Synchrotron X-ray Powder Data Research Articles

On symmetry of rare-earth scandium borate RESc3(BO3)4 (RE = Ce, Nd) laser crystals

Symmetry, structural features, and point defects in the Czochralski-grown single crystals with the initial compositions CeSc3(BO3)4 (CSB-1.0) and Nd1.25Sc2.75(BO3)4 (NSB-1.25) are studied by the full-profile Rietveld method on synchrotron X-ray powder data. The space group P321 is first established and confirmed for CSB-1.0 and NSB-1.25 crystals, respectively. The features of synchrotron diffraction patterns that distinguish the space group Р321 from the R32 one are given. The site occupancies in both structures are refined and resulted in the stoichiometric composition for CeSc3(BO3)4, which coincides with the nominal composition, and the composition [Nd0.455(1)Sc0.045(39)(1)]Nd0.500(2)Sc3(BO3)4 ((Nd0.955Sc0.045(39))Sc3(BO3)4), which differs from the initial Nd1.25Sc2.75(BO3)4 one, with partial occurrence of Sc ions in one of the trigonal-prismatic sites. An analysis of literature data on single crystals with the general formula RESc3(BO3)4 (RE ​= ​Ce, Pr, Nd) together with the results of present study of powdered single crystals gives reason to exclude a crystallization of these compounds and solid solutions based on them in the space group R32. The correlation between the refined compositions of RESc3(BO3)4 crystals (RE ​= ​La, Ce, Pr, Nd), having a specific space group, and compositions of initial charge is given.

Crystal structure of the thortveitite-related M phase, (MnxZn1-x)2V2O7 (0.75 < x < 0.913): a combined synchrotron powder and single-crystal X-ray study.

The determination of the crystal structure of the M phase, (MnxZn1-x)2V2O7 (0.75 < x < 0.913), in the pseudobinary Mn2V2O7-Zn2V2O7 system for x ≃ 0.8 shows that the previously published triclinic unit-cell parameters for this thortveitite-related phase do not describe a true lattice for this phase. Instead, single-crystal X-ray data and Rietveld refinement of synchrotron X-ray powder data show that the M phase has a different triclinic structure in the space group P-1 with Z = 2. As prior work has suggested, the crystal structure can be described as a distorted version of the thortveitite crystal structure of β-Mn2V2O7. A twofold superstructure in diffraction patterns of crystals of the M phase used for single-crystal X-ray diffraction work arises from twinning by reticular pseudomerohedry. This superstructure can be described as a commensurate modulation of a pseudo-monoclinic basis structure closely related to the crystal structure of β-Mn2V2O7. In comparison with the distortions introduced when β-Mn2V2O7 transforms at low temperature to α-Mn2V2O7, the distortions which give rise to the M phase from the β-Mn2V2O7 prototype are noticeably less pronounced.

Metastable Ni7B3: A New Paramagnetic Boride from Solution Chemistry, Its Crystal Structure and Magnetic Properties.

We trapped an unknown metastable boride by applying low-temperature solution synthesis. Single-phase nickel boride, Ni7B3, was obtained as bulk samples of microcrystalline powders when annealing the amorphous, nanoscale precipitate that is formed in aqueous solution of nickel chloride upon reaction with sodium tetrahydridoborate. Its crystal structure was solved based on a disordered Th7Fe3-type model (hexagonal crystal system, space group P63mc, no. 186, a = 696.836(4) pm, c = 439.402(4) pm), using synchrotron X-ray powder data. Magnetic measurements reveal paramagnetism, which is in accordance with quantum chemical calculations. According to high-temperature X-ray diffraction and differential scanning calorimetry this nickel boride phase has a narrow stability window between 300 and 424 °C. It crystallizes at ca. 350 °C, then starts decomposing to form Ni3B and Ni2B above 375 °C, and shows an exothermic effect at 424 °C.

An α-rhombohedral boron-related compound with sulfur: Synthesis, structure and thermoelectric properties

The structural and thermoelectric properties of a series of icosahedral boron-rich solids, B6S1 − x obtained from the elements in a Ga melt were studied to estimate their potential as thermoelectric materials. Rietveld analysis of synchrotron and conventional X-ray powder data displayed the rhombohedral structure with the α-rhombohedral boron framework of B12 icosahedra and a narrow filling fraction range for sulfur atoms in the interstitial voids (0.37 < x ⩽ 0.40). The samples obtained exhibited p-type semiconducting behavior with large Seebeck coefficients, reaching a maximum of ∼220 μV K−1.

Crystal Chemistry and Cation Ordering in Zirconolite 2M

ABSTRACTStructural studies of single phase or nearly single phase zirconolite ceramic samples have been conducted using electron microscopy and microanalysis, X-ray diffraction, neutron diffraction, and spectroscopic methods. We show that it is possible to produce a complete series of zirconolite 2M samples with substitution of 2Ti by Nb+Fe in the HTB layer. The samples are single phase up to about 80% Nb +Fe substitution, with the appearance of a minor perovskite phase at higher Nb+Fe levels. Electron probe microanalysis reveals that the samples are homogeneous and close to their nominal compositions, except for those containing perovskite, which have a slight excess of Zr and a deficiency in the Fe content. The lattice parameters and the positions of certain Raman bands are non-linear as a function of composition, suggesting the possibility of cation ordering over the three available Ti sites within the HTB layer. Rietveld refinement of Synchrotron X-ray powder data for the Nb+Fe end-member have been conducted for the disordered case and for six trial models each with a different ordering scheme. Results of this exercise indicate that Fe preferentially occupies the Ti2 (split) site with partial ordering of Nb and the remaining Fe over the Ti1 and Ti2 octahedra. The preference of Fe for the five coordinated Ti2 site has been confirmed by 57Fe Mossbauer spectroscopy.

Accuracy in Rietveld quantitative phase analysis of Portland cements

The polymorphs that constitute most Portland cements have been synthesized: tricalcium silicate, dicalcium silicate, aluminate, ferrite, gypsum, bassanite and calcite. They have been used to prepare artificial mixtures,i.e.white Portland clinker, grey Portland clinker and two types of grey Portland cements. Quantitative mineralogical analyses of these mixtures have been obtained by laboratory X-ray powder diffraction (λ = 1.54 Å) and the Rietveld method. To assess the accuracy of these analyses, high-energy synchrotron X-ray powder data (λ = 0.40 Å) for the same mixtures have also been studied. Furthermore, synchrotron X-ray powder data were collected for binary mixtures of the polymorphs and a corundum standard. This was done to determine the presence of impurity crystalline phases in the synthesized samples and to check the presence of non-negligible amorphous phase contents. The errors in the synchrotron X-ray analyses are quite low (usually smaller than 1 wt%). The relative errors in the laboratory X-ray analyses are of the order of 2% for the main phases and increase to approximately 5–10% for the low-content components. These errors are acceptable in the factory environment and the routine application of this methodology in the cement industry is being implemented.

The incommensurate modulated structure of Sr 14−xCa xC 24O 41 as a function of temperature and composition

From synchrotron x-ray powder data collected on the BM16 beam line at ESRF, we have determined the incommensurate modulated structure of Sr 14−xCa xCu 24O 41 (x=6, 13.6) from 10K to 300K, using the XND software. No major structural change occurs as a function of temperature. The comparison of the atomic positions and CuO interatomic distances between the two compounds indicates a charge transfer from the chain to the ladder sublattice as the Ca content increases. Furthermore, whereas the charges appear localized in both sublattices for the x=6 compound, those on the ladder sublattice of the x=13.6 metallic compound are delocalized. This behavior has been correlated with the degree of disorder induced by the substitution in the adjacent Sr/Ca layer.

Direct Observation of Orbital Order in Manganites by MEM Charge-Density Study

We have investigated the charge-density distribution of an antiferromagnetic bi-layered manganite, NdSr 2 Mn 2 O 7 , by the maximum entropy method (MEM) using the synchrotron X-ray powder data. The magnetic structure below T N =150 K is known to be a layered-type (A-type). We have found a significant change of the electron distribution around the Mn site below T N . Our observation indicates d x 2 - y 2 -orbital polarization in the A-type antiferromagnetic state.

Synthesis, Characterization, and Structure Solution of CIT-5, a New, High-Silica, Extra-Large-Pore Molecular Sieve

The synthesis, structure solution, and characterization of the high-silica molecular sieve CIT-5 (California Institute of Technology Number 5) is described. CIT-5 is synthesized at hydrothermal conditions in the presence of N(16)-methylsparteinium and preferrably lithium cations. The structural solution of CIT-5 shows that it contains one-dimensional pores circumscribed by 14 tetrahedral atoms (14 MR). Rietveld refinement of the synchrotron X-ray powder data gives a symmetry and space group assignment for the structure of Pmn2_1 (no. 31) with refined unit cell parameters of a = 13.6738(8) A, b = 5.0216(3) A, and c = 25.4883(7) A (V = 1750.1 A^3). Electron diffraction and transmission electron microscopy confirm the space group and the topology of the structure viewed along the [010] direction. Solid-state ^(29)Si NMR spectroscopy results are consistent with the space group assignment. The thermal/hydrothermal stability of CIT-5 compares well to that of other large- and extra-large-pore, high-silica molecular sieves. The acid form of CIT-5 is able to perform hydrocarbon reactions such as cracking and alkylation and shows behaviors that are different from other zeolites.

Rietveld refinement of Ca2TiSiO6perovskite

The structure of the high-pressure cubic double perovskite Ca 2 TiSiO 6 was refined using synchrotron X-ray powder data [S.G. Fm3m, a = 7.4105(1) Aa, Z = 4]. Ti and Si are fully ordered into alternating octahedra in the perovskite framework, with Ca occupying a tetrahedrally distorted cuboctahedron. The cation-anion bond lengths are Ti-O = 1.947(3) Aa, Si-O = 1.758(3) Aa, and Ca-O = 2.622(1) Aa.

Structures and Phase Transition of Bi2CdO2[GeO4

The structure of Bi2CdO2[GeO4] was solvedab initioon the basis of laboratory and synchrotron X-ray powder data. The high temperature orthorhombic phase, stable at room temperature, crystallizes inPbcn(60),Z= 8,a= 10.4969(1) Å,b= 10.0944(1) Å,c= 5.60601(3) Å, andV= 594.01(1)Å3with 14 refinable positional parameters. The structure consists of [GeO4] tetrahedra, strongly distorted trigonal [CdO6] prisms and irregular [BiO4+2] polyhedra. At low temperatures, the material undergoes a small distortion to monoclinic symmetry (γ = 90.37° at 20 K).

VPI-8: A High-Silica Molecular Sieve with a Novel “Pinwheel” Building Unit and Its Implications for the Synthesis of Extra-Large Pore Molecular Sieves

Relatively large (up to 4 μm), needle-like crystals of the high-silica molecular sieve VPI-8 are synthesized by hydrothermal methods and are subsequently calcined to remove the pore-filling organic TEA+ (tetraethylammonium cation). Numerous physicochemical techniques are used to determine and characterize the structure of organic-free VPI-8. The structure is discussed with the use of two ordered topologies (1a and 1b) and an average structure (1c) that accounts for the possibility of structural disorder in one of the T-atoms (Si1). The symmetry and space group of the ordered structure type 1a (tetragonal, P4̄ (no. 81)) is supported by electron diffraction data and the Rietveld refinement of the synchrotron X-ray powder data based on 1a converges with the lowest agreement factors. The pore system of VPI-8 consists of one-dimensional channels containing 12-membered T-atom rings (12MRs) that run down the c-axis. HRTEM images along [001] confirm the general structure topology (the possible disorder occurs only along c). The structure of VPI-8 contains five unique T-atoms in atomic ratios of 1:4:4:4:4 per unit cell. These number densities are consistent with the data obtained from 29Si solid-state NMR spectra. The main feature of the VPI-8 structure is a novel “pinwheel” framework building unit that has not been observed before in microporous materials and that seems well-suited for forming extra-large pore molecular sieves. Hypothetical framework models of extra-large pore molecular sieves containing one-dimensional 14MR (2), 16MR (3), 18MR (4), and 20MR (5) channel systems based on this new building unit are presented and discussed.

Cs(TiAs)O 5 and Cs(TiP)O 5: A Disordered Parent Structure of ABOCO 4 Compounds

The structures of the two isostructural compounds Cs (TiAs)O 5 (CTA) and Cs(TiP)O 5 (CTP) were solved and refined by application of the Rietveld profile technique to high-resolution synchrotron X-ray and neutron powder data. The structures are cubic, space group F d3̄ m with a = 10.23415(5) Å (CTA) and a = 10.08733(2) Å (CTP), and are characterized by an unusual framework consisting of randomly disordered TiO 6 octahedra and XO 4 ( X = As, P) tetrahedra with the Cs + cation occupying large cavities within this framework. The lack of long-range order between octahedra and tetrahedra simulates partially occupied octahedra shared by Ti and X with large displacement parameters for both cations and oxygens. The Ti, X-site can be resolved as a split-atom position where the tetrahedral metal is shifted from the center toward the edge formed by the octahedral oxygens. Split-oxygen positions were refined from a combination of X-ray and neutron data for CTA. The cubic structure is related to the orthorhombic KTiOPO 4-type structure in that the latter can be derived from the former by ordering of the TiO 6 octahedra and PO 4 tetrahedra. Since different ordering schemes applied to the cubic CTA/CTP structure lead to other known ordered structures of the same stoichiometry such as K(Mg 1/3Nb 2/3)OPO 4 and γ-NaTiOPO 4, cubic CTA/CTP can be viewed as the "mother of all ABOCO 4-structures." The elucidation of the cubic structure allows us to propose a quantified "threatened structure model," which helps in the understanding of the mechanism of this phase transition.

Suppression of superconductivity in Hg-1223 and Hg-1234 by partial replacement of Hg by carbon

Hg based 1223 and 1234 phases were prepared by a high-pressure high-temperature technique using precursors obtained by the decomposition of BaO 2, CaCO 3 and CuO in oxygen flow at 925°C. These phases were found to exhibit much lower T c's (64 K for 1223 phase, and no superconductivity for the 1234 phase) than the corresponding Hg based samples prepared by different precursors and this even after heat treatments in oxygen flow. The structural refinement of this 1223 phase carried out by using synchrotron X-ray powder data revealed a large deficiency on the Hg site; the occupancy factor for this site was found to be 0.69(1). This may be due to the replacement of some of the Hg cations by C. By IR spectroscopy the characteristic frequencies corresponding to the vibrations of the CO 3 −2 group were found in these samples. Large amounts of intergrowth defects were defected by electron microscopy and no superstructure spots were found in the diffraction patterns. The replacement of Hg by C in the structures of the Hg based superconductors induces a decrease of the concentration of the extra oxygen in the (Hg, C) layer bonded to the Hg atoms. Since it is this oxygen which oxidizes the (CuO 2) layers, its decrease causes a reduction of these layers, which is responsible for the decrease of the superconducting transition temperatures.

Tetrahedral-Framework Lithium Zinc Phosphate Phases: Location of Light-Atom Positions in LiZnPO 4 · H 2O by Powder Neutron Diffraction and Structure Determination of LiZnPO 4 by ab initio Methods

The syntheses, crystal structures, and certain properties of two lithium zinc phosphate phases, LiZnPO 4 · H 2O and LiZnPO 4, are reported. LiZnPO 4 · H 2O is an isostructure of the Li- A-type zeolite LiAlSiO 4 · H 2O and consists of a fully ordered three-dimensional network of vertex-sharing ZnO 4 and PO 4 tetrahedral units surrounding 8-, 6-, and 4-ring windows. The extraframework lithium cation and water molecule are located in this cavity system. These nonframework species (including protons) were unambiguously located by Rietveld refinement against powder neutron data. The structure of LiZnPO 4 was solved ab initio using synchrotron X-ray powder data and consists of a new "semicondensed" tetrahedral-framework structure, incorporating the guest lithium cations in squashed 6-ring channels. LiZnPO 4 may be prepared from LiZnPO 4 · H 2O by a first-order phase transition which involves Zn/P/O bond breaking/making. This transformation is briefly discussed and related to similar transformations in other framework systems. Crystal data: LiZnPO 4 · H 2O: M r = 185.31, orthorhombic, space group Pna 2 1 (No. 33), a = 10.575(2) Å, b = 8.0759(9) Å, c = 4.9937(6) Å, V = 426.5(2) Å 3, Z = 4, T = 15(2) K, R p = 3.95%, R wp = 5.02%, χ 2 = 2.33. LiZnPO 4: M r = 167.29, orthorhombic, Pn 2 1 a (No. 33), a = 10.0207(2) Å, b = 6.6731(2) Å, and c = 4.96548(8) Å, V = 332.04(2) Å 3, Z = 4, T = 298(1) K, R p = 11.63%, R wp = 15.22%, χ 2 = 2.82.

The structure of zeolite ZSM-23 (MTT) refined from synchrotron X-ray powder data

The structure of zeolite ZSM-23 (structure-type code MTT) loaded with NH4F [ao = 11.129 (1), bo = 5.025 (1) and Co = 21.519 (1) Å and β = 89.85 (4)°] was refined from high-resolution synchrotron X-ray powder data. The Rietveld refinement converged in space group P1211 (No. 4) to RF = 0.090 and Rwp = 0.085, confirming the proposed model of the silicate framework. The use of complementary methods (synchrotron X-ray powder diffraction, 29Si magic angle-spinning nuclear-magnetic-resonance spectroscopy, and distance and angle least-squares refinements) was essential for the determination of the true space-group symmetry, a prerequisite for the successful Rietveld structure refinement.

Structures of C 60 intercalation compounds

In the past two years, many intercalation compounds of C 60, have been prepared and characterized by X-ray powder diffraction techniques. The M-C 60 systems with M = Li, Na, K, Rb, Cs and Ca have been studied, and the structures of several compounds have been determined in considerable detail, including the face-centered cubic superconductors K 3C 60 and Rb 3C 60, body-centered tetragonal K 4C 60, body-centered cubic M 6C 60 with M = K, Rb and Cs, simple hexagonal C 60I 4, and the monoclinic molecular ferromagnet tetrakis-dimethylamino-ethylene (TDAE) C 2N 4(CH 3) 8C 60. The Rietveld profile technique for structure refinement coupled with high resolution synchrotron X-ray powder data has played an important role in providing accurate structural data for many of these compounds. In this article, a detailed review of the literature dealing with structural studies on the intercalated compounds of C 60 will be given.

The structure of cimetidine (C10H16N6S) solved from synchrotron-radiation X-ray powder diffraction data

The crystal structure of cimetidine (C10H16N6S) has been solved and refined from high-resolution synchrotron X-ray powder data collected on station 8.3 at Daresbury Laboratory. The structure is monoclinic, P21/n, Z = 4, a = 10.7029 (3), b = 18.8262 (1), c = 6.8266 (2) Å, β = 111.306 (2)°. The data were auto-indexed and the integrated intensities were obtained by pattern decomposition. The structure was solved by direct methods followed by iterative cycles of least-squares refinement and Fourier syntheses. All 17 non-H atoms were located. The structure was then refined using the method of Rietveld [J. Appl. Cryst. (1969), 2, 65–71]. A difference Fourier synthesis after the convergence of the non-H-atom model showed positive electron density at the expected positions of all 16 H atoms, ten of which were significantly above the background. The H atoms were allowed to refine unconstrained and the final agreement factors were RI = 1.9, Rwp = 8.5 and Rex = 6.9%; the equivalent R-factors without the H atoms were RI = 10.3 and Rwp = 16.2%, demonstrating the significant contribution of the H atoms to the profile.

The combination of synchrotron powder diffraction and high-resolution solid-state NMR experiments

Structural investigations on three zeolites, ZSM-11, ZSM-12 and ZSM-5, have been carried out by a combination of high-resolution solid-state 29Si NMR experiments and Rietveld refinement of synchrotron X-ray powder data. These techniques, being sensitive to short- and long-range order, respectively, gave a most complete picture of the structure of the solid state. Variable temperature NMR experiments of ZSM-11 revealed a displacive phase transition with a change in symmetry at about 300 K. A Rietveld refinement of a data set recorded at 373 K confirmed the proposed structure model. The structure of ZSM-12 was refined from room temperature synchrotron powder data. Space group ambiguities because of psuedo-symmetry of the silica framework of the zeolite were resolved with the aid of 29Si NMR experiments leading to C2/ c as the true space group symmetry. 29Si NMR and X-ray studies of ZSM-5 showed that the symmetry of the crystal structure depends on the temperature, the concentration of the guest molecule, and on the number of Al atoms in the framework. A full pattern refinement of silica-ZSM-5 loaded with two molecules p-dichlorobenzene per unit cell showed that the site occupied by the sorbed molecule is not at the intersection of the channels but shifted by 1 4 along b 0.

High resolution synchrotron X-ray powder diffraction study of the structure of Tℓ 2 Ba 2 CaCu 2 O 8

Abstract Polycrystalline material of approximate composition Tl 2 Ba 2 Ca Cu 2 O 8 was synthesized by melting oxides together, pressing pellets and heating in sealed quartz tubes. High resolution synchrotron X-ray powder data obtained at beam-line X7A at the National Synchrotron Light Source revealed a tetragonal phase with a = 3.8545, c = 29.322 A , and a few weak impurity peaks. Rietveld refinement based on space group I4/mmm gave results in essential agreement with those obtained in previous x-ray single crystal and neutron powder determinations, including appreciable positional disorder of the thallium and oxygen atoms in the Tl O planes. The refined site occupancies were found to be high for Ca and low for Tl, consistent with partial substitution of the type (Tl 1.65 Ca 0.35 )Ba 2 (Ca 0.85 Tl 0.15 )Cu 2 O 8 . However, the possibility that here is some Cu on the Tl sites cannot be excluded.