Vibrational spectra of [(CH3)(3)NH](3)Sb2Cl9, [(CH3)(3)NH](3)Bi2Cl9 and their mixed crystals

The IR and Raman spectra of [(CH3)(3)NH](3)Sb2Cl9 (A), [(CH3)(3)NH](3)Bi2Cl 9 (B) and two of their mixed crystals containing respectively 33% (AB.33) a nd 42% Bi (AB.42) are analyzed and compared. A and AB.33 show ferroelectric -paraelectric phase transition at 364 K and 344 K, respectively. AB.42 and B are paraelectric in the temperature range between 90 and 365 K. Most of t he vibrational modes show continuous changes, with the temperature, in the IR frequencies or intensities with no soft mode behavior. However, characte ristic nu(NH ... Cl) and delta(N ... Cl) vibrations of weakly hydrogen-bond ed species are only observed in A and AB.33 below the temperature of the ph ase transition and are related to the ferroelectricity, The evolution of th e IR spectra with the temperature suggests that the ferroelectric propertie s are connected with the reorientation of the cations which needs a breakin g of the weak NH ... Cl hydrogen bonds in the paraelectric phase. (C) 1999 Elsevier Science B.V. All rights reserved.