Experimental Study of the Low Temperature Phase Transition of Rb₂ZnCl₄

Abstract

The low temperature phase transition of Rb₂ZnCl₄ was studied by measuring the Raman spectra, dielectric constants and spontaneous polarization. A soft mode (the B mode), whose frequency tends to zero as T₃=74.6 K is approached from below, was observed in addition to the totally symmetric soft mode which had been already observed by Francke et al. No remarkable anomaly in the dielectric constants was seen around T₃. The spontaneous polarization along the a-axis was observed both below and above T₃. From these experimental results and group-theoretical consideration, the phase transition at T₃ is considered to be induced by a doubly-degenerate phonon mode at the Brillouin zone boundary of the phase III, and the space group of the phase IV below T₃ is guessed to be P2₁11 (Z=24).