Correction for preferred orientation in Rietveld refinement

Mechanosynthesis of Stable Salt Hydrates of Allopurinol with Enhanced Dissolution, Diffusion, and Pharmacokinetics

Allopurinol (ALO) is a medication that treats gout and kidney stones by lowering uric acid synthesis in the blood. The biopharmaceutics classification system (BCS) IV drug exhibits poor aqueous solubility, permeability, and bioavailability. To overcome the bottlenecks of ALO, salts with maleic acid (MLE) and oxalic acid (OXA) were synthesized using the solvent-assisted grinding method. The novel multicomponent solids were characterized by PXRD, DSC, TGA, FT-IR, and SEM images. The crystal structures of these salts with variable stoichiometry were obtained using Rietveld refinement from the high-resolution PXRD data. The proton from the dicarboxylic acid is transferred to the most basic pyrimidine "N" of ALO. The N-H···N hydrogen-bonded ALO homodimer is replaced by the N+-H···O- ionic interactions in ALO-OXA (2:1:0.4) and ALO-MLE (1:1:1) salt hydrates. The organic salts improved solubility and dissolution up to 5-fold and the diffusion permeability up to 12 times compared to the native drug in a luminal pH 6.8 phosphate buffer medium. The salt hydrates were exceptionally stable during storage at 30 ± 5 °C and 75 ± 5% relative humidity. Superior dissolution and diffusion permeability of the ALO-MLE salt resulted in improved pharmacokinetics (peak plasma concentration) that offers a promising solid dosage form with enhanced bioavailability and lower dosage formulation.

Preferred orientation and its effects on intensity-correlation measurements

Intensity-correlation measurements allow access to nanostructural information on a range of ordered and disordered materials beyond traditional pair-correlation methods. In real space, this information can be expressed in terms of a pair-angle distribution function (PADF) which encodes three- and four-body distances and angles. To date, correlation-based techniques have not been applied to the analysis of microstructural effects, such as preferred orientation, which are typically investigated by texture analysis. Preferred orientation is regarded as a potential source of error in intensity-correlation experiments and complicates interpretation of the results. Here, the theory of preferred orientation in intensity-correlation techniques is developed, connecting it to the established theory of texture analysis. The preferred-orientation effect is found to scale with the number of crystalline domains in the beam, surpassing the nanostructural signal when the number of domains becomes large. Experimental demonstrations are presented of the orientation-dominant and nanostructure-dominant cases using PADF analysis. The results show that even minor deviations from uniform orientation produce the strongest angular correlation signals when the number of crystalline domains in the beam is large.

High-precision face-centered cubic–hexagonal close-packed volume-change determination in high-Mn steels by X-ray diffraction data refinements

High-Mn steels attract attention because of their various technological properties. These are mainly mechanical and functional, such as the shape-memory effect, high damping capacity, high strength with simultaneous large ductility, the TRIP/TWIP (transformation- and twinning-induced plasticity) effect, low cycle fatigue and high work hardening capacity. All these phenomena are associated with the face-centered cubic (f.c.c.)–hexagonal close-packed (h.c.p.) martensitic transformation which takes place in these alloys. During this phase transition defects are introduced, mainly due to the large volume change between austenite and martensite. Knowing this volume change is key to understanding the mechanical behavior of these metallic systems. In the present article, a full-pattern refinement method is presented. The proposed method uses data obtained by means of conventional X-ray diffraction from regular bulk samples and allows a high-precision calculation of the lattice parameters of both phases, f.c.c. and h.c.p., under conditions very different from randomly oriented (powder) materials. In this work, the method is used to study the effect of chemical composition on the volume change between the two structures. By applying empirical models, the results enabled the design and fabrication of Fe–Mn-based alloys with a small volume change, showing the potential of this new tool in the search for improved materials.

Applications of Rietveld Analysis to Materials Characterization in Solid-State Chemistry, Physics and Mineralogy

The utilization and optimization of the properties of materials follows most effectively from a detailed knowledge and understanding of the positions and energetics of their constituent atoms, generally obtained from scattering/diffraction experiments involving electrons, neutrons or electromagnetic radiation. For the most part, these experiments are undertaken on individual crystals of the material, thereby preserving the resolution (and information content) of the three-dimensional reciprocal lattice. However, many of the substances of greatest academic and technical importance either do not crystallize with dimensions large enough for single-crystal studies, or display the properties of maximal interest only when present in finely-divided (powdered) form. In a diffraction experiment, the reciprocal lattice is then collapsed on to the single dimension of the 2θ scale.

Cryochemically Obtained Nanoforms of Antimicrobial Drug Substance Dioxidine and Their Physico-chemical and Structural Properties

Nanoforms of the antimicrobial drug substance 2,3-bis-(hydroxymethyl) quinoxaline-N,N′-dioxide with particles sizes between 50 and 300 nm were obtained by cryochemical modification of the initial pharmaceutical substance using a freeze-drying technique and were characterized by different physicochemical methods (FTIR, UV-Vis, 1H-NMR, DSC, TG and X-ray diffraction) and transmission electron microscopy (TEM). The data obtained from FTIR- and UV–Vis-spectroscopy confirmed the unaltered chemical structure of dioxidine molecules due to the cryochemical modification method. At the same time, X-ray diffraction and thermal analysis data show the change of the crystal structure compared to the parameters of the initial pharmaceutical dioxidine substance. A higher dissolution rate was revealed for cryomodified dioxidine nanoforms. The existence of three polymorphic crystal phases was established for cryomodified dioxidine samples possessed by some thermal activation processes: two anhydrous polymorphic phases, triclinic (T) and monoclinic (M), and one hydrated form (H).

Hidden Oceans? Unraveling the Structure of Hydrous Defects in the Earth's Deep Interior

High-pressure silicates making up the main proportion of the earth's interior can incorporate a significant amount of water in the form of OH defects. Generally, they are charge balanced by removing low-valent cations such as Mg²⁺. By combining high-resolution multidimensional single- and double-quantum ¹H solid-state NMR spectroscopy with density functional theory calculations, we show that, for ringwoodite (γ-Mg₂SiO₄), additionally, Si⁴⁺ vacancies are formed, even at a water content as low as 0.1 wt %. They are charge balanced by either four protons or one Mg²⁺ and two protons. Surprisingly, also a significant proportion of coupled Mg and Si vacancies are present. Furthermore, all defect types feature a pronounced orientational disorder of the OH groups, which results in a significant range of OH···O bond distributions. As such, we are able to present unique insight into the defect chemistry of ringwoodite's spinel structure, which not only accounts for a potentially large fraction of the earth's entire water budget, but will also control transport properties in the mantle. We expect that our results will even impact other hydrous spinel-type materials, helping to understand properties such as ion conduction and heterogeneous catalysis.

Crystal structure of chlorido-{1-(2,3-dimethyl-5-oxido-1-phenyl-1H-pyrazol-2-ium-4-yl-κO)-2-[3-methyl-5-oxo-1-phenyl-4,5-di-hydro-1H-pyrazol-4-yl-idene-κO]hydrazin-1-ido-κN (1)}copper(II) from laboratory X-ray powder data

In the title compound, [Cu(C21H19N6O2)Cl], the Cu(II) atom is in a slightly distorted square-planar coordination involving two O atoms from the pyrazolone rings [Cu-O = 2.088 (10) and 1.975 (10) Å], an N atom of the azo group [Cu-N = 2.048 (13) Å] and a chloride anion [Cu-Cl = 2.183 (5) Å]. The organic anions act as tridentate chelating ligands. The mol-ecules stack in columns along the c axis.

Fast quantitative analysis of strong uniaxial texture using a March–Dollase approach

Interrelations between the degree of uniaxial preferred orientation and the intensities and widths of selected X-ray diffraction peaks are analyzed within the March–Dollase approach. Simple analytical expressions are developed which relate the degree of preferred orientation to the rocking curve width of the strongest diffraction peak or the intensity ratio of two diffraction peaks, one of them being originated in the preferably orientated atomic planes.

Crystal structure of 2-(4-methylphenyl)-2-oxo-1-(phenyliodonio)-1-(triphenylarsonio)ethanid tetrafluoroborate solved from X-ray powder data

The crystal structure of 2-(4-methylphenyl)-2-oxo-1-(phenyliodonio)-1-(triphenylarsonio)ethanid tetrafluoroborate, C33H27AsIO+BF4 –, was solved and refined from laboratory X-ray powder diffraction data in combination with a DFT calculation (CoK α1, a=24.427(2) Å, b = 13.3412(18) Å, c=22.399(2) Å, β = 119.49(2)°, S.G. C2/c, R p/R wp = 0.030/0.039). The main features of the C33H27AsIO+BF4 – structure are similar to those reported previously for arsenium and phosphorous organic compounds. The distinctive features are short I … O and I … F (2.743(19) and 2.916(17) Å, respectively) intermolecular contacts, also observed previously in the same type of compound, refined from single crystal data.

Zoledronic acid: monoclinic and triclinic polymorphs from powder diffraction data

The crystal structures of the monoclinic and triclinic polymorphs of zoledronic acid, C5H10N2O7P2, have been established from laboratory powder X-ray diffraction data. The molecules in both polymorphs are described as zwitterions, namely 1-(2-hydroxy-2-phosphonato-2-phosphonoethyl)-1H-imidazol-3-ium. Strong intermolecular hydrogen bonds (with donor–acceptor distances of 2.60 Å or less) link the molecules into layers, parallel to the (100) plane in the monoclinic polymorph and to the (1\overline{1}0) plane in the triclinic polymorph. The phosphonic acid groups form the inner side of each layer, while the imidazolium groups lie to the outside of the layer, protruding in opposite directions. In both polymorphs, layers related by translation along [100] interact through weak hydrogen bonds (with donor–acceptor distances greater than 2.70 Å), forming three-dimensional layered structures. In the monoclinic polymorph, there are hydrogen-bonded centrosymmetric dimers linked by four strong O—H...O hydrogen bonds, which are not present in the triclinic polymorph.

Crystal structures of pyrazolo[1,5-a]pyrimidine derivatives solved from powder diffraction data

The molecular crystal structures of 3-amino-4-nitro-6-mefhyl-8-oxopyrazolo[1,5-a]pyrimidine (C7H7N5O3; space group P21/n; Z = 4; a = 18.920(4) Å, b = 8.441(2) Å, c = 5.210(1) Å, β = 90.82(2)°) and 3-amino-4-nitro-6,8-dimethylpyrazolo[1,5-a]pyrimidine (C8H9N5O2; space group P[unk]; Z = 2; a = 7.643(2) Å, b = 9.142(3) Å, c = 7.492(1) Å, α = 111.12(2)°, β = 100.66(2)°, γ = 102.58(2)°) have been determined from X-ray and neutron powder diffraction data using grid search procedure. The hydrogen-bonded molecules of the former compound form chains directed along the diagonals of the bc plane, while the latter crystal structure adopts dimeric arrangement.

A grid search procedure of positioning a known molecule in an unknown crystal structure with the use of powder diffraction data

An efficient grid search procedure successfully applied to the solution of three unknown molecular structures from X-ray and neutron powder diffraction data is presented.

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 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.

The crystal structure of sodium oxamate NaC2O3NH2 from powder diffraction data

The crystal structure of sodium oxamate, NaC2O3NH2, has been determined from synchrotron powder diffraction data. First Guinier-Johannson photographs have been taken to derive the unit cell. Grid search and Rietveld refinement have been used to determine the structure. The crystal structure is monoclinic, P21/c, Z = 4, a = 3.5513(1) Å, b = 5.2561(2) Å, c = 20.4451(8) Å, β = 99.329(2)° and V = 376.58 Å3. The final RP value obtained with Rietveld refinement was 6.5 %