EPR study of the low temperature ferroelectric phase transition in Cu2+ doped Rb2ZnCl4 single crystals

Temperature dependent EPR measurements on copper doped Rb2ZnCl4 single crystals allowed us to evidence and study the P21cn↔C1c1 structural phase transition that takes place in this compound at 74.6 K. From the two types of Cu2+ centers localized at different anionic sites, called Cu2+(I) and Cu2+(II), which are formed in this compound, only the Cu2+(II) centers exhibit observable changes in their EPR spectra, attributable to the symmetry lowering. The observed changes have been related to the soft-mode responsible for the structural phase transition.