| Electromagnetic property of Al2O3 crystal will be changed when doped with transition-metal-ion Fe3+. A lot of study have shown that the EPR ground-state zero-field splitting of the doped system have a close relation with lattice local structure distortion after doping. Many suppositions about the lattice local structure have ever been proposed in previous theoretical and experimental studies; however, these suppositions can just explain part of reasonable EPR parameters, which show that the previous structure models can not effectively describe the true distortion of the lattice local structure after doping. In the present work, firstly we introduce method of building complete energy matrix of d5 configuration ion in trigonal symmetry as well as EPR theory. Then we propose two different theoretical models (displacement model and elongation model) to describe lattice local structure distortion of Al2O3:Fe3+ system. By diagonlizing of the complete energy matrix, we calculate the ground-state zero-field splitting of Al2O3:Fe3+ system. Contrasting two models, we find that only elongation model can calculate well reasonable EPR parameters and calculated values are also in very good agreement with experimental data. Therefore, one conclusion can be drew that Fe3+ ion occupy the exact site of Al3+ ion in host lattice and lead to the upwards and downwards elongated distortion of lattice local structure along the trigonal symmetry axis. This description of elongation model is in accordance with the lattice structure distortion trend of Al2O3:Cu3+, Al2O3:Ni2+, Al2O3:Cr3+ and KZnF3:Fe3+ systems that have the same lattice structure.
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