Unveiling Jahn-Teller-Modulated Structural Reversibility in Organic–Inorganic Hybrid Compound ( p -Bromoanilinium)2CuCl4 (PBrAnCuCl) with Temperature by Raman Spectroscopy and X-ray Diffraction Studies

Organic-Inorganic Hybrid Compounds (OIHCs) of the A2CuCl4 class having tunable multifunctional properties are made up of a 2D layered framework consisting of (CuCl6)4– octahedra sandwiched between organic cations (A+). In this study, single-crystal analysis of (p-bromoanilinium)2CuCl4 at 100 K reveals a phase transition from the ambient space group Pna21 to Pca21. Temperature-dependent powder X-ray measurements from 100 to 400 K showed noticeable structural changes near the low 2θ region (between 5.2° and 11.8°) indicating re-orientation of both organic and inorganic moieties around 260 K. Temperature-dependent Raman studies show peak splits in both organic and inorganic lattice mode regions at 80 K. Polarization-dependent Raman data suggest that the observed peak splits for these modes are new Raman modes due to the phase transition from Pna21 to Pca21 and appeared as resolved frequencies that can be attributed to orientational effects, deformation vibrations of (CuCl6)4– octahedra (189 cm–1), and reduced thermal broadening. Preservation of framework stability was observed via compression of a long Cu─Cl mode as seen in blue shifting of 180 cm–1, followed by blue-shifting of Cu─Cl bridging (286 cm–1) and C─Br (225 cm–1) modes above 260 K, thus governing the structure partially back to space group Pna21 at 300 K.