Halide-dependent vibrational dynamics in DMAPbBr_3-xCl_x perovskites

We present a comprehensive spectroscopic and computational investigation of the mixed-halide hybrid perovskite system DMAPbBr_3-xCl_x (x = 0, 0.8, 1.6, 1.9, 2.5, 3), revealing unprecedented insights into halide-mediated structural dynamics. Through advanced antisolvent vapor-assisted crystallization, we synthesized high-quality single crystals for comprehensive characterization via complementary Brillouin, Raman, and Fourier-transform infrared (FTIR) spectroscopy techniques. Our room-temperature Brillouin analysis uncovers a pronounced composition-driven symmetry transformation from hexagonal (P6₃/mmc) to orthorhombic (P2₁2₁2₁) between x = 1.9 and 2.5, marked by an abrupt enhancement in longitudinal acoustic phonon frequency. Systematic Raman and FTIR investigations demonstrate definitive correlations between halide substitution and dimethylammonium cation vibrational signatures, elucidating critical relationships between halide mass, bond strength, and hydrogen bonding interactions. Remarkably, temperature-dependent Raman spectroscopy reveals anomalous phonon hardening with increasing temperature—a phenomenon we attribute to quartic anharmonicity within the face-sharing PbX₆ octahedral framework. The distinctive redshift behavior of the ν(CNC) mode provides compelling evidence of thermal weakening in cation-framework coupling mechanisms. These discoveries establish fundamental structure-property relationships in DMA-based halide perovskites and demonstrate the strategic potential of halide substitution for precise engineering of vibrational and elastic properties in next-generation low-symmetry hybrid perovskite systems.