Studies of EPR parameters and defect structures for the tetragonal Ni2+ centers in AgX (X=Cl, Br) crystals from the two-mechanism model

Abstract

The high-order perturbation formulas of electron paramagnetic resonance (EPR) parameters (g factors g∥, g ⊥ and zero-field splitting D) based on the two-mechanism (the crystal-field (CF) mechanism and the charge-transfer (CT) mechanism, the latter is neglected in the CF theory) models for 3d8 ions in tetragonal octahedral clusters are established from a cluster approach. By using these formulas and the parameters obtained from the optical spectra, the EPR parameters g∥, g ⊥ and D of the tetragonal Ni2+ centers in AgX (X=Cl, Br) crystals are calculated. The results are in reasonable agreement with the experimental values. The calculations show that the sign of the EPR parameter Q CT [Q=Δ g∥(=g∥−g s ), Δ g ⊥ (=g ⊥−g s ) or D, where g s ≈ 2.0023 is the value of free electron] due to CT mechanism is the same as that of the corresponding Q CT due to the CF mechanism in both AgCl: Ni2+ and AgBr: Ni2+ crystals, but the relative importance of the CT mechanism (characterized by Q CT/Q CF) for Q=Δ g∥, Δ g ⊥ and D is ∼ 7%, 8% and 1.4% in AgCl: Ni2+, respectively, and those in AgBr: Ni2+ are ∼ 46%, 56% and 44%, respectively. It appears that for the 3d n MX6 clusters in crystals with ligands of large atomic number, the theoretical explanations of the EPR parameters should take both CF and CT mechanisms into account. The defect structures of the tetragonal Ni2+ centers in AgX (X=Cl, Br) crystals are also obtained from the calculations. The results are discussed.