Kinetics of Formation and Crystallization of Ln2Ti2O7 (Ln = Gd, Lu) Pyrochlores from Nanoparticulate Precursors

The kinetics of formation and crystallization of Ln2Ti2O7 (Ln = Gd, Lu) pyrochlores from nanoparticulate precursors prepared via coprecipitation have been analyzed using X-ray diffraction, Raman spectroscopy, luminescence spectroscopy, scanning electron microscopy, calorimetry (differential scanning calorimetry (DSC)), mass spectrometry, and quantitative thermogravimetry (TG). The results demonstrate that the formation of the pyrochlores proceeds through crystallization of a nanoparticulate fluorite phase. The starting mixtures have been shown to consist in considerable measure of hydroxides and hydroxycarbonates, rather than of oxides. The first synthesis step at temperatures below 550–650°C is decomposition of the starting compounds to titanium oxide and lutetium (or gadolinium) dioxycarbonate. The second step is the synthesis of final compounds, also accompanied by CO2 release. Thus, “high-temperature” CO2 release makes it possible to visualize the synthesis kinetics. Specially designed experiments involving prolonged heat treatment at low temperatures (540 h at 550°C and 216 h at 700°C) have shown that Lu2Ti2O7 can be synthesized almost completely from starting mixtures even at 550°C. The high degree of conversion at 550°C has been confirmed by quantitative TG. Raman and luminescence spectroscopy results demonstrate that the disordered nanooxide synthesized at 550°C, with a crystallite size of 15 Å, has the fluorite structure. Prolonged heat treatment at 700°C was accompanied by an increase in crystallite size and a fluorite-to-pyrochlore phase transition. During heating of the starting precursor at a high rate, 10°C/min (DSC and TG), all of the processes were shifted to higher temperatures. The formation of the final Ln2Ti2O7 (Ln = Gd, Lu) pyrochlores through a nanoparticulate fluorite phase is characteristic of both systems. Thus, all of the Ln2M2O7 (M = Ti, Zr, Hf) 3+/4+ pyrochlores are formed as a result of an order–disorder transition from a nanoparticulate fluorite phase to a pyrochlore phase.