Temperature and composition dependent phase transitions of lead-free piezoelectric (Bi0.5Na0.5)TiO3-BaTiO3 thin films.

Extensive studies on (Bi0.5Na0.5)TiO3-BaTiO3 piezoceramics have aroused growing interest in developing high-performance piezoelectric films based on this promising lead-free piezoelectric system. High quality (1 - x%) (Bi0.5Na0.5)TiO3-x% BaTiO3 (x = 0-15) thin films were synthesized on Pt(111)/Ti/SiO2/Si(100) substrates via a sol-gel method. The phase transitions as functions of composition and temperature were investigated by X-ray diffraction (XRD) and temperature-dependent Raman spectra analysis. A morphotropic phase boundary (MPB) located at 5 ≤ x ≤ 10 was found in the films, being identical to that in bulk materials. Additionally, complicated structural changes with temperature in a wide composition range were observed, which correspond to the evolution from a low-symmetry ferroelectric phase or a relaxor phase to high-symmetry relaxor phases. The ferroelectricity and piezoelectricity were also systematically studied with an emphasis on the influence of composition and phase structure. A phase diagram focusing on the above two kinds of phase transitions has been proposed in this work.