THE IMPORTANCE OF THE SPECIMEN DISPLACEMENT CORRECTION IN RIETVELD PATTERN FITTING WITH SYMMETRIC REFLECTION-OPTICS DIFFRACTION DATA, B. O'Connor, D. Li, B. Hunter, pp. 96-102

Displacement of the specimen surface from the instrument rotation axis in reflection optics diffractometry introduces bias into the measured Bragg peak positions thereby causing systematic shifts in the 2θ-dependent Rietveld parameters. Specimen transparency also results in 2θ-bias, which tends to mimic that from specimen displacement. The authors have investigated the influence of specimen displacement bias on Rietveld lattice parameters when Bragg-Brentano XRD optics are employed. A strategy is recommended for refining lattice parameters using a specimen displacement correction and with the zero-point correction being determined independently of the Rietveld zero-point correction to cope with the strong correlations between lattice parameters, zero-point and specimen displacement. INTRODUCTION The Curtin University Materials Research Group has gained extensive experience in the use of Rietveld analysis for material characterisation with particular reference to advanced ceramics characterisation of phase composition, linear strain (lattice parameter changes), non-linear strain and crystallite size (line broadening), and texture. The present study is the latest in a series of Advances in X-ray Analysis publications by authors B O’Connor and D Li on Rietveld analysis methodology [1,2]. This particular study was conducted in view of their experience in various studies of linear strain by Rietveld analysis with Bragg-Brentano XRD data that the lattice parameters have poor reproducibility. This experience is consistent with the IUCr Rietveld round robin study on m-ZrO2 data which found that the accuracy of lattice parameters was x16 worse than the esd estimates from Rietveld refinements [3]. SPECIMEN DISPLACEMENT BIAS A specimen displacement (subsequently designated SD) ∆R of the specimen surface from the instrument rotation axis for Bragg-Brentano optics results in the bias of peak positions by ___________________________________________________________ Address: ANSTO, Lucas Heights Research Laboratories, Private Mail Bag 1, Menai, NSW 2234. Australia Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol.44 1 96