Development and Certification of NIST Standard Reference Materials for Powder Diffraction

In the 12 years since the last Accuracy in Powder Diffraction meeting, there have been significant advances. Just an example: a search of the Protein Data Bank for “powder” shows just 2 hits for before 2000, 19 for 2000‐2005, 27 for 2005‐2010 and 31 for 2010‐present. The current Cambridge Crystallographic Data Base (CCDC) shows 2354 molecular crystal structures determined via powder diffraction (out of some 0.6M structures determined by single crystal methods!); most have been determined in the last 10 years Similarly, most synchrotron sources now have high resolution multi‐detector powder diffractometers either in operation or under construction; 10 years ago there was only one (at ESRF). Software developments have also kept pace with, for example, the addition of extensive restraints and rigid bodies in GSAS for support of protein refinements and the continued development of the fundamental parameters approach (FPA) in TOPAS. These developments come with a new set of questions about accuracy. For example, what is the impact of constraints/restraints on the results; should restraints be considered as “observations”? What instrumental deficiencies now appear with high resolution multi‐detector data? Are they accurately modeled in Rietveld refinements? Is our approach to validate powder diffraction results adequate? This talk will summarize these and other developments and their impact on the accuracy of powder diffraction results.