Formation mechanism and characteristics of lanthanum-doped BaTiO3 powders and ceramics prepared by the sol-gel process

Pure and lanthanum-doped barium titanate nanopowders described by two different formulae, as Ba1-xLaxTiO3, for lower La concentrations (0 <= x <= 0.005) and Ba1-xLaxTi1-xO3 for higher La concentration (x = 0.025) were prepared by an alkoxide sol-gel method. Single phase compositions were obtained after annealing at 900 degrees C for 2 h, in air. The increase of the lanthanum content causes structural and morphological changes in the oxide powders, including the evolution of the unit cell from tetragonal toward a cubic symmetry, the particle size decrease and a higher aggregation tendency. SEM investigations of the ceramics sintered at 1300 degrees C for 4 h indicate significant changes of the microstructural features (strong decrease of the average grain size and increase of the intergranular porosity) with the raise of La amount Lanthanum addition to barium titanate prepared by sol-gel induces a more significant shift of the Curie temperature toward lower values, than that one reported in literature for ceramics of similar compositions, but processed by the conventional solid state method. The compositions with smaller La amount (x <= 0.005) show semiconducting properties at room temperature and high relative dielectric permittivity values, while the undoped ceramics and those doped with higher La content (x = 0.025) are good dielectrics. The ceramic with x = 0.025 exhibits acceptable low losses, a very diffuse ferroelectric-paraelectric transition and Curie temperature closed to the room temperature, being thus susceptible for high tunability applications. (C) 2015 Published by Elsevier Inc.