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

The interplay between lattice distortions, magnetism, and spin-orbit coupling 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 Neel temperature (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.

💡 Research Summary

This paper presents a comprehensive investigation of the interplay between lattice distortions, magnetism, and strong spin‑orbit coupling (SOC) in the antifluorite halide K₂IrBr₆. Using a combination of temperature‑dependent Raman spectroscopy, electron paramagnetic resonance (EPR), and first‑principles density‑functional theory (DFT) calculations with Hubbard‑U and SOC, the authors elucidate how successive structural phase transitions—cubic → tetragonal at ~170 K and tetragonal → monoclinic at ~122 K—affect phonon behavior and the local magnetic environment.

X‑ray diffraction confirms high‑quality cubic Fm 3̅ m symmetry at room temperature and reproduces the known symmetry‑lowering transitions driven by cooperative distortions of the IrBr₆ octahedra. Magnetic susceptibility measurements reveal antiferromagnetic ordering below the Néel temperature (T_N ≈ 16 K) and a Curie–Weiss temperature of –88 K, indicating significant antiferromagnetic correlations well above T_N.

Raman spectra recorded from 11 K to 300 K display three prominent phonon peaks at 111.3, 174.7, and 212.3 cm⁻¹, which are assigned to the internal T₂g, E_g, and A₁g modes, respectively, based on DFT‑DFPT calculations. As temperature approaches the structural transitions, the phonon frequencies deviate from the conventional anharmonic hardening/softening trend, and the linewidths broaden dramatically, especially near 170 K and 122 K. This anomalous linewidth broadening cannot be explained solely by standard anharmonic phonon‑phonon decay; instead, it signals strong dynamic spin‑phonon coupling that persists well into the paramagnetic regime.

EPR measurements performed between 100 K and 300 K show anisotropic g‑factors and resonance‑field shifts that track the symmetry changes. In the low‑symmetry monoclinic phase, the EPR linewidth narrows, reflecting the emergence of static spin‑lattice correlations mediated by the entangled J_eff = 1/2 Ir⁴⁺ moments. Even above the magnetic ordering temperature, the temperature dependence of the g‑factor indicates that short‑range antiferromagnetic fluctuations continue to interact with the lattice.

First‑principles calculations employ VASP with PBE‑GGA, a plane‑wave cutoff of 500 eV, and a Hubbard U = 1.8 eV on Ir 5d states. SOC is explicitly included. The computed electronic structure yields small band gaps (0.27–0.31 eV) for all three phases, confirming the insulating nature of K₂IrBr₆. Phonon calculations using DFPT and the Phonopy package reproduce the Raman‑active modes and predict the full set of symmetry‑allowed vibrations: the cubic phase hosts A₁g + E_g + 2T₂g modes; the tetragonal phase splits into 3A₁g + 3B₁g + 2B₂g + 6E_g; the monoclinic phase further expands to 12A_g + 12B_g. Mode‑resolved displacement patterns reveal that the low‑frequency external T₂g mode (≈61 cm⁻¹) corresponds to K‑ion breathing against a rigid IrBr₆ framework, while the internal T₂g (≈108 cm⁻¹) involves Br‑Br shear within the octahedra. The higher‑energy E_g and A₁g modes represent symmetric Br stretching that transiently distorts the IrBr₆ octahedra.

The combined experimental and theoretical results lead to several key insights: (i) symmetry‑lowering structural transitions dramatically renormalize phonon energies and lifetimes, (ii) dynamic spin‑phonon coupling is already significant well above T_N, as evidenced by phonon linewidth anomalies and EPR g‑factor evolution, (iii) static spin‑lattice correlations develop in the monoclinic phase, manifested by EPR linewidth narrowing, and (iv) the strong SOC of Ir⁴⁺ intertwines spin and lattice degrees of freedom, making K₂IrBr₆ an exemplary platform for studying SOC‑driven spin‑lattice physics.

Overall, the work establishes K₂IrBr₆ as a model system where halide ligand fields, octahedral distortions, and spin‑orbit entanglement collectively govern spin‑lattice coupling. The findings suggest that chemical manipulation of halide composition or external pressure could be used to tune the balance between structural symmetry, magnetic interactions, and lattice dynamics, opening pathways toward designing quantum materials with controllable magnetic and phononic responses.