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Broadband Brillouin scattering study of ferroelectric instability of barium sodium niobate

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, $T_{\text{C}}=560$°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.