Cr³⁺ in pure and Mg-doped LiNbO₃: analysis of the EPR and optical spectra

The electron paramagnetic resonance spectra of Cr³⁺ in LiNbO₃ and heavily Mg-doped LiNbO₃, have been measured and analysed according to the superposition model. The axial spectrum of Cr³⁺ in LiNbO₃ is consistent with Nb substitution. On the other hand, the dominant isotropic spectrum found in Mg-doped LiNbO₃ can be accounted for if an appropriate ( approximately=0.1 AA) relaxation of the Cr³⁺ along the c-axis is allowed. Moreover, a point-ion calculation shows that the axial shift of the impurity ion is also qualitatively consistent with recent observations of an enhanced pi -polarization character of the optical transitions in LiNbO₃:Mg, Cr.