Rietveld refinement of LaB6: data from mXRD

The Rietveld method of crystal structure refinement was an important breakthrough, allowing crystal structural information to be obtained from powder diffraction data. One remaining challenge is to collect Rietveld-quality data for polycrystalline minerals in situ, using laboratory-based micro X-ray diffraction (mXRD) techniques. Here a new data collection method is presented, called 'multiframes', which produces high-quality data, suitable for Rietveld refinement, using the Bruker D8 DISCOVER micro X-ray diffractometer. 91 frames of two-dimensional X-ray diffraction data were collected for powdered NIST SRM 660 LaB6 standard material, using a general area-detector diffraction system (GADDS), at intervals of 0.8DG 2theta. For each frame, only the central 1DG 20 was integrated and merged to produce a diffraction profile from 17 to 90DG 2theta. Rietveld refinement of this data using TOPAS2 gave a unit-cell parameter (ao) and atomic position of boron (x) for LaB6 of 4.1549 (1) A and 0.1991 (9), respectively (Rwp = 4.26, RBragg = 3.21). The corresponding La-B bond length was calculated to be 3.0522 A. These parameters are in good agreement with the literature values for LaB6. These encouraging results suggest that Rietveld-quality micro X-ray diffraction data can be collected from the Bruker D8 DISCOVER diffractometer, provided that the GADDS detector is stepped in small increments, for each frame only the central 1DG 2theta is integrated at constant arc length, and counting time is sufficient to yield adequate intensity (~10000 counts).