Anomalous lattice anharmonicity and spin-lattice coupling in spin-orbit coupled halide K₂IrBr₆

The interplay between lattice distortions, magnetism, and spin-orbit coupling (SOC) in 5d transition-metal halides offers a fertile platform for exploring correlated spin-lattice dynamics. Here, we investigate the impact of structural symmetry breaking on lattice vibrations and local spin environments in the antifluorite compound K₂IrBr₆ using temperature-dependent Raman spectroscopy, electron paramagnetic resonance (EPR), and first-principles lattice dynamics calculations. K₂IrBr₆ undergoes successive cubic-to-tetragonal and tetragonal-to-monoclinic phase transitions at ∼170 K and ∼122 K, respectively, driven by cooperative distortions of the IrBr₆ octahedra. Raman spectroscopy reveals anomalous phonon linewidth broadening and unconventional temperature dependence of phonon energies near these transitions, indicating that dynamic spin-phonon coupling is significant well above the Néel temperature (T_N ≈ 16 K). First-principles phonon calculations support the mode assignments and demonstrate that symmetry-lowering distortions significantly renormalize vibrational modes, consistent with the experimental observations. Complementary EPR measurements detect anisotropic g-factors, resonance field shifts, and linewidth narrowing across the structural transitions, reflecting the emergence of static spin-lattice correlations mediated by spin-orbit entanglement. These findings establish K₂IrBr₆ as a model system where halide ligand fields, octahedral distortions, and SOC collaboratively govern spin-lattice coupling, providing chemical pathways to engineer quantum materials with tunable magnetic and lattice responses.