Transmission electron microscopy study of n= 1–5 Sr_n+1Ti_nO_3n+1 epitaxial thin films

Epitaxial Sr_n+1Ti_nO_3n+1 thin films with n = 1–5 were synthesized on (001) SrTiO₃ substrates by reactive molecular beam epitaxy. The structure and microstructure of the films were investigated by x-ray diffraction, transmission electron microbeam diffraction, and high-resolution transmission electron microscopy (HRTEM) in combination with computer image simulations. Both diffraction and HRTEM studies revealed that all the films are epitaxially oriented with their c axis perpendicular to the (001) SrTiO₃ plane of the substrate. Detailed investigations using quantitative HRTEM methods indicated that the films have the expected n = 1–5 structures of the Ruddlesden–Popper Sr_n+1Ti_nO_3n+1 homologous series. Among these films, Sr₂TiO₄, Sr₃Ti₂O₇, and Sr₄Ti₃O₁₀ thin films are nearly free of intergrowths, while Sr₅Ti₄O₁₃ and Sr₆Ti₅O₁₆ thin films contain noticeably more antiphase boundaries in their perovskite sheets and intergrowth defects. We show that these results are consistent with what is known about the thermodynamics of Sr_n+1Ti_nO_3n+1 phases.