X-ray diffraction study of the phase transitions of (CH3)(4)NCdCl3 between293 and 80 K: a quantitative analysis of the ferroelastic domains distribution below 118 K

X-ray diffraction patterns of [N(CH3)(4)][CdCl3], tetramethylammonium trich lorocadmate(II), have been investigated in the temperature range 80-293 K, which includes two phase transitions at 118 and 104 K, respectively. The ma in interest in this compound is to establish the mechanism of the structura l phase transitions common to other members of the isostructural family [(C H3)(4)N][MX3]. It is supposed to be related to the ordering of the organic part together with some small distortion of the inorganic chains. The origi n of the order-disorder mechanism would be the orientationally disordered d istribution of the tetramethylammonium tetrahedra at room temperature. Maxi mum Entropy Methods suggest that the most probable distribution of the orga nic groups can be described through the so-called two-well model, in which one threefold axis of the tetramethylammonium tetrahedron coincides with th e crystallographic threefold axis of the structure. Below 118 K the reflect ions are split. However, the splitting cannot be fully explained by the fer roelastic domains expected to appear after the phase transitions. Recent NM R results [Mulla-Osman et al. (1998). J. Phys. Condensed Matter, 10, 2465-2 476] corroborate the existence of more domains than expected from symmetry considerations. A model of ferroelastic domains which is in agreement with both X-ray diffraction diagram and NMR measurements is proposed.