Morphotropic phase boundary in solution derived (Bi0.5Na0.5)1-xBaxTiO3 thin films: part I crystalline structure and compositional depth profile

Resumen:
In this work, ferroelectric (Bi0.5Na0.5)(1-x)BaxTiO3 thin films were fabricated by chemical solution deposition (CSD) with compositions x=0.050-0.150. Stoichiometric thin films (hereinafter BNBT) and others containing 10mol% excesses of Bi3+ and Na+ (BNBTxs) were spin coated onto Pt/TiO2/SiO2/(100)Si substrates and crystallized by rapid thermal processing at 650 degrees C for 60s in oxygen atmosphere. Crystalline structure is studied by X-ray diffraction using Cu anode ((Cu)=1.5406) and synchrotron radiation (=0.97354). Rietveld refinement showed the coexistence of rhombohedral/tetragonal phases in the BNBT films for x values close to those reported for (Bi0.5Na0.5)(1-x)BaxTiO3 bulk ceramics. Different volume fractions of the rhombohedral/tetragonal phases are detected as a function of the Ba2+ content. An apparent shift of the position of the morphotropic phase boundary (MPB) is observed in the BNBTxs films. Here, the MPB region appears for nominal Ba2+ molar values of x similar to 0.10 and the experiments using a grazing-incidence synchrotron radiation indicate the existence of a crystalline phase with pyrochlore structure at the film surface. Rutherford backscattering experiments (RBS) revealed that the bismuth excess is not volatilized during the crystallization of the BNBTxs films which present inhomogeneous compositional depth profile and thick BixPt bottom interfaces. The MPB BNBT films with x similar to 0.055 have a homogeneous compositional depth profile without appreciable bottom interfaces. Scanning electron micrographs reveal less porosity and higher grain sizes in the stoichiometric films than in those with Bi3+ and Na+ excesses.