Crystal structure, dielectric properties and molecular motions in (i-C4H9NH3)(3)Bi2Br9

The crystal structure of (i-C4H9NH3)(3)Bi2Br9 at room temperature has been determined and refined to R = 0.036. The crystal is orthorhombic, space gro up Ama2. The structure is built up of the i-butylammonium cations and isola ted Bi2Br93- anions. The complex dielectric permittivity along the a-axis h as been measured between 500 Hz and 1000 MHz in the vicinity of two phase t ransitions at 252 and 263 Ii. The dielectric response close to 252 K is wel l described by the Debye equation. The activation energy of the reorientati on of the i-butylammonium cations is found to be 0.68 eV. The temperature d ependencies of the proton relaxation time T-1 and of the second moment of H -1 NMR has been studied between 120 and 390 K. The relaxation time T-1 reve als one minimum due to the motion of both NH3 and CH3 groups. Analysis of t he relaxation data yields the activation energy barriers for the NH3 and CH 3 groups motion of 0.113 and 0.037 eV, respectively. The NMR studies sugges t that the 263 K phase transition is connected with the reorientation of th e cations as a whole. (C) 2000 Elsevier Science Ltd. All rights reserved.