Theoretical investigations of the EPR parameters for Cr3+ and Mn4+ ions in PbTiO3 crystals

The complete high-order perturbation formulas of EPR parameters (g factors g(||), g(perpendicular to) and zero-field splitting D), containing the crystal-field (CF) mechanism and charge-transfer (CT) mechanism (the latter is omitted in crystal-field theory which is often used to study the EPR parameters), are established from a cluster approach for 3d(3) ions in tetragonal octahedral sites. According to the calculations based on these formulas, the EPR parameters g(||), g(perpendicular to) and zero-field splitting D for Cr3+ and Mn4+ ions in PbTiO3 crystals are explained reasonably. The calculations show that (i) the sign of g-shift Delta g(CT)(i) (=g(i) -g(s), where g(s) = 2.0023 is free-electron value and i = || and perpendicular to) in CT mechanism is opposite to, but that of D-CT is the same as, the corresponding signs in the CF mechanism and (ii) the relative importance of CT mechanism for the high valence state 3d(3) ion (e.g., Mn4+) is large and so the contributions to EPR parameters from CT mechanism should be taken into account. The different sign of splitting D and the different defect structure for Cr3+ and Mn4+ impurity centers in PbTiO3 crystals are also suggested from the calculations. The results are discussed. (C) 2007 Elsevier B.V. All rights reserved.