Christopher L. Farrow

PDFgetX3: a rapid and highly automatable program for processing powder diffraction data into total scattering pair distribution functions

PDFgetX3 is a new software application for converting X-ray powder diffraction data to an atomic pair distribution function (PDF). PDFgetX3 has been designed for ease of use, speed and automated operation. The software can readily process hundreds of X-ray patterns within a few seconds and is thus useful for high-throughput PDF studies that measure numerous data sets as a function of time, temperature or other environmental parameters. In comparison to the preceding programs, PDFgetX3 requires fewer inputs and less user experience and it can be readily adopted by novice users. The live-plotting interactive feature allows the user to assess the effects of calculation parameters and select their optimum values. PDFgetX3 uses an ad hoc data correction method, where the slowly changing structure-independent signal is filtered out to obtain coherent X-ray intensities that contain structure informaxadtion. The output from PDFgetX3 has been verified by processing experimental PDFs from inorganic, organic and nanosized samples and comparing them with their counterparts from a previous established software. In spite of the different algorithm, the obtained PDFs were nearly identical and yielded highly similar results when used in structure refinement. PDFgetX3 is written in the Python language and features a well documented reusable code base. The software can be used either as a standalone application or as a library of PDF processing functions that can be called from other Python scripts. The software is free for open academic research but requires paid license for commercial use.

Complex modeling: a strategy and software program for combining multiple information sources to solve ill posed structure and nanostructure inverse problems.

A strategy is described for regularizing ill posed structure and nanostructure scattering inverse problems (i.e. structure solution) from complex material structures. This paper describes both the philosophy and strategy of the approach, and a software implementation, DiffPy Complex Modeling Infrastructure (DiffPy-CMI).

Nyquist-Shannon sampling theorem applied to refinements of the atomic pair distribution function

We have systematically studied the optimal real-space sampling of atomic pair distribution (PDF) data by comparing refinement results from oversampled and resampled data. Based on nickel and a complex perovskite system, we show that not only is the optimal sampling bounded by the Nyquist interval described by the Nyquist-Shannon (NS) sampling theorem as expected, but near this sampling interval, the data points in the PDF are minimally correlated, which results in more reliable uncertainty estimates in the modeling. Surprisingly, we find that PDF refinements quickly become unstable for data on coarser grids. Although the Nyquist-Shannon sampling theorem is well known, it has not been applied to PDF refinements, despite the growing popularity of the PDF method and its adoption in a growing number of communities. Here, we give explicit expressions for the application of NS sampling theorem to the PDF case, and establish through modeling that it is working in practice, which lays the groundwork for this to become more widely adopted. This has implications for the speed and complexity of possible refinements that can be carried out many times faster than currently with no loss of information, and it establishes a theoretically sound limit on the amount of informationmorexa0» contained in the PDF that will prevent over-parametrization during modeling.«xa0less

Robust structure and morphology parameters for CdS nanoparticles by combining small-angle X-ray scattering and atomic pair distribution function data in a complex modeling framework

In this work, the concept of complex modeling (CM) is tested by carrying out a co-refinement of the atomic pair distribution function and small-angle X-ray scattering data from CdS nanoparticles. It is shown that, compared with either single technique alone, the CM approach yields a more accurate and robust structural insight into the atomic structure and morphology of nanoparticles. This work opens the door for the application of CM to a wider class of nanomaterials and for the incorporation of additional experimental and theoretical techniques into these studies.

Towards a robust ad hoc data correction approach that yields reliable atomic pair distribution functions from powder diffraction data.

We examine the equations to obtain atomic pair distribution functions (PDFs) from x-ray, neutron and electron powder diffraction data with a view to obtaining reliable and accurate PDFs from the raw data using a largely ad hoc correction process. We find that this should be possible under certain circumstances that hold, to a reasonably good approximation, in many modern experiments. We describe a variational approach that could be applied to find data correction parameters that is highly automatable and should require little in the way of user inputs yet results in quantitatively reliable PDFs, modulo unknown scale factors that are often not of scientific interest when profile fitting models are applied to the data with scale factor as a parameter. We have worked on a particular implementation of these ideas and demonstrate that it yields PDFs that are of comparable quality to those obtained with the existing x-ray data reduction program PDFgetX2. This opens the door to rapid and highly automated processing of raw data to obtain PDFs.

SrRietveld: a program for automating Rietveld refinements for high-throughput powder diffraction studies

SrRietveld is a highly automated software toolkit for Rietveld refinement. Compared to traditional refinement programs, it is more efficient to use and easier to learn. It is designed for modern high-throughput diffractometers and is capable of processing large numbers of data sets with minimal effort. The software currently uses conventional Rietveld refinement engines, automating GSAS and FullProf refinements. However, as well as automating and extending many tasks associated with these programs, it is designed in a flexible and extensible way so that in the future these engines can be replaced with new refinement engines as they become available. SrRietveld is an open-source software package developed in Python.

Quantitative nanoparticle structures from electron crystallography data

We describe the quantitative refinement of nanoparticle structures from gold nanoparticles probed by electron diffraction in the ultrafast electron crystallography UEC geometry. We establish the equivalence between the modified radial distribution function employed in UEC and the atomic pair distribution function PDF used in x-ray and neutron powder-diffraction analysis. By leveraging PDF refinement techniques, we demonstrate that UEC data are of sufficient quality to differentiate between cuboctahedral, decahedral, and icosahedral nanoparticle models for the ground-state dark structures of the gold nanoparticles. Furthermore, we identify the signatures of systematic errors that may occur during data reduction and show that atomic positions refined from UEC are robust to these errors. This work serves as a foundation for reliable quantitative structural analysis of time-resolved laser-excited nanoparticle states.