We report the results of a line-broadening study on a ceria sample prepared for a size-strain round robin. The sample was prepared from a recursor hydrated ceria by heating in a silica crucible at 650 C for 45 hours. Another ceria sample wasprepared to correct for the effects of instrumental broadening by annealing commercially obtained ceria at 1300 C for 3 h and slowly cooling it in the furnace. The diffraction measurements were carried out on two laboratory and two synchrotron x-ray sources, two constant-wavelength neutron and a TOF neutron source. SEM micrographs have shown predominantly spherical grain shape with a lognormal size distribution. Diffraction measurements were analyzed by three methods: the model assuming a lognormal size distribution of spherical crystallites, Warren-Averbach analysis, and Rietveld refinement. The last two methodshave detected a relatively small strain in the sample, as opposed to the first method. Assuming a strain-free sample, the results from all three methods agree well. The average real crystallite size, on the assumption of a spherical crystallite shape, is about 180 A. The scatter of results given by different instruments is relatively small, although significantly larger than estimated standard uncertainties. The Rietveld-refinement results for this ceria sample indicate that the diffraction peaks can be successfully approximated with a pseudo-Voigt function. In a common approximation used in Rietveld-refinement programs, this implies that the size-broadened profile has to be a Voigt or pseudo-Voigt function with a significant Gaussian component.
Citation: Journal of Applied Crystallography
Issue: Part 6
Pub Type: Journals
line broadening, Rietveld refinement, size-strain separation, Warren-Averbach analysis