Anomalous low-energy phonons in nearly tetragonal BiFeO3 thin films

TL;DR

This study reveals a coupled structural and ferroelectric phase transition in BiFeO3 thin films near 360 K, characterized by anomalous low-energy phonon behaviors indicating strong octahedral tilting interactions.

Contribution

It provides new insights into the phonon anomalies and the coupling between structural and ferroelectric transitions in BiFeO3 thin films near the tetragonal phase.

Findings

Large hardening of low-energy phonons by 15 cm$^{-1}$

Increase in phonon mode intensity by an order of magnitude

Appearance of new phonon modes around 360 K

Abstract

We present evidence for a concomitant structural and ferroelectric transformation around $T_S\sim 360$ K in multiferroic BiFeO$_3$/LaAlO$_3$ thin films close to the tetragonal phase. Phonon excitations are investigated by using Raman scattering as a function of temperature. The low-energy phonon modes at 180-260 cm$^{-1}$ related to the FeO$_6$ octahedron tilting show anomalous behaviors upon cooling through $T_S$; (i) a large hardening amounting to 15 cm$^{-1}$, (ii) an increase of intensity by one order of magnitude, and (iii) an appearance of a dozen new modes. In contrast, the high-frequency modes exhibit only weak anomalies. This suggests an intimate coupling of octahedron tilting to ferroelectricity leading to a simultaneous change of structural and ferroelectric properties.