Phase transition of some ferroelectric niobate crystals with tungsten-bronze structure at low temperatures

Abstract

From 15 K to room temperature, dielectric and pyroelectric properties have been measured in six ferroelectric niobate single crystals with tungsten-bronze (TB) structure. Dielectric and pyroelectric experiments show that in the three kinds of ferroelectric niobate single crystals ${\mathrm{Sr}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ba}}_{\mathrm{x}}$${\mathrm{Nb}}_{2}$${\mathrm{O}}_{6}$, ${\mathrm{Pb}}_{\mathrm{x}}$${\mathrm{Ba}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Nb}}_{2}$${\mathrm{O}}_{6}$, and (${\mathrm{K}}_{\mathrm{x}}$${\mathrm{Na}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${)}_{0.4}$(${\mathrm{Sr}}_{\mathrm{y}}$Ba $_{1\mathrm{\ensuremath{-}}\mathrm{y}}$${)}_{0.8}$${\mathrm{Nb}}_{2}$${\mathrm{O}}_{6}$ with tetragonal TB structure, there is a newly identified phase transition between 60 and 80 K which has been supported by x-ray analysis. The crystal's symmetry changes from point group 4mm to point group m, and the direction of the ferroelectric polar axis tilts away from the c axis in the tetragonal lattice cell to the a axis in the monoclinic lattice cell as temperature decreases. The difference between the high-frequency dielectric constants and the low-frequency dielectric constants indicates that the phase transition is of a diffuse nature. According to the specific-heat experimental data, it is suggested that this phase transition is not a first-order transition. A model of structural change has been suggested to explain this phase transition.