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Rietveld Refinement

Rietveld Refinement. Why do Reitveld Refinement?. What are the limitations of Rietveld ?. Phase detection limit of 1-5% Impurity atom determination New/unknown crystal structures Determination of disorder parameter Multiple parameters can have same effect diffraction patterns

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Rietveld Refinement

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  1. Rietveld Refinement

  2. Why do Reitveld Refinement?

  3. What are the limitations of Rietveld? • Phase detection limit of 1-5% • Impurity atom determination • New/unknown crystal structures • Determination of disorder parameter • Multiple parameters can have same effect diffraction patterns • Equipment (powder vs. single crystal, x-rays vs. neutrons) • Garbage in, garbage out! (noisy data, poor quality sample, poor sample prep)

  4. Basic refinement procedure • Refine: • Background • Lattice parameters • Peak intensities • Peak shapes • Peak positions • Phase fractions Experimental diffraction pattern Starting crystal structure (.cif, ICSD) Refinement program: GSAS, FullProf • Assess: • Goodness of fit/R factors • Impurity phases • Peak/background shapes • Difference pattern Instrument file (.inst, LaB6 standard)

  5. Some common challenges/problems • False minimas • Refinement diverges (“blows up”) • Which goodness of fit to choose? R vs. Chi sq? • Preferred orientation • Large amorphous background • Incorrect starting crystal structure or lattice parameter • Poor quality data!

  6. Resources and links • GSAS/EXPGUI: http://www.ncnr.nist.gov/programs/crystallography/software/gsas.html • GSAS tutorials: http://www.aps.anl.gov/Xray_Science_Division/Powder_Diffraction_Crystallography/ • FullProf: https://www.ill.eu/sites/fullprof/ • PowDLL (file conversion): http://users.uoi.gr/nkourkou/powdll/ • Lots of info about fitting parameters: http://www.ing.unitn.it/~luttero/laboratoriomateriali/RietveldRefinements.pdf • People: LaurynBaranowski (lbaranow@mines.edu), Lakshmi Krishna (lkrishna@mines.edu)

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