Defect structure of β-LiAl

Lattice parameter and density measurements have been made on a series of Li-Al alloys extending over the LiAl β-phase. The lattice parameter of the slowly cooled β-phase alloys varied almost linearly with composition within the phase field. Theoretical density calculations for several single-defect models from lattice parameter data did not yield a good fit with the experimental density data. Rather, a model based on the coexistence of two types of defects, namely, vacancies in the lithium sublattice and lithium antistructure atoms in the aluminum sublattice, characterized the defect structure. Calculated concentrations of these defects varied with departure from stoichiometry and showed good agreement with the concentration of lithium vacancies estimated from NMR measurements. Compositional dependence of the vacancy concentration and the lithium diffusion coefficient produced a good correlation and strongly suggested that the lithium atoms diffuse via a vacancy mechanism. It is hypothesized that the defect structure of this compound may be related to the electronic structure of this so-called NaTl phase.     

A determination of the structure of liquid Ga2Te3 using combined X-ray diffraction and neutron diffraction with isotopic substitution

The partial structure factors of liquid Ga₂Te₃ have been determined using a combination of neutron diffraction with isotopic substitution (NDIS) and anomalous X-ray diffraction around the Te K edge. The inclusion of X-ray diffraction data significantly improves the statistical accuracy of the partial structure factors obtained compared with those obtained by the NDIS method alone. The results show that the liquid is hetero-coordinated in a way similar to that of typical two-component glasses or ionic liquids. There is no evidence, within the accuracy of this experiment, for substantial numbers of homopolar bonds as suggested by recent ab initio molecular dynamics simulations. The bond distances and coordination numbers show strong similarities to the tetrahedrally bonded defective zinc blende structure of the crystalline solid before melting.     

Crystal structure determination of Pb2Sr2YCu3O8+δ (δ=0, 1.67) by powder neutron diffraction

Crystal structures of Pb₂Sr₂YCu₃O_8+δ(δ=0.0 and 1.67) were determined by powder neutron diffraction. For δ=0, it had been indicated by powder X-ray diffraction that the structure was a monoclinic one. The R factors of the present analysis for the monoclinic structure were about 3/4 of those for the previous orthorhombic ones. The structure was almost the same as the previous X-ray result except z(O(2)). For the annealed phase, the tetragonal and the orthorhombic structures had been presented by X-ray diffraction and by neutron diffraction, respectively. The R factors of the present analysis for the orthorhombic structure were about 5/6 of those for the tetragonal structure. The distance between the Pb plane and the oxygen-deficient Cu plane was found to become larger in proportion to δ, though there occurs a phase separation in the low-δ region.     

High-resolution neutron powder diffraction study on the structure of BaPbO3

Using high-resolution time-of-flight neutron powder diffraction, the crystal structure of BaPbO₃ has been reinvestigated at room temperature and 4.2 K. By comparing different structural models, i.e. the orthorhombic Imma and the monoclinic I2/m, it is concluded that the former one describes correctly the structure of BaPbO₃, and no Imma→I2/m phase transition exists in the temperature range investigated. The apparent monoclinic distortion is likely due to the existence of twins that introduce the micro strain resulting in anisotropic line broadening of the observed profiles.     

Defect characterization of slow-cooled and quenched samples of calcium-copper-titanate through positron annihilation spectroscopy

Abstract

Positron annihilation lifetime and Doppler-broadened gamma-ray spectra have been analyzed for slow-cooled and thermally quenched polycrystalline samples of calcium-copper-titanate. Two positron lifetimes revealing the characteristic defects in the respective samples were carefully analyzed to compare and contrast the significance of their origin and implication. A third component arising from positronium formation at the powdered particle surfaces has been considered in the analysis although its significance is lost in its very small intensity (～ 1.1–1.2%). In the quenched sample, the defect-specific long positron lifetime (τ₂) is found to larger and the mean lifetime smaller while its intensity I₂ is found drastically smaller and the concentration of defects less by an order of magnitude as compared to the slow-cooled sample. The observed changes in electrical parameters of slow-cooled and quenched samples were found to have correlations with the positron annihilation lifetime and Doppler-broadened lineshape parameters.     

Crystal structure determination of Pb2Sr2YCu3O8+δ (δ=0, 1.67) by powder neutron diffraction

Crystal structures of Pb₂Sr₂YCu₃O_8+δ(δ=0.0 and 1.67) were determined by powder neutron diffraction. For δ=0, it had been indicated by powder X-ray diffraction that the structure was a monoclinic one. The R factors of the present analysis for the monoclinic structure were about of those for the previous orthorhombic ones. The structure was almost the same as the previous X-ray result except z(O(2)). For the annealed phase, the tetragonal and the orthorhombic structures had been presented by X-ray diffraction and by neutron diffraction, respectively. The R factors of the present analysis for the orthorhombic structure were about of those for the tetragonal structure. The distance between the Pb plane and the oxygen-deficient Cu plane was found to become larger in proportion to δ, though there occurs a phase separation in the low-δ region.     

Structural behaviour of AgNO3 at low temperatures by neutron diffraction

Structural behaviour of silver nitrate (AgNO₃) at low temperatures has been investigated by neutron powder diffraction and differential scanning calorimetry (DSC). Analysis showed abnormal changes in the rotations of nitrate (NO₃) anions and thermal displacement parameters of the atoms near 220 K and 125 K. However, the basic lattice is compatible with the orthorhombic symmetry (space group Pbca) till 12 K. The fine, small-scale structural anomalies probably originate from freezing of reorientation of NO₃ ions from high-temperature disordered phase.      

3D structure of dendritic and hyper-branched macromolecules by X-ray diffraction

The atomic ordering in dendritic and hyper-branched macromolecules has been determined by X-ray diffraction. The approach of the atomic pair distribution function technique has been used due to the lack of 3D periodicity in these polymeric materials. Dendrimers are found to possess a semi-regular structure riddled with nanosize cavities. The cavities are joined into channels connecting dendrimer's surface and core. In contrast, hyper-branched polymers are rather irregular at the atomic scale and with less accessible interior.     

Crystal structure of Fe-doped lanthanum gallate for oxygen partial pressures studied by neutron powder diffraction

Neutron powder diffraction experiments were carried out to investigate a change in a crystal structure of La_0.8Sr_0.2Ga_0.65Fe_0.35O₃ for oxygen partial pressures, P_O2, at 800 °C. The crystal structure was refined on the basis of the R3?c symmetry for the P_O2 range from 10^? 1 to 10^? 20 atm, by the Rietveld analysis. It was found that lattice parameters, a and c, monotonically expand with decreasing P_O2, and then both expansions are rapidly suppressed below 10^? 4 atm. In the meantime, l_M–O and l_O–O(2) also discontinuously increased with decreasing P_O2, while l_O–O(1) did not change at all P_O2, where l_M–O, l_O–O(1) and l_O–O(2) are the bond lengths within a MO₆ octahedron (M = Ga_0.65Fe_0.35). This result indicates that the l_M–O and the l_O–O(2) are more important than the l_O–O(1) for such a complicated lattice expansion for P_O2.     

Effects of proton irradiation on structure of NdFeB permanent magnets studied by X-ray diffraction and X-ray absorption fine structure

The effects of proton irradiation on the structure of NdFeB permanent magnet were investigated by X-ray diffraction and X-ray absorption fine structure (XAFS). The results reveal that proton irradiation has no effect on the long-range structure, but significantly affects the atomic local structure of the NdFeB magnet. The alignment degree of the magnet decreases and the internal stress of the lattice increases after proton irradiation. XAFS results show that the coordination number of Fe-Nd in the first neighboring coordination shell of the Fe atoms decreases and the disorder degree increases.     

The crystal structure of the prototypic ceramic superconductor BaPbO3: An X-ray and neutron diffraction study

The structure of the distorted perovskite BaPbO₃ was studied with high-resolution X-ray diffraction at 300 K and 26 K and with neutron diffraction at 300 K. Simultaneous refinement of the neutron and X-ray data sets (300 K) using the Rietveld method yields a monoclinic structure with the space groupI 2/m and lattice parametersa=6.0278 (1) Å,b=6.0664(1) Å,c=8.5109(1) Å, and =90.083 (2)^o. The tilting of the oxygen octahedra is given asa ^– a ^– c ⁰ in Glazer's notation [11]. The monoclinic angle corresponds to the angle between the cubic directions [110] _c and _c . This is in contrast to the observations in BaBiO₃. The structure of BaBiO₃ has the same space groupI 2/m, the similar dimensions of the unit cell and the same tilt system, but = ([110] _c , [001] _c ) as the monoclinic angle. As a consequence there is only one type and size of, PbO₆ octahedra but two types of octahedra in BaBiO₃. This fact may influence the occurrence of superconductivity in solid solutions (Ba(Pb_1–x Bi _x )O₃ containing a large fraction of lead by enhancing valence fluctuations.     

Crystal structure, electrical and magnetic properties of La1 − xSrxCoO3 − y

The crystal structure and properties of La_{1 − x}Sr_xCoO_{3 − y} with strontium contents ranging from x = 0.1 to x = 0.7 have been studied. The lattice parameters were measured as a function of temperature (4.2–400 K) and the crystal structure was found to change from rhombohedral (at low temperatures and values of x) to cubic. While LaCoO₃ is paramagnetic the oxides in the composition range 0.2 < x < 0.6 are soft ferromagnets. The strontium additions are compensated by the formation of Co⁴⁺ (cobalt ions with one positive effective charge, Co_Co^.) and oxygen vacancies (V_o^..). From the results it is concluded that the relative importance of oxygen vacancies increases with increasing temperature and decreasing oxygen activity. As a result the concentration of electronic charge carriers — and the resultant electrical conductivity — decrease with increasing temperature. The defect structure is discussed and it is concluded that defect associations — probably between oxygen vacancies and strontium ions — and formation of microdomains of perovskite-related phases are important aspects of the overall structure of these perovskite phases.     

Perovskite-like system (Sr,La)(Fe,Ga)O3−δ: structure and ionic transport under oxidizing conditions

The maximum solid solubility of gallium in the perovskite-type La_1−xSr_xFe_1−yGa_yO_3−δ (x=0.40–0.80; y=0–0.60) was found to vary in the approximate range y=0.25–0.45, decreasing when x increases. Crystal lattice of the perovskite phases, formed in atmospheric air, was studied by X-ray diffraction (XRD) and neutron diffraction and identified as cubic. Doping with Ga results in increasing unit cell volume, while the thermal expansion and total conductivity of (La,Sr)(Fe,Ga)O_3−δ in air decrease with gallium additions. The average thermal expansion coefficients (TECs) are in the range (11.7–16.0)×10⁻⁶ K⁻¹ at 300–800 K and (19.3–26.7)×10⁻⁶ K⁻¹ at 800–1100 K. At oxygen partial pressures close to atmospheric air, the oxygen permeation fluxes through La_1−xSr_xFe_1−yGa_yO_3−δ (x=0.7–0.8; y=0.2–0.4) membranes are determined by the bulk ambipolar conductivity; the limiting effect of the oxygen surface exchange was found negligible. Decreasing strontium and gallium concentrations leads to a greater role of the exchange processes. As for many other perovskite systems, the oxygen ionic conductivity of La_1−xSr_xFe_1−yGa_yO_3−δ increases with strontium content up to x=0.70 and decreases on further doping, probably due to association of oxygen vacancies. Incorporation of moderate amounts of gallium into the B sublattice results in increasing structural disorder, higher ionic conductivity at temperatures below 1170 K, and lower activation energy for the ionic transport.     

The application of neutron powder diffraction techniques for crystallographic studies at high pressures

Abstract

For phase transition studies, neutron powder diffraction offers a number of important advantages over x-ray based techniques, for example ab-initio structural determination. There are two distinct methods using either monochromatic angular dispersive geometry on a reactor cold source or time-of-flight energy-dispersive techniques requiring a pulsed neutron source. Both techniques offer comparable resolution but have differing advantages for high pressure studies. Recent studies illustrate the benefits of the two methods and the application of these to solve unknown crystal structures.     

Study of the atomic structure and dynamics of Cu-Fe nanocomposites by X-ray diffraction and inelastic neutron scattering

The atomic structure and the dynamics of Cu_{1 ? x} Fe _x samples (x = 0.15, 0.5, and 0.85) obtained by melt spinning from the elements that are immiscible under standard conditions has been investigated. The change in the physical properties of the system obtained in this way is due to the formation of nanocrystallites.     

Correlation of ferromagnetism and structure in Fe observed by high resolution X-ray diffraction

In order to find evidence of the ferromagnetic interaction, the crystal structure and the magnetic property of pure Fe are investigated from room temperature to 950 °C by measuring X-ray diffraction patterns and magnetizations. The ferro–paramagnetic transition occurs near 770 °C with large stretch and contraction of lattice constants, which is an evidence of correlation of ferromagnetism and crystal structure. The structural phase transition between α-bcc and γ-fcc is observed above 850 °C. The d-spaces between (011) and (101) planes, observed with a high resolution triple-axis X-ray diffractometer, differ at RT; Δd≡d₁₀₁–d₀₁₁≈0.45%. The structure of Fe is pseudocubic, showing a body-centered tetragonal structure of c/a<1 generated by magnetostriction (exchange striction) induced by spontaneous magnetization.     

Phonon–phonon interactions in Ce0.85Gd0.15O2−δ nanocrystals studied by Raman spectroscopy

Phonon–phonon interactions and phase stability of Gd‐doped ceria nanocrystals were examined over the temperature range 293–1100 K by Raman spectroscopy. The phonon confinement model (PCM) based on size, inhomogeneous strain and anharmonic effects was used to properly describe the anharmonic interactions in this system. The interplay between size and anharmonic effects influenced different phonon decay channels in nano grains than in larger grains. After the gradual cooling down to room temperature (RT), the Raman study revealed the phase separation in this system pointing to the phase instability of Ce_0.85Gd_0.15O_2−δ nanocrystals after heat treatment. The concentration of extrinsic (intrinsic) oxygen vacancies was also studied by Raman spectroscopy during the heat treatment of the Ce_0.85Gd_0.15O_2−δ nanocrystalline sample. Copyright © 2009 John Wiley & Sons, Ltd.     

Structural modifications of dodecatungstophosphoric acid hexahydrate induced by temperature in the 10–358 K range. In situ high-resolution neutron powder diffraction investigation

Dodecatungstophosphoric acid hexahydrate H₃PW₁₂O₄₀·6H₂O crystal structure has been investigated by neutron powder diffraction (NPD) at different temperatures in the 10–358 K range. A nonconvergent reversible phase transition has been noticed at about 320 K. This transition is associated with a change in dynamic equilibrium of hydrate species and partial reduction/oxidation (redox) W⁶⁺↔W⁵⁺. Expressive structure changes lie in the P---O bonding inside Keggin's anion and the H₅O₂⁺ conformational angle.