Structural peculiarities and phonon spectra of A 3 B 2 C 9 compounds: a Raman-scattering diagnostic

In presented work the Raman spectra of the family of ferroelectric crystals A₃B₂C₉, (here A equals Cs, Rb; B equals Sb, Bi; C equals Br) are given and the assignment of some high frequency bands is fulfilled. The conclusion about possible difference of structure of the crystals Rb₃Bi₂Br₉ and Rb₃Sb₂Br₉ from well known and detaily investigated structure of Cs₃Bi₂Br₉ crystal is made. We also suggest that significant variation of spectra in the temperature region 110 - 300 K can be not only due to phase transition effects but also owing to anharmonic Fermi resonance between one- and two phonon excitations.