Home About us Contact
|
Home About us Contact | ||
|
|
||
Rietveld Refinement (rietveld + refinement)
Kinds of Rietveld Refinement Selected AbstractsTricalcium Silicate T1 and T2 Polymorphic Investigations: Rietveld Refinement at Various Temperatures Using Synchrotron Powder DiffractionJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2004Vanessa K. Peterson The lattice parameters, cell volume, and structure of a sample of phase pure triclinic tricalcium silicate were determined using in situ, high-temperature synchrotron powder diffraction and full-profile Rietveld refinement. The temperature range covered was from ambient to 740°C. Evidence of superstructure was found. The T2 type structure with disordered SiO4 tetrahedra was observed, and an average structure for the subcell (P, a= 11.7416(2) Å, b= 14.2785(2) Å, c= 13.7732(2) Å, ,= 105.129(1)°, ,= 94.415(1)°, and ,= 89.889(1)°) is presented. Differential thermal analysis and X-ray fluorescence was also performed. [source] ChemInform Abstract: Rietveld Refinement of the Structure of Nd2Zn15Ge2.CHEMINFORM, Issue 17 2001P. Demchenko Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Transformation of haematite and Al-poor goethite to Al-rich goethite and associated yellowing in a ferralitic clay soil profile of the middle Amazon Basin (Manaus, Brazil)EUROPEAN JOURNAL OF SOIL SCIENCE, Issue 5 2005E. Fritsch Summary The red and yellow colours of ferralitic soils in the tropics have for long intrigued pedologists. We have investigated the upward yellowing in a 10-m thick profile representative of the Ferralsols of the plateaux of the Manaus region of Brazil. We determined changes in the nature and crystal chemistry of their Fe oxides by optical and Mössbauer spectroscopy as well as Rietveld refinement of X-ray diffraction patterns. We attribute the upward yellowing of the soil to a progressive transformation of the Fe oxides at nearly invariant iron contents. Aluminium in contrast is strongly mobilized in the uppermost clay-depleted topsoil where there is preferential dissolution of kaolinite and crystallization of gibbsite. Haematite decreases from 35 to 10% of the Fe oxides from the bottom to the top of the profile and the particles become smaller (75,10 nm). Its Al for Fe-substitution remains almost unchanged (10,15 mol %). The average Al-substitution rate of goethite increases from 25 to 33 mol %, and its mean crystal diameter remains in the range 20,40 nm. The proportion of Al-rich goethite (33 mol %) increases at the expense of less Al-substituted Fe oxides (haematite and goethite). This conversion with restricted transfer of iron means that the amount of Al stored in Fe oxides gradually increases. Kaolinite, haematite and Al-poor goethite are thus witnesses of earlier stages of ferralitization of the soil. In contrast, Al-rich goethite and gibbsite initiate the alitization (or bauxitization) of the soil. They correspond to the last generation of soil minerals, which most likely reflects the present-day weathering conditions. The progressive replacement of kaolinite, haematite and Al-poor goethite by new generations of Al-rich goethite and gibbsite attests to greater activities of water and aluminium and smaller activity of aqueous silica in the topsoil than in the subsoil. We interpret this as a consequence of longer periods of wetting in the topsoil that could result from soil aging, more humid climate or both. [source] Bioinspired Design of SrAl2O4:Eu2+ PhosphorADVANCED FUNCTIONAL MATERIALS, Issue 4 2009Mariya H. Kostova Abstract A phosphor based on Sr0.97Al2O4:Eu0.03 with a biomorphous morphology is manufactured via vacuum assisted infiltration of wood tissue (Pinus sylvestris) with a precursor nitrate solution. The nitrate solution penetrates homogeneously into the uniform arrangement of rectangular shaped tracheidal cells of the wood tissue. According to scanning electron microscopy, the original wood cell walls are completely transformed retaining the original wood structure. The major crystalline phase is monoclinic SrAl2O4, detected by X-ray diffraction and confirmed by Rietveld refinement. Energy-dispersive X-ray analysis proves the homogeneous conversion of the original wood cell wall into Sr0.97Al2O4:Eu0.03 struts. The optical properties of the resulting phosphor material are determined by photoluminescence and cathode-luminescence spectroscopy in scanning electron microscopy. The biotemplated Sr0.97Al2O4:Eu0.03 shows a characteristic green emission at 530,nm (2.34,eV). Shaping biomorphous SrAl2O4:Eu2+ phosphor with a microstructure pseudomorphous to the bioorganic template anatomy offers a novel approach for designing micropatterned phosphor materials. [source] Self-Assembled Nanostructures: Role of Water in Directing Diphenylalanine Assembly into Nanotubes and Nanowires (Adv. Mater.ADVANCED MATERIALS, Issue 5 20105/2010) The controllable assembly behavior of diphenylalanine molecules to form nanowires (NWs) and nanotubes (NTs) and their structural details are presented. Hyotcherl Ihee, Sang Ouk Kim and co-workers show on p. 583 that the nanoscale morphologies are closely related to molecular arrangements of diphenylalanine as revealed by Rietveld refinement of powder X-ray diffraction patterns and electron-density distributions in NTs and NWs via the maximum entropy method analysis. [source] Role of Water in Directing Diphenylalanine Assembly into Nanotubes and NanowiresADVANCED MATERIALS, Issue 5 2010Jangbae Kim The controllable assembly behavior of diphenylalanine molecules to form nanowires (NWs) and nanotubes (NTs) and their structural details are presented (see figure). The nanoscale morphologies are closely related to molecular arrangements of diphenylalanine as revealed by Rietveld refinement of powder X-ray diffraction patterns and electron-density distributions in NTs and NWs via the maximum entropy method analysis. [source] Parametric Rietveld refinement for the evaluation of powder diffraction patterns collected as a function of pressureJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3 2010Ivan Halasz Under the assumption that the structural parameters of a crystalline phase change `smoothly' with increasing pressure, the evolution of the parameters can be parameterized as a function of pressure using continuous monotonic functions. Four different approaches to determine the structural evolution of As2O5 with increasing pressure from a set of powder diffraction patterns collected over the pressure range from 2.5 to 19.5,GPa have been investigated. Approach (A) was the common sequential refinement of atomic coordinates with restraints on the geometry and was compared with three parameterization approaches. Approach (B) used direct parameterization by low-order polynomials of each crystallographically distinct atomic coordinate, (C) described the atoms of the asymmetric unit as a rigid body and allowed the internal degrees of freedom of the rigid body to vary with the change in pressure using rigid unit modes, and (D) described the crystal structure as a distortion of the higher-symmetry structure of As2O5 (which is here also a high-temperature phase) by using symmetry-adapted distortion modes. Approach (D) offers the possibility to directly introduce an order parameter into Rietveld refinement through an empirical power law derived from Landau theory and thus to obtain the value of the critical exponent. In contrast, the rigid-body approach did not fit the data as well. All parameterizations greatly reduce the number of required parameters. [source] Crystal structure of cobalt-substituted calcium hydroxyapatite nanopowders prepared by hydrothermal processingJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2 2010Ljiljana Veselinovi A series of cobalt-exchanged hydroxyapatite (CoHAp) powders with different Ca/Co ratios and nominal unit-cell contents Ca10,xCox(PO4)6(OH)2, x = 0, 0.5, 1.0, 1.5 and 2.0, were synthesized by hydrothermal treatment of a precipitate at 473,K for 8,h. Based on ICP (inductively coupled plasma) emission spectroscopy analysis, it was established that the maximum amount of cobalt incorporation saturated at ,12,at.% under these conditions. The effects of cobalt content on the CoHAp powders were investigated using ICP emission spectroscopy, particle size analysis, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) analyses as well as X-ray powder diffraction (XRPD) including Rietveld analysis. According to XRPD, all the materials are single-phase HAp and CoHAp of low crystallinity. Rietveld analysis shows that Co enrichment causes the c cell parameter to decrease at a faster rate than the a cell parameter. A microstructural analysis showed anisotropic X-ray line broadening due to crystallite size reduction. In CoHAp there is significant crystal elongation in [001], and the average size decreases with increasing cobalt content. The crystallite morphology transforms from rod-like for the pure HAp to lamellae at the highest degree of Co substitution. The results of Rietveld refinement (symmetry, size and morphology of the crystallites) were confirmed by TEM and HRTEM analysis. [source] A comparative in situ Rietveld refinement study: thermal decomposition and transformation of CoAl and CoZnAl layered double hydroxidesJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 6 2008Rune E. Johnsen Rietveld refinement based on in situ X-ray powder diffraction (XRPD) data was combined with thermogravimetric analysis (TGA) and mass spectrometry (MS) to study and compare the phase transformations, thermal stability, microstructural and structural changes of two cobalt-containing nitrate-based layered double hydroxides (LDHs) upon heating in a controlled inert atmosphere of nitrogen. The XRPD data were collected, using synchrotron X-ray radiation, with a time resolution of 107,s, which made it possible to carry out detailed structural studies of the initial layered double hydroxides as well as their decomposition products: spinel for a CoAl,NO3 LDH and spinel/zincite for a CoZnAl,NO3 LDH. Correlating these data with those from the TGA,MS analyses gives us information about the transformation mechanisms. Rietveld refinements of the two spinel phases reveal remarkable differences. The a axis of the spinel formed by decomposition of the CoAl,NO3 LDH increases almost linearly from approximately 598 to 1163,K, mainly due to the dominating thermal expansion, whereas the a axis of the spinel formed by decomposition of the CoZnAl,NO3 shows a more complex temperature dependency. Between approximately 698 and 1073,K, the a axis is almost constant due to pronounced chemical interaction with an additional amorphous phase and the zincite phase, whereas from 1073 up to 1163,K it increases linearly. Calculations, based on the results of the Rietveld refinements, of the size of the octahedral and tetrahedral coordination polyhedra in the spinel show that the octahedra shrink and the tetrahedra expand with increasing temperature. The unusual thermal behaviour of the octahedra is discussed and attributed to the low formation temperature of the cobalt aluminium spinel phase. Finally, the intensity of a low-angle scattering (LAS) signal observed in the XRPD patterns was correlated with the decomposition of the LDH, and determination of the specific surface areas gave the temperature-dependent BET surface areas. [source] Crystal structure prediction of organic pigments: quinacridone as an exampleJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 1 2007N. Panina The structures of the ,, , and , polymorphs of quinacridone (Pigment Violet 19) were predicted using Polymorph Predictor software in combination with X-ray powder diffraction patterns of limited quality. After generation and energy minimization of the possible structures, their powder patterns were compared with the experimental ones. On this basis, candidate structures for the polymorphs were chosen from the list of all structures. Rietveld refinement was used to validate the choice of structures. The predicted structure of the , polymorph is in accordance with the experimental structure published previously. Three possible structures for the , polymorph are proposed on the basis of X-ray powder patterns comparison. It is shown that the , structure in the Cambridge Structural Database is likely to be in error, and a new , structure is proposed. The present work demonstrates a method to obtain crystal structures of industrially important pigments when only a low-quality X-ray powder diffraction pattern is available. [source] Chlorartinite, a volcanic exhalation product also found in industrial magnesia screedJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 5 2006Kunihisa Sugimoto The volcanic exhalation product chlorartinite, [Mg2(CO3)(H2O)(OH)]Cl·H2O, has recently been found to be a minor, and in some samples a major, component of magnesia floors for industrial use. In order to be able to perform quantitative phase analysis using the Rietveld method, its crystal structure was determined from high-resolution synchrotron powder diffraction data by the global optimization technique of simulated annealing and Rietveld refinement. The final Rp and Rwp values are 5.23% and 6.56%, respectively. Chlorartinite crystallizes in the rhombohedral space group R3c (No. 161), with a = 23.14422,(16),Å, c = 7.22333,(5),Å, V = 3350.84,(5),Å3, Z = 18. The building units of chlorartinite consist of MgO6 octahedra forming 15-membered puckered rings which are interconnected by CO3 triangular moieties. The rings are stacked to form a honeycomb-like three-dimensional framework structure with large isolated channels. Within the channels, free chlorine atoms and disordered water molecules are located. [source] A new technique for angle-dispersive powder diffraction using an energy-dispersive setup and synchrotron radiationJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 6 2004Yanbin Wang A new diffraction technique for combined angle- and energy-dispersive structural analysis and refinement (CAESAR), by collecting angle-dispersive data using a solid-state detector (SSD) and white synchrotron radiation, is introduced. By step scanning a well calibrated SSD over a limited 2, range, a series of one-dimensional energy-dispersive data (intensity versus energy) are obtained as a function of 2,. The entire intensity (Int) data set consists of several thousand channels covering a range of photon energies, E (up to ,150,keV), at each of the ,1000 2, steps, forming a 2,4 mega-element two-dimensional array, Int(E, 2,). These intensity data are then regrouped according to photon energies, which are defined in the multichannel SSD as individual channels, yielding a large number of intensity versus 2, (angle-dispersive) data sets, Int(E = const., 2,), each of which corresponds to a given photon energy or wavelength. The entire data set, selected subsets or composite scans can be used for multiple data set Rietveld refinement. Data collected both on ,-Al2O3 (a NIST diffraction standard) at ambient conditions and on a mixture of MgO and Au at high pressure were analyzed using the Rietveld technique, with varying schemes of data treatment. Furthermore, it is demonstrated that data within certain energy bands (,E/E = ±10%) may be binned together to improve counting statistics in a composite angle-dispersive scan, even when collected with much coarser scan steps of 0.1 or 0.2°. This technique is useful for high-pressure as well as general purpose powder diffraction studies that have limited X-ray access to the sample using synchrotron radiation. Several advantages are discussed. [source] Observation of orientational disorder in the hexagonal stuffed tridymite Sr0.864Eu0.136Al2O4 by the maximum-entropy methodJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 5 2004H. Yamada The crystal structure of a strontium europium aluminate, Sr0.864Eu0.136Al2O4, with a novel hexagonal form was investigated by a combination of Rietveld analysis and the maximum-entropy method (MEM) with synchrotron X-ray powder diffraction data. The electron density image calculated by the MEM/Rietveld method revealed that the apical oxygen ion in the AlO4 tetrahedron has a broad distribution corresponding to an extraordinarily large atomic displacement parameter. This structure could be expressed by a split-atom model, with which the Rietveld refinement gave Rwp = 2.99% and RB = 4.16%. Subsequently, MEM-based pattern fitting (MPF) decreased the R factors to Rwp = 2.81% and RB = 2.34% and the electron density image clearly showed that the apical oxygen ions of the AlO4 tetrahedra are split over three sites around a threefold axis involving an elongated distribution of the residual O ions along the c axis. These results suggest that AlO4 tetrahedra in Sr0.864Eu0.136Al2O4 are orientationally disordered. [source] Performance of a new furnace for high-resolution synchrotron powder diffraction up to 1900,K: application to determine electron density distribution of the cubic CaTiO3 perovskite at 1674,KJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 5 2004Masatomo Yashima Accurate crystal structure analysis at high temperatures is an important challenge in science and technology. A new electric furnace for the measurement of high-resolution (,d/d = 0.03%) synchrotron radiation powder diffraction profiles from materials at high temperatures (up to 1900,K in air) has been designed and fabricated. This furnace consists of a ceramic refractory with MoSi2 heaters, an aluminium body cooled by flowing water, and a sample stage with a spinner and a controller for sample-height adjustment. In situ synchrotron powder diffraction measurement for a calcium titanate perovskite specimen at 1674,K has been performed using the furnace at beamline 3A of the Photon Factory. The electron density distribution of the cubic perovskite at 1674,K was successfully obtained using a combination of Rietveld refinement, the maximum-entropy method (MEM) and MEM-based pattern-fitting techniques. The Ti atoms exhibit covalent bonding with the O atoms in the cubic CaTiO3 perovskite at this temperature, while the Ca atoms are ionic. These results indicate that the new furnace yields high-quality data for accurate crystal structure analysis. [source] Limitations of asymmetric parallel-beam geometryJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3 2004N. A. Raftery Bragg diffraction peak profiles and intensities in asymmetric (,,2,) diffraction using a mirror-based parallel-beam geometry were compared with symmetric parallel-beam (,,2,) and conventional Bragg,Brentano (,,2,) diffraction for a powdered quartz sample and the NIST standard reference material (SRM) 660a (LaB6, lanthanum hexaboride). A comparison of the intensities and line widths (full width at half-maximum, FWHM) of these techniques demonstrated that low incident angles (,,<,5°) are preferable for the parallel-beam setup. For higher , values, if 2,,<,2,, mass absorption reduces the intensities significantly compared with the Bragg,Brentano setup. The diffraction peak shapes for the mirror geometry are more asymmetric and have larger FWHM values than corresponding peaks recorded with a Bragg,Brentano geometry. An asymmetric mirror-based parallel-beam geometry offers some advantages in respect of intensity when compared with symmetric geometries, and hence may be well suited to quantitative studies, such as those involving Rietveld analysis. A trial Rietveld refinement of a 50% quartz,50% corundum mixture was performed and produced adequate results. [source] Bridging the gap , structure determination of the red polymorph of tetrahexylsexithiophene by Monte Carlo simulated annealing, first-principles DFT calculations and Rietveld refinementJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3 2002Marcus A. Neumann The crystal structure of the red polymorph of tetrahexylsexithiophene (THST) is solved from X-ray powder diffraction data by a direct-space Monte Carlo simulated-annealing approach. First-principles density functional theory (DFT) calculations are used to distinguish between three nearly identical solutions in the space groups C2/m, C2 and P and to improve the overall accuracy of the crystal structure. The correct space group is found to be C2/m. In all space groups, the thiophene backbone is planar and the hexyl side chains assume an all- trans conformation except for two terminal methyl residues, which adopt a gauche orientation. The ability of first-principles DFT calculations to provide atomic coordinates of single-crystal quality is demonstrated by lattice-energy minimization of the known crystal structure of the yellow polymorph of THST. The combination of Monte Carlo simulated annealing, first-principles DFT calculations and Rietveld refinement presented in this paper is generally applicable. It provides a powerful alternative to standard approaches in cases where the information content of the powder diffraction pattern alone is insufficient to distinguish between different structure solutions. DFT calculations can also provide invaluable guidance in Rietveld refinement. [source] Robust Rietveld refinement in the presence of impurity phasesJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 6 2001W. I. F. David A modified least-squares analysis is presented that allows reliable structural parameters to be extracted from a powder diffraction pattern even in the presence of a substantial unmodelled impurity contribution. The algorithm is developed within the context of Bayesian probability theory. Experimental points that fall above those calculated, and are thus more probably from impurity peaks, are systematically down-weighted. This approach is illustrated with a two-phase example. [source] Elastic strain and stress determination by Rietveld refinement: generalized treatment for textured polycrystals for all Laue classesJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2 2001N. C. Popa A novel approach to model diffraction line shifts caused by elastic residual or applied stresses in textured polycrystals is proposed. The model yields the complete strain and stress tensors as a function of crystallite orientation, as well as the average values of the macroscopic strain and stress tensors. It is particularly suitable for implementation in Rietveld refinement programs. The requirements on refinable parameters for all crystal Laue classes are given. The effects of sample symmetry are also included and the conditions for strain invariance to both the sample symmetries (texture and stress/strain) are discussed. [source] Rietveld quantitative amorphous content analysisJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2 2001A. G. De La Torre A procedure for Rietveld quantitative amorphous content analysis (RQACA) is outlined, in which the effects of systematic errors in the powder patterns are studied. The method derives the amorphous content from the small overestimation of an internal crystalline standard in a Rietveld refinement of an appropriate mixture. Of several standards studied, Al2O3 gave the best results. The statistical analysis of standard mixtures with a known amount of amorphous content indicated that this is a precise and accurate tool. It enables the measurement of the amorphous content with an accuracy close to 1%. Sample preparation and Rietveld analysis need to be optimized in order to minimize the systematic errors. The analysis of samples with phases displaying strong preferred orientation effects gives very high errors in the amorphous content. Samples with different absorption coefficients have also been studied in order to evaluate the importance of microabsorption. This plays an important role but it can be adequately corrected if the absorption coefficients of the standard and the sample are not very different. RQACA has been applied to tricalcium silicate, C3S, which is the main component of Portland cement. The average amorphous content of C3S, after microabsorption correction using two standards of higher and lower absorption coefficients, was found to be 19%. [source] Diffraction-line shift caused by residual stress in polycrystal for all Laue groups in classical approximationsJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 1 2000N. C. Popa Analytical formulae for all Laue groups are derived, giving the dependence of the residual elastic strain measured by X-ray and neutron diffraction on the direction in the sample and on the Miller indices. These formulae are valid for isotropic polycrystals in the limits of the Reuss and Voigt approximations and are appropriate for Rietveld refinement. [source] Quantitative high-pressure pair distribution function analysisJOURNAL OF SYNCHROTRON RADIATION, Issue 5 2005John B. Parise The collection of scattering data at high pressure and temperature is now relatively straightforward thanks to developments at high-brightness synchrotron radiation facilities. Reliable data from powders, that are suitable for structure determination and Rietveld refinement, are routinely collected up to about 30,GPa in either a large-volume high-pressure apparatus or diamond anvil cell. In those cases where the total elastic scattering is of interest, as it is in the case of nano-crystalline and glassy materials, technical developments, including the use of focused high-energy X-rays (>80,keV), are advantageous. Recently completed experiments on nano-crystalline materials at the 1-ID beamline at the Advanced Photon Source suggest that quantitative data, suitable for pair distribution function analysis, can be obtained. [source] 4d Electronic structure analysis of ruthenium in the perovskite oxides by Ru K - and L -edge XASJOURNAL OF SYNCHROTRON RADIATION, Issue 2 2001Jong-Young Kim The 4d electronic structure of ruthenium in the perovskite oxides, La2MRuIVO6 (M = Zn, Mg, and Li) and Ba2YRuVO6, has been investigated by the Ru K-and L-edge XANES and EXAFS analyses. Such X-ray absorption spectroscopic results clarify that the RuIV (d4) and RuV (d3) ions are stabilized in nearly regular Oh site. Comparing the Ru L-edge XANES spectra of perovskites containing isovalent ruthenium, it has been found that the t2g state is mainly influenced by A site cation, whereas the eg is mainly affected by neighboring B site cation. The experimental EXAFS spectra in the range of R ,,4.5 Å are well reproduced by ab-initio calculation based on crystallographic data, which supports the long-range structure presented by Rietveld refinement. [source] Chemistry and Morphology of Hydrogarnets Formed in Cement-Based CASH Hydroceramics Cured at 200° to 350°CJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2009Konstantinos Kyritsis We have studied the chemistry and the morphology of hydrogarnet crystals produced in cement-based hydroceramic materials at elevated temperatures (200°,350°C) with silica and alumina additions. Such materials lie within the hydrothermal CaO,Al2O3,SiO2,H2O (CASH) system. Hydrogarnet Ca3Al2(SiO4)3,y(OH)4y is the dominant aluminum bearing phase formed and its composition is influenced mainly by the curing temperature and to a lesser degree by the addition of silica. The composition parameter y was estimated by Rietveld refinement of X-ray diffraction (XRD) data. Electron probe microanalysis (EPMA) shows that the hydrogarnets incorporate minor elements such as Fe, Mg, and S. EPMA data confirmed the hydrogarnet composition estimated from XRD. Both octahedral and icositetrahedral forms are observed. The icositetrahedral form is associated with higher minor element content. [source] Low-Temperature Aging Behavior of Alumina-Toughened ZirconiaJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2008Jens Schneider The corrosion of alumina-toughened zirconia (ATZ) as a consequence of hydrothermal treatment was investigated, especially the transformation kinetics from tetragonal zirconia to the monoclinic phase. For this purpose, polished ATZ specimens (Ra<5 nm) were aged in water vapor at different temperatures ranging from 70° to 134°C. The fraction of the monoclinic phase was determined using X-ray diffraction and Rietveld refinement. The isothermal transformation curves obtained were fitted to the Mehl,Johnson,Avrami equation by least squares. An Arrhenius plot of the fitted transformation rates was used to determine the activation energy and the pre-exponential factor. Following this procedure, the kinetic parameters of the phase transformation were extrapolated down to body temperature and the formation of the monoclinic phase was simulated. In addition, optical interferometry on well-polished specimens (Ra<2 nm) was alternatively used to calculate the monoclinic fraction from the histogram dataset. The results agree very well with those of the X-ray measurements. Additionally, the development of surface roughness with increasing aging time is discussed. [source] Synthesis and Crystal Structure of a New Layered Carbide ZrAl4C4JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2008Tomoyuki Iwata A new ternary layered carbide, ZrAl4C4, has been synthesized and characterized by X-ray powder diffraction. The crystal structure was successfully determined using direct methods and further refined by the Rietveld method. The crystal is trigonal (space group P3m1, Z=2) with lattice dimensions a=0.332471(3) nm, c=2.19717(2) nm, and V=0.210330(3) nm3. The final reliability indices calculated from the Rietveld refinement were Rwp=6.56% (S=1.58), Rp=4.92%, RB=1.90%, and RF=0.98%. The compound shows an intergrowth structure with NaCl-type [Zr2C3] thin slabs separated by Al4C3 -type [Al8C7] layers. [source] Tricalcium Silicate T1 and T2 Polymorphic Investigations: Rietveld Refinement at Various Temperatures Using Synchrotron Powder DiffractionJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2004Vanessa K. Peterson The lattice parameters, cell volume, and structure of a sample of phase pure triclinic tricalcium silicate were determined using in situ, high-temperature synchrotron powder diffraction and full-profile Rietveld refinement. The temperature range covered was from ambient to 740°C. Evidence of superstructure was found. The T2 type structure with disordered SiO4 tetrahedra was observed, and an average structure for the subcell (P, a= 11.7416(2) Å, b= 14.2785(2) Å, c= 13.7732(2) Å, ,= 105.129(1)°, ,= 94.415(1)°, and ,= 89.889(1)°) is presented. Differential thermal analysis and X-ray fluorescence was also performed. [source] Real-time XRD Investigations During the Formation of Cu(IngGa)Se2 Thin FilmsPARTICLE & PARTICLE SYSTEMS CHARACTERIZATION, Issue 6 2005Frank Hergert Abstract Knowledge of the solid-state reactions providing the synthesis of the absorber material Cu(IngGa)Se2 well below its melting point is an essential prerequisite for the further optimization of the technologically relevant production processes. Therefore, powder XRD has been applied as nondestructive tool to follow chemical solide-state reactions in-situ. Subsequent Rietveld refinement provides the quantitative phase evolution time. [source] Superexchange interactions in inverse spinel lithium ferritesPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 12 2007Sung Wook Hyun Abstract Spinel ferrites, MFe2O4 (M = Ni, Mg, Co, Li) samples were prepared by sol-gel method. It has been studied by X-ray diffraction, Mössbauer spectroscopy. X-ray diffraction patterns were analyzed by the Rietveld refinement. The samples have been cubic spinel structure with the lattice constant (a0) is 8.326 , 8.390 Å. The temperature dependence of the magnetic hyperfine field is analyzed by the Néel theory of ferrimagnetism. The intersublattice A-O-B and intrasublattice A-O-A superexchange interactions are found to be antiferromagnetic while the intrasublattice B-O-B superexchange interaction is ferromagnetic for the MFe2O4 (M = Ni, Mg, Co) samples as shown in Table 1. On the other hand, the intersublattice superexchange interaction is found to be antiferromagnetic while the intrasublattice superexchange interactions are ferromagnetic for the lithium ferrite sample. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Multipole electron-density modelling of synchrotron powder diffraction data: the case of diamondACTA CRYSTALLOGRAPHICA SECTION A, Issue 4 2010H. Svendsen Accurate structure factors are extracted from synchrotron powder diffraction data measured on crystalline diamond based on a novel multipole model division of overlapping reflection intensities. The approach limits the spherical-atom bias in structure factors extracted from overlapping powder data using conventional spherical-atom Rietveld refinement. The structure factors are subsequently used for multipole electron-density modelling, and both the structure factors and the derived density are compared with results from ab initio theoretical calculations. Overall, excellent agreement is obtained between experiment and theory, and the study therefore demonstrates that synchrotron powder diffraction can indeed provide accurate structure-factor values based on data measured in minutes with limited sample preparation. Thus, potential systematic errors such as extinction and twinning commonly encountered in single-crystal studies of small-unit-cell inorganic structures can be overcome with synchrotron powder diffraction. It is shown that the standard Hansen,Coppens multipole model is not flexible enough to fit the static theoretical structure factors, whereas fitting of thermally smeared structure factors has much lower residuals. If thermally smeared structure factors (experimental or theoretical) are fitted with a slightly wrong radial model (s2p2 instead of sp3) the radial scaling parameters (`,' parameters) are found to be inadequate and the `error' is absorbed into the atomic displacement parameter. This directly exposes a correlation between electron density and thermal parameters even for a light atom such as carbon, and it also underlines that in organic systems proper deconvolution of thermal motion is important for obtaining correct static electron densities. [source] Octahedral tilting in Pb-based relaxor ferroelectrics at high pressureACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2010Bernd J. Maier We have employed a combination of powder neutron diffraction and single-crystal synchrotron X-ray diffraction to characterize the pressure-induced phase transitions that occur in the perovskite-type relaxor ferroelectric PbSc0.5Ta0.5O3 (PST) and Pb0.78Ba0.22Sc0.5Ta0.5O3 (PST-Ba). At ambient pressure the symmetry of the average structure for both compounds is as a result of partial ordering of the Sc and Ta cations on the octahedral sites. At pressures above the phase transition both the neutron and X-ray diffraction patterns exhibit an increase in the intensities of h,k,l = all odd reflections and no appearance of additional Bragg reflections. Synchrotron single-crystal X-ray diffraction data show that the intensity of hhh peaks, h,=,2n,+,1, does not change with pressure. This indicates that the structural distortion arising from the phase transition has a glide-plane pseudo-symmetry along the ,111, cubic directions. Rietveld refinement to the neutron powder data shows that the high-pressure phase has either or symmetry, depending on whether the presence of 1:1 octahedral cation ordering is neglected or taken into account, and comprises octahedral tilts of the type a,a,a, that continuously evolve with pressure. The cubic-to-rhombohedral transition is also marked by a large increase in the anisotropy of the displacement ellipsoids of the Pb cations, indicating larger displacements of Pb cations along the rhombohedral threefold axis rather than within the perpendicular plane. For PST the anisotropy of the Pb displacement parameters decreases at approximately 3,GPa above the phase-transition pressure. For both PST and PST-Ba the average magnitudes of Pb-cation displacements expressed in terms of isotropic displacement ellipsoids gradually decrease over the entire pressure range from ambient to 7.35,GPa. [source] | ||||||||||||||||