On the determination of accurate intensities from powder diffraction data. I. Whole-pattern fitting with a least-squares procedure

New Insights in the Hydrothermal Synthesis of Rare-Earth Carbonates

The rare-earth carbonates represent a class of materials with great research interest owing to their intrinsic properties and because they can be used as template materials for the formation of other rare earth phases, particularly of rare-earth oxides. However, most of the literature is focused on the synthesis and characterization of hydroxycarbonates. Conversely, in the present study we have synthesized both rare-earth carbonates—with the chemical formula RE₂(CO₃)₃·2-3H₂O, in which RE represents a generic rare-earth element, and a tengerite-type structure with a peculiar morphology—and rare-earth hydroxycarbonates with the chemical formula RECO₃OH, by hydrothermal treatment at low temperature (120 °C), using metal nitrates and ammonium carbonates as raw materials, and without using any additive or template. We found that the nature of the rare-earth used plays a crucial role in relation to the formed phases, as predicted by the contraction law of lanthanides. In particular, the hydrothermal synthesis of rare-earth carbonates with a tengerite-type structure was obtained for the lanthanides from neodymium to erbium. A possible explanation of the different behaviors of lighter and heavier rare-earths is given.

Application of matrix decomposition algorithms for singular matrices to the Pawley method inZ-Rietveld

Z-Rietveldis a program suite for Rietveld analysis and the Pawley method; it was developed for analyses of powder diffraction data in the Materials and Life Science Facility of the Japan Proton Accelerator Research Complex. Improvements have been made to the nonlinear least-squares algorithms ofZ-Rietveldso that it can deal with singular matrices and intensity non-negativity constraints. Owing to these improvements,Z-Rietveldsuccessfully executes the Pawley method without requiring any constraints on the integrated intensities, even in the case of severely or exactly overlapping peaks. In this paper, details of these improvements are presented and their advantages discussed. A new approach to estimate the number of independent reflections contained in a powder pattern is introduced, and the concept of good reflections proposed by Sivia [J. Appl. Cryst.(2000),33, 1295–1301] is shown to be explained by the presence of intensity non-negativity constraints, not the intensity linear constraints.

Ab initio crystal structure determination of β-sodium acetate from powder data

The crystal structure of β-sodium acetate, NaC2H3O2, has been determined from X-ray powder diffraction data. Grid search and Rietveld refinement have been used to determine the structure. The crystal symmetry is orthorhombic (space group Pmn21, Z = 2) and the unit cell parameters are a = 3.4517(5) Å, b = 9.9123(11) Å and c = 5.1864(6) Å. Soft constraints have been applied to the molecule. The final RF value obtained was 7.5%.

Application of Direct Methods to powder data. A weighting scheme for intensities in the optimal symbolic addition program SIMPEL88

Application of Direct Methods to reflection intensities obtained by powder diffraction is often obstructed by the fact that not all intensities are reliable. In this paper a weighting scheme is proposed to overcome this problem. The scheme is implemented in the Direct Methods program SIMPEL88 in such a way that phase relations containing overlapping reflections are less probable than they would have been containing very reliable reflections only. Preliminary test results show that this way of tackling the problem is very promising.

Crystal structure determination of a series of benzene derivatives from powder data

The crystal structures of 4-(hydroxyl-phenyl)-acetonitrile (A) 4-(nitro-phenyl)-acetonitrile (B) and 2-(4-methoxy-phenoxy)-ethanol (C), I-bromomethyl-4-nitro-benzene (D) and I,4-dichloro-2-nitro-benzene (E) have been determined from X-ray powder diffraction data. Grid search and Rietveld refinement have been used to determine the structure. The crystals of (A) and (B) are monoclinic, space group P21/c, Z = 4 with cell parameters a = 6.771 Å, b = 8.568 Å, c = 11.766 Å and β = 93.47° for (A) and a = 8.444 Å, b = 5.942 Å, c = 15.780 Å and β = 100.06° for (B); the crystals of (C) and (D) are orthorhombic, space group Pbcn, Z = 8, a = 38.00 Å, b = 7.158 Å and c = 6.493 Å for (C) and space group P212121, Z = 4, a = 6.506 Å, b = 25.49 Å and c = 4.735 Å for (D). The crystals for (E) are triclinic, space group P[unk], Z = 2, a = 7.404 Å, b = 8.273 Å, c = 7.234 Å, α = 109.6°, β = 112.9° and γ = 73.2°. Chemical diagrams of all five compounds are depicted in Fig. 1. Soft constraints have been applied to the molecules during Rietveld refinement. The final Rp values obtained were 4.3 (A, Alpha1), 8.8 (A), 5.0 (B), 8.7 (C), 6.6 (D) and 5.7% (E), respectively. Compound (A) was measured both on a Bragg-Brentano Alpha1 diffractometer and on a Guinier camera. The other the structures were only measured with the Guinier camera.

Crystal structure of new Ni(II) complex with non-symmetrical bis-enaminone from powder diffraction data

The crystal structure of C22H32BrN2NiO2 was ab initio solved from conventional X-ray powder data by combination of few powder diffraction techniques. After the intensity estimating procedure based on texture method, the orientation and approximate position of the molecule was found by the Patterson methods. Next, Patterson and direct method search program PATSEE was used to locate the molecule more precisely. Missing atoms of flexible groups and final refinement was performed by Rietveld method. The structure consists of flat molecules connected by van der Waals forces. The compound crystallises in the monoclinic space group P21/c (No. 14) with a=10.362(3) Å, b=18.468(3) Å, c=12.066(3) Å, β = 124.53(1)°, Z=4, and contains 28 atoms in asymmetric unit.

Application of simulated annealing approach for structure solution of molecular crystals from X-ray laboratory powder data

Simulated annealing approach was successfully applied to solve three unknown molecular structures from X-ray laboratory powder data using a priory known structural fragments. Some possible developments of the method are discussed.

Crystal structure determination of a series of small organic compounds from powder data

The crystal structures of 2,4-di-bromo-aniline (A; C6H5NBr2 ), 4-iodo-anisole (B; C7H7OI), 2-iodo-benzenemethanol (C; C7H7OI), 2-amino-benzothiazole (D; C7H6N2S) and 2-amino, 5-bromo-pyridine (E; C5H5N2Br) have been determined from X-ray powder diffraction data. Grid search and Rietveld refinement have been used to determine the structures. The crystals of (A) and (B) are orthorhombic, the crystals of (C), (D) and (E) are monoclinic. (A): Space group P212121, Z=4 with cell parameters a = 11.18(1), b = 16.17(1), c = 4.110(3)Å; (B): Space group Pca21, Z = 4, a = 6.288(4), b = 7.361(4), c = 16.93(1)Å; (C): Space group P21/n, Z = 4, a = 13.23(1), b = 4.652(3), c = 12.82(1)Å, β = 109.69(4)°; (D): Space group P21/c, Z = 4, a = 14.58(2), b = 4.094(4), c = 11.62(1)Å, β = 94.12(6)°; (E): Space group P21/c, Z = 4, a = 13.80(1), b = 5.839(5), c = 7.687(7)Å, β = 106.04(5)°. Chemical diagrams of all five compounds are depicted in Fig. 1. Soft constraints have been applied to the molecules during Rietveld refinement. The final R p values obtained were 7.3 (A), 2.9 (B), 11.1 (C), 16.4 (D) and 7.9% (E) respectively. All compounds were measured on a Guinier camera. In addition, the structure of compound (A) was confirmed by single-crystal structure determination.

Crystal structure of 5′-phenyl-1,1′:3′,1″-terphenyl-4-carboxylic acid, a 27 atoms organic compound by powder method

The crystal structure of a complex organic compound containing 27 independent non-hydrogen atoms in asymmetric unit has been solved from powder diffraction data collected at ESRF Grenoble. Structure model was found using PATSEE program. By Rietveld method the structure was completed and refined to final values of discrepancy factors RF and Rwp equal to 14.6 and 13.2%, respectively. The space group is P21/c, unit cell parameters a = 11.0769(5), b = 22.387(1), c = 7.3885(3) Å, β = 91.923(4)°.

Crystal structure of the complex of 1,8-bis(dimethylamino)naphthalene with p-nitrosophenol by powder diffraction methods

Crystal structure solution of organic compounds without heavy atoms by powder diffraction method is still a challenging task, particularly when there is not a rigid group in the compound, or when there are more than one molecule in the asymmetric unit. In this paper the crystal structure solution of the complex of “proton sponge” 1,8-bis(dimethylamino)naphthalene (DMAN) and p-nitrosophenol: C14H19N2 + ‧ C6H4(NO)O– ‧ C6H5(NO)OH is presented. This structure, with 25 atoms and three independent molecules in the asymmetric unit, was solved from powder data by “pseudo-atom” method. The space group Pnma, Z=4, a=12.2125(5) Å, b=10.7524(7) Å, c=18.6199(14) Å, R wp and R F are 12.8 and 16.0%, respectively.