Structure and properties of tris(tetramethylammonium) nonabromodiarsenate(III), [(CH3)4N]3[As2Br9]

The new tetramethylammonium bromoarsenate( III) crystal [( CH3)(4)N](3) [ As2Br9] has been synthesized and its phase behaviour established by differential scanning calorimetry and dilatometry. Three phase transitions are found: at 346/346 K ( I <-> II), at 165/ 171 K ( II <-> III) and at 157/ 165 K ( III <-> IV) on cooling/ heating. Single- crystal x- ray diffraction studies of phases I ( at 370 K) and II ( at 298 K) have shown that the structure comprises discrete [ As2Br9](3-) anions and disordered [( CH3)(4)N](+) cations. Phase II crystallizes in the polar space group P31c; its polar nature was confirmed by pyroelectric measurements. Phase I has higher symmetry, P63mc or P (6) over bar 2c. Dielectric dispersion measurements revealed a relaxation process in the crystal. The ferroic ( ferroelastic) properties were found below 165 K ( the II -> III transition). The NMR studies indicate the subtle change in the motional state of the tetramethylammonium cations through all observed phase transitions.