rietveld refinement. why do reitveld 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• Equipment (powder vs. single crystal, x-rays vs.

neutrons)• Garbage in, garbage out! (noisy data, poor quality

sample, poor sample prep)

Basic refinement procedure

Experimental diffraction pattern

Starting crystal structure (.cif, ICSD)

Instrument file (.inst, LaB6 standard)

Refinement program: GSAS, FullProf

Refine:• Background• Lattice parameters• Peak intensities• Peak shapes• Peak positions• Phase fractions

Assess:• Goodness of fit/R factors• Impurity phases• Peak/background shapes• Difference pattern

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!

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: Lauryn Baranowski (lbaranow@mines.edu), Lakshmi Krishna (lkrishna@mines.edu)