a Faculty of Chemistry, University of Wroc?aw, Joliot Curie 14, 50-383 Wroc?aw, Poland
b Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Okólna 2, P.O. Box 937, 50-950 Wroc?aw 2, Poland
Abstract:
Crystals from the rich family of alkylammonium halogenoantimonates(III) and bismuthates(III) containing small bioctahedra as well as infinite two- or one-dimensional polyoctahedral units attract particular attention from the point of view of possible applications. Such crystals exhibit a wealth of phase transitions including those to ferroelectric and ferroelastic phases. The analysis of vibrational spectra performed in this paper with respect to modes assigned to alkylammonium group shows that their interactions with polyanionic sublattice is of medium strength. The absorption pattern and particularly the splitting of bands on cooling very well correlates with other anomalies of physical properties and particularly with the behaviour of second moment of PMR lines and T1 and T1ρ relaxation times as functions of temperature. The temperature behaviour of modes is well described in terms of pseudospin–phonon coupling model that we presented for different modes in various crystals. The role of anionic dynamics is not sufficiently recognised but in one case, namely for the low temperature transition in (MA)3Bi2Br9 crystal this role is predominant, according to vibrational and NQR spectra of methylammonium and isostructural caesium salt.