DISORDER AND THE MECHANISM OF PHASE TRANSITION OF [(CH₃)₂CHNH₃]₂BiCl_5. Jacek Zaleski and Adam Pietraszko^#, Institute of Chemistry, University of Opole, 45-951 Opole, Oleska 48, Poland, ^#Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wroclaw, Okólna 2, Poland

The alkylammonium halogenoantimonates (111) and bismuthates (111) are ionic-molecular salts. Recently the physical properties of the family attracted considerable attention. Many compounds of this group show interesting ferroic properties. They possess complicated sequences of phase transitions, some of them to polar phases.

[(CH₃)₂CHNH₃]₂BiCl₅ crystallises at 298 K in P2₁/c space group. Its anionic sublattice is composed of isolated Bi₂Cl₁₀^4- units, composed of two BiC1₆^3- octahedra sharing an edge. There are two crystallographically non-equivalent cations in crystal structure. One ordered and one disordered by splitting the position of tertiary carbon atom between two sites with occupancy factors 0.5.

The analysis of orientation and eigenvalues of tensor of moment of inertia of isopropylammonium cation with relation to the observed disorder is discussed. Results lead to the conclusion that disorder is associated with the overall rotation of the cation between two positions. We may thus expect freezing of rotation of the cation on decreasing temperature.

The temperature dependence of lattice parameters, DSC and dielectric dispersion studies indicate that [(CH₃)₂CHNH₃]₂BiCl₅ undergoes an order-disorder phase transition at 164 K. Dielectric dispersion studies reveal low frequency relaxation process described by the Cole-Cole formula with [[alpha]][[circleplus]]0.15-0.20 in the phase transition temperature region. Results of these studies confirm the postulated mechanism of freezing of reorientation motion of the disordered isopropylammonium cation at the phase transition temperature.