Direct observation of a-site vacancies and a twin boundary structure in La2/3TiO3-based ceramics using HAADF/STEM

Ceramics of 0.9La2/3TiO3-0.1LaAlO3 prepared by the mixed oxide route were sintered at 1400°C in air for 4 h and cooled at rates 180°-1°C/h. The products were at least 97% dense with grain sizes of 7-10 μm and relative permittivities 66.9±0.7 at 4 GHz. Rapidly cooled samples (180°C/h) were single phase and exhibited the highest dielectric Q × f values of 16 300±900 GHz. High-resolution transmission electron microscopy revealed microtwins and domains in all the ceramics, with the highest density of both features in the rapidly cooled samples. A-site vacancies in rapidly cooled 0.9La2/3TiO 3-0.1LaAlO3 ceramics were imaged by aberration-corrected scanning transmission electron microscopy. It is shown that vacancies and La1 atoms occupy the 4(h) sites and the 4(g) site is fully occupied by La2 atoms in the orthorhombic Cmmm structure of lanthum titanate. The microtwin boundaries in the microstructure of La2/3TiO 3 stabilized by LaAlO3 lie on the La2 atomic planes. On twinning, La2 and La1 atoms share the same site. The sharing of the 4(h) and 4(g) atomic sites at the twin boundaries is a mechanism to reduce lattice strain upon the continuous transformation from a high-temperature simple cubic Fm3m form to a low-temperature orthorhombic Cmmm form, which involves vacancy/cation ordering and anti-phase tilting of the oxygen octahedra. © 2010 The American Ceramic Society.