Theoretical calculations of spin-Hamiltonian parameters for CsCdX3:Ni2+ (X = Cl, Br) crystals from the two-mechanism model

The high-order perturbation formulas of spin-Hamiltonian (SH) parameters (g factors g∥, g⊥ and zero-field splitting D) for 3d8 ions in trigonal octahedral sites of crystals are derived considering not only the crystal-field (CF) mechanism, but also the charge-transfer (CT) mechanism (which is neglected in the extensively used CF theory). From these formulas and by considering the suitable impurity-induced local lattice relaxation, the SH parameters of CsCdX3:Ni2+ (X = Cl, Br) crystals are calculated. The results are in reasonable agreement with the experimental values. The sign of QCT (Q = Δg∥, Δg⊥ or D, where the g-shift Δgi = gi − ge, ge ≈ 2.0023 is the free-electron value) due to CT mechanism is the same as that of the corresponding QCF due to CF mechanism. The relative importance of CT mechanism (characterized by QCT/QCF) increases with the increasing atomic number of ligand X. So, for 3dn ion clusters in crystals with heavy element ligand ion (e.g., Br−), the reasonable explanations of SH parameters should contain the contributions from both CF and CT mechanisms.