The crystal structure and ferroelectric and piezoelectric properties of the (1-x)(Na0.5Bi0.5)TiO(3)xBaTiO(3) (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 mu m. 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 = 0.060.07. Nonetheless, the piezoelectric coefficient, -epsilon(31)*, 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/m(2), respectively. These -epsilon(31)* values are the highest found for lead-free piezoelectric thin films to date, and larger than those of Pb(Zr,Ti)O-3 thin films.