Ruddlesden–Popper Planar Faults and Nanotwins in Heteroepitaxial Nonstoichiometric Barium Titanate Thin Films

Nonstoichiometric 10-mol%-excess-BaO–BaTiO₃ (Ba_1.1TiO_3.1) thin film grown on a (100) SrTiO₃ substrate consisted of heteroepitaxial c -axis-oriented BaTiO₃ perovskite crystals including Ruddlesden–Popper planar faults and nanometer-scale multiple (111) twin lamellae. High-resolution electron microscopy and electron energy-loss spectroscopy revealed that nanotwins with coherent (111) Σ3 coincident site lattice boundaries were terminated in the BaTiO₃ matrix to form incoherent (211) Σ3 boundaries accommodating excess barium ions. Both Ruddlesden–Popper planar fault and incoherent (211) boundary formation were proposed as possible accommodation mechanisms of excess barium ions in the perovskite film.