Densification retardation in the sintering of La2O3-doped barium titanate ceramic

La2O3-doped TiO2-excess BaTiO3 has been studied for its pressureless-sintering behaviour. The resultant microstructure was analyzed by using both scanning and transmission electron microscopy. Second-phases of polytitanates, notably Ba6Ti17O40 and La2Ti2O7 (La2O3·2TiO2) are formed by solid-state reaction upon heating to the sintering temperature. Grain growth inhibition occurs at a threshold donor concentration of ≈0.30 mol.% La2O3 in the solid-state when samples are pressureless-sintered at 1215°C. The second-phase of monoclinic-La2Ti2O7 of the continuous nature located along grain-boundaries has been identified in the laser-sintered samples at the doping level of 0.50 mol.% La2O3. Densification retardation found in the La2O3-doped compositions is attributed to the presence of La2Ti2O7 that has effectively blocked the diffusion path for neck formation in the initial stage of sintering. The role of La2O3 on the solid-state sintering of BaTiO3 is discussed in favour of the solid-solution defect model.