Tetrahedron, 5-ion Spin Hamiltonian Parameters Of Theoretical Research

Semiconductors doped with 3d5 (e.g., Fe3+ and Mn2+) ions exhibit unique magnetic, electrical conductivity, optical and photocatalytic properties and are often adopted as important functional materials. Normally, these properties are closely related to the local structures (e.g., occupation, local lattice distortions) of the impurity ions, which can be conveniently investigated by means of electron paramagnetic resonance (EPR) technique. The EPR studies have been performed on some 3d5 ions doped II-VI semiconductors, and the spin Hamiltonian parameters (zero-field splitting, g factors and the hyperfine structure constants) were experimentally measured. Up to now, however, these observed results have not been satisfactorily interpreted. For example, the previous theoretical studies on the g factors were usually based on the simple g formulas, by adopting various adjustable parameters to describe impurity-ligand covalency. However, the above calculations included only the contributions from the conventional crystal-field mechanism, while those from the charge transfer mechanism were neglected. These treatments are unsuitable for the ligands (e.g., Se2?, Te2?) with very strong covalency and spin-orbit coupling interactions and can induce significant errors for the theoretical results. As for the trigonally distorted tetrahedral 3d5 centers, the influences of the local lattice distortions on the spin Hamiltonian parameters were not taken into account, and the EPR analysis failed to connect with the local structures of the systems. In order to overcome the shortcomings of the previous studies, the improved perturbation formulas of the g factor are established in this work for a 3d5 ion under ideal tetrahedra, by considering both the crystal-field and charge transfer contributions from the cluster approach. The related molecular orbital coefficients are correlated to the optical spectral data of the studied systems. These formulas are applied to the cubic Fe3+ centers in ZnX (X=O, S, Se, Te). Meanwhile, the perturbation formulas of zero-field splitting D, g factors and hyperfine structure constants based on the crystal-field mechanism are adopted for a trigonally distorted tetrahedral 3d5 cluster with the ligands (e.g., S2?) of moderate covalency and spin-orbit coupling interaction.The trigonal field parameters are connected with the local structures around the impurities. The above formulas are applied to the EPR and local structure analyses for Mn2+ in the Cd1-xMnxS quantum dots.(1) For the Fe3+ centers in ZnX, the contributions from the charge transfer mechanism are found to increase rapidly with increasing the atomic number of the ligand. The charge transfer contributions to the g factor should be considered in view of the obvious covalency of Fe3+ and the relatively lower charge transfer levels (especially for X= Se and Te).(2) For the Cd1-xMnxS quantum dots, the impurity Mn2+ is found not to occupy exactly the host cation site but to experience the displacement (≈0.041 ?) towards the ligand triangle along the C3 axis. In addition, the above impurity displacement and the core polarization constant show slight increases with increasing the Mn concentration x and make the dominant contributions to the increases of the experimental D and A factors, respectively.