Site-Dependent Crystal Fields of Nd3+, Er3+, and Yb3+ in the Chalcogenide Perovskite BaZrS3: Spectroscopic and Magnetic Signatures from Embedded-Cluster CASSCF/NEVPT2 Calculations

BaZrS3 doped with Nd3+, Er3+, or Yb3+ shows structured near-infrared emission whose interpretation requires distinguishing the local coordination environments. Embedded-cluster CASSCF/NEVPT2 calculations with spin–orbit coupling in the complete 4fn configuration space compare Nd3+, Er3+, and Yb3+ in six-coordinate Zr-site and eight-coordinate Ba-site models, and breathing-coordinate scans quantify the sensitivity to the local Ln–S distance. Validation on Cs2NaLnCl6 shows that the complete state space brings the Er3+4I13/2 and 4I11/2 energies to within 0.5% of experiment, where the highest-multiplicity restriction leaves larger, multiplet-dependent errors. The two sites give comparable ground-multiplet widths but contrasting level patterns: the Zr-site ligand field is dominated by rank-four terms and the Ba-site field by rank-two terms, with opposite responses to the local bond length. Published low-temperature emission data are compatible with a Zr-like environment for the dominant Er3+ and Nd3+ emitters and, tentatively, a contracted Ba-like environment for the broadest Yb3+ population; ground-doublet g-tensors and susceptibilities provide discriminants for EPR and magnetometry, and restricted-active-space calculations calibrated on Er3+ in LaCl3 show that, within the static electronic models, the electric-dipole strength of Er3+ and Yb3+ is symmetry-suppressed at the Zr site whereas the Ba-site ions acquire a forced electric-dipole channel of comparable or larger strength.