Synthesis and characterization of ultrafine Ln2Ti2O7 (Ln = Sm, Gd, Dy, Er) pyrochlore oxides by stearic acid method

Stearic acid method (SAM) was developed to synthesize series of pyrochlore Ln2Ti2O7 (Ln = Sm, Gd, Dy, Er) nanocrystals. The synthesis process was monitored by X-ray diffraction, Thermal-gravimetric-differential thermal analysis and Fourier Transform InfraRed methods. Comparing with traditional solid-state reaction (SSR), Ln2Ti2O7 can be synthesized at relatively low temperature (700-800 °C) with shortened reaction time (2-4 h). The average particle size of Ln2Ti2O7 was greatly reduced (ca. 40 nm) and the BET surface area was increased (ca. 12 m2/g) by using SAM. From the X-ray diffraction patterns, we found that Ln has an effect on the crystal structure of Ln2Ti2O7, every lattice peak shifted to larger angle slightly with the increasing atomic number of Ln. Also, the lattice constant of Ln2Ti2O7 was calculated by Jade.5 and found it decreased along with the decrease of ionic radius of Ln3+. The morphology of obtained Ln2Ti2O7 was determined by transmission electron microscopy technique. Results showed that the obtained Ln2Ti2O7 were all square-like and the interplanar distance of Ln2Ti2O7 (Ln = Sm, Gd, Dy, Er) according to (111) plane was 0.65, 0.64, 0.63, and 0.62 nm respectively, which was measured from High Resolution Transmission Electron Microscopy images. Possible reason for this phenomenon was presented.