Investigations of the axial displacements of Co<sup>2+</sup>and Ni<sup>3+</sup> in Al<sub>2</sub>O<sub>3</sub>

Abstract

The axial displacements for Co2+ and Ni3+ in Al2O3 are 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=1/2) in trigonal symmetry, respectively. Based upon these studies, the Co2+ is found to shift towards the center of the oxygen octahedron by an amount ZCo (≈ 0.03 Å) along the C3 axis, while the Ni3+ may suffer another axial displacement ZNi (≈ −0.38 Å) 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.