Conventional Powder Diffraction Research Articles

Pinpointing and Quantifying the Aluminum Distribution in Zeolite Catalysts Using Anomalous Scattering at the Al Absorption Edge.

The location of aluminum in a zeolite framework structure defines the accessibility and geometry of the catalytically active sites, but determining this location crystallographically is fraught with difficulties. Typical zeolite catalysts contain only a small amount of aluminum, and the X-ray scattering factors for silicon and aluminum are very similar. To address this problem, we have exploited the properties of resonant X-ray powder diffraction across the Al K edge, where the aluminum scattering factor changes dramatically. By combining conventional synchrotron powder diffraction data with those collected at energies near the X-ray absorption edge, aluminum is highlighted. In this way, the different distributions of aluminum in two FER-type zeolites with identical chemical compositions but different catalytic properties could be determined unambiguously. The results are consistent with previous studies, but quantitative. This approach constitutes a major advance in our fundamental understanding of the relationship between zeolite structure and catalytic activity.

Structure Determination of Er Doped Ti-Al-Nb Alloy by Neutron Diffraction Analysis.

Neutron diffraction experiments with both conventional powder diffraction setup and under in-situ compressive loading were conducted to investigate the structural origins of large strength and toughness enhancements in Ti-16Al-27Nb alloy after Er addition. The primary phase is determined to be the ordered B2 structure form, in agreement with the previous electron microscopy study. Lattice strains of {210} and {100} planes were measured as a function of applied stress, and elastic anisotropy was found for both, and strong plastic nonlinearity was discovered for (210) reflection. The grain refinement during plastic deformation was proposed by both the 2D diffraction intensity distribution and SEM observations, while stress-induced martensitic phase transition was not observed in this study. It is believed that the activation of different slip systems and grain refinement might be the structural origin of the novel mechanical properties of this alloy.

Using a machine learning approach to determine the space group of a structure from the atomic pair distribution function.

A method is presented for predicting the space group of a structure given a calculated or measured atomic pair distribution function (PDF) from that structure. The method utilizes machine learning models trained on more than 100 000 PDFs calculated from structures in the 45 most heavily represented space groups. In particular, a convolutional neural network (CNN) model is presented which yields a promising result in that it correctly identifies the space group among the top-6 estimates 91.9% of the time. The CNN model also successfully identifies space groups for 12 out of 15 experimental PDFs. Interesting aspects of the failed estimates are discussed, which indicate that the CNN is failing in similar ways as conventional indexing algorithms applied to conventional powder diffraction data. This preliminary success of the CNN model shows the possibility of model-independent assessment of PDF data on a wide class of materials.

Coexistence of ferroelectric phases and electric field induced structural transformation in sodium potassium bismuth titanate ceramics

Lead-free sodium potassium bismuth titanate (1−x)[Na0.5Bi0.5TiO3]-x[K0.5Bi0.5TiO3] (NKBTx) ceramics have been believed to be a rhombohedral crystal system with R3c symmetry up to x = 0.16 mole fraction, right now. For the first time, on the contrary, we have disclosed the existence of a low-symmetry (MA)-type monoclinic phase (69%) along with rhombohedral R3c (31%) in the NKBT0.10 sample from Rietveld studies on conventional and high-energy synchrotron powder diffraction data in the unpoled state. The peak splitting and reduced FWHM of {110}pc and {111}pc reflections after poling indicate the field-induced structural transformation. Rietveld analysis reveals that the monoclinic (Cc) phase is partially transformed to rhombohedral (R3c) at 50 kV/cm. Two distinct dielectric anomalies are noticed in the dielectric curve at 120 and 310 °C corresponding to freezing temperature (Tf) and glass-like transition temperature (Tm), respectively. The dielectric diffusivity (γ) estimated from the modified Curie-Weiss law reveals that the sample has a relaxor behavior. Structural inhomogeneity in the nanoscale region due to cation disorder in the A-site causes relaxing nature. A well-saturated square-like ferroelectric hysteresis loop is obtained with a remnant polarization and a coercive field of 35.8 μC/cm2 and 47 kV/cm, respectively. A piezoelectric constant (d33) of 85 pC/N is obtained for the sample which is somewhat higher than that of pure Na0.5Bi0.5TiO3 ceramics.

Focus on test, measurement, and analytical equipment

Focus on test, measurement, and analytical equipment

Whole-pattern fitting technique in serial femtosecond nanocrystallography.

Serial femtosecond X-ray crystallography (SFX) has created new opportunities in the field of structural analysis of protein nanocrystals. The intensity and timescale characteristics of the X-ray free-electron laser sources used in SFX experiments necessitate the analysis of a large collection of individual crystals of variable shape and quality to ultimately solve a single, average crystal structure. Ensembles of crystals are commonly encountered in powder diffraction, but serial crystallography is different because each crystal is measured individually and can be oriented via indexing and merged into a three-dimensional data set, as is done for conventional crystallography data. In this way, serial femtosecond crystallography data lie in between conventional crystallography data and powder diffraction data, sharing features of both. The extremely small sizes of nanocrystals, as well as the possible imperfections of their crystallite structure, significantly affect the diffraction pattern and raise the question of how best to extract accurate structure-factor moduli from serial crystallography data. Here it is demonstrated that whole-pattern fitting techniques established for one-dimensional powder diffraction analysis can be feasibly extended to higher dimensions for the analysis of merged SFX diffraction data. It is shown that for very small crystals, whole-pattern fitting methods are more accurate than Monte Carlo integration methods that are currently used.

The crystal structure of kiddcreekite solved using micro X-ray diffraction and the EPCryst program

AbstractAbundant kiddcreekite grains were identified in the Zijinshan Cu-Au epithermal deposit in Fujian Province, China. The mineral occurs as polycrystalline grains, 5–300 μm in size, associated with colusite, enargite, stannoidite, mawsonite, vinciennite, hemusite, tennantite and wolframite in a predominantly covellite ore. Based on electron microprobe analysis, the empirical formula of the kiddcreekite is Cu6.2Sn0.97W0.95S7.83, without significant Se or Te contents. The crystal structure of kiddcreekite was solved using the direct-space method (EPCryst) from laboratory micro X-ray diffraction (μXRD) data and refined by the Rietveld method. The R values of the final Rietveld refinement were Rp = 9.06%, Rwp = 8.31%, RB = 3.16 and RF = 2.17%. Kiddcreekite has a cubic structure, space group F3m and lattice parameter a = 10.8178(3) Å (Z = 4, V = 1265.95(6) Å3). In the unit cell, W, Sn and Cu atoms occupy the 4a, 4c and 24f Wyckoff positions, respectively, and S atoms occupy two sets of 16e Wyckoff positions. The structure of kiddcreekite consists of stacked double MeS4 layers (giving a W–Sn–Cu tier and a Cu–vacancy tier) as in the sphalerite substructure. This study also demonstrates the possibility of using laboratory µXRD data coupled with the direct-space method to solve inorganic structures in cases where samples are too small for conventional powder and single-crystal diffraction.

Why was the structure of the niobium silicate AM-11 so difficult to solve?

The synthesis of the polycrystalline niobium silicate catalyst AM-11 was first reported in 1998 [1], but its structure proved to be elusive. In 2007 we received two samples from the Aveiro group. At the time, we were looking for a material suitable for the application of the texture method of structure solution, and AM-11 seemed to be ideal for this purpose. One of the samples had needle and the other platelet morphology, and textured samples could be prepared in both cases. The conventional powder diffraction pattern could be indexed on a hexagonal, an orthorhombic or a monoclinic unit cell, so this was the first issue to be resolved. The texture measurements quickly revealed that the crystal system was orthorhombic, but the structure resisted solution. We then tried applying the precession electron diffraction technique in combination with high-resolution powder diffraction data, but beyond confirming the orthorhombic symmetry, these data did not help us to solve the structure. Another attempt was made with a new sample and an improved texture setup, but to no avail. Rotation electron diffraction data and high-resolution transmission electron microscopy images showed that some disorder was present and helped to define the space group, but the structure remained a mystery. The powder charge-flipping routine in Superflip [2], yielded tantalizingly clear electron density maps, but they could not be interpreted sensibly. The unit cell parameters were seen to be related to those of the titanium silicate zorite [3] (one axis doubled in AM-11), so the problem was taken up once again last year. By starting with a simplified zorite framework structure with Nb in place of Ti, and performing what amounts to manual Fourier recycling, the surprisingly simple structure (1Nb, 3 Si, 9 O), which is significantly different from zorite, finally revealed itself. There is some stacking disorder, but the structure is otherwise innocuous. What made it so difficult to solve?

Testing the Debye Function Approach on a Laboratory X-ray Powder Diffraction Equipment. A Critical Study.

Total Scattering Methods are nowadays widely used for the characterization of defective and nanosized materials. They commonly rely on highly accurate neutron and synchrotron diffraction data collected at dedicated beamlines. Here, we compare the results obtained on conventional laboratory equipment and synchrotron radiation when adopting the Debye Function Analysis method on a simple nanocrystalline material (a synthetic iron oxide with average particle size near to 10 nm). Such comparison, which includes the cubic lattice parameter, the sample stoichiometry and the microstructural (size-distribution) analyses, highlights the limitations, but also some strengthening points, of dealing with conventional powder diffraction data collections on nanocrystalline materials.

Synthesis, structural and spectroscopic characterization and biomimetic properties of new copper, manganese, zinc complexes: Identification of possible superoxide-dismutase mimics bearing hydroxyl radical generating/scavenging abilities

A series of Cu(II), Zn(II) and Mn(II) coordination compounds has been synthesized by reaction of the corresponding metal salts and pyrazolyl-based ligands, i.e. the neutral 1-(2-(4-((2,2,2-tri(1H-pyrazol-1-yl)ethoxy)methyl)benzyloxy)-1,1-di(1H-pyrazol-1-yl)ethyl)-1H-pyrazole { p-C 6H 4[CH 2OCH 2C(pz) 3] 2, (L 1), and the anionic hydridotris(3-phenyl-5-methylpyrazolyl)borate (L 2) −, bis(pyrazolyl)acetate (L 3) and bis(3,5-dimethylpyrazolyl)acetate (L 4) −: the species [L 1(CuCl 2) 2] ( 1), [L 1(Cu(OAc) 2) 2] ( 2), [L 1(Zn(OAc) 2) 2] ( 3), [(CuCl(L 2)(Hpz Ph,Me)] ( 4), [Mn(L 3) 2]·2H 2O, ( 5), [ZnCl(L 3)(imH)]·MeOH [CuCl(L 4)(imH)]·2H 2O ( 7) have been obtained (Hpz Ph,Me = 3-phenyl-5-methylpyrazole, imH = imidazole). Complexes 1 and 4 have been structurally characterized, also using less conventional powder diffraction methods. The superoxide scavenging activity has been characterized by indirect assays including EPR analysis. All complexes exhibit superoxide scavenging activity with IC 50 in the µM range and they protect against the oxidative action of peroxynitrite in different ways. 1, 4 and 7 exert both an anti- and pro-oxidant effect depending on their concentration as evaluated by EPR and fluorescence methods. The pro-oxidative effects are absent in Zn(II) and Mn(II) complexes.

Neutron polarization analysis study of the frustrated magnetic ground state ofβ-Mn1−xAlx

We have performed a neutron polarization analysis study of the short-range nuclear and magnetic correlations present in the dilute alloy, $\ensuremath{\beta}{\text{-Mn}}_{1\ensuremath{-}x}{\text{Al}}_{x}$ with $0.03\ensuremath{\le}x\ensuremath{\le}0.16$, in order to study the evolution of the magnetic ground state of this system as it achieves static spin-glass order at concentrations $x>0.09$. To this end we have developed a reverse--Monte Carlo algorithm which has enabled us to extract Warren-Cowley nuclear short-range order parameters and magnetic spin correlations. Using conventional neutron powder diffraction, we show that the nonmagnetic Al substituents preferentially occupy the magnetic site II Wyckoff positions in the $\ensuremath{\beta}\text{-Mn}$ structure---resulting in a reduction of the magnetic topological frustration of the Mn atoms. These Al impurities are found to display strong anticlustering behavior. The magnetic spin correlations are predominantly antiferromagnetic, persisting over a short range which is similar for all the samples studied---above and below the spin-liquid-spin-glass boundary---while the observed static (disordered) moment is shown to increase with increasing Al concentration.

Structure elucidation of cyameluric acid by combining solid-state NMR spectroscopy, molecular modeling and direct-space methods

We present the structure solution for solvent-free cyameluric acid which has first been synthesized more than 150 years ago. By densely intertwining the complementary methods solid-state NMR spectroscopy, molecular modeling as well as direct-space methods for the analysis of the X-ray powder diffraction data we succeeded where conventional powder diffraction analysis alone failed. In a first step, the correct tautomer was identified by combining solid-state NMR and ab initio calculations. The quantum-chemically optimized molecule was further employed in direct-space methods for the crystal structure solution. After a subsequent Rietveld refinement the positions of the hydrogen atoms in the crystal were determined by comparing experimental and calculated NMR chemical shift parameters. In this context, the sensitivity of the anisotropy δ aniso and the asymmetry parameter η of the carbonyl 13C towards the hydrogen bonding environment proved to be very valuable. Cyameluric acid crystallizes in space group P2 12 12 1 ( Z = 4, a = 6.4701(5) Å, b = 9.9340(6) Å, c = 12.0985(7) Å) with one complete molecule in the asymmetric unit. It consists of the symmetric trioxo tautomer which is arranged in a three-dimensional hydrogen bond network where all three NH groups interact with carbonyl groups.

Quantitative Rietveld analysis of CAC clinker phases using synchrotron radiation

The quantitative Rietveld analyses of twenty samples of CAC from four different manufacturers over the world, one synthetic mixture and a NIST standard were performed using synchrotron radiation. As compared with conventional XRD, synchrotron powder diffraction permitted to find new minor phases, improve the characterization of solid solutions of iron rich CAC phases and reduce preferential orientation and microabsorption effects. Diffraction data were complemented with XRF and TG/DT analyses. Synchrotron results were used as a reference test to improve the performance of conventional powder diffraction, by an accurate selection of refinable profile and structural parameters, and permitted to extract several recommendations for conventional quantitative Rietveld procedures. It is shown that with these recommendations in mind, conventional XRD based Rietveld analyses are comparable to those obtained from synchrotron data. In summary, quantitative XRD Rietveld analysis is confirmed as an excellent tool for the CAC cement industry.

Enhanced analysis of biomaterials by synchrotron diffraction

There is an increasing body of evidence that prosthetic implants based upon titanium alloys attain improved performance when coated with calcium hydroxyapatite (HAP). Such coatings appear to promote osteointegration and bone in-growth. Plasma spraying is the most frequently employed route to coating fabrication. Detailed chemical and structural characterisation of these coatings is essential for (i) coating technology development, (ii) ensuring consistent material quality and (iii) assessing coating performance. The work presented here employed combined conventional powder diffraction and glancing angle synchrotron diffraction to examine the in vitro performance of apatite coatings formed by plasma spraying. Coatings were exposed to simulated body fluid and foetal calf serum, and changes to the coating chemistry and structure determined. A new analysis method, synchrotron depth profiling tomography, has been applied to obtain structural features through the coating depth. The dissolution and re-precipitation behaviour of the coatings was found to be significantly different for each media. For the first time, it has been possible to identify and quantify the formation of a nanocrystalline, carbonated HAP phase. It has been possible to simultaneously model all coating phases apparent within the X-ray diffraction data and thus quantify changes to film composition. DPT has also revealed subtle changes in coating features with depth and these may have a significant impact on coating dissolution. The findings are discussed in the context of kinematic models for the coating behaviour and implications for the performance of such coatings in vivo.

Study of Submicrocrystalline Materials by Conventional Powder Diffraction and Diffuse Scattering in Transmitted Wave

Two methods were used for the analysis of submicro-crystalline copper prepared by high-pressure (6 GPa) severe plastic deformation. Conventional powder XRD study was carried in terms of the modified Williamson-Hall (WH) plots and shown large line-broadening anisotropy. A new high-resolution small-angle transmission diffuse scattering method was applied for determination of crystallite size. Two methods were used for the evaluation of the diffuse scattering - calculation of the autocorrelation function of the crystalline shape by the transformation of the measured signal and by the direct fitting. Bimodal block size distribution has to be assumed for annealed samples. This was also confirmed by classical back-reflection film method as well as by the long tails of conventional diffraction profiles.

Kinetics of the Decomposition of Crocidolite Asbestos: A Preliminary Real-Time X-Ray Powder Diffraction Study

This work is a preliminary kinetic study of the crocidolite decomposition followed in situ at high temperature using real time conventional powder diffraction and DTA in the temperature range 720-795 °C. The data analysis using the Avrami models indicates that the rate limiting step of the reaction is monodimensional ion diffusion (n=0.5) with an activation energy of 129 (10)kcal/mole.

Kinetic study of the dehydroxylation of chrysotile asbestos with temperature by in situ XRPD

The kinetics of the dehydroxylation of chrysotile was followed in situ at high temperature using real-time conventional and synchrotron powder diffraction (XRPD). This is the first time kinetics parameters have been calculated for the dehydroxylation of chrysotile. The value of the order of the reaction mechanism calculated using the Avrami model indicates that the rate-limiting step of the reaction is a one-dimensional diffusion with an instantaneous nucleation or a deceleratory rate of nucleation of the reaction product. Hence, the rate-limiting step is the one-dimensional diffusion of the water molecules formed in the interlayer region by direct condensation of two hydrogen atoms and an oxygen atom. The calculated apparent activation energy of the reaction in the temperature range 620–750 °C is 184 kJ mol−1. The diffusion path is along the axis of the fibrils forming the fibers. The amorphous or short-range ordered dehydroxylate of chrysotile is extremely unstable because forsterite readily nucleates in the Mg-rich regions. Moreover, it is less stable than the dehydroxylate of kaolinite, the so-called metakaolinite, which forms mullite at about 950 °C. This difference is interpreted in terms of the different nature of the two ions Mg2+ and Al3+ and their function as glass modifier and glass-forming ion, respectively.

Ab initio crystal structure determination of Na 2Si 3O 7 from conventional powder diffraction data

The crystal structure of Na 2Si 3O 7 has been determined by direct methods using integrated intensities of conventional X-ray powder diffraction data and subsequently refined with the Rietveld technique. The title compound was prepared from Na 2Si 3O 7×H 2O by careful thermal decomposition at 440°C. Sodium trisilicate adopts monoclinic symmetry, space group P2 1/ c with unit cell parameters a=7.1924(5) Å, b=10.6039(8) Å, c=9.8049(7) Å, β=120.2478(4)°, V=646.0(9) Å 3 and Z=4. It belongs to the group of interrupted framework silicates of four- and three-connected [SiO 4]-tetrahedra with a ratio of Q 3:Q 4=2:1. Within the framework the sodium atoms are coordinated by 4 to 6 oxygen ligands. The porous character of the new phase is reflected in a framework density FD=18.6 T-atoms/1000 Å 3, a value which is comparable to those observed in zeolitic materials. The topology of the tetrahedral network is identical to the one observed in the hydrous sodium silicate Na 2Si 3O 7×H 2O. Differences can be attributed to tetrahedral rotations. In agreement with the structure analysis the 29Si MAS NMR spectrum of Na 2Si 3O 7 shows two peaks at −91.9 ppm and −102.5 ppm with an intensity ratio of 2:1 which are assigned to three- and four-connected [SiO 4]-tetrahedra, respectively.

Structure determination from conventional powder diffraction data: Application to hydrates, hydrochloride salts, and metastable polymorphs

Structure determination from conventional powder diffraction data: Application to hydrates, hydrochloride salts, and metastable polymorphs

Structure Determination from Conventional Powder Diffraction Data: Application to Hydrates, Hydrochloride Salts, and Metastable Polymorphs

Recent advances in crystallographic computing have made it possible to solve by powder diffraction methods structures that have not been possible to solve by single‐crystal methods. Although there is vast improvement in the quality of data obtained from high‐intensity synchrotron radiation, we found that surprisingly reliable results can be obtained from conventional laboratory sources. In this article we examine the application of Monte Carlo/simulated annealing methods for the determination of structures ranging in complexity from 9 to 15 degrees of freedom. We re‐determine the structures of papaverine hydrochloride and erythromycin A dihydrate by the powder diffraction method and compare the structures to those determined by single‐crystal diffraction methods. The structure of a metastable polymorphic form of acetohexamide, form B, is solved and examined spectroscopically. Its structure has not previously been solved by single‐crystal techniques because of the small size of its crystals. © 2000 Wiley‐Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 89:958–966, 2000