Effect of post-annealing on the chemical state and crystalline structure of PLD Ba0.5Sr0.5TiO3 films analyzed by combined synchrotron X-ray diffraction and X-ray photoelectron spectroscopy

Abstract A Ba0.5Sr0.5TiO3 (BSTO) thin film was grown on a MgO (100) substrate by pulsed laser deposition. The interrelation between the results derived from X-ray photoelectron spectroscopy (XPS) and 2D-reciprocal space maps (2D RSMs) recorded in high resolution diffraction produce complementary information about the chemical state and about the crystalline structure. Both methods confirm the presence of two BSTO1 and BSTO2 phases in the as-deposited and in the post-annealed states, having two chemical environments, two out-of-plane lattice parameters c1 and c2 and two tetragonal distortion ratios D1 and D2. Using the advantages of a combined study, the BSTO1 phase corresponds to the perovskite phase while the BSTO2 phase belongs to a BSTO phase having high tetragonal distortion with respect to the perovskite BSTO1 phase. The post-annealing has induced a transformation from BSTO1 phase to the BSTO2 phase in the surface layer and in the bulk of the BSTO film in addition to the appearance of a further crystalline phase corresponding to a third chemical state. This represents an important and a novel finding in the understanding of the influence of the post-thermal treatment on the structure and the chemical states of the BSTO film which could be accordingly monitored by XRD and XPS. In fact, upon post-annealing, a proportion of BSTO1 perovskite phase decreases while a strained BSTO2 phase increases and a newly formed BaTiO3-like phase appears.