Broadband Brillouin scattering study of ferroelectric instability of barium sodium niobate

TL;DR

This study uses broadband Brillouin scattering to investigate the ferroelectric instability in barium sodium niobate, revealing critical slowing down near the Curie temperature and indicating an order-disorder transition.

Contribution

First broadband Brillouin scattering analysis of BNN revealing ferroelectric phase transition dynamics and polarization fluctuations.

Findings

Observation of an intense central peak near T_C

Critical slowing down of polarization fluctuations

Confirmation of order-disorder type ferroelectric transition

Abstract

The barium sodium niobate (BNN) with tungsten-bronze structure is one of well-known optical crystals for electro-optic and nonlinear optic applications. This paper reviews the ferroelectric instability of BNN crystals. BNN is a uniaxial ferroelectric with a spontaneous polarization along the tetragonal $c$-axis. There is no report on the observation of an optical soft mode responsible for a ferroelectric phase transition. In the vicinity of the Curie temperature, $T_{\text{C}}=560${\deg}C, an intense central peak (CP) related to the polarization fluctuations along the $c$-axis was observed by the broadband Brillouin scattering experiment. The relaxation time determined by the CP width shows the critical slowing down towards $T_{\text{C}}$. This fact indicates that the ferroelectric instability of BNN is an order-disorder type.