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 °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° 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° and 1.6°, respectively, from surface normal due to the formation of a 90° 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° 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° domain structures for films grown on (110) and (111)SrTiO₃. This differs from our previous data for tetragonal Pb(Zr,Ti)O₃ 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° 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° 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)O₃ films.