Evaluating experimental methods and techniques in X-ray diffraction using 280 000 data sets in the Powder Diffraction File

Abstract

Release 2003 of the Powder Diffraction File (PDF) contains ∼280 000 unique entries organized in a series of tables in a relational database format. The PDF is available in two products, PDF-4/Full File and PDF-4/Organics, which allow users to access and query over 150 million filled entry fields. An editorial database is used to generate the commercial products that contain tables of experimental details and statistical evaluation criteria used by the editors to evaluate quality and determine quality marks for each entry in the PDF. This editorial database has nearly doubled the searchable entry fields. This database was mined to evaluate experimental methods in X-ray diffraction. Both experimental powder diffraction data and data calculated from predominantly single crystal X-ray structural analyses were assigned statistical quality criteria. For experimental powder data, the average delta two theta values for all d-spacings in the entry set were used. Calculated data were evaluated using R factor values as the primary quality criteria. A Quality Index, which measures the errors in refined unit cell parameters divided by the magnitude of the cell parameter, can be used to compare all types of data. Experimental variables were then analyzed versus these criteria. Variables include optic configuration, (i.e., Seeman–Bohlin, Guinier, Debye–Scherrer), use of internal and external standards, use of monochromators, wavelength divergence, wavelength selection, equipment radius, specimen transparency, and specimen absorption. This study significantly differs from prior round robin analyses in that the use of the database allows us to study very large population sets for every variable analyzed.