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 BNN crystals. a uniaxial spontaneous polarization along tetragonal $c$-axis. There no report on observation an soft mode responsible phase transition. In vicinity Curie temperature, $T_{\text{C}}=560${\deg}C, intense central peak (CP) related to fluctuations $c$-axis was observed by broadband Brillouin scattering experiment. relaxation time determined CP width shows critical slowing down towards $T_{\text{C}}$. fact indicates that order-disorder type.