Ferroelectric phase transitions in tensile-strained NaNbO₃ epitaxial films probed by in situ x-ray diffraction

In this study, in situ synchrotron x-ray diffraction was used to investigate a high-temperature phase transition in tensile-strained epitaxial NaNbO3/(110)DyScO3 films. A ferro-to-ferroelectric phase transition was found in the temperature range between 230 and 350 °C, with a very pronounced thermal hysteresis. While at room temperature the well known monoclinic a1a2 in-plane ferroelectric phase is present, here, a slightly disturbed orthorhombic ferroelectric a1/a2 phase with exclusive in-plane polarization was found at high temperatures. This phase appeared in four variants, indicating the occurrence of four types of superdomains. The orthorhombic symmetry and the corresponding domain morphology observed at high temperatures showed a striking similarity to K0.9Na0.1NbO3 thin films grown on a (110) NdScO3 substrate.