Crystal field analysis of the 3d N ions at low symmetry sites including the ''imaginary'' terms

No abstract

The energy levels and wavefunctions including the two lowest-l~ng levels. namely 4A2 and :E, for which reliable cxperim~tal data exist for C ? ' ions at C3 s-etry sites in L i W . are calculated using the complete m&r diagonalization method wiiilhin the 3d3 configuration. The Hamiltonian eonnid&d includes thc eleamstatic term, the Trees correction, the rpin-orbit interachon and the crystal-field interaction. The role of the additional Iowsymmetry rrystal-field term ET3Oi3 (in the Stevens operator notation). neglected in he C% approximation used so far in the literahlre, is studied. The superposition model is developed for 3d3 ions at C3 ~ymmelly sites and applied to study the site occupancy d C?+ in L i m . Analysis of the optical data i n d i e s that C? ions substitute at Nb sites and Li sites simultaneously. The present considerations offer an improvement avertheearlierapproximarians using C3" symmetry wly. The zero-field splitting predicted by the crystal-field calculations for C ? ' at the Nb site matches the experimental value from EPR mdies very well. This is contrary to the earlier prediction by the superposition model analysis of the spin-Hamiltonian parameters indicating that the 2ero-Beld splilting for C?' ions at Li rites matches the experimental zerofield splitling better than that for C?+ ai Nb sites. Since the present calculations involve fitting not only the zero-field splitting but also the energies of the : E state. the present predictions may be more reliablc than the previous predidionns.

. R u~o w~c z~) (a) X-band EPR spectra of Fe3+ in LiNbO, single crystals at room temperature are analyzed with a spin-Hamiltonian of C, symmetry. First the zero-field splitting (ZFS) parameters b:, bt, b:, and b4 are calculated using the superposition model (SPM). The SPM-ZFS results for Fe3+ at the Nb site agree better with the experimental ones than those for Fe3+ at the Li site. Next the crystal field (CF) parameters are predicted using SPM and two sets of available structural data for LiNbO,. Diagonalization of the full Hamiltonian, including the C F Hamiltonian, within the whole 3d5 configuration yields the ZF splittings for Fe3 + centres. The Z F splittings obtained from the theoretical SPM-CF calculations for Fe3+ at the Nb site agree better with the experimental EPR data than those for Fe3+ at the Li site. The present results indicate that the observed EPR spectra of Fe3+ : LiNbO, can be attributed to Fe3+ ions at the Nb sites.

crystal field (CF) parameters are obtained from the superposition model using two sets of available structural data. Diagonalization of the C F Hamiltonian within the whole 3d5 configuration yields the theoretical Z F splittings. The results for Mn2+ at the Nb site agree better with the experimental data than those for MnZ+ at the Li site. Hence the superposition model analyses of the observed EPR spectra indicate that the spectra can be most likely attributed to Mn'+ ions at Nb sites. The ways to reconcile this conclusion with the opposite one from the low temperature ENDOR study are discussed.