Morphotropic Phase Boundary in Solution‐Derived ( Bi _0.5 Na _0.5 ) _{1− x} Ba _x TiO ₃ Thin Films: Part I Crystalline Structure and Compositional Depth Profile

Abstract

In this work, ferroelectric ( Bi 0.5 Na 0.5 ) 1− x Ba x TiO 3 thin films were fabricated by chemical solution deposition ( CSD ) with compositions x = 0.050–0.150. Stoichiometric thin films (hereinafter BNBT ) and others containing 10 mol% excesses of Bi 3+ and Na + ( BNBTxs ) were spin coated onto Pt / TiO 2 / SiO 2 /(100) Si substrates and crystallized by rapid thermal processing at 650°C for 60 s in oxygen atmosphere. Crystalline structure is studied by X‐ray diffraction using Cu anode (λ Cu = 1.5406 Ǻ) and synchrotron radiation (λ = 0.97354 Ǻ). Rietveld refinement showed the coexistence of rhombohedral/tetragonal phases in the BNBT films for x values close to those reported for ( Bi 0.5 Na 0.5 ) 1− x Ba x TiO 3 bulk ceramics. Different volume fractions of the rhombohedral/tetragonal phases are detected as a function of the Ba 2+ 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 Ba 2+ molar values of x ~ 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 Bi x Pt bottom interfaces. The MPB BNBT films with x ~ 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 Bi 3+ and Na + excesses.