title: An Implementation of the Fundamental Parameters Approach for Analysis of X-ray Powder Diffraction Line Profiles.
abstract: This work presents an open implementation of the Fundamental Parameters Approach (FPA) models for analysis of X-ray powder diffraction line profiles. The original literature describing these models was examined and code was developed to allow for their use within a Python based least squares refinement algorithm. The NIST interest in the FPA method is specific to its ability to account for the optical aberrations of the powder diffraction experiment allowing for an accurate assessment of lattice parameter values. Lattice parameters are one of the primary certified measurands of NIST Standard Reference Materials (SRMs) for powder diffraction. Lattice parameter values obtained from analysis of data from SRMs 640e and 660c using both the NIST FPA Python code and the proprietary, commercial code Topas, that constitutes the only other actively supported, complete implementation of FPA models within a least-squares data analysis environment, agreed to within 2 fm. This level of agreement demonstrates that both the NIST code and Topas constitute an accurate implementation of published FPA models.
authors.affiliation: National Institute of Standards and Technology, Gaithersburg, MD 20899 USA.
authors.forename: Marcus H
authors.initials: MH
authors.lastname: Mendenhall
authors.affiliation: University of California Los Angeles, Department of Statistics, 8125 Math Sciences Bldg., Los Angeles, CA 90095-1554 USA.
authors.forename: Katharine
authors.initials: K
authors.lastname: Mullen
authors.affiliation: National Institute of Standards and Technology, Gaithersburg, MD 20899 USA.
authors.forename: James P
authors.initials: JP
authors.lastname: Cline
keywords: Fundamental Parameters Approach; Python; Standard Reference Material; X-ray line profile; powder diffraction