Precise implications for real-space pair distribution function modeling of effects intrinsic to modern time-of-flight neutron diffractometers

Abstract

Total scattering and pair distribution function (PDF) methods allow for detailed study of local atomic order and disorder, including materials for which Rietveld refinements are not traditionally possible (amorphous materials, liquids, glasses and nanoparticles). With the advent of modern neutron time-of-flight (TOF) instrumentation, total scattering studies are capable of producing PDFs with ranges upwards of 100-200 Å, covering the correlation length scales of interest for many materials under study. Despite this, the refinement and subsequent analysis of data are often limited by confounding factors that are not rigorously accounted for in conventional analysis programs. While many of these artifacts are known and recognized by experts in the field, their effects and any associated mitigation strategies largely exist as passed-down `tribal' knowledge in the community, and have not been concisely demonstrated and compared in a unified presentation. This article aims to explicitly demonstrate, through reviews of previous literature, simulated analysis and real-world case studies, the effects of resolution, binning, bounds, peak shape, peak asymmetry, inconsistent conversion of TOF to d spacing and merging of multiple banks in neutron TOF data as they directly relate to real-space PDF analysis. Suggestions for best practice in analysis of data from modern neutron TOF total scattering instruments when using conventional analysis programs are made, as well as recommendations for improved analysis methods and future instrument design.

Local atomic structure of thin and ultrathin films via rapid high-energy X-ray total scattering at grazing incidence

“…The limited Q-space resolution in turn affects the PDF by damping the G(r) function, i.e. reducing the PDF peak intensities with increasing r (Petkov, 2012;Olds et al, 2018). This effect is accounted for by fitting the parameter Q damp to data from a calibrant.…”

Exact and fast calculation of the X-ray pair distribution function

“…Q-dependent resolution function. Even though most PDF beamlines operate at settings that result in a rather broad resolution function, these two parameters are not really sufficient to describe these effects, especially in cases of complex resolution functions like the ones found in neutron time-of-flight diffractometers (Olds et al, 2018). To deal with complex resolution functions, Tucker et al (2001) modified the inverse transformation method originally developed by Pusztai & McGreevy (1997).…”

POWGEN: rebuild of a third-generation powder diffractometer at the Spallation Neutron Source

“…This results in the lining up of all the diffraction peaks in effective TOF. Previous studies have shown that failing to align the individual spectra well manifests as an unusual peak shape in the summed spectra (Olds et al, 2018). Then the single summed spectrum is treated according to the equation scheme…”

“…The quality of refinement matches or exceeds that available at other modern neutron total scattering beamlines. Factors influencing the quality of neutron TOF PDF data were recently reviewed (Olds et al, 2018). POWGEN's overall high resolution results in observable intensities exceeding 200 Å À1 in real space, while the influence of its near linear Ád/d (equivalent to ÁQ/Q) resolution behavior displayed in Fig.…”