Investigations of the Axial Displacements of Co~(2+) and Ni~(3+) in AI_2O_3

摘要

The axial displacements for Co~(2+) and Ni~(3+) in Al_2O_3 ai'e theoretically investigated starting from the perturbation formulas of the EPR parameters for a 3d7 ion with high spin (S=3/2) and low spin (S=//2) in trigonal symmetry, respectively. Based upon these studies, the Co~(2+) is found to shift towards the center of the oxygen octahedron by an amount AZco (≈0.03 A) along the C_3 axis, while the Ni~(3+) may suffer another axial displacement AZN, (≈-0.38 A) away from the center of the octahedron. The calculated EPR parameters based upon the above axial displacements show good agreement with the observed values. The differences in the EPR parameters and the axial displacements for the two 3d7 ions are discussed.