A new structural model for ferrihydrite that challenges the standard ferrihydrite model established by X-ray diffraction and confirmed by neutron diffraction and single-crystal electron nanodiffraction was recently proposed by Michel et al. (2007a) from the simulation of the pair distribution function obtained by Fourier transformation of diffraction data measured at λ = 0.137 Å. The new ferrihydrite model is isostructural to akdalaite (Al10O14(OH)2), a mineral having the Baker-Figgis δ-isomer of the Al13-Keggin structure as its structural motif. The new model is unrealistic because: (1) it is completely periodic (i.e. defect-free); (2), it has 20% tetravalent octahedral iron (VIFe4+), 20% divalent tetrahedral iron (IVFe2+), and some IVFe–O distances equal to or larger than the VIFe3+–O distances, thus violating Pauling's 2nd rule; (3) it does not describe X-ray diffraction and EXAFS spectroscopic data; and, (4) it is inconsistent with electron microscopy results and contradicts previous X-ray scattering studies.

The incomplete miscibility of Al into the ferrihydrite structure has been a recurring issue in understanding the environmental geochemistry of this important oxyhydroxide. During co-precipitation from acidic aqueous solution, ferrihydrite has been observed to accept only up to ∼25 at.% Al without the formation of multi-phasic Al and Fe oxyhydroxides. Using basic chemistry and crystal-chemical relationships we propose here that the saturation limit of Al substitution in the structure of Fe oxyhydroxides is controlled by Al – Al avoidance in a manner that conforms to Pauling's distortion rule. Employing this hypothesis, we show that the predicted miscibility limit for Al incorporation is 25 at.% in ferrihydrite and 33 at.% in goethite, in agreement with previous observations. These results indicate that the classical f-phase model for ferrihydrite best represents observations. Incorrect assignment of Fe site occupancy and other shortcomings of the akdalaite/tohdite model for ferrihydrite are also discussed.

The risk of overfitting pair distribution function (PDF) data for highly defective material (Farrow et al., 2007) is illuminated with the example of the nanocrystalline hydrous ferric oxyhydroxide, ferrihydrite. Two structural models have been published by Michel et al. (2007, 2010) using this method, both of which contradict the standard ‘ferrihydrits’ model established by X-ray diffraction (Drits et al., 1993), and confirmed by single-crystal electron nanodiffraction (Janney et al., 2001) and neutron diffraction (Jansen et al., 2002). Although PDF data are reproduced equally well with the two regression models, neither model is realistic: the first (fhyd6) violates Pauling's 2nd rule, and the second (ferrifh), Pauling's 3rd rule.

Characterization of semi and noncrystalline materials, monitoring structural phase transitions in situ, and obtaining structural information together with spatial distribution of the investigated material are only a few applications that hugely benefitted from the combination of high-energy X-rays and modern algorithms for data processing. This work examines the possibility of advancing these applications by shortening the data acquisition and improving the data quality by using the new high-energy PILATUS3 CdTe detector.

As we celebrate the 75th anniversary of the Powder Diffraction File, the PDF® is still a method for chemical and material analyses. The database and embedded software are designed to solve a range of solid-state material analysis problems that includes phase identification, quantitative analysis, crystallinity, and crystallite size measurements. A versatile platform allows users to interpret X-ray, electron, neutron, or synchrotron diffraction patterns for their analyses. Over several decades as diffraction hardware and software continued to improve, the International Centre for Diffraction Data continues to improve the methods and the PDF database, offering unprecedented analysis capabilities to the modern user.

Total scattering data of nanocrystalline gahnite (ZnAl2O4, 2–3 nm) have been collected with three of the most commonly used instruments: (i) ID31 high-resolution diffractometer at the European Synchrotron Radiation Facility (ESRF) (Q max = 22 Å−1); (ii) ID11 high-energy beamline at the ESRF (Q max = 26.6 Å−1); and (iii) Empyrean laboratory diffractometer by PANalytical with molybdenum anode X-ray tube (Q max = 17.1 Å−1). Pair distribution functions (PDFs) for each instrument data-set have been obtained, changing some of the parameters, by PDFgetX3 software, with the aim of testing the software in the treatment of different total scattering data. The material under analysis has been chosen for its nanometric (and possibly disordered) nature, to give rise to a challenge for all the diffractometers involved. None of the latter should have a clear advantage. The PDF and F(Q) functions have been visually compared, and then the three PDF sets have been used for refinements by means of PDFgui suite. All the refinements have been made exactly in the same way for the sake of a fair comparison. Small differences could be observed in the experimental PDFs and the derived results, but none of them seemed to be significant.

ID11 is a multi-purpose high-energy beamline at the European Synchrotron Radiation Facility (ESRF). Owing to the high-energy X-ray source (up to 140 keV) and flexible, high-precision sample mounting which allows small sample–detector distances to be achieved, experiments such as total scattering in transmission geometry are possible. This permits the exploration of a wide Q range and so provides high real-space resolution. A range of samples (glasses and crystalline powders) have been measured at 78 keV, first putting the detector as close as possible to the sample (~10 cm), and then moving it vertically and laterally with respect to the beam in order to have circular and quarter circle sections of diffraction rings, with consequent Q MAX at the edge of the detector of about 16 and 28 Å−1, respectively. Data were integrated using FIT2D, and then normalized and corrected with PDFgetX3. Results have been compared to see the effects of Q-range and counting statistics on the atomic pair distribution functions of the different samples. A Q of at least 20 Å−1 was essential to have sufficient real-space resolution for both type of samples while statistics appeared more important for glass samples rather than for crystalline samples.

A new software application has been developed to provide a convenient method for the submission of diffraction data directly to the International Centre for Diffraction Data, for inclusion in the Powder Diffraction File (PDF)™. The application, titled Genie, acts as a portal for the merging of data from separate sources allowing the user to quickly and easily submit their data for the PDF. Using Java Web Start technology Genie can be run using all common computer platforms. Genie is flexible and can be used to read a variety of common file formats. Furthermore, Genie can also be used for submission of New Diffraction Data for publication in the Powder Diffraction Journal.

X-ray diffraction and scattering on a single multipurpose X-ray platform have been used to probe the structure, composition, and thermal behavior of TiO2 nanoparticles ranging in size from 1 to 10 nm. Ambient and non-ambient Bragg diffraction, small-angle X-ray scattering (SAXS), as well as total scattering and pair-distribution function (PDF) analysis are combined to obtain a comprehensive picture of the samples. At these ultrasmall particle-size dimensions, SAXS and PDF prove powerful in distinguishing the salient features of the materials, in particular the size distribution of the primary particles (SAXS) and the identification of the TiO2 polymorphs (PDF). Structural features determined by X-ray scattering techniques are corroborated by high-resolution transmission electron microscopy. The elemental make-up of the materials has been measured using X-ray fluorescence spectrometry and energy-dispersive X-ray analysis.

Eighty specimens of cellulosic materials were analyzed over a period of several years to study the diffraction characteristics resulting from polymorphism, crystallinity, and chemical substitution. The aim of the study was to produce and verify the quality of reference data useful for the diffraction analyses of cellulosic materials. These reference data can be used for material identification, polymorphism, and crystallinity measurements. Overall 13 new references have been characterized for publication in the Powder Diffraction File (PDF) and several others are in the process of publication.

The family of zeolitic imidazolate framework (ZIF) compounds is efficient sorbent materials that can be used for catalytic, ion exchange, gas storage, and gas separation applications. A high-resolution reference X-ray powder diffraction pattern for one of the ZIF members, bis(2-methylimidazolyl)-zinc, C8H10N4Zn (commonly known as ZIF-8), was determined using synchrotron diffraction data obtained at the Advanced Photon Source (APS) in Argonne, IL. The sample was confirmed to be cubic I-43m, with a = 17.01162(6) Å, V = 4932.08 Å3, and Z = 12. The reference X-ray powder diffraction pattern has been submitted for inclusion in the Powder Diffraction File (PDF).

Synthetic analogs of naturally occurring nanoparticles have been studied by a range of X-ray techniques to determine their structure and chemistry, and relate these to their novel chemical properties and physical behavior. ZnS nanoparticles, formed in large concentrations naturally bymicrobial action, have an interesting core-shell structure with a highly distorted and strained outer layer. The strain propagates through the particles and produces unusual stiffness but can be relieved by changing the nature of the surface ligand binding. Weaker bound ligands allow high surface distortion, but strongly bound ligands relax this structure and reduce the overall strain. Only small amounts of ligand exchange causes transformations from the strained to the relaxed state. Most remarkably, minor point contacts between strained nanoparticles also relax the strain. Fe oxyhydroxide nanoparticles appear to go through structural transformations dependent on their size and formation conditions, and display a crystallographically oriented form of aggregation at the nanoscale that alters growth kinetics. At least one Fe oxyhydroxide mineral may only be stable on the nanoscale, and nonstoichiometry observed on the hematite surface suggests that for this phase and possibly other natural metal oxides, chemistry may be size dependent. Numerous questions exist on nanominerals formed in acid mine drainage sites and by reactions at interfaces.

The mixing properties of various orders of fluctuation correlations are investigated in the present paper for a rarefied gas free shear layer at hypersonic speed. The molecular kinetic theory and the direct simulation Monte Carlo (DSMC) method are employed for the present calculations. The Reynolds average process is assumed in obtaining the correlation functions. The results show that flow field structure was very similar to that of continuum flow ones at high Reynolds numbers. The probability density functions (pdf) in velocity space f(u′),f(v′), and f(u′, v′) are also calculated to counter explain the distributions of the correlation functions in the mixing layer. From the calculated distributions of the fluctuation correlation functions, <u′v′>, <u′2v′>, and <v′2u′>, one can find that the distributions behave similar to the turbulent transport phenomena in that of a continuum flow one. The distributions of the fluctuation correlation functions, <u′, v′> is described via the joint probability density function, f(u′, v′). The behavior of the higher-order fluctuation correlation functions, <u′2v′>, <u′3> and <u′4 >, are also explained via the probability density function.