Broadband Brillouin scattering study of ferroelectric instability of barium sodium niobate

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_C = 560°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_C. This fact indicates that the ferroelectric instability of BNN is an order-disorder type.

Keywords

• brillouin scattering
• ferroelectric
• order-disorder
• central peak
• barium sodium niobate