Introduction of strain-relaxed 90 domain structure by lattice mismatch in tetragonal ferroelectric (Bi,K)TiO₃ epitaxial films grown hydrothermally below Curie temperature

Epitaxial (Bi, K)TiO3 films with 800- to 900-nm thicknesses were grown hydrothermally at 200 degrees C on SrTiO3 substrates covered with SrRuO3 layers. Perfectly (001)-oriented films grew on (100)SrTiO3 due to good lattice matching. Films on (110)SrTiO3 had mixed orientations of dominant (110) and minor (101), while three types of (111) orientations with in-plane 120 degrees rotation were observed for the film on (111)SrTiO3. The (101) and (110) orientations of the film deposited on (110)SrTiO3 were tilted by approximately 2.6 degrees and 1.6 degrees, respectively, from surface normal due to the formation of a 90 degrees domain with a twinning plane. The plane-view measurement for the film deposited on (111)SrTiO3 showed nine spots. These are explained by the presence of 15 possible spots resulting from the relaxed 90 degrees domain combination and by overlapping. The lattice parameters of these films explain the tilting angles of these domains. These results reveal the formation of perfectly relaxed 90 degrees domain structures for films grown on (110) and (111)SrTiO3. This differs from our previous data for tetragonal Pb(Zr,Ti)O-3 films grown above T-C on (110) and (111)SrTiO3 because the present films directly grow the ferroelectric films below T-C without phase change. The tilting angle of the polar-axis and the volume fraction of the 90 degrees domain can explain the piezoelectric responses of these films assuming that films have purely an up-state. This suggests that these films show almost pure up-state polarization without 180 degrees domains, at least along surface-normal directions. These data show that these films have domain structures different from the well-known ones for the tetragonal Pb(Zr,Ti)O-3 films.