Synchrotron X-ray Powder Diffraction Methods Research Articles

Long-range A-site cation disorder in NaA(MO4)2 (M = Mo, W) double scheelite oxides

Synchrotron X-ray and neutron powder diffraction methods have been used to obtain accurate long-range average structures of some double scheelite compounds of the type NaA(BO4)2 (A ​= ​La, Pr, Nd, Sm, Lu, and Bi; B = Mo, W) at room temperature. Phase pure samples were synthesized using standard solid-state methods. Rietveld refinements using combined synchrotron X-ray diffraction (SXRD) and neutron diffraction (NPD) revealed a random distribution of the Na and A-type cations regardless of the presence of 6s2 lone pairs (such as Bi3+) and the difference in oxidation states and ionic radii between the cations. The NaA(BO4)2 (A ​= ​La, Pr, Nd, Sm, Lu, and Bi) series displayed linear trends in lattice parameters and AO8 polyhedra volume with the ionic radius of the A-type cation for the lanthanoids, but a deviation from the trend was observed for A ​= ​Bi3+. The NaBi(BO4)2 structure has a smaller than expected unit cell volume than based on extrapolation from the corresponding NaLn(BO4)2 series, possibly due to short-range ordering of the 6s2 lone pair electrons.

Beyond the ionic radii: A multifaceted approach to understand differences between the structures of LnNbO4 and LnTaO4 fergusonites

Synchrotron X-ray powder diffraction methods have been used to obtain accurate structures of the lanthanoid tantalates, LnTaO4, at room temperature. Three different structures are observed, depending on the size of the Ln cation: P21/c (Ln = La, Pr), I2/a (Ln = Nd-Ho), and P2/c (Ln = Tb-Lu). BVS analysis indicated that TaV is six-coordinate in these structures, with four short bonds and two longer bonds. Synchrotron X-ray powder diffraction methods were also used to observe the impact of Ta doping on the orthoniobates, Ln(Nb1-xTax)O4 (Ln = Pr, Nd, Sm, Gd, Tb, Dy, Ho, Yb, and Lu). Where both the niobate and tantalate oxide were isostructural (fergusonite structure, space group I2/a), complete solid solutions were prepared. In these solid solutions, the unit cell volume decreases as the Ta content increases. The subtle interaction evident between the LnO8 and BO6 sublattices in the fergusonite-type oxides was not observed in the related pyrochlore oxides. A combined synchrotron X-ray and neutron powder diffraction study of the series Ho(Nb1-xTax)O4 was used to determine accurate atomic positions of the anions, and hence, bond lengths. This revealed a change in the (Nb/Ta)-O bond lengths, reflective of the difference in the valence orbitals of Nb(4d) and Ta(5d). Examination of the partial density of states demonstrates differences in the electronics between Nb and Ta, leading to a difference in the bandgap. This study highlights the importance of the long B-O contacts in the fergusonite structures, and its potential impact on the I2/a to I41/a phase transition.

Composition, Thermal Expansion and Phase Transitions in Framework Silicates: Revisitation and Review of Natural and Synthetic Analogues of Nepheline-, Feldspar- and Leucite-Mineral Groups

Framework silicates form about 70% of the Earth’s crust, mainly feldspars ~50–60% and quartz ~10–15%. Less-abundant feldspathoids include nepheline-, leucite-, and sodalite-group minerals, rich in structurally challenging properties. This review paper deals with anhydrous feldspar-, nepheline-, and leucite/pollucite groups, emphasising the importance of parallel studies on natural and synthetic samples. Four topics are covered. For decades, petrologists have analysed nephelines and recalculated their compositions as endmember molecules but, by not following rules of stuffed-tridymite crystal chemistry, have not estimated reliably the excess SiO2 present in solid solution. Some materials scientists make similar mistakes, and a new approach is described here. Synthesis studies of analogue feldspars, nephelines, and leucite/pollucites led to collaborative studies, mainly using laboratory and synchrotron X-ray powder diffraction methods at room and elevated temperatures, to study thermal expansion and displacive phase transitions. Such work was recently expanded to address the spontaneous strain relations. Topics covered here include work on nepheline/kalsilite analogues in the system SrAl2O4—BaAl2O4; thermal expansion of (K,Na)Al-, RbAl-, RbGa-, and SrAl-feldspars; and thermal expansion and phase transitions in analogue leucites KGaSi2O6 (tetragonal to cubic) and K2MgSi5O12 (monoclinic to orthorhombic). Results are reviewed in the context of research published in mineralogical and more-widely in physical sciences journals.

Studies of the fergusonite to scheelite phase transition in LnNbO4 orthoniobates

Synchrotron X-ray powder diffraction methods have been used to obtain accurate structures of the lanthanoid orthoniobates, LnNbO4 at room temperature. The observed decrease in the unit cell volume in going from Ln = La to Lu correlates with a decrease in the volume of the LnO8 polyhedra due to the changing size of the Ln3+ cation. In all cases the Nb is in a highly distorted six-coordinate geometry with two long, ~2.5 Å, and four short ~1.9 Å Nb–O distances. The importance of the long Nb–O bonds is evident from Bond Valence Sum Calculations. Variable temperature structural studies of three examples, Ln = La, Sm and Gd, showed each transformed to a high temperature tetragonal scheelite type structure in space group I41/a. In the high temperature structure the Nb are tetrahedrally coordinated and it appears that monoclinic – tetragonal transition is best described as a reconstructive transition in which the long Nb–O(1) bond is broken upon heating.

Thermal expansion of pyrolusite, β-MnO2; a synchrotron X-ray diffraction study

High resolution synchrotron X-ray powder diffraction (S-XRD) methods have been used to study the temperature dependence of the structure of β-MnO2 between room temperature and 400 °C. Rietveld analysis of the S-XRD data provided accurate and precise until cell parameters from which the thermal expansion coefficients (TEC) were determined, αa = 7.8 × 10−6 K−1 and αc =7.2 × 10−6 K−1. The difference in the anisotropy in the TEC and the temperature dependence of the distortion of MnO6 and TiO2 suggests MnMn interactions across the shared edge are significant.

Structural studies of the high temperature phases of AgTaO3

The temperature dependence of the structure of a polycrystalline sample of AgTaO3 has been determined using in-situ Synchrotron X-ray powder diffraction methods. This work finds no evidence for the presence of a monoclinic phase, rather three phase transitions have been identified, namely R3c↔390°CCmcm↔465°CP4/mbm↔580°CPm3̅m. The rhombohedral phase was further studied at room temperature by neutron powder diffraction. The co-existence of the rhombohedral and orthorhombic phases around 380–400 ° C indicates that the transition between these is first order, and gives rise to unusual peak shapes in the diffraction patterns.

Magnetic and Structural Studies of Sc Containing Ruthenate Double Perovskites A2ScRuO6 (A = Ba, Sr)

Ruthenium-containing double perovskites A2ScRuO6 have been synthesized as polycrystalline powders and structurally characterized using a combination of synchrotron X-ray and neutron powder diffraction methods. When A = Ba, a hexagonal 6L perovskite structure is obtained if the synthesis is conducted at ambient pressure and a rock-salt ordered cubic structure is obtained if the sample is quenched from high pressures. The Sr oxide Sr2ScRuO6 is obtained with a rock-salt ordered corner sharing topology. Heat capacity and bulk magnetic susceptibility measurements show that the three oxides are antiferromagnets. Cubic Ba2ScRuO6 undergoes a metal-insulator transition near 270 K and hexagonal Ba2ScRuO6 is a semiconductor with an activation energy of 0.207 eV. The magnetic structures of the two rock-salt ordered double perovskites were established using powder neutron diffraction and are described by k = (0,0,1) and k = (0,0,0) for the Ba and Sr oxides, respectively, corresponding to type I antiferromagnetic structures, with ferromagnetic layers stacked antiferromagnetically. The ambient-pressure hexagonal polymorph of Ba2ScRuO6 has partial Sc-Ru ordering at both the face-sharing B2O9 dimer and corner-sharing BO6 sites. The magnetic structure is described by k = (1/2,0,0) with the basis vector belonging to the irreducible representation Γ3.

Structural and Magnetic Properties of the Osmium Double Perovskites Ba2-xSrxYOsO6.

The crystal and magnetic structures of double perovskites of the type Ba2-xSrxYOsO6 were studied by synchrotron X-ray and neutron powder diffraction methods, bulk magnetic susceptibility measurements, and X-ray absorption spectroscopy. The structures were refined using combined neutron and synchrotron data sets based on an ordered array of corner-sharing YO6 and OsO6 octahedra, with the Ba/Sr cations being completely disordered. The structure evolves from cubic to monoclinic Fm3̅m (x ≈ 0.6) → I4/m (x ≈ 1.0) → I2/m (x ≈ 1.6) → P21/n as the Sr content is increased, due to the introduction of cooperative tilting of the octahedra. Bulk magnetic susceptibility measurements demonstrate the oxides are all anti-ferromagnets. The decrease in symmetry results in a nonlinear increase in the Neel temperature. Low-temperature neutron diffraction measurements of selected examples show these to be type-I anti-ferromagnets. X-ray absorption spectra collected at the Os L3- and L2-edges confirm the Os is pentavalent in all cases, and there is no detectable change in the covalency of the Os cation as the A-cation changes. Analysis of the L3/L2 branching ratio shows that the spin-orbit coupling is constant and insignificant across the series.

Crystal structures and phase transitions in Sr doped Ba2InTaO6 perovskites

The crystal structures of the double perovskites of the type Ba2−xSrxInTaO6 have been studied by synchrotron X-ray and neutron powder diffraction methods. The structures are based on ordered array of the tilted InO6 and TaO6 octahedra, with the Ba/Sr cations being completely disordered. The structure evolves from cubic to monoclinic Fm3¯m→x~0.6I4/m→x~1.2I2/m→x~1.6P21/n. The changes in the symmetry induced by replacing the Ba with Sr have been identified by a symmetry-mode analysis. Variable temperature S-XRD studies of selected examples have shown that heating the samples induces the same sequence of transitions observed in the composition dependent studies.

Does Local Disorder Occur in the Pyrochlore Zirconates?

The zirconates Ln(2)Zr(2)O(7) (Ln = lanthanoid) have been studied using a combination of Zr L-edge X-ray absorption near edge structure (XANES) and synchrotron X-ray and neutron powder diffraction methods. These studies demonstrate that as the size of the lanthanoid cation decreases, the local structure evolves smoothly from the ideal pyrochlore toward the defect fluorite rather than undergoing an abrupt transformation. The Zr L-edge spectrum is found to be extremely sensitive to changes in the local coordination environment and demonstrates an increase in local disorder across the pyrochlore oxides. The sensitivity of the XANES measurements enables us to identify the progressive nature of the transition that could not be detected using bulk diffraction techniques.

Crystal structure and phase transitions in the uranium perovskite, Ba2SrUO6

The structure of one of the oxides proposed to be present in the grey phase of irradiated mixed oxide fuel, the double perovskite Ba2SrUO6 has been investigated from room temperature to 1300K using synchrotron X-ray powder diffraction methods. The divalent strontium and hexavalent uranium are found to be fully ordered in the double-perovskite arrangement of alternating octahedra sharing corner oxygen atoms. At room temperature Ba2SrUO6 adopts a monoclinic structure in space group P21/n. Heating to above 900K induces a first order transition to a rhombohedral structure, and further heating to above 1200K results in a continuous transition to a cubic structure. The sequence of structures is associated with the progressive loss of cooperative tilting of the corner sharing SrO6 and UO6 octahedra.

The effect of tocopherol on the structure and permeability of phosphatidylcholine liposomes

There are numerous phospholipid formulations that incorporate α-tocopherol as a stabilizing agent but there are few studies of the effect of α-tocopherol on phospholipid structure and bilayer permeability. This study uses synchrotron X-ray powder diffraction methods to investigate how α-tocopherol changes the structure of distearoylphosphatidylcholines bilayers. Increasing proportions of α-tocopherol up to 20mol% induces ripple structures in the bilayers. Two types of ripple structure are produced which are seen in electron micrographs of freeze-fracture replicas with periodicities of 16 and 12nm, respectively. The stoichiometry of phospholipid: α-tocopherol in the ripple structures at 37°C is 8:1. The presence of α-tocopherol tends to reduce the angle of tilt of the hydrocarbon chains of the phospholipid in the gel phase from about 34° to the bilayer normal at 20°C into a more vertical orientation. Increasing proportions of α-tocopherol progressively decrease the temperature of the gel to liquid-crystal phase transition of the phospholipid. The presence of up to 20mol% α-tocopherol in 1-palmitoyl-2-oleoyl-phosphocholine inhibits leakage of phenol red dye from liposomes. The effect of 7mol% α-tocopherol on leakage was compared with phospholipid liposomes containing 50mol% cholesterol. The cholesterol-containing liposomes inhibited leakage to a greater extent than the vesicles incorporating α-tocopherol but the effect of α-tocopherol at equivalent molar proportions was comparable to cholesterol.

The structure of complexes between phosphatidylethanolamine and glucosylceramide: A matrix for membrane rafts

Interaction between membrane lipids creates lateral domains within which essential membrane processes like trans-membrane signaling, differentiation etc. take place. Attention has focused on liquid-ordered phases formed by sphingomyelin and cholesterol but formation of ordered domains on the cytoplasmic membrane surfaces has largely been neglected. Synchrotron X-ray powder diffraction methods were used to investigate the interaction between two components of the cytoplasmic leaflet of the plasma membrane, phosphatidylethanolamine and glucosylceramide. Multilamellar dispersions of binary mixtures of different molecular species of phosphatidylethanolamine and glucosylceramide were examined. Stoichiometric complexes are formed when the phosphatidylethanolamine has at least one unsaturated fatty acid. The stoichiometry of the complexes was 2.0 fluid phospholipids per glucosylceramide with C22/24 N-acyl chains and 1.8 with C-12 chains. Saturated molecular species of phosphatidylethanolamines were immiscible with glucosylceramide. The complexes formed with unsaturated phosphatidylethanolamines and glucosylceramide are stable above physiological temperatures. A putative role of these matrices in membrane rafts is considered.

A synchrotron X‐ray diffraction characterization of the structure of complexes formed between sphingomyelin and cerebroside

Specific lipid-lipid interactions are believed to be responsible for lateral domain formation in the lipid bilayer matrix of cell membranes. The miscibility of glucocerebroside and sphingomyelin extracted from biological tissues has been examined by synchrotron X-ray powder diffraction methods. Fully hydrated binary mixtures of egg-sphingomyelin codispersed with glucosylceramide rich in saturated C22 and C24 N-acyl fatty acids were subjected to heating scans between 20 and 90 °C at 2 °C·min(-1). X-ray scattering intensity profiles were recorded at 1 °C intervals simultaneously in both small-angle and wide-angle scattering regions. A gel phase characterized by a single symmetric peak in the wide-angle scattering region was transformed in all mixtures examined to a fluid phase at about 40 °C, similar to dispersions of pure egg-sphingomyelin. A coexisting lamellar structure was identified at temperatures up to about 75 °C which was characterized by a broad Bragg reflection. The scattering intensity of this structure increased relative to the structure assigned as bilayers of pure sphingomyelin with increasing proportions of glucosylceramide in the mixture. The relationship between the scattering intensities of the two peaks and the relative mass fractions of the two lipids showed that the bilayers assigned to a glucosylceramide-rich structure were composed of sphingomyelin and glucosylceramide in molar ratios of 1 : 1 and 2 : 1, respectively, at temperatures below and above the order-disorder phase transition temperature of the sphingomyelin (40 °C).

Structure and cation ordering in spinel-type TcCo2O4. An example of a trivalent technetium oxide

The structure of TcCo(2)O(4) has been determined using a combination of synchrotron X-ray and neutron powder diffraction methods. It has an inverse spinel structure where the Tc occupies the octahedral sites. Both the refined Tc-O distance and X-ray absorption spectra suggest the Tc is predominantly trivalent.

An X‐ray diffraction study of model membrane raft structures

Protein sorting and assembly in membrane biogenesis and function involves the creation of ordered domains of lipids known as membrane rafts. The rafts are comprised of all the major classes of lipids, including glycerophospholipids, sphingolipids and sterol. Cholesterol is known to interact with sphingomyelin to form a liquid-ordered bilayer phase. Domains formed by sphingomyelin and cholesterol, however, represent relatively small proportions of the lipids found in membrane rafts and the properties of other raft lipids are not well characterized. We examined the structure of lipid bilayers comprised of aqueous dispersions of ternary mixtures of phosphatidylcholines and sphingomyelins from tissue extracts and cholesterol using synchrotron X-ray powder diffraction methods. Analysis of the Bragg reflections using peak-fitting methods enables the distinction of three coexisting bilayer structures: (a) a quasicrystalline structure comprised of equimolar proportions of phosphatidylcholine and sphingomyelin, (b) a liquid-ordered bilayer of phospholipid and cholesterol, and (c) fluid phospholipid bilayers. The structures have been assigned on the basis of lamellar repeat spacings, relative scattering intensities and bilayer thickness of binary and ternary lipid mixtures of varying composition subjected to thermal scans between 20 and 50 °C. The results suggest that the order created by the quasicrystalline phase may provide an appropriate scaffold for the organization and assembly of raft proteins on both sides of the membrane. Co-existing liquid-ordered structures comprised of phospholipid and cholesterol provides an additional membrane environment for assembly of different raft proteins.

Structural and Physical Properties of Heavily Doped Yttrium Vanadate: Y0.6Cd0.4VO3

Structural properties of Y0.6Cd0.4VO3 were investigated by electron diffraction and laboratory and synchrotron X-ray powder diffraction methods. Y0.6Cd0.4VO3 crystallizes in space group Pnma (GdFeO3-type perovskite structure) between 12 and 300 K (a = 5.45887(3) A, b = 7.57250(4) A, and c = 5.27643(2) A at 300 K). The lattice parameters showed anomalous behavior on temperature. The c parameter linearly decreased from 12 to 120 K, and then it lineally increased from 160 to 300 K. The b parameter was constant between 12 and 120 K, demonstrated a drop from 120 to 200 K, and then lineally increased from 200 to 300 K. The c/a ratio had a rather sharp maximum at 150 K. In Y0.6Cd0.4VO3 the V−O distances in the ac plane began to split to shorter and longer ones below 150 K, indicating that orbital fluctuations are involved. The phase transition near 150 K in Y0.6Cd0.4VO3 is accompanied by a broad anomaly on the specific heat and change of the slope of the inverse magnetic susceptibility. Other members of the Y1-x...

Investigation of the origin of deep levels in CdTe doped with Bi

Combining optical (low temperature photoluminescence), electrical (thermoelectric effect spectroscopy), and structural (synchrotron X-ray powder diffraction) methods, the defect structure of CdTe doped with Bi was studied in crystals with dopant concentration in the range of 1017–1019at.∕cm3. The semi-insulating state observed in crystals with low Bi concentration is assigned to the formation of a shallow donor level and a deep donor recombination center. Studying the evolution of lattice parameter with temperature, we postulate that the deep center is formed by a Te–Te dimer and their formation is explained by a tetrahedral to octahedral distortion, due to the introduction of Bi in the CdTe lattice. We also shows that this model agrees with the electrical, optical, and transport charge properties of the samples.

Phase segregation in mixed Nb–Sb double perovskites Ba 2LnNb 1−xSb xO 6

The phase composition of two series of mixed Nb 5+–Sb 5+ double perovskites formed between the pairs Ba 2EuNbO 6–Ba 2PrSbO 6 and Ba 2NdSbO 6–Ba 2NdNbO 6 have been studied using synchrotron X-ray powder diffraction methods. In both series extensive phase segregation is observed demonstrating limited solubility of Sb 5+ in these Nb 5+ perovskites, irrespective of the precise structures of the double perovskite. Evidence for a monoclinic I2/ m phase in the series formed between tetragonal I4/ m Ba 2EuNbO 6 and rhombohedral R 3 ¯ Ba 2EuNbO 6 is presented. It is postulated that this phase segregation is a consequence of competing bonding requirements of the Nb 5+ and Sb 5+ cations associated with their electronic configurations.

Combined neutron and synchrotron X-ray diffraction study of Sr/Mg-doped lanthanum gallates up to high temperatures

Abstract Combined neutron diffraction and high-resolution synchrotron X-ray powder diffraction methods have been used to examine the crystal structures of two sample sets of Sr/Mg-doped Lanthanum gallate with the compositions La0.9Sr0.1Ga1−yMgyO3−0.5(0.1+y) ( y = 0 , 0.1, 0.2) and La0.8Sr0.2Ga1−yMgyO3−0.5(0.2+y) ( y = 0.15 , 0.2) up to 900 °C. At room temperature all samples of the first series exhibit orthorhombic structures with space group Imma: La0.9Sr0.1GaO2.95: a = 5.4904 ( 1 ) A , b = 7.7757 ( 1 ) A , c = 5.5229 ( 1 ) A ; La0.9Sr0.1Ga0.9Mg0.1O2.9: a = 5.5100 ( 1 ) A , b = 7.8080 ( 1 ) A , c = 5.5411 ( 1 ) A ; La0.9Sr0.1Ga0.8Mg0.2O2.85: a = 5.5269 ( 1 ) A , b = 7.8318 ( 2 ) A , c = 5.5459 ( 1 ) A . The samples of the second series have the cubic perovskite structure with space group Pm 3 ¯ m at room temperature: La0.8Sr0.2Ga0.85Mg0.15O2.825: a = 3.9160 ( 1 ) A ; La0.8Sr0.2Ga0.8Mg0.20O2.80: a = 3.9195 ( 1 ) A . Samples of the first series transform from the orthorhombic to a rhombohedral (Imma→ R 3 ¯ c ) structure at ∼170 °C for La0.9Sr0.1GaO2.95, at ∼430 °C for La0.9Sr0.1Ga0.9Mg0.1O2.9, and between 600 and 700 °C for La0.9Sr0.1Ga0.8Mg0.2O2.85. Both La0.8Sr0.2Ga0.85Mg0.15O2.825 and La0.8Sr0.2Ga0.8Mg0.2 show no structural deviations from the cubic aristotype over the whole temperature range. The room temperature Imma structures of the first series are justified by a domain model and are rationalized in terms of static disorder increasing with Mg content, thus driving the phase transition temperatures to higher values in agreement with tolerance factor considerations. The distortion of the rhombohedral high-temperature phases (octahedra tilting and compression) and the effect of phase transitions on the ionic conductivity are discussed.