Magnetic Properties and Spin-Orbit Coupling Driven Jahn-Teller Distortions in K2ReX6 (X = Cl, Br, I) with Half-filled 5d-Shell

Abstract

Vacancy-ordered antifluorite halides hosting 5d transition metals have garnered increased attention in recent years due to the interplay between crystal field splitting, electronic correlations, and spin-orbit coupling (SOC), which collectively promote the emergence of novel quantum phenomena. In this work, we perform first-principles density functional theory calculations to comprehensively investigate the structural, electronic and magnetic properties of cubic K2ReX6 (X = Cl, Br, I) with aelectronic configuration. Our result, in excellent agreement with the experimental findings, reveal that all three compounds adopt a type-I antiferromagnetic ordering, resulting from moderate nearest-neighbor antiferromagnetic interactions and weaker next-nearest-neighbor ferromagnetic couplings, despite the presence of geometric frustration in the face-centered cubic (fcc) framework. Furthermore, the systems are well described as insulators in the high-spin S = 3/2 state with a quenched orbital moment, rather than a spin-orbit entangled Jeff = 3/2 state. Theoretically, the presence of SOC effect can stabilize the spin-orbit entangled Jeff = 3/2 state and activate Jahn-Teller effect, leading to lattice distortion in K2ReX6 with a half-filled 5d-shell. However, even when the SOC strength is artificially increased to 2.5 times its self-consistent value - bringing the system closer to the ideal the spin-orbit entangled Jeff = 3/2 state - the resulting SOC-driven Jahn-Teller distortion within the ReX6 octahedron remains subtle. While the magnitude (~ 0.01 Å) is significantly smaller than that observed in typical Jahn-Teller systems, the trend (both in magnitude and mode) of SOC-driven Jahn-Teller distortion across the halide series provides a crucial insight into the paradoxical distortion within ReX6 octahedron observed experimentally in the low-temperature phases. These findings contribute to a broader understanding of intrinsic SOC-induced Jahn-Teller distortions in spin-orbit entangled systems, while simultaneously revealing the experimental challenges associated with detecting such subtle lattice displacements.