Evidence for low temperature phase transition in Rb2ZnCl4 from dielectric constant and birefringence measurements

Temperature dependences of the dielectric constants ?_i and of the birefringences Δn_i for light propagation directions along all the three crystallographic axes (i = a, b, c) have been measured between 5 and 350 K. The optical measurements clearly reveal a new phase transition at T_L = 75 K, below which the crystal structure is transformed from the ferroelectric Pna2₁ phase into an unknown low temperature phase, possibly the monoclinic space group P112₁. Small peaks in the dielectric constants ?_a and ?_b have been observed at this transition temperature.