Microstructural Evolution from Praseodymium-Containing Zircon Gels to Prx-ZrSiO4 Solid Solutions

Structural evolution and coupled microstructural transformations occurred on annealing mineralizer-free Pr-containing ZrSiO4 gels up to final praseodymium-doped zircon yellow pigments are reported. Gels with nominal compositions Pr-x-ZrSiO4, 0 <= x <= 0.125, were prepared from mixtures of zirconium and silicon alkoxides and praseodymium acetylacetonate. Crystallization pathway and microstructural changes of thermally treated dried gels were followed by infrared spectroscopy, X-ray diffraction, field emission scanning and transmission electron microscopies, respectively. Results indicated that the crystallization pathway through the whole process of final Pr-x-ZrSiO4 solid solutions formation displayed three well-defined steps. The crystallization of tetragonal Pr-containing ZrO2 nanocrystals occurred first, followed by their phase transformation to the monoclinic form. The last step was the reaction between the monoclinic Pr-zirconia and the amorphous silica and its kinetics was dependent on the amount of the nominal praseodymium content. The microstructure before the Pr-zircon formation consisted of an arrangement of tetragonal-or monoclinic Pr-containing ZrO2 particles coated by an amorphous silica layer, with sizes lower than 100 nm and between 200 and 400 nm, respectively. The final Prx-ZrSiO4 solid solution products were particles sized in the range between 200 nm and 1 mu m. Microstructural changes revealed that the whole formation process to final Prx-ZrSiO4 non-aggregated particles was led by the synthetic procedure used for the preparation of gel precursors.