Figure 1.

(Top right) Structure of 1. Atom labels are Er (green), N (blue), Si (orange), and C (dark gray). (Bottom) Electronic interactions in an Er³⁺ ion due to electron repulsion, spin–orbit coupling, and crystal-field contributions. Red arrows represent the relative energies of the M_J = ±15/2 → ±13/2 transition at 0 T and the −15/2 → −13/2 transition under applied magnetic fields. It should be noted that labeling the crystal-field states by a single-eigenvector component M_J here is an approximation, as Jank et al. pointed out.³⁰ The labeling here does not include, e.g., third-order terms in the crystal-field Hamiltonian which may mix some of the states here.