Pattern Fitting Technique Research Articles

Synchrotron X-ray and neutron investigation of the structure and thermal expansion of the monoclinic Al13Cr2 phase

The crystal structure of the Al13Cr2 intermetallic phase at 298 K was studied by synchrotron X-ray and neutron diffraction through the analysis of data collected on bulk samples, one in the as-cast condition, one after annealing at 773 K for 24 h and one for 168 h. Furthermore the thermal expansion behaviour of the structure was investigated for the first time in-situ up to 773 K. The diffraction patterns were analysed using whole pattern fitting techniques using the Le Bail and Pawley methods as well as the Rietveld method. The structure was found to be monoclinic with space group C2/m and lattice parameters a = 25.3101 Å, b = 7.6081 Å, c = 10.9689 Å and β = 128.704∘ at 298 K in the as-cast condition. From the temperature variation of the unit cell parameters the principal axes of the thermal expansion tensor were calculated. The largest principal axis has a magnitude of 1.66×10−5 K−1 and lies parallel to b, the second largest lies in the a-c plane at an angle of ∼37° from c in the direction of -a with a value 1.64×10−5 K−1 whereas the smallest lies at ∼53° to c in the direction of +a and has a value of 1.31×10−5 K−1. Analysis of the behaviour of the icosahedra revealed that their expansion is highly anisotropic and the level of anisotropy is related to the point symmetry of the central Cr atoms.

COMBUSTION AND LEACHING BEHAVIOR OF TRACE ELEMENTS IN LIGNITE AND COMBUSTION BY-PRODUCTS FROM THE MUĞLA BASIN, SW TURKEY

The Muğla Basin is one of the most well-documented coal basins of Anatolia, SW Turkey. Previous studies mainly focused on coal geology, as well as on the environmental impacts from trace elements emitted into the atmosphere during coal combustion. However, the environmental impacts from coal utilization also include groundwater contamination from hazardous trace elements leached from exposed lignite stockpiles or ash disposal dumps. In the present study a comparative assessment of the combustion, as well as the leaching behaviour of trace elements from sixteen lignite, fly ash and bottom ash samples under various pH conditions is attempted. The samples were picked up from three regions in the Muğla Basin, namely, these of Yeniköy, Kemerköy and Yatağan. Proximate and ultimate analyses were performed on all samples. Quantitative mineralogical analysis was carried out using a Rietveld-based full pattern fitting technique. The elements Ag, As, B, Ba, Be, Co, Cr, Cu, Fe, Ga, Hf, Li, Mn, Mo, Ni, Pb, Sr, U, V and Zn were grouped according to their volatility during combustion and their leachability in the various types of samples. The pH of the leaching agent little affected the leaching trends of most elements and the mode of occurrence proved to be the major factor controlling primarily combustion and to a lesser extent leaching. The elements were classified into 7 classes with increasing environmental significance with Mo, Sr and V being the most potentially hazardous trace elements in the Muğla region.

Analysis of light scattering by red blood cells in ektacytometry using global pattern fitting

Ektacytometry measures the shape of red blood cells under shear stress by analyzing the diffraction pattern of laser light passing through a thin layer of suspended cells. Here we model the diffraction pattern using a combination of Bessel and anomalous scattering functions, and employ a global pattern-fitting technique over nine different shear stresses to determine the separate mechanical properties of normal and non-deformable cells. This technique is capable of yielding the correct elongation index of the normal cells over a range of shear stresses even when they are mixed with as much as 50% non-deformable cells. Additionally, the relative concentrations of normal and non-deformable cells can be determined.

Comparative study of natural and man-made polymers using whole powder pattern fitting technique

Natural polymers like chitosan, hemp and man-made polymers like poly(ethylene oxide)(PEO) and hydroxypropyl methylcellulose (HPMC) were investigated using wide angle X-ray scattering (WAXS) method. The merits and demerits of natural and man-made polymers were quantified in terms of microcrystalline parameters, determined by whole powder pattern fitting technique, which is an extension of single order method developed by us.

Structural and Mechanistic Studies of the Dehydration of MoO2PO3OH·H2O and the In situ Identification of Two New Molybdenum Phosphates

We report investigations into the dehydration pathway of the precursor material MoO2PO3OH·H2O to γ-(MoO2)2P2O7. The reaction occurs in three distinct stages via the formation of two new previously unidentified molybdenum phosphate phases, β-MoOPO4 and δ-(MoO2)2P2O7. Conditions for the isolation of these phases were identified by a whole powder pattern fitting technique to follow phase evolution versus time and temperature and later verified by full Rietveld refinement. Structural refinement of β-MoOPO4 was performed against X-ray and neutron data. The new phase has lattice parameters a = 7.4043(3) A, b = 7.2128(3) A, c = 7.2876(3) A, β = 118.346(2)°, and volume 342.53(3) A3 at room temperature, containing 7 unique atoms in space group Cc. δ-(MoO2)2P2O7 forms on slowly heating the precursor material to 793 K. Lattice parameters at room temperature are a = 16.2213(11) A, b = 3.8936(3) A, and c = 6.2772(4) A and volume 396.46(5) A3, in space group C2221. A transformation mechanism is proposed for the dehydra...

Computation of stacking and twin faults in varieties of cotton fibers using whole powder pattern fitting technique

The extent of stacking and twin faults has been estimated in different varieties of cotton fibers grown in the Karnataka State of India using wide-angle X-ray scattering (WAXS) data. Further, the microstructural parameters such as crystal size 〈N〉 and lattice strain (g in %) have been determined by a whole powder pattern fitting technique developed by us. In all these cases we note that the stacking and twin faults are quite significant in determining the properties of cotton fibers and their microstructural parameters.

Characterization of Irradiated Microstructure by X-Ray Diffraction Line Profile Analysis

Zirconium-based alloys have been irradiated with 11 and 15 MeV proton and 116 MeV oxygen ions at different doses. The changes in the microstructure have been studied for the ion-irradiated alloys as a function of dose using X-ray diffraction line profile analysis (XRDLPA) based on the whole powder pattern fitting technique. It was observed that the microstructural parameters such as domain size, microstrain within the domain, and dislocation density did not change significantly with the increase in dose for proton-irradiated samples. A clear change was noticed in these microstructural parameters as a function of dose for oxygen-irradiated samples. There was a drastic decrease in domain size at a dose of 1 × 1017 O5+/m2, but these values reached a plateau with increasing dose. The values of microstrain and dislocation density increased significantly with the dose of irradiation.

X-ray Diffraction Study on Cooling-Rate-Induced γ fcc→ε hcp Martensitic Transformation in Cast-Homogenized Fe-26Mn-0.14C Austenitic Steel

The thermal stability of a cast homogenized austenitic steel with basic composition Fe-26Mn-0.14C has been studied using the Rietveld entire X-ray diffraction (XRD) pattern fitting technique. The decomposition of austenite was observed to be very sensitive to the cooling rate. A very high proportion (49 to 70 pct) of e-martensites was found in the differently cooled specimens as well in the as-cast specimen. The density of various planar (stacking and twin) faults was also estimated in both e-martensites and retained austenite. The significant variation in the extent of γ fcc→e hcp martensitic transformation was interpreted in terms of athermal and isothermal martensites formed during cooling and grain size of austenite. The austenite grains were found to be comprised of a high to moderate density of stacking faults (∼10−3 to 10−2) and negligible twin fault probability (∼10−5). On the other hand, in the e-martensites, the dominant planar fault was twins (∼10−3). Approximate values of the dislocation density within the retained austenite were also evaluated using the crystallite size and root-mean-square (rms) strain values obtained from the “size-strain-shape” analyses.

Performance of a new furnace for high-resolution synchrotron powder diffraction up to 1900 K: application to determine electron density distribution of the cubic CaTiO3perovskite at 1674 K

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 (δd/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 MoSi2heaters, an aluminium body cooled by flowing water, and a sample stage with a spinner and a controller for sample-height adjustment.In situsynchrotron 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 CaTiO3perovskite 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.

Advances in Powder Diffraction Analysis

Powder diffraction offers a wide spectrum of applications to solid-state scientists. The method traditionally used for phase analysis and the study of structural imperfections has benefited, in the last twenty years, from great advances in the instrumentation and computer-based approaches for pattern indexing and modelling. The factors at the origin of the metamorphosis of the method are presented. The major modern applications reported include quantitative analysis and the extraction of three-dimensional structural and microstructural properties. The use of pattern-fitting techniques for the characterization of the microstructure is discussed through applications to nanocrystalline materials. Remarkable results achieved in the solution of crystal structures are presented, as well as the impact in solid-state chemistry of powder crystallography, particularly for elucidating the crystal chemistry of families of compounds for which only powders are available. New strategies for solving the phase problem have been introduced and new classes of solids are being studied, such as drugs, coordination and organic compounds.

Characterisation of microstructure of isothermal martensite in Fe–23Ni–3.8Mn by Rietveld method

The lattice defect-related features of the microstructure evoluted due to lattice invariant shear of martensitic transformations at low temperature (77 K and 143 K) in Fe–23Ni–3.8Mn alloy have been characterised by analysing the X-ray profiles originating from the parent and isothermally transformed martensite phases. The X-ray profiles have been analyzed employing Rietveld's whole powder pattern fitting technique, incorporating preferred orientations of the crystallites. The martensite phase has lower values of the crystallite sizes and higher value r.m.s. strain and higher dislocation densities compared to those of the austenite phase, both in the bulk and deformation induced transformed powder samples. The annealed powders, however, could not be transformed isothermally at low temperatures (77 K and 143 K), unlike the bulk specimens. The bulk martensite has an increased value of hardness compared to the austenite.

Quantitative analysis of SiC polytype distributions by the Rietveld method

The quantitative determination of SiC polytype distributions is very difficult using traditional X-ray powder diffraction quantitative analysis methods, because the diffraction patterns of the various polytypes partially superimpose. The whole pattern fitting technique encapsulated in the Rietveld method is introduced to solve this problem. The detection limits for each polytype can be estimated based on their standard deviations. The results of both synthesized and experimental diffraction data show that the Rietveld method can give precise and accurate percentage compositions of the four most common SiC polytypes. This approach provides a practical tool to relate the preparation conditions to performance properties of SiC-based materials.

Powder Diffraction Studies in the YONO3-Y2O3 System

The crystal structure of yttrium oxide nitrate and the microstructure of the oxide obtained from its thermal decomposition have been investigated by modern powder diffraction methods. Sequential high-temperature diffraction studies of the thermal decomposition of two precursors of yttrium oxide nitrate, i.e., neutral yttrium nitrate and yttrium hydroxide nitrate, are reported. The structure of YONO3 has been determined ab initio from conventional powder diffractometry using monochromatic X-rays. The pattern was indexed by the successive dichotomy method yielding tetragonal unit cell dimensions: a = 3.8590(1) Å and c = 9.7161(6) Å. The space group is P4/nmm and Z = 2. Most of the atoms were located by direct methods and the remaining ones from a Fourier map. Refinement of the complete diffraction profile parameters converged to final agreement factors Rp = 0.12, Rwp = 0.16 and RF = 0.039. The structure is closely related to the matlockite PbFCl-type structure. The microstructure of ex-oxide-nitrate yttrium oxide has been analysed by total pattern fitting techniques, from which strain-free coherently diffracting nanodomains with a rectangular parallelepipedic shape have been obtained.

Determination of structure factors of copper by convergent beam electron diffraction

There has been considerable recent interest in the determination of structure factors from convergent-beam electron diffraction (CBED) patterns and the ultimate goal is the ability to retrieve the crystal structure of an unknown crystal by inversion of a CBED pattern. There are a number of different methods that have been used to extract structure factor information. The zone-axis pattern fitting technique of Bird and Saunders has recently been used to obtain structure factors for silicon that compare well with those obtained by X-ray methods. This work extends the techniques to f.c.c. metals, specifically copper.CBED patterns were recorded from [110] zone axes of electropolished foils of pure copper (99.999% purity) in the Philips EM420T at Argonne National Laboratory. The patterns were energy-filtered by scanning the whole pattern across the entrance aperture of a Gatan #607 serial energy loss spectrometer and collecting the zero loss intensity only (energy window ∼5eV).

Intercalation of AlCl 3 into FeCl 3-graphite intercalation compounds and occurrence of BI-intercalation

The formation process of the ternary AlCl 3-FeCl 3-graphite intercalation compounds (GICs) was studied by using three different reaction paths at 150°C. In the reaction of stage 1, FeCl 3-GICs with AlCl 3, the exchange of intercalated FeCl 3 for AlCl 3 occurred and, consequently, the formation of the “mixed”-type ternary GIC was observed at the edge of the flakes. The reaction of stage 2 FeCl 3-GIC with AlCl 3 however, gave the “bi-intercalation”-type GIC due to intercalation of Aid, into unoccupied graphite galleries. In the reaction of the host graphite with an equimolar mixture of AlCl 3 and FeCl 3 the preferential intercalation of AlCl 3, was observed in the beginning of the reaction, and then the exchange of AlCl 3, for FeCl 3 in the stage 1 compound occurred. Through the determination of the charge density distribution and the structural refinement by a pattern-fitting technique on mixedand bi-intercalation-type GICs, the former was found to have more defective intercalate layers than the latter.