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

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 group Ama2. The structure is built up of the i-butylammonium cations and isolated Bi2Br93- anions. The complex dielectric permittivity along the a-axis has been measured between 500 Hz and 1000 MHz in the vicinity of two phase transitions at 252 and 263 Ii. The dielectric response close to 252 K is well described by the Debye equation. The activation energy of the reorientation of the i-butylammonium cations is found to be 0.68 eV. The temperature dependencies 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 reveals one minimum due to the motion of both NH3 and CH3 groups. Analysis of the relaxation data yields the activation energy barriers for the NH3 and CH3 groups motion of 0.113 and 0.037 eV, respectively. The NMR studies suggest that the 263 K phase transition is connected with the reorientation of the cations as a whole. (C) 2000 Elsevier Science Ltd. All rights reserved.