Progressive Fluorination-Controlled Steric Effects in Low-Dimensional Bismuth Bromides

Abstract

Although fluorination introduces tunable lattice rigidity and steric matching that influence molecular dynamics, the use of progressive fluorination as an effective method for structure−property tuning remains limited. Here, we report structural, thermal, dielectric, vibrational, and optical investigations of two members of zero-dimensional (0D) hybrid bismuth bromides with the general formula A3BiBr6, where A denotes 2,2-difluoroethylammonium and 2,2,2-trifluoroethylammonium cations. Differential scanning calorimetry reveals that the difluorinated compound undergoes a first-order phase transition at 184.3/177.5 K (heating/cooling), similar to the monofluorinated analogue, whereas its trifluorinated counterpart exhibits no structural phase transitions within the studied temperature range. A single-crystal X-ray diffraction study shows that the trifluorinated compound adopts trigonal symmetry (space group P3̅). In contrast, the difluorinated analogue crystallizes in the orthorhombic space group Pnnm at room temperature and transforms upon cooling into a monoclinic phase described by the P21/n space group. Temperature-dependent infrared and Raman spectroscopies indicate that the phase transition mechanism is governed by order−disorder processes. The noncovalent interaction calculations support that the F···F intermolecular interactions originate from sterically constrained packing rather than attractive interactions, contributing to dense crystal arrangement. Optical studies reveal that band gap energies correlate with octahedral deformation and level of fluorination. The difluorinated compound exhibits orange luminescence of excitonic origin, which is still observable at room temperature. The presented data demonstrate that degree of fluorination is a versatile design strategy for tuning dipolar degrees of freedom, intermolecular interactions, the strength of organic−inorganic coupling, and optical properties in hybrid halides.