Thermal Expansion and Dielectric Properties of Triangular Lattice Antiferromagnet RbCoBr3

Thermal expansion and dielectric measurements of RbCoBr3 were performed to examine the lattice distortion accompanied by structural phase transitions. It was confirmed that RbCoBr3 undergoes structural phase transitions at 90, 37, and 10 K because its linear thermal expansion and the temperature dependence of its linear thermal expansion coefficients showed clear anomalies at these temperatures. The pyroelectric charge measurement showed that a spontaneous electric polarization appears between 37 and 31 K and below 10 K. We propose a successive structural phase transition sequence and crystal symmetries at each phase.