Ligand Substitutional Effects on Molecular Dynamics and Broadband Near-White Emission in 2D 2‑Fluoroethylammonium Cadmium Chlorobromide

Fine-tuning the physicochemical
properties of halide organic–inorganic
hybrids has recently become crucial for materials design. In this
article, structural, thermal, phonon, and optical effects of partial
halide ligand replacement in two-dimensional cadmium chloride templated
by 2-fluoroethylammonium cations are discussed. The substituted compound
exhibits at 266.2/271.2 K a first-order phase transition from a disordered
orthorhombic Cmce phase to an ordered monoclinic P21/c structure. The mechanism
of this order–disorder transformation accompanied by unusually
high-entropy changes involves displacements within the inorganic layers.
Thermal studies revealed the presence of a second, more continuous
transformation, overlapping with the main phase transition, which
was attributed to changes within the hydrogen bonding network. Presented
studies showed that partial chloride-to-bromide ligand replacement
resulted in (i) stronger octahedral distortions and displacements
of the inorganic layers, (ii) a decrease in transition temperature
and the change in entropy, (iii) a reduction of negative thermal expansion
effect and octahedral tilting, and (iv) a lowering of organic cation
confinement in the chloride–cadmium subnetwork, which leads
to weakening of the contribution of the second phase transformation.
Optical studies showed that ligand substitution causes narrowing in
bandgap energy from 5.12 to 4.28 eV and triggering of white to yellow-green
emission thermally quenched above 240 K.