Structural properties and phase transformation of sol–gel prepared lanthanum tantalates

Abstract

Perovskite lanthanum tantalate La1/3TaO3 (LT) precursors were prepared by a new polymeric complex sol–gel method using Ta-citrate or -tartrate complexes in different solvents (ethanol or methanol) and calcination between 750 and 1050 °C. The structural properties of precursors were characterised by X-ray diffraction (XRD), differential scanning calorimetry/thermogravimetric analysis (DSC/TG), Fourier transform infrared (FTIR) and Raman spectroscopy, scanning and transmission electron microscopy (SEM, TEM). The fluorite orthorhombic tungsten bronze (TB) of La4.67Ta22O62 and perovskite La1/3TaO3 phases with a small amount of pyrochlore monoclinic LaTaO4 phase were formed after calcination at 900–1050 °C respectively, irregardless of solvent. The particle morphology changed from larger agglomerates composed of spherical 5–20 nm amorphous clusters without visible crystallization at 750 °C to plate-like shaped of well-ordered LT nanoparticles (ethanol) or individual spherical clusters with the fine inner structure (methanol) at 1050 °C. The same effect of alcohol solvent on phase transformation during annealing was shown, whereas the effect of chelating agent was insignificant. Selected area electron diffraction (SAED) patterns of La1/3TaO3 precursors after calcination at 900 °C (a) metastable fluorite La4.67Ta22O62, (b) perovskite La1/3TaO3 and (c) LaTaO4 phases and (d) multidomain structure of super cell of La2/3Ta2O9 .