An original non-hydrolytic sol-gel approach, using 2-ethyl-1,3-hexanediol as reactive solvent, was proposed to synthetize nanostructured magnetite. Iron-oxide nanoparticles were prepared and studied ...as a function of the precursor-to-solvent ratio. The crystallization degree of nanoparticles was followed by the combined Rietveld and Reference Intensity Ratio method. This procedure has allowed the determination of both amorphous and crystalline content of nanomagnetite, using hematite as suitable internal standard. The results of Rietveld method show that the crystalline content decreases as the precursor-to-solvent is increased, ranging from 67 to 60 wt%. Information on the crystallite size-strain distribution and microstructural evolution of nanocrystals was supplied by line profile analysis of the powder diffraction patterns, employing the Whole Powder Pattern Modeling analysis: the obtained log-normal distribution curves become increasingly narrow and symmetrical, while nanoparticle microstrain increases as the precursor concentration is increased. The dimensional analysis of the Transmission Electron Microscopy images has allowed to obtain the nanoparticle grain-size distribution. Nanoparticle dimensions decreases from 15 to 9 nm increasing the precursor concentration. The comparison between the results of X-ray diffraction and microscopic characterization techniques highlighted the effect of several factors, such as size, shape and microstructure of magnetite nanoparticles, on their functional magnetic response. Magnetic characterizations show that magnetite nanoparticles are not in the superparamagnetic phase even at room temperature, independent of the precursor concentration. On the other hand, the room-temperature saturation magnetization, ranging from 73 to 60 emu/g, is a function of the nanoparticle average size, decreasing as the precursor concentration increases.
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•Magnetite nanoparticles are obtained by non-hydrolytic sol-gel synthesis.•Nanoparticle properties are analyzed as a function of the precursor-to-solvent ratio.•Diffraction analysis are used to providing information on microstructural evolution of nanocrystals.•Nanoparticles grain distribution is estimated by dimensional analysis of the TEM images.•Correlation of the nanoparticle microstructure with their magnetic properties is shown.
The efficiency of the successive dichotomy method for powder diffraction pattern indexing Louër & Louër (1972). J. Appl. Cryst. 5, 271–275 has been proved over more than 30 years of usage. Features ...implemented in the new version of the computer program DICVOL04 include (i) a tolerance to the presence of impurity (or inaccurately measured) diffraction lines, (ii) a refinement of the `zero‐point' position, (iii) a reviewing of all input lines from the solution found from, generally, the first 20 lines, (iv) a cell analysis, based on the concept of the reduced cell, to identify equivalent monoclinic and triclinic solutions, and (v) an optional analysis of input powder data to detect the presence of a significant `zero‐point' offset. New search strategies have also been introduced, e.g. each crystal system is scanned separately, within the input volume limits, to limit the risk of missing a solution characterized by a metric lattice singularity. The default values in the input file have been extended to 25 Å for the linear parameters and 2500 Å3 for the cell volume. The search is carried out exhaustively within the input parameter limits and the absolute error on peak position measurements. Many tests with data from the literature and from powder data of pharmaceutical materials, collected with the capillary technique and laboratory monochromatic X‐rays, have been performed with a high success rate, covering all crystal symmetries from cubic to triclinic. Some examples reported as `difficult' cases are also discussed. Additionally, a few recommendations for the correct practice of powder pattern indexing are reported.
Powder pattern indexing can still be a challenge, despite the great recent advances in theoretical approaches, computer speed and experimental devices. More plausible unit cells, belonging to ...different crystal systems, are frequently found by the indexing programs, and recognition of the correct one may not be trivial. The task is, however, of extreme importance: in case of failure a lot of effort and computing time may be wasted. The classical figures of merit for estimating the unit‐cell reliability {i.e.M20 de Wolff (1968). J. Appl. Cryst.1, 108–113 and FN Smith & Snyder (1979). J. Appl. Cryst.12, 60–65} sometimes fail. For this reason, a new figure of merit has been introduced in N‐TREOR09, the updated version of the indexing package N‐TREOR Altomare, Giacovazzo, Guagliardi, Moliterni, Rizzi & Werner (2000). J. Appl. Cryst. 33, 1180–1186, combining the information supplied by M20 with additional parameters such as the number of unindexed lines, the degree of overlap in the pattern (the so‐called number of statistically independent observations), the symmetry deriving from the automatic evaluation of the extinction group, and the agreement between the calculated and observed profiles. The use of the new parameters requires a dramatic modification of the procedures used worldwide: in the approach presented here, extinction symbol and unit‐cell determination are simultaneously estimated. N‐TREOR09 benefits also from an improved indexing procedure in the triclinic system and has been integrated into EXPO2009, the updated version of EXPO2004 Altomare, Caliandro, Camalli, Cuocci, Giacovazzo, Moliterni & Rizzi (2004). J. Appl. Cryst. 37, 1025–1028. The application of the new procedure to a large set of test structures is described.
The monoclinic structures of vanadium dioxide are widely studied as appealing systems due to a plethora of functional properties in several technological fields. In particular, the possibility to ...obtain the VO2 material in the form of thin film with a high control of structure and morphology represents a key issue for their use in THz devices and sensors. Herein, a fine control of the crystal habit has been addressed through an in-depth study of the metal organic chemical vapor deposition (MOCVD) synthetic approach. The focus is devoted to the key operative parameters such as deposition temperature inside the reactor in order to stabilize the P21/c or the C2/m monoclinic VO2 structures. Furthermore, the compositional purity, the morphology and the thickness of the VO2 films have been assessed through energy dispersive X-ray (EDX) analyses and field-emission scanning electron microscopy (FE-SEM), respectively. THz time domain spectroscopy is used to validate at very high frequency the functional properties of the as-prepared VO2 films.
Basic principles and recent developments of the Whole Powder Pattern Modelling (WPPM) approach are briefly reviewed and illustrated by a typical case study: a highly deformed metal powder (AISI 434) ...produced by high energy ball milling. Microstructural parameters, like crystalline domain size distribution, dislocation density, and arrangement parameters, are directly obtained by modelling the experimental pattern, without using arbitrary analytical profile functions.
Some improvements have been introduced in the current computer program for powder diffraction pattern indexing using the dichotomy algorithm. The resulting version, DICVOL14, includes optimizations ...and extension of scanning limits for triclinic cases, a detailed review of the input data from the indexing solutions, cell centering tests and a new approach for zero-point offset evaluation. The performance of the new version is illustrated with many examples, such as triclinic cases with long axes and dominant zones. Some important parameters in pattern indexing based on the dichotomy algorithm are commented upon, e.g. the precision of data and spurious lines.
Room‐temperature synchrotron X‐ray diffraction and subsequent detailed line profile analysis of Fe powder were performed for microstructural characterization. The peak shapes of the diffraction ...pattern of Fe were found to be super‐Lorentzian in nature and the peak widths were anisotropically broadened. These peak profile features of the diffraction pattern are related to the microstructural parameters of the material. In order to elucidate these features of the diffraction pattern, detailed line (peak) profile analyses were performed using the Rietveld method, modified Williamson–Hall plots and whole powder pattern modelling (WPPM), and related microstructural parameters were determined. Profile fitting using the Rietveld and WPPM methods with a single microstructural (unimodal) model shows systematic deviation from the experimentally observed diffraction pattern. On the basis of Rietveld analysis and microstructural modelling it is revealed that the microstructure of Fe consists of two components (bimodal profile). The microstructural parameters of crystallite/domain size distribution, dislocation density, nature of dislocations and phase fraction were evaluated for both components. The results obtained using different methods are compared, and it is shown that diffraction peak profile analysis is capable of modelling such inhomogeneous bimodal microstructures.
Synchrotron X‐ray diffraction data of Fe powder have a super‐Lorentzian peak shape with anisotropic peak broadening. Detailed microstructural (unimodal and bimodal) analyses were carried out using the Rietveld method, modified Williamson–Hall plots and whole powder pattern modelling to investigate these features of the diffraction pattern. The bimodal analysis approach gives improved profile fitting, with the two components of the microstructure having different dislocation densities and a broad lognormal crystalline domain size distribution.
A new linear function for modelling the background in whole‐powder‐pattern fitting has been derived by applying LASSO (least absolute shrinkage and selection operator) and the technique of tree ...search. The background function (BGF) consists of terms bnL(2gθ/180)−n/2 and bnH(1 − 2gθ/180)−n/2 for the low‐ and high‐angle sides, respectively. Some variable parameters of the BGF should be fixed at zero while others should be varied in order to find the best fit for a given data set without inducing overfitting. The LASSO algorithm can automatically select the variables in linear regression analysis. However, it finds the best‐fit BGF with a set of adjustable parameters for a given data set while it derives a different set of parameters for a different data set. Thus, LASSO derives multiple solutions depending on the data set used. By regarding the individual solutions from LASSO as nodes of trees, tree structures were constructed from these solutions. The root node has the maximum number of adjustable parameters, P. P decreases with descending levels of the tree one by one, and leaf nodes have just one parameter. By evaluating individual solutions (nodes) by their χ2 index, the best‐fit single path from a root node to a leaf node was found. The present BGF can be used simply by varying P in the range 1–10. The BGF thus derived as a final single solution was incorporated into computer programs for Pawley‐based whole‐powder‐pattern decomposition and Rietveld refinement, and the performance of the BGF was tested in comparison with the polynomials currently widely used as the BGF. The present BGF has been demonstrated to be stable and to give an excellent fit, comparable to polynomials but with a smaller number of adjustable parameters and without introducing undulation into the calculated background curve. Basic algorithms used in statistics and machine learning have been demonstrated to be useful in developing an analytical model in X‐ray crystallography.
The linear background function to be used in whole‐powder‐pattern fitting is derived by applying least absolute shrinkage and selection operator (LASSO) and tree search. It gives excellent fits with a smaller number of adjustable parameters than the currently used polynomials, without introducing undulation into the calculated background curve. Basic algorithms used in statistics and machine learning are demonstrated to be useful in developing an analytical model in X‐ray crystallography.
Ferritic ODS steel elemental powder compositions with various Zr content (0.3-1.0 wt.%), ground in a Pulverisette 6 planetary ball mill, were extensively studied by X-ray diffraction line profile ...analysis, microscopic observations, microhardness testing and particle size measurements. A characteristic three-stage process of flattening the soft powders, formation of convoluted lamellae and, finally, formation of nanocrystalline grains was observed. In order to quantify the microstructural properties, expressed mainly in terms of crystallite size and dislocation density, a methodology for detailed and accurate microstructure analysis of nanosized and severely deformed materials was proposed by the Whole Powder Pattern Modelling (WPPM) approach. In the case of the proposed ODS alloy composition, the overlapping of Fe and Cr Bragg reflections makes the microstructure analysis certainly more complicated. The results showed that the microstructure of powders evolved towards the nanocrystalline state consisting of fine (diameter of ~15 nm) and narrowly dispersed domains, with extensive dislocation density exceeding 1016 m
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