X-RAY-DIFFRACTION STUDY OF THE FERROELECTRIC PHASE-TRANSITION OF (CH3)4NCDBR3 (TMCB)

The crystal structure of tetramethylammonium tribromocadmate (II), [N(CH3)4][CdBr3] (TMCB), has been studied at 295 and 85 K, in order to analyze the structural changes connected with the improper para-ferroelectric phase transition occurring at approximately 160 K. The space group of the paraelectric (room-temperature) phase is P6(3)/m and that of the ferroelectric (low-temperature) phase is P6(1)-P6(5). Final agreement factors are R (wR) = 0.062 (0.075) and R (wR) = 0.029 (0.038) for the data collected at 295 and 85 K respectively. It is shown that this phase transition is related to an orientational order-disorder process of the tetramethylammonium (TMA) groups, which leads to a tripling of the lattice constant along the c (hexad) direction; this process can be described by the freezing of a pseudo-spin coordinate defined in the frame of a Frenkel-type model. In addition, the helical-type structure found for the ferroelectric phase occurs as a result of a bi-linear coupling of this pseudo-spin variable with the transverse acoustic mode at point DELTA(0,0,1/3). The spontaneous polarization comes from the combination of a small distortion of the ordered TMA groups and an anti-translation of CdBr3 octahedra chains against the TMA groups. Crystal data: M(r) = 426.27, lambda(Mo Kalpha) = 0.7107 angstrom. At 295 K: P6(3)/m, Z = 2, F(000) = 392, a = 9.388 (2), c = 6.991 (5) angstrom, V = 533.6 (6) angstrom3, D(x) = 2.653 (3) g cm-3 mu = 131.03 cm-1. At 85 K: P6(1)(P6(5)), Z = 6, F(000) = 1176, a = 9.219 (4), c = 20.86 (1) angstrom, V = 1536 (2) angstrom3, D(x) = 2.765 (4) g cm-3, mu = 136.56 cm-1.