Structural Modification and Large Piezoelectricity of Strained (Na, Bi) TiO₃–BaTiO₃ Epitaxial Thin Films

The crystal structure and ferroelectric and piezoelectric properties of the (1−x)(Na_0.5Bi_0.5)TiO₃–xBaTiO₃ (NBT−BT) thin films grown on Pt/MgO(100), (111), and (110) substrates were systematically investigated around the morphotropic phase boundary (MPB) composition (x = 0.05–0.15). X-ray diffraction and transmission electron microscopy revealed the successful epitaxial growth of high-quality thin films of NBT–BT with a thickness of 3 μm. Due to compressive thermal strain caused by the difference in thermal expansion between NBT–BT and MgO planes, all the NBT–BT thin films grown on MgO(100), (111), and (110) substrates were distorted to form single-crystal lattices with tetragonal, rhombohedral, and orthorhombic symmetries, respectively. This is quite different from the case of free bulk NBT–BT in which rhombohedral-to-tetragonal transformation of the crystal lattice occurs at MPB compositions of x = 0.06–0.07. Nonetheless, the piezoelectric coefficient, −e₃₁*, of the present NBT–BT thin films was largest when x = 0.07 for tetragonal and rhombohedral thin films and 0.09 for orthorhombic ones, with values as high as 18.1, 15.0, and 20.6 C/m², respectively. These −e₃₁* values are the highest found for lead-free piezoelectric thin films to date, and larger than those of Pb(Zr,Ti)O₃ thin films.