Extending the Reach of Powder Diffraction Modelling

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Authors: John S.O. Evans
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.
Authors: Pamela S. Whitfield, Isobel J. Davidson, Lyndon D. Mitchell, Siobhan A. Wilson, Stuart J. Mills
Abstract: The TOPAS macro language can be a powerful tool for increasing the capabilities of X-ray powder diffraction analysis. New corrections and constraints can be implemented without altering the program's code, allowing for experimentation with new ideas and approaches. Examples are given, exposing the power and flexibility of the macro language to help solving problems with a few lines of code. The use of simulated annealing for structure solution of an organic material from data exhibiting preferential orientation is one example. Another one is about extraction of useful structural information in Rietveld refinement of natural hydrotalcite-group minerals, a problematic case that would normally be regarded as over-parameterized for the data available.
Authors: Kevin H. Stone, P.W. Stephens
Abstract: A robust refinement procedure is implemented in the program TOPAS through an iterative reweighting of the data. Examples are given of the procedure as applied to fitting partially overlapped peaks by full and partial models and also of the structures of ibuprofen and acetaminophen in the presence of unmodeled impurity contributions.
Authors: Ian C. Madsen, Ian E. Grey, Stuart J. Mills
Abstract: A study of the thermal decomposition sequence of a sample of natural arsenian plumbojarosite has been undertaken using in situ X-ray diffraction. The sample was heated to 900°C using an Anton-Paar heating stage fitted to an INEL CPS120 diffractometer. The data were analysed using a whole-pattern, Rietveld based approach for the extraction of quantitative phase abundances. The instrument configuration used required the development and application of algorithms to correct for aberrations in the (i) peak intensities due to differing path lengths of incident and diffracted beams in the sample and (ii) peak positions due to sample displacement. Details of the structural models used were refined at selected steps in the pattern and then fixed for subsequent analysis. The data sequence consists of some 110 individual data sets which were analysed sequentially with the output of each run forming the input for analysis of the next data set. The results of the analysis show a complex breakdown and recrystallisation sequence including the formation of a major amount of amorphous material after initial breakdown of the plumbojarosite.
Authors: Ivan Halasz, Robert E. Dinnebier
Abstract: Molecular motion by refinement of TLS matrices from high resolution laboratory powder diffraction data: implementation in the program TOPAS and application to crystalline naphthalene
Authors: Ivan Halasz, Robert E. Dinnebier
Abstract: A simulated annealing like optimization method has been successfully applied for the determination of global minimum of equation of state (EoS) parameters. The approach is tested using the program TOPAS on a high precision variable pressure data set of quartz [R.J. Angel, D.R. Allan, R. Miletich, L.W. Finger: J. Appl. Cryst. Vol 30 (1997) p. 461] It is evidenced that least-squares refinement finds the global minimum for all EoS except for the Birch-Murnaghan EoS of fourth order for which it is shown that a clearly distinguished global minimum does not exist, at least for the used data set. The method presented herein serves as a fast test for the global minimum verification for the EoS parameters but should be applicable also to other optimisation problems.
Authors: Melanie Müller, Robert E. Dinnebier, Naveed Z. Ali, Branton J. Campbell, Martin Jansen
Abstract: The first order phase transition of CsFeO2 was investigated using synchrotron powder diffraction data as a function of temperature. Two alternative approaches were used to describe the deviation of the framework crystal structure relative to the high-symmetry parent structure: symmetry (a.k.a. distortion) modes and polyhedral-tilt parameters. In both cases, the relevant parameters were refined as a function of temperature using the method of parametric Rietveld refinement. We demonstrate a semi-automated and generally applicable method for the determination of spontaneous lattice strain variations, order parameters and power-law exponents as derived from Landau theory.
Authors: Paneerselvam Rajiv, Robert E. Dinnebier, Martin Jansen
Abstract: A new program to perform fast sequential and parametric whole powder profile refinement of in situ time-resolved powder diffraction data is presented. The program interacts with the launch mode kernel of the total powder pattern analysis software suite Topas® for doing the refinements. The program provides a graphical interface platform, upon which the huge Topas input command files necessary to perform sequential and parametric refinements can be easily prepared and executed. This program requires the user license dongle for Topas academic version 3 or higher.
Authors: Ali Samy, Robert E. Dinnebier, Pavel E. Kazin, Sander van Smaalen, Martin Jansen
Abstract: A set of macros for the powerful least squares and global optimization program TOPAS has been written in order to create a user friendly interface to the maximum entropy method (MEM) program BayMem. As a case study, the crystal structures of pure strontium hydroxyl apatite and strontium hydroxyl apatite with approximately 10% of the hydroxyl anions substituted by peroxide anions were analyzed by TOPAS and BayMem. Although the concentration of peroxide is relatively small, distinct differences in the two crystal structures could be made visible by the MEM.

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