An X-ray Powder Diffraction Study of the Microstructure and Growth Kinetics of Nanoscale Crystallites Obtained from Hydrated Cerium Oxides

Abstract

A detailed analysis of the microstructural properties of nanocrystalline cerium(IV) oxide powders produced by thermal dehydration of two hydrated ceria precipitated from the ceric ammonium and sulfate salts in ammonia solutions is described. In addition, it is shown that the diffraction pattern of hydrated ceria can be modeled with the structure data of CeO 2 and that the average crystallite size for the precursor CeO 2 ·1.6H 2 O is 19 Å. The analysis of nanocrystalline CeO 2 is based on the modern developments of X-ray powder diffraction line broadening analysis, using 16 Bragg reflections. It is shown that, on average, the crystallites have a spherical shape with volume-weighted and area-weighted diameters in the ranges 264−1349 Å and 178−1079 Å for CeO 2 ( am -CeO 2 ) obtained from the ammonium-based ceria and 263−1756 Å and 195−927 Å for CeO 2 ( sul -CeO 2 ) obtained from the sulfate-based ceria. The early stages of the crystallite growth have been studied up to 775 °C and 900 °C, respectively. While sul -CeO 2 is strain-free over the temperature range investigated, am -CeO 2 contains a small amount of strains decreasing with temperature until 700 °C, where strains are negligible. The kinetics of the crystallite growth has been studied for the two samples, from which activation energies have been calculated. Crystallite sizes are also compared to the values from SEM and BET measurements. Strain-free sul -CeO 2 is suggested as a potential candidate as reference material for crystallite size in powder diffraction.