Structural phase transitions in AgTa_0.5Nb_0.5O ₃ thin films

Octahedral tilt transitions in epitaxial AgTa_0.5Nb _0.5O₃ (ATN) films grown on (001)_p (where p=pseudocubic) oriented SrRuO₃/LaAlO₃ and LaAlO ₃ substrates were characterized by electron diffraction and high resolution x-ray diffraction. It was found that the ATN films exhibited octahedral rotations characteristic of the Pbcm space group, similar to those seen in bulk materials; however, the temperature of the M₃↔ M₂ phase transition has been suppressed by ∼250 K due to the fact that the correlation length for rotations about c_p was significantly reduced. The average off-center B-cation displacements, which signify the degree of long-range order for these local cation positions, were negligibly small compared to bulk materials, as inferred from the near-zero intensity of the 1/4(00l)_p-type reflections. On cooling, pronounced ordering of B-cation displacements occurred at ≈60 K which is significantly lower compared to bulk (≈310 K). The onset of this ordering coincides with a broad maximum in relative permittivity as a function of temperature. It is believed that point and planar defects in thin ATN films disrupt the complex sequence of in-phase and antiphase rotations around c_p thereby reducing the effective strength of interactions between the tilting and cation displacements.