Experience with powder X-ray diffractometer Bruker D8 Advance: Automatic divergence slit, variable counting time data collection

M. Čurda, F. Laufek

Czech Geological Survey, Geologická 6, 152 00 Prague 5, Czech Republic
michal.curda@gelology.cz

The importance of careful sample preparation and data collection in powder X-ray diffraction studies have been described on several previous occasions. Laboratory instruments normally have favoured configuration which is used for a wide range of experiments. In many laboratories, custom and/or time pressure, rather than a considered approach, often determine the data collection strategy [1]. Before collection of powder diffraction pattern, it is a good idea to know what information you might get out of it and consider the appropriate data collection strategy. Parameters which can be considered include angular range, step size, counting time, wavelength etc. For example, the qualitative phase identification generally requires only relatively small 2θ range containing the strongest reflections of the sample, the meaningful Rietveld refinement (i.e. for the purpose of quantitative phase analysis) requires high quality data measured to small d values [2]. In this presentation, two techniques which can improve the data quality in Bragg-Brentano geometry, will be presented. The conventional Bruker D8 Advance diffractometer with the CuKα tube and the Lynx Eye XE detector was used for the data collection.  

The application of the automatic divergence slit (ADS) (also called Theta compensating slit) ensures that the irradiated sample length is kept equal over a measured range of 2θ. This in turn leads to higher diffracted intensities in middle and higher area of 2θ. As is shown in Fig. 1, this is the main advantage of ADS slit in comparison to the traditional fixed divergence slit (FDS). Moreover, the application of automatic divergence slit may prevent the undesired effect of beam overflow at low diffraction angles, which may occur when large fixed diverge slit (typically 1° or larger) is used.

The diffracted intensity in conventional X-ray diffraction patterns decrease with the increased diffraction angle. The main physical factors which cause this variation include the drop atomic scattering factors, thermal vibrations and the Lorentz-polarisation factor. Variable counting time data collection strategy (VCT) may significantly improve the statistic quality of data at the middle and high diffraction angles.     

Fig. 1. Comparison of diffraction patterns of LaB₆ collected with automatic and fixed divergence slits.

1. R. D. Hill & I. C. Madsen, in Structure Determination from Powder Diffraction Data, edited by W. I. F. David, K. Shankland, L. B. McCusker & Ch. Baerlocher (Oxford University press), 2002, pp. 98-117.

2. J. K. Cockcroft & A. N. Fitch, in Powder Diffraction Theory and Practice, edited by R. E. Dinnebier & S. J. L. Billinge (Cambridge: The Royal Society of Chemistry), 2008, pp. 20-57.