Tetrahedron 3d 9 Ions Epr Parameters Of The Theoretical Research

Semiconductors doped with 3d⁹(e.g., Cu²⁺and Ni⁺) ions are often adopted as important luminescence materials. The properties of these systems uauslly sensitively depend upon local structures (e.g., occupation, lattice distortion) of the impurity ions, which canbe conveniently investigated by means of electron paramagnetic resonance (EPR) technique. Experimentally, extensive EPR studies have been carried out for the semiconductors containing 3d⁹ dopants, and the spin Hamiltonian parameters (anisotropic g factors and the hypefine structrue constants) were measured. Unfortuantelly, theoretical explanations to these parameters are unsatisfactory yet. For example, previous studies on the g factors were largely based on the simple perturbation formulas, and various adjustable parameters were applied to fit a few experimental results. Meanwhile, the ligand orbital and spin-orbit coupling contributions were not taken into account, which may induce some errors for these systems with strong covalency. In addition, local lattice distortions (e.g., displacements of the impurity ions along C3 axis) around impurity ions were also ignored as well. In order to overcome the above shortcomings, the perturbation formuals of the spin Hamiltonian parameters are established in this work, and the ligand orbital and spin-orbit coupling contributions are considered here based on the cluster approach. Further, some important parameters (e.g., trigonal field parameters, molecular orbital coefficeints) are correlated to the local strcutreus and optical data of studied systems. The above formuals are applied to the substitutional Cu²⁺ centers in GaN, ZnO and CdS (and also the interstitial Cu²⁺ center in CdSe nanocrystals), and the experimental EPR spectra of these systems are satisfactorily interpreted.(1) For Cu²⁺ in GaN, ZnO and CdS, the impurity Cu²⁺ are found not to occupy exactly the ideal cation sites but to shift along C₃ axis by about -0.004,0.01 and -0.12 A for GaN, ZnO and CdS, respectively (here the displacement direction towards the ligand triangle is defined as positive). In addition, covaleny factors ( 0.6 - 0.7 ) and the the significant orbital admixture coefficients (λ_π≈0.15-0.45,λ_σ≈0.6-0.8andλ_s≈0.55-0.8) reveal considerable covalency of these systems. Therefore, the contributions from the ligand orbitals and spin-orbit coupling interactions are important and should be considered.(2) For Cu²⁺ in CdSe nanocrystals, Cu²⁺ is found to occupy the octahedral interasitial site (i.e., [CuSe6]10- cluster), instead of the previously assumed substitutional tetrahedral Cd²⁺ site. For this Jahn-Teller cluster, it can suffer the Jahn-Teller effect by stretching two parallel Cu²⁺-Se^2- bonds by about 0.18 A along [100] (C4) axis and then lead to a tetragonally elongated octahedron. Despite of the large Cu²⁺-Se^2- bond length, this system also exhibits some covalency effect, the ligand contributions should also be taken into account. Additionally, the experimental optical emission band assigned to the Cu²⁺ center in CdSe nanocrystals is theoretically and reasonably verified in this work.