At present the parameter a of the Fe3+ ion in a crystal has still not been determined. In this paper we discuss this problem by diagonalizing the complete matrices for a ligand-field spin-orbit-coupling perturbation. The results obtained are in good agreement with experimental findings. Furthermore, possible resonances for the difference between our results and previous ones are given.

It is shown that Low's conjecture, that the Stark level 4TI(G) in MgO:Mn2+ would be higher than 14000 cm-l from the ground state, is in accord with EPR experiments. Furthermore, possi-ble reasons for the difference between our results and previous ones are given.

An analysis of the relationship between the EPR trigonal-field parameters and the local crystal structure of octahedral Mn2+ centre indiamagnetic garnets is presented by diagonalizing the complete energy matrices for a d5 configuration ion in a trigonal ligand-field. Boththe second-order and fourth-order EPR parameters D and (a-F) are taken simultaneously into the structural investigation. It is shownthat the local lattice structure around an octahedrally coordinated Mn2+ centre in diamagnetic garnets have expansion distortions, whichmay be attributed to the fact that the radius of Mn2+ ion is larger than that of Al3+ or Ga3+ ion, and the Mn2+ ion will push the oxygenligands outwards. The local lattice structure parameters R and h for octahedral Mn2+ ion centre in the crystals are determined, respectively.Meanwhile, it is also demonstrated that the empirical impurity-ligand distance R RH

The complete energy matrix including all the spin states is constructed for d4ions in tetragonal symmetry within a strong-field representation. The excitationlevels, fine-structure splitting, and EPR parameters in Rb2CrCl4 and CrF2 arestudied with the energy matrix. The contributions of the spin singlets to zerofieldsplitting (ZFS) are investigated for the first time. The results show thatthe spin-singlet contribution to D is negligible, but the contributions to a and Fcannot be neglected.

A theoretical method for describing magnetic-exchange-interactions in covalent molecular systems is established byintroducing covalent effect into Girerd-Journaux-Kahn’s (GJK) magnetic formula and adopting our Double-Slater-Function (DSF) calculation method. The calculations show that the exchange coupling parameter J depends sensitivelyon the covalent factor N; the stronger the covalent effect the stronger the antiferromagnetic exchange interaction; thedirect-and kinetic-exchange interactions are both important. The calculation for MnIII-O-MnIII spin-clusters inMn2Oð5-NO2saldienÞ2 reveals that about 30% of the antiferromagnetic contribution comes from covalent effect, andthat the theoretical values J ¼111:02 to138:85 cm 1 agree well with the experimental findings ðJðexpt:Þ ¼ 120 cm 1Þ.