A-site cation-vacancy ordering in Sr1-3x/2LaxTiO3:: A study by HRTEM

The system Sr1-3x/2LaxTiO3 (0.25 less than or equal to x less than or equal to 0.6) has been studied by high-resolution transmission electron microscopy, allowing the direct observation of vacant sites on the Sr/La sublattice of the perovskite structure. Isolated vacancies are observed in 2D structural projections of samples having x similar to 0.3. Vacancy pairing occurs with increasing La concentration, followed by the formation of vacancy squares of side 2a(0) and vacancy rectangles 2a(0) x a(0). The latter signify vacancy ordering in an orthorhombic superstructure, and the former are considered to derive from orthogonal superstructure microdomains, The details of the ordering are sensitive to the heat treatment used during preparation Of the sample. (C) 2000 Academic Press.