(X = Cl, Br, and I) using High-resolution X-ray Scattering*

Chromium tri-halides CrX₃ (X = Cl, Br, and I) exhibit low-temperature, layer-dependent magnetic properties that can be manipulated by external stimuli, making them essential candidates for spintronics applications. Their magnetic ground states depend keenly on electronic parameters such as spin-orbit coupling (SOC), Hund’s coupling (J_H), p−d covalency, and interorbital Coulomb interactions. Accurately determining these parameters is paramount for understanding the CrX₃ physics.  We have used high-resolution resonant inelastic X-ray scattering (RIXS) spectroscopy to study CrX₃ across phase transition temperatures. Ligand field multiplet calculations were used to determine the electronic structure parameters by incorporating the crystal field interactions in a distorted octahedral orientation with C₃ symmetry. The crystal field distortion parameters Dσ and Dτ, and the energies of d orbitals, have been reported. The measurements reveal an energy separation between spin-allowed quartet states and spin-forbidden doublet states, which increases from CrCl₃ to CrI₃. The determined 10Dq values are in good agreement with the spectrochemical series; the Racah B parameter follows the expected Nephelauxetic effect. This study validates the role of SOC in Cr 2p spin-flip excitations. Such precise measurements offer insights into the energy design of spintronic devices that utilize quantum state tuning and the effect of halides in determining spin-flip excitation energies in 2D magnets.