Papers by Keyword: Powder Diffraction

Abstract: The fundamental parameters approach (FPA) as implemented in TOPAS is investigated for analyses of conventional X-ray powder diffraction (XRPD) data. The FPA involves the convolution of a series of models, each one constituting an individual contribution to the geometric portion of the instrument profile function (IPF). Parameters within each model are refined by least squares to yield a presumably accurate description of the experiment. If one wishes to interrogate the functionality of said models, a diffractometer wherein the uncertainties in optical character are minimized is required. To this end, a diffractometer was built at NIST which featured conventional divergent beam optics in conjunction with a well aligned, stiff, and accurate goniometer assembly. Initial results indicated that the detector arm was flexing; this problem has been addressed with the fabrication and installation of a new arm and counterweight assembly. Data collected from NIST Standard Reference Material (SRM) 660a, lanthanum hexaboride, are analyzed using the FPA method to yield conclusions on the validity of the models with respect to shape and position of the diffraction profiles.

Abstract: The basics of the Whole Powder Pattern Modeling and its implementation in the PM2K software are briefly reviewed. The main features and functionalities, and most common line broadening models are introduced with the aid of working examples related to the instrumental profile and to a plastically deformed metal. A summary of the main expressions is reported in the appendix, together with a list of useful references.

Abstract: Whole Powder Pattern Modeling (WPPM) is especially useful to study line defects in powder and polycrystalline materials. As a result of recent progresses in this field, dislocation studies can be carried out for any slip system and crystal symmetry of the studied phases. Basic theory and procedures are described with the help of some representative cases of study. The use of the dislocation-related broadening in the PM2K software implementing the WPPM approach is shown with practical examples.

Abstract: Microstrain broadening due to composition variations in -FeN0.407 has been evaluated using different evaluation procedures ranging from pattern decomposition to whole-powder-pattern modelling. Details how to set up these procedures in the TOPAS suite of programs have been described. The different procedures yield essentially the same results, but they have their advantages and disadvantages, which are discussed.

Abstract: Topas is a powerful and flexible software suite for the analysis of powder diffraction (and other) data. In this paper we describe methods to interact with the software in efficient and customisable ways. We also provide brief details of a wiki site for sharing ideas and methods for this software (topas.dur.ac.uk). Finally we give an overview of the potential advantages of parametric refinement methods within Topas and exemplify them with a quantitative analysis study of the phase transitions in WO3.

Abstract: The implementation of a physically-sound size broadening model for peak profiles in the Rietveld method is presented. TOPAS macros are provided and the results compared with analogous modelling performed according to advanced analysis methods such as the Whole Powder Pattern Modelling.

Abstract: Magnetic and structural properties in multifunctional FSMA (Ferromagnetic Shape Memory Alloys) belonging to Heusler family are frequently related to the occurrence of structural modulation in martensitic phases. The highest MFIS (Magnetic Field Induced Strain) effect has been observed in Ni-Mn-Ga alloys showing martensitic modulated structures. Depending on the composition, pressure and temperature conditions, this periodic structural distortion, consisting of shuffling of atomic layers along specific crystallographic directions, accompanies the martensitic transformation. Over the years, different modulated martensitic structures have been observed and classified depending upon the periodicity of corresponding superstructure (nM with n=3, 5, 6, 7, 12 etc). On the other hand, it has been demonstrated that in most cases such structural modulation is incommensurate and the crystal structure can be solved by applying superspace approach. The crystallographic representation of different modulated structures, obtained by structure refinement on powder diffraction data, suggests a unified description where every different “nM” periodicity can be straightforwardly represented. It will be presented an overview illustrating structural features of several displacive modulated martensitic lattices. For a specific Ni-Mn-Ga composition, the evolution of structural modulation upon temperature change will be illustrated.

Abstract: The recent evolution of powder diffraction line profile analysis toward full pattern methods is discussed. Specific reference is made to the Whole Powder Pattern Modelling (WPPM), as applied to metals and ceramics subjected to strong plastic deformation. Examples concerning three different materials science studies are shown to illustrate features and potentialities of the WPPM approach.

Abstract: A multi-purpose pattern-fitting system, RIETAN-2000, has been extensively utilized to contribute to many structural studies. It offers a sophisticated structure-refinement technique of whole-pattern fitting based on the maximum-entropy method (MEM) in combination with a MEM analysis program PRIMA. We have recently completed a successor system, RIETAN-FP, to RIETAN-2000, adding new features such as standardization of crystal-structure data, an extended March-Dollase preferred-orientation function, and automation of imposing restraints on bond lengths and angles. Further, we have been developing a new three-dimensional visualization system, VESTA, using wxWidgets as a C++ application framework. VESTA excels in visualization, rendering, and manipulation of crystal structures and electron/nuclear densities determined by X-ray/ neutron diffraction and electronic-structure calculations. VESTA also enables us to display wave functions and electrostatic potentials calculated with part of these programs.