Increase In Unit Cell Volume Research Articles

Perovskite-like system (Sr,La)(Fe,Ga)O 3− δ: structure and ionic transport under oxidizing conditions

The maximum solid solubility of gallium in the perovskite-type La 1− x Sr x Fe 1− y Ga y O 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− x Sr x Fe 1− y Ga y O 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− x Sr x Fe 1− y Ga y O 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.

Thermal expansion anomaly of Pr2AlFe13Mn3 compound near its Curie temperature and magnetic properties of Pr2AlFe16−xMnx compounds

Structure and magnetic properties of Pr2AlFe16−xMnx (x=0–10) compounds have been investigated by means of x-ray diffraction and magnetization measurements. The Pr2AlFe16−xMnx compounds have a rhombohedral Th2Zn17-type structure. Their unit-cell volume increase slowly with increasing x, then rapidly with the further increase of x. This implies a strong positive spontaneous volume magnetostriction exists in the magnetic state for Pr2AlFe16−xMnx compounds. X-ray diffraction of Pr2AlFe13Mn3 compound from 160 to 300 K shows that a negative coefficient of thermal expansion exists, ᾱ≈−3.6×10−4/K, from 180 to 198 K. The Curie temperature and the saturation magnetization of Pr2AlFe16−xMnx compounds show a rapid drop with increasing x.

Alpha-Decay Radiation Damage Study of a Glass-Bonded Sodalite Ceramicwaste form

ABSTRACTEffects of alpha decay on the ceramic waste form used to immobilize fission products and actinides accumulated from electrometallurgical treatment of Department of Energy metallic spent nuclear fuel is being studied. This involved the preparation of a surrogate ceramic waste form containing two-weight percent plutonium-238 and monitoring changes to the waste form resulting from alpha-decay damage. The phase assembly of the ceramic waste form consists of glass-bonded sodalite with small quantities of nepheline, halite and plutonium dioxide. The waste form has been monitored for three years and has acquired an internal alpha-decay dose of 1 × 1018 disintegrations per gram of material. Methods used to monitor the material include: density determination, chemical durability measured by immersion testing, microscopy and powder xray diffraction. The investigation has, to date, found little alteration to the waste form due to alpha-decay damage. X-ray diffraction analysis has detected a unit cell volume increase of 0.7 percent of the plutonium dioxide phase. Furthermore, bubbles and/or voids have been observed by transmission electron microscopy in the sodalite and glass phase. The origin of these defects is unclear and under investigation.

Characteristics of Mg 2Ni 0.75M 0.25 (M=Ti, Cr, Mn, Fe, Co, Ni, Cu and Zn) alloys after surface treatment

The ternary Mg 2Ni 0.75M 0.25 (M=Ti, Cr, Mn, Fe, Co, Ni, Cu and Zn) alloys have been successfully synthesized by the ball milling diffusion method (BDM). XRD results show that they all have a hexagonal crystal structure. We show that the alloys after fluorination treatment show good performance: the hydrogen desorption capacity reaches the highest value after only two adsorption–desorption cycles. The ratio of H/M is about 1.18 (≈3.3 wt.%) on desorption at 250°C. The hydrides have monoclinic phases. The larger the unit volume, the more unstable is the hydride. The dissociation enthalpies and temperatures of the hydrides in an open system are decreased with increasing unit cell volume. Replacement of Ni in Mg 2Ni by Cr, Mn and Co has the same effect on Mg 2Ni: they lower the decomposition plateau pressure; Ti and Cu have the opposite effect and Fe and Zn have little effect.

STRUCTURAL AND SUPERCONDUCTING PROPERTIES OF La2CuO4+δ OXIDIZED BY KMnO4 UNDER HYDROTHERMAL CONDITIONS

A series of samples of La2CuO4+δ chemically oxidized in KMnO 4 solution under hydro-thermal conditions have been prepared. A large orthorhombic distortion and the increased unit cell volume of the oxidized sample have been observed by means of X-ray powder diffraction, compared with that of the starting material. Selected-area electron diffraction investigations reveal that two kinds of incommensurate modulation structures exist in the oxidized samples. Magnetic susceptibility measurements indicate that the oxidized samples are multiphase with two superconducting phases, one is dominant with onset of superconducting transition temperature at 43 K or 40 K, and the other at 19 K. Up to now, a T c of 43 K is the highest value achieved in KMnO 4-treated samples. The excess oxygen content determined by iodometric titration increases with the oxidation temperature. Average grain sizes observed by scanning electron microscope have no obvious changes for the samples before and after oxidation.

Magnetic and structural properties of SmCo 7− xTi x magnets

The main phase in as-cast SmCo 7− x Ti x ( x=0.3, 0.4, 0.5, 0.6) magnets is found to have the TbCu 7-type structure. The c/ a ratio decreases with increasing Ti concentration x, but the unit cell volume increase with x. A minor amount of a rhombohedral Th 2Zn 17-type phase was detected at x=0.3, 0.4, and an another Ce 2Ni 7-type phase with low Curie temperature appeared when x exceeds 0.5. No more than 0.1 T of intrinsic coercivity was obtained in the as-cast samples. After annealing, the TbCu 7-type SmCo 7− x Ti x magnets decomposed into 1 : 5 and 2 : 17 phases, with a slight increase in coercivity compared with the corresponding as-cast samples up to a maximum of 0.24 T. The as-milled amorphous SmCo 7− x Ti x magnets recrystallised with TbCu 7-type structure after annealing at 750°C for 2 h. High intrinsic coercivities of more than 2.0 T were obtained for almost all compositions, and maximum coercivity of 2.5 T was found for the ball-milled SmCo 6.6Ti 0.4 magnet. The high coercivity of the ball-milled SmCo 7− x Ti x magnets is attributed to the strong pinning of the walls of the ‘interaction domains’ by the network of grain boundaries between the nanocrystalline grains.

Specific protein dynamics near the solvent glass transition assayed by radiation-induced structural changes.

The nature of the dynamical coupling between a protein and its surrounding solvent is an important, yet open issue. Here we used temperature-dependent protein crystallography to study structural alterations that arise in the enzyme acetylcholinesterase upon X-ray irradiation at two temperatures: below and above the glass transition of the crystal solvent. A buried disulfide bond, a buried cysteine, and solvent exposed methionine residues show drastically increased radiation damage at 155 K, in comparison to 100 K. Additionally, the irradiation-induced unit cell volume increase is linear at 100 K, but not at 155 K, which is attributed to the increased solvent mobility at 155 K. Most importantly, we observed conformational changes in the catalytic triad at the active site at 155 K but not at 100 K. These changes lead to an inactive catalytic triad conformation and represent, therefore, the observation of radiation-inactivation of an enzyme at the atomic level. Our results show that at 155 K, the protein has acquired--at least locally--sufficient conformational flexibility to adapt to irradiation-induced alterations in the conformational energy landscape. The increased protein flexibility may be a direct consequence of the solvent glass transition, which expresses as dynamical changes in the enzyme's environment. Our results reveal the importance of protein and solvent dynamics in specific radiation damage to biological macromolecules, which in turn can serve as a tool to study protein flexibility and its relation to changes in a protein's environment.

Structure and magnetic properties of Nd 2(Co, Mn) 17 compounds

X-ray diffraction (XRD) and magnetization measurements were carried out to investigate the effect of Mn substitution for Co on the structure and magnetic properties of Nd 2Co 17− x Mn x compounds with x=0–8. XRD patterns show that all samples are single phase with the rhombohedral Th 2Zn 17-type structure except the samples with x=6–8 which contain a small amount of impurity. The substitution of Mn for Co leads to a monotonic increase in unit cell volume. The saturation magnetic moment first increases slightly for x=0–2 and then decreases quickly with increasing Mn concentration. The Curie temperature T C decreases monotonously with increasing Mn content. XRD measurements on magnetically aligned samples with x=0–6 exhibit an easy-plane type of magnetocrystalline anisotropy at room temperature. For x=1–5, a spin-reorientation transition is observed above room temperature in M– T curve. This is explained by that Mn substitution for Co leads to a change of the magnetocrystalline anisotropy of the 3d-sublattice from basal plane to c-axis and to temperature-induced competition between the 3d-sublattice and Nd-sublattice anisotropy. The magnetic phase diagram is given. The magnetocrystalline anisotropy field μ 0 H A is found to decrease sharply with Mn substitution for x⩽2 and then remains nearly unchanged with increasing Mn concentration, the anisotropy constant K 1 is negative for x=1–5 with the absolute value decreases sharply.

The structure and magnetic properties of Gd 2Co 17− xMn x ( x=0–4) compounds

X-Ray diffraction (XRD) and magnetization measurements were employed to investigate the effect of substitution of Mn for Co on the structure and magnetic properties of Gd 2Co 17− x Mn x ( x=0–4) compounds. XRD patterns show that all samples are single phase with rhombohedral Th 2Zn 17-type structure. The substitution of Mn for Co leads to a monotonic increase in unit cell volume. The saturation magnetization increases slightly for x≤2, then decreases slowly. The Curie temperature T C is found to decrease monotonically with increasing Mn concentration. The samples with x=1–4 exhibit a uniaxial anisotropy at room temperature as determined from the XRD measurement on magnetically aligned samples. The magnetocrystalline anisotropy field H A increases with increasing Mn concentration, attaining a maximum value of 43.7 kOe at x=4. Spin reorientation phenomena are observed in Gd 2Co 17 and off-stoichiometric Gd 2Co 16 compounds, which may be due to the different temperature dependence of the anisotropy of the Co sites.

Solvent behaviour in flash-cooled protein crystals at cryogenic temperatures

The solvent behaviour of flash-cooled protein crystals was studied in the range 100--180 K by X-ray diffraction. If the solvent is within large channels it crystallizes at 155 K, as identified by a sharp change in the increase of unit-cell volume upon temperature increase. In contrast, if a similar amount of solvent is confined to narrow channels and/or individual cavities it does not crystallize in the studied temperature range. It is concluded that the solvent in large channels behaves similarly to bulk water, whereas when confined to narrow channels it is mainly protein-associated. The analogy with the behaviour of pure bulk water provides circumstantial evidence that only solvent in large channels undergoes a glass transition in the 100--180 K temperature range. These studies reveal that flash-cooled protein crystals are arrested in a metastable state up to at least 155 K, thus providing an upper temperature limit for their storage and handling. The results are pertinent to the development of rational crystal annealing procedures and to the study of temperature-dependent radiation damage to proteins. Furthermore, they suggest an experimental paradigm for studying the correlation between solvent behaviour, protein dynamics and protein function.

Atomic resolution structures of trypsin provide insight into structural radiation damage

Radiation damage is an inherent problem in protein X-ray crystallography and the process has recently been shown to be highly specific, exhibiting features such as cleavage of disulfide bonds, decarboxylation of acidic residues, increase in atomic B factors and increase in unit-cell volume. Reported here are two trypsin structures at atomic resolution (1.00 and 0.95 A), the data for which were collected at a third-generation synchrotron (ESRF) at two different beamlines. Both trypsin structures exhibit broken disulfide bonds; in particular, the bond from Cys191 to Cys220 is very sensitive to synchrotron radiation. The data set collected at the most intense beamline (ID14-EH4) shows increased structural radiation damage in terms of lower occupancies for cysteine residues, more breakage in the six disulfide bonds and more alternate conformations. It appears that high intensity and not only the total X-ray dose is most harmful to protein crystals.

Crystal structure of Sm 2Fe 17− yM y compounds with M=Al, Si, Ga

The lattice parameters and atomic positions of the atoms, in particular the M-atoms, for arc-melted Sm 2Fe 17− y M y compounds with M=Ga, Al, Si and y=0, 1, 2 were determined using X-ray diffraction methods and subsequent Rietveld refinement. An increasing unit cell volume was found for M=Ga and Al, and a decreasing cell volume for substituting Fe by Si. Ga occupies the atomic position 18 h in the rhombohedral R-3m Zn 2Th 17-type structure in all cases, whereas Al and Si occupy the 18 h position up to a concentration of 1 atom per formula unit and for a larger concentration, additionally, the 18 f position. Despite of the different behaviour of the lattice constants, the nearest neighbour distances were found to behave similarly for all substituting elements in Sm 2Fe 17− y M y because of different shifts of the atomic sites.

Influence of Mn substitution on the structure and magnetic propertiesof Sm2Co17 compounds

The effect of Mn substitution for Co on the structure and magnetic propertiesof Sm2Co17-xMnx compounds with x = 0-8 were investigated by meansof x-ray diffraction (XRD) and magnetization measurements. XRD patterns showthat all samples are single phase with rhombohedral Th2Zn17-typestructure. The substitution of Mn for Co leads to a monotonic increase in unitcell volume. The saturation magnetization increases slightly for x⩽2, thendecreases quickly with increasing Mn content. The Curie temperature TC isfound to decrease monotonically with increasing Mn concentration. XRDmeasurement on magnetically aligned samples exhibits an easy-axis type ofmagnetic anisotropy at room temperature with x = 1-7. The magnetocrystallineanisotropy field µ0HA is found to increase with increasing Mnconcentration, attaining a maximum value of 14 T at x = 1.

ChemInform Abstract: X-Ray High-Temperature in situ Investigation of the Aluminide TiAl2 (HfGa2 Type).

Abstract X-ray high-temperature powder diffraction investigations were made in situ on the aluminide TiAl 2 (Pearson symbol tI 24, space group I 4 1 / amd , HfGa 2 type) in the temperature range 20–763°C. The unit cell parameters and the unit cell volume increase linearly with temperature, whereas the axial ratio c / a hardly changes with temperature. Metallographical and X-ray powder diffraction investigations on the heat-treated and subsequently quenched alloys show that TiAl 2 is stable up to 1200°C. In the as-cast alloy TiAl 2 , a metastable orthorhombic modification TiAl 2 (m) has been observed ( oC 12, Cmmm , ZrGa 2 type). X-ray powder diffraction data for the stable TiAl 2 modification are presented.

Electrical resistivity of the Kondo systems (Ce 1− xLa x)TSi 3[formula omitted

The electrical resistivities of the pseudo-ternary alloys (Ce 1− x La x )TSi 3 ( T= Rh, Ir; 0≤x≤1) are reported. The 4f-derived part of their resistivities, ρ 4f, is found by subtracting the temperature-dependent part of the resistivity of LaTSi 3. A maximum, characteristic of dense Kondo systems, is obtained in ρ 4f at a temperature T max =105 K for CeRhSi 3 and at 130 K for CeIrSi 3. T max decreases for both compounds with increased La concentration, x. X-ray powder diffraction was used to measure the increase in tetragonal unit cell volume V for the (Ce 1− x La x )TSi 3 alloys with increase in x. The compressible Kondo model is applied to describe our results of T max( x) in terms of the on-site Kondo exchange interaction J and the electronic density of states N( E F) at the Fermi level. The experimental results yield | JN( E F)| x=0 =0.072±0.016 for CeRhSi 3 and 0.070±0.001 for CeIrSi 3.

Structure and magnetic properties of ErFe11-xGaxTi

ErFe11-xGaxTi compounds with ThMn12-type structure were synthesized for x = 0,1, 2, 3, 4, 5. The crystalline structure and Ga occupancy in the compoundswere analysed in terms of the Rietveld method. With increasing Ga content, thelattice constant and unit-cell volume increase monotonically. Ti atoms arefound to occupy 8i sites for all the compounds. Bond length and Ga occupancyfactor are given. The Curie temperature increases first, going through amaximum and then decreases with increasing x while saturation magnetizationdecreases monotonically. Spin re-orientation from the uniaxial to the easy-conetype is detected for all the compounds as the temperature decreases from 300to 5 K. A tentative spin phase diagram is constructed for the ErFe11-xGaxTi. Inthe ErFe11-xGaxTi compounds, a magneto-history effect is observed which can beattributed to irreversible movement of narrow domain walls that are easilypinned.

Structure and properties of the MlNi 5 system with tin substitution

The hydriding performance, electrochemical properties and structure of MlNi 5− x Sn x ( x=0∼0.5) and LaNi 5− x Sn x ( x=0–0.2) hydrogen storage alloys were investigated by pressure–composition isotherms, electrochemical measurements, XRD and atomic parameters. The substitution of Ni by Sn leads to an increase of the unit cell volume and decrease of the electron concentration. The unit cell volume increase decreases the plateau pressure and improves the hydride stability and charge–discharge cycle life. The main factor which influences the standard enthalpy of the hydriding reaction is the number of the outer orbit electrons and not the atomic size factor. With a small amount of tin substitution (such as Sn=0.2), the cycle life increases by 52% (0.5C) and 42% (1.0C), but maximum discharge capacity decreases only by 3.0 and 3.5%, respectively.

Neutron diffraction studies of La 4BaCu 4MO 13+δ (M = Ni and Co) oxides

The structures of La 4BaCu 4NiO 13.20 and La 4BaCu 4CoO 13.35 oxides synthesized in NaOH–KOH flux at 450°C were studied using powder neutron diffraction Rietveld analysis. These oxides crystallize in a tetragonal structure in the P4/ m space group with Ni 3+ or Co 3+ partially occupying 6-coordinated (1a) and 5-coordinated (4j) sites of the La 4BaCu 5O 13+δ system. Increase in oxygen content with Ni and Co substitution is reflected in the increase in unit-cell volume, compared with that of the parent La 4BaCu 5O 13+δ.

X-ray high-temperature in situ investigation of the aluminide TiAl 2 (HfGa 2 type)

X-ray high-temperature powder diffraction investigations were made in situ on the aluminide TiAl 2 (Pearson symbol tI24, space group I4 1/ amd, HfGa 2 type) in the temperature range 20–763°C. The unit cell parameters and the unit cell volume increase linearly with temperature, whereas the axial ratio c/ a hardly changes with temperature. Metallographical and X-ray powder diffraction investigations on the heat-treated and subsequently quenched alloys show that TiAl 2 is stable up to 1200°C. In the as-cast alloy TiAl 2, a metastable orthorhombic modification TiAl 2(m) has been observed ( oC12, Cmmm, ZrGa 2 type). X-ray powder diffraction data for the stable TiAl 2 modification are presented.

Structure and magnetic properties of Tb2Co17−xMnx compounds

X-ray diffraction (XRD) and magnetization measurements were employed to investigate the effects of Mn substitution for Co on the structure and magnetic properties of Tb2Co17−xMnx compounds with x=0–7. XRD patterns show that all samples are single phase except the samples with x=6 and 7 which contain a small amount of impurity. The substitution of Mn for Co leads to a monotonic increase in unit cell volume. The saturation magnetization at 5 K remains nearly unchanged upon Mn substitution, while the Curie temperature decreases monotonically. XRD measurements on magnetically aligned powders with x=0–7 exhibit an easy-plane type of magnetic anisotropy at room temperature. For x=2, 3, and 4, spin reorientation transitions above room temperature are observed in the M–T curves. The magnetic phase diagram is given.