Successive structural phase transitions in a hexagonal linear-chain ferroelectric crystalRbMnBr3

Birefringence and dielectric measurements were performed on a triangular lattice antiferromagnet ${\mathrm{RbMnBr}}_3$. Successive structural phase transitions at ${\mathit{T}}_4$ (∼220 K), ${\mathit{T}}_3$ (=230 K), and ${\mathit{T}}_2$ (=444 K) were found. Through D-E hysteresis loop observations, the phases IV (${\mathit{T}}_4$<T<${\mathit{T}}_3$), III (${\mathit{T}}_3$<T<${\mathit{T}}_2$), and II (${\mathit{T}}_2$<T<${\mathit{T}}_{\mathrm{melt}}$) were determined to be ferroelectric. The spontaneous polarization is along the c axis, which disappears abruptly at ${\mathit{T}}_4$ with decreasing temperature. Below ${\mathit{T}}_4$ (phase V) the crystal is proposed to be antiferroelectric. The dielectric behavior of ${\mathrm{RbMnBr}}_3$ is very similar to that of ${\mathrm{KNiCl}}_3$; however, a higher-temperature phase transition at ${\mathit{T}}_1$ assigned for ${\mathrm{KNiCl}}_3$ was not detected in differential-scanning-calorimetry data on ${\mathrm{RbMnBr}}_3$.