Phase transition between two different orientations of the Q phase in the NaNbO$_3$ thin film

TL;DR

This study investigates the temperature-induced phase transition in NaNbO$_3$ thin films, revealing a reorientation of the ferroelectric Q phase's polarization from an angle to parallel with the interface, supported by dielectric, structural, and spectroscopic data.

Contribution

It provides experimental evidence of a temperature-driven transition between two orientations of the ferroelectric Q phase in NaNbO$_3$ thin films, which was previously only theoretically anticipated.

Findings

Reorientation of the ferroelectric polarization from 45° to parallel with the interface.

Observation of diffuse phase transition around 440-500 K.

Detection of structural changes via x-ray and Raman spectroscopy.

Abstract

Temperature evolution of dielectric response, atomic structure, and lattice dynamics in thin film of sodium niobate in the epitaxial NaNbO$_3$/SrRuO$_3$/(001)MgO heterostructure is studied by dielectric measurements, x-ray diffraction, and Raman spectroscopy. It is found that at room temperature NaNbO$_3$ is in ferroelectric state, whereas the temperature-dependent dielectric constant experiences a broad maximum at 440~K on cooling and at 500~K on heating and reveals a diffuse phase transition. Reciprocal space mapping shows the presence of both anti-phase and in-phase tilting of oxygen octahedra. The temperature dependence of the M-point reflections suggests reorientation of the in-phase octahedra tilting axis from being parallel to the substrate at room temperature to perpendicular orientation at high temperatures. The temperature evolution of the shape of the Raman spectra reveal the decrease of the number of constituting peaks on heating. These results are interpreted as indicating a temperature-driven transition between two different orientations of the bulk ferroelectric Q phase with respect to the interface, namely between the state with electric polarization pointing at $\approx45^{\rm o}$ to the normal at room temperature to the state with polarization parallel to the interface above the transition. Transitions of this kind can be anticipated from theoretical considerations, while the experimental evidences of such are yet scarce.