Anharmonic effects and temperature dependence of the lattice spectra of ferroelectric crystals A3B2C9

In the present work the Raman spectra of the family of ferroelectric crystals 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. The conclusion about the possible differences of the structures of the crystals Rb 3 Bi 2 Br 9 and Rb 3 Sb 2 Br 9 from the well-known and thoroughly investigated structure of the Cs 3 Bi 2 Br 9 crystal is made. It is shown that anharmonic interaction of vibrations can give rise to anomalous T -dependence of spectra in the low frequency region. For a crystal having methyl-ammonium (MA) groups the strong anharmonic effects can generate a very complicated structure around internal fundamental frequencies of the MA group which may be considered as a result of the phase transition effects.