Perovskites based on sodium bismuth titanate doped with barium titanate – Na0.5Bi0.5TiO3-xBaTiO3 (abbreviated NBT-BT) are considered the most promising lead-free candidate materials to substitute Pb(Zr1-xTix)O3 (PZT) materials in devices designed to respect standards and environmental laws. Taking into account the toxicity of lead-based systems, there is an urgent need to develop environmental friendly materials and numerous lead-free piezoelectric materials are under investigation in worldwide spread laboratories for replacing PZT in future devices. However, it is difficult to match or surpass the applicability of PZT in devices such as ferroelectric access memories (FeRAMs), piezoelectric ultrasonic transducers or pyroelectric infrared (IR) sensors, due to very high dielectric, piezoelectric and pyroelectric properties coupled with a high phase transition temperature.
Structural and polar transformations in NBT-BT are more complicated than in other perovskite solid solutions, also due to the strong disorder of the A-sites occupied by Na+, Bi3+ or Ba2+ ions, with different valence, mass and ionic radius. NBT transforms successively, from the high temperature cubic paraelectric into tetragonal antiferroelectric (or ferroelectric) and further into a rhombohedral ferroelectric phase. In solid solution with BT, the ground ferroelectric phase changes from rhombohedral R3c to tetragonal ferroelectric P4mm, at the so-called morphotropic phase boundary (MPB) (x ≈ 0.06-0.07). However, despite the fact that ferroelectric materials with MPB have enhanced ferroelectric and piezoelectric properties, it is difficult to transpose them in thin films since MPB is limited to a small composition range. The goal of this study was to investigate the optical, structural, dielectric, pyroelectric and ferroelectric properties of NBT-BT thin films obtained by pulsed laser deposition as a function of composition, from pure NBT across and beyond morphotropic phase boundary (MPB) (x=0, 0.04, 0.06, 0.08). Dielectric and ferroelectric measurements were performed using an impedance analyzer HP 4294A and RT 66A Ferroelectric Test System. XRD, SEM, HR-TEM and AFM techniques have been used for morphologic and structural characterizations of NBT-BT films. Pyroelectric properties were investigated with a Woollam Variable Angle Spectroscopic Ellipsometer (VASE) system under different temperature conditions.