Annealing effect on microstructure of iron-substituted strontium titanate revealed by high resolution transmission electron microscopy

摘要

Oxides with perovskite-related structure (generally formulated as ABO3) are of great potential in numerous applications due to their electrical properties (varying from insulators to superconductors) and magnetic properties [1-3]. These properties are strongly dependent on the stoichiometry of the crystal used. The structural defects due to the deviation from the stoichiometry of perovskite resulting from cationic doping are very important for the electrical transport because the high conductivity can often be ascribed to high concentrations of point defects obtained by heavy doping with aliovalent cations. Different substitutions made in this structure have resulted in a great number of oxides still belonging to the perovskite structure family [4-6]. Cationic substitution with ions of a different oxidation state allows for the formation of stoichiometric and non-stoichiometric compounds with compositions in a range of ABO3 and ABO2.5, changing from a complete anionic sublattice, i.e. perovskite structure, to one with 1/6 of the anionic sites unoccupied, i.e. the brownmillerite structure [7, 8]. Other intermediate phases between these two structures have been predicted with general formula A,^,^,,^, in which x = 1/rc is the number of oxygen vacancies in the perovskite structure. These compounds follow a structural model in which (n - 1) octahedral planes alternate with a tetrahedral one. Up to now the structures with n = 3 and 4 have been reported [9, 10]. Since advances in the development of ceramics rely on an understanding of the mechanisms that contribute to the formation of the desired micro-structural features or control the electrical, magnetic and mechanical responses of the material, it is very important from the scientific and technical viewpoint to control the defects in the matrix with the objective of obtaining the desired electrical, magnetic and mechanical properties. In the present study the influence of heat treatment routes on the microstructure in iron-substituted strontium titanate was investigated through transmission electron microscopic (TEM) observation.