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The defect structures and the g factors for the two orthorhombic Ti3+ centers (A and B) in CaYAlO4 are theoretically investigated from the perturbation formulas of the g factors g, g, and g for a 3d(1) ion in orthorhombically distorted octahedra. The centers may be attributed to V, locating on the Al3+ site associated with one nearest-neighbouring oxygen vacancy (V-O) along the z (or c) axis and additional next-nearest-neighbouring Ti4+ replacing the host Al3+ (Ti-Al, i.e., center A) or Al3+, vacancy (V-Al, i.e., center B) perpendicular to the c axis, respectively. Due to the electrostatic interactions arising from the local charge mismatch, the central Ti3+ undergoes a large displacement Delta Z away from the V-O in the c axis, while the ligand O2- intervening in the Ti3+ and TiAl (or V-Al suffers a small outward (or inward) shift Delta X in the corresponding c plane related to the center of oxygen octahedron. The theoretical g factors for both centers based on the above displacements are in reasonable agreement with the observed values. (C) 2007 Elsevier B.V. All rights reserved.