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.