THE RAMAN-SPECTRA OF THE FAMILY CRYSTALS CS3BI2BR9, RB3BI2BR9 AND RB3SB2BR9

In the present work the Raman spectra of the family of ferroelectric c rystals A(3)B(2)C(9) (where A = Cs, Rb; B = Sb, Bi; C = Br) are given and the assignment of some high-frequency bands is fulfilled. Conclusi ons about possible differences between the structure of Rb3Bi2Br9 and Rb3Sb2Br9 crystals from the well-known and detailed structure of Cs3Bi 2Br9 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 resonanc e between one-and two-phonon excitations. (C) 1997 Elsevier Science B. V.