Influence of Bromination on the Phase Transitions, Structural, Phonon, and Optical Properties Explored for 2-Bromoethylammonium Bismuth Bromide

Abstract

The physicochemical characteristics of a zero-dimensional (0D) hybrid organic-inorganic bismuth bromide templated by 2-bromoethylammonium cations (BEA)3BiBr6 are investigated. Single-crystal X-ray diffraction data showed that the examined material crystallizes in a triclinic P1̅ structure at room temperature with organic cation partial disorder involving one of the three independent BEA+ cation types. The (BEA)3BiBr6 crystal undergoes a first-order structural phase transition during cooling (256 K) to the completely ordered low-temperature P1̅ phase. Upon heating, two structural transformations during heating occur at 342 and 371 K to the monoclinic P2/n phase. According to the X-ray diffraction data and vibrational studies, the high-temperature and intermediate phases exhibit the same pattern of structural disorder including two independent BEA+ cations. The dielectric, thermal, and vibrational investigations confirmed that the observed phase transitions are essentially related to the thermal evolution of cation disorder; however, they also involve changes in hydrogen bonds, distortion of halogenobismuthate ions, and conformational changes of BEA+ cations. Optical experiments showed that the investigated material has a wide-bandgap energy of 3.07 eV and generates blue light below ca. 165 K when excited at 375 nm. Furthermore, research indicates that bromination of organic cations could become a new and effective method to fine-tune the optoelectronic properties.