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The EPR g factors, g(parallel to) and g(perpendicular to), for the isoelectronic 3d(9) ions Ni(+) and Cu(2+) at the tetragonal Cu(+) site of the CuGaSe(2) crystal are calculated from the high-order perturbation formulas based on a two-spin-orbit-parameter model. In the model, both the contributions to g factors from the spin-orbit parameter of central 3d(9) ion and that of ligand ion are contained. The calculated results appear to be consistent with the experimental values. The tetragonal distortions (characterized by theta - theta(0), where theta is the angle between the metal-ligand bond and C(4) axis, and theta(0)approximate to E54.74 degrees is the same angle in cubic symmetry) of Ni(+) and Cu(2+) centers, which are different from the corresponding angle in the host CuGaSe(2) crystal and from impurity to impurity, are obtained from the calculations. The difference of the sign of g(parallel to) - g(perpendicular to) between the isoelectronic Ni(+) and Cu(2+) centers is found to be due to the different tetragonal distortions of both centers in the CuGaSe(2) crystal. (C) 2010 Elsevier B.V. All rights reserved.