SUPERPOSITION MODEL AND CRYSTAL-FIELD ANALYSIS OF THE (4)A(2) AND (2)(A)E STATES OF CR-3+ IONS AT C3 SITES IN LINBO3

The energy levels and wavefunctions including the two lowest-lying lev els, namely 4A2 and 2(a)E, for which reliable experimental data exist for Cr3+ ions at C3 symmetry sites in LiNbO3, are calculated using the complete matrix diagonalization method within the 3d3 configuration. The Hamiltonian considered includes the electrostatic term, the Trees correction, the spin-orbit interaction and the crystal-field interacti on. The role of the additional low-symmetry crystal-field term B4-3O4- 3 (in the Stevens operator notation), neglected in the C3v approximati on used so far in the literature, is studied. The superposition model is developed for 3d3 ions at C3 symmetry sites and applied to study th e site occupancy of Cr3+ in LiNbO3. Analysis of the optical data indic ates that Cr3+ ions substitute at Nb sites and Li sites simultaneously . The present considerations offer an improvement over the earlier app roximations using C3v symmetry only. The zero-field splitting predicte d by the crystal-field calculations for Cr3+ at the Nb site matches th e experimental value from EPR studies very well. This is contrary to t he earlier prediction by the superposition model analysis of the spin- Hamiltonian parameters indicating that the zero-field splitting for Cr 3+ ions at Li sites matches the experimental zero-field splitting bett er than that for Cr3+ at Nb sites. Since the present calculations invo lve fitting not only the zero-field splitting but also the energies of the 2(a)E state, the present predictions may be more reliable than th e previous predictions.