STRUCTURAL PHASE-TRANSITIONS IN (N-C3H7NH3)(2)BIBR5 AND (N-C3H7NH3)(3)BIBR6

The (n-C3H7NH2)(2)BiBr5 crystal reveals a complex sequence of phase tr ansitions (on cooling); 1st-order at 334, 235 and 170 K (probably cont inuous). The (n-C3H7NH3)(3)BiBr6 crystal undergoes a 1st-order phase t ransition at 281 K. Differential scanning calorimetry, linear thermal expansion and dielectric studies allow us to characterize these phase transitions in both crystals. Molecular motion in (n-C3H7NH3)(2)BiBr5 was studied by measuring the temperature dependencies of the proton sp in-lattice relaxation time, T-1, and the second moment M(2). All phase transitions having an ''order-disorder'' nature are due to both the r otation of the alkyl chains of (n-C3H7NH3)(+) cations and the C-3 moti on of NH3+ and CH3 groups.