High temperature ferro-paraelectric phase transition in tris(trimethylammonium) nonachlorodiantimonate(III) (TMACA) studied by X-ray diffraction method

The structure of [NH(CH3)(3)](3)Sb2Cl9, tris(trimethylammonium) nonachlorodiantimonate(III) (TMACA) has been determined at 295 K and 373 K, below and above the high temperature ferro-paraelectric phase transition. In both phases the anionic sublattice of TMACA is built of characteristic two-dimensional (Sb2Cl93-)(n) polyanionic layers lying in the be plane. In room temperature, ferroelectric phase (monoclinic, Pe space group) there are three crystallographically non-equivalent trimethylammonium [NH(CH3)(3)](+) cations. Two of them are located between polyanionic layers and the third one, disordered, inside the cavity formed by six SbCl63- octahedra. In the high temperature paraelectric phase (monoclinic, P2(1)/c space group) there are only two independent trimethylammonium cations in the structure. One of them is located between the inorganic layers, while the other one is placed inside the polyanionic cavity, Both cations are disordered. Temperature dependencies of the lattice parameters, determined between 295 and 375 K, confirmed the presence of phase transition(s) in TMACA in the high temperature region. It was found that the ferro-paraelectric phase transition is associated with the triggering of the overall reorientation of trimethylammonium cations located inside the polyanionic cavities. (C) 2001 Elsevier Science Ltd. All rights reserved.