Layered-structural monoclinic-orthorhombic perovskite La_2Ti_2O_7 to orthorhombic LaTiO_3 phase transition and their microstructure characterization

摘要

The layered-structural ceramics, such as lanthanum titanate (La_2Ti_2O_7), have been known for their good temperature and low dielectric loss at microwave frequencies that make them good candidate materials for high frequency applications. However, few studies have been conducted on the synthesis optimization by sol gel reaction, in particular by acrylamide polymerization route. The interest in La_2Ti_2O_7 ceramic has been greatly increased recently due to the effect of oriented grains. This anisotropy of the microstructure leads to anisotropy in dielectric, electrical and mechanical properties. In this study, grain oriented lanthanum titanate was produced by the sol-gel acrylamide polymerization route. The characterizations of the samples were achieved by thermal analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). X-ray diffraction indicates that the formation of monoclinic perovskite La_2Ti_2O_7 nanocrystals is a necessary first step to obtain orthorhombic LaTiO_3 nanocomposites (with space group Pbnm). In this work we identified that the monoclinic perovskite La_2Ti_2O_7 with space group P2_1 transforms its structure into one with the orthorhombic space group Cmc2_1, at approximately 1073 K. The microstructure associated consisted of flaky monoclinic La_2Ti_2O_7 nanocomposites in comparison with round-shaped LaTiO_3 nanocomposites.