analogous anomaly exists in the pressure dependence of the bond polarity, as measured by the asymmetry of the static valence charge.