The raman spectra of the family crystals Cs 3Bi 2Br 9, Rb 3Bi 2Br 9 and Rb 3Sb 2Br 9

Abstract

In the present work the Raman spectra of the family of ferroelectric crystals A 3B 2C 9 (where A = Cs, Rb; B = Sb, Bi; C = Br) are given and the assignment of some high-frequency bands is fulfilled. Conclusions about possible differences between the structure of Rb 3Bi 2Br 9 and Rb 3Sb 2Br 9 crystals from the well-known and detailed structure of Cs 3Bi 2Br 9 crystals are made. We also suggest that the significant variation between the spectra in the temperature range 77–300 K can be not only due to phase transition effects but also to anharmonic Fermi resonance between one- and two-phonon excitations.