Ceramics of 0.9La(2/3)TiO(3)-0.1LaAlO(3) prepared by the mixed oxide route were sintered at 1400 degrees C in air for 4 h and cooled at rates 180 degrees-1 degrees C/h. The products were at least 97% dense with grain sizes of 7-10 mu m and relative permittivities 66.9 +/- 0.7 at 4 GHz. Rapidly cooled samples (180 degrees C/h) were single phase and exhibited the highest dielectric Q x 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 0.9La(2/3)TiO(3)-0.1LaAlO(3) ceramics were imaged by aberration-corrected scanning transmission electron microscopy. It is shown that vacancies and La-1 atoms occupy the 4(h) sites and the 4(g) site is fully occupied by La-2 atoms in the orthorhombic Cmmm structure of lanthum titanate. The microtwin boundaries in the microstructure of La2/3TiO3 stabilized by LaAlO3 lie on the La-2 atomic planes. On twinning, La-2 and La-1 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.