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

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 ${\mathrm{Sr}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Ba}}_{\mathrm{x}}$ ${\mathrm{Nb}}_2$ ${\mathrm{O}}_6$, ${\mathrm{Pb}}_{\mathrm{x}}$ ${\mathrm{Ba}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Nb}}_2$ ${\mathrm{O}}_6$, and (${\mathrm{K}}_{\mathrm{x}}$ ${\mathrm{Na}}_{1\mathrm{-}\mathrm{x}}$ ${)}_{0.4}$(${\mathrm{Sr}}_{\mathrm{y}}$Ba ${}_{1\mathrm{-}\mathrm{y}}{}^{}{}_{}{}^{}$ ${)}_{0.8}$ ${\mathrm{Nb}}_2$ ${\mathrm{O}}_6$ 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.