Crystal structure, phase transitions and ferroelastic properties of [(CH3)2NH2]3[Bi2Cl9]

A sequence of structural phase transitions in [(CH3)(2)NH2](3)[Bi2Cl9] (DMACB) is established on the basis of differential scanning calorimetry (DSC) and dilatometric studies. Four phase transitions are found: at 367/369, 340/341, 323/325 and 285/292K (on cooling/heating). The crystal structure of DMACB is determined at 350 K. It crystallizes in monoclinic space group P2(1)/n: a = 8.062(2), b = 21.810(4), c = 14.072(3) Angstrom, beta = 92.63(3)degrees, Z = 4, R-1 = 0.0575, wR(2) = 0.1486. The crystal is built of the double chain anions ("pleated ribbon structure") and the dimethylammonium cations. Dielectric studies in the frequency range 75 kHz-900 MHz indicate relatively fast reorientation of the dimethylammonium cations over the I, II, III and IV phases. Infrared spectra are recorded in the temperature range 40-300 K and analyzed in region assigned to the symmetric and asymmetric NC2 stretching vibrations. Optical observations show the existence of the ferroelastic domain structure over all phases below 367 K. The possible mechanisms of phase transitions are discussed on the basis of presented results. (C) 2003 Elsevier Science (USA). All rights reserved.