IMPROVING THE ACCURACY OF RIETVELD-DERIVED LATTICE PARAMETERS BY AN ORDER OF MAGNITUDE, B. O'Connor, S. Pratapa, pp. 158-165

This is the fourth in a series of papers on Rietveld materials characterisation metrology presented at recent Denver X-ray Conferences [refs 1-3]. Reference 3 examined the management of the zero-point and specimen displacement corrections in extracting lattice parameters from BraggBrentano x-ray diffraction data by Rietveld analysis. This paper emphasises the importance of pre-determining the zero-point of the 2θ-scale, o θ 2 , prior to, and independently of, the Rietveld calculations. In addition, the specimen displacement correction must be refined in the Rietveld computations with Bragg-Brentano data to minimise systematic errors in the lattice parameters. A zero-point determination procedure is described using line-position standard reference materials (SRMs) to extrapolate a plot of bias θ 2 versus obs cosθ to θ = 90°, with 2θbias = 2θtrue 2θobs for 2θtrue calculated with the known lattice parameters and 2θobs being the measured value. Excellent agreement was obtained for o θ 2 values determined with NIST SRM 660a LaB6 and NIST Si 640 standards. The effectiveness of this approach when used with the co-refinement of the lattice parameters and specimen displacement refinement is demonstrated with data sets for CeO2, Al2O3 and TiO2 (rutile) powders from the NIST SRM 674a set. The accuracy of lattice parameters determined by this Rietveld approach is approximately 1 part in 50,000.