A theoretical model of the ground state and the magnetic moment of spin crossover of Fe(III) in a cu- bic ligand field has been established by diagonalizing the complete matrices for electron-electron-repulsion, ligand-field, spin-orbit-coupling, and Zeeman interactions. It is shown that the temperature dependence or unusual magnetic moment of Fe(III) in molecules is due to a large zero-field splitting and an asymmetrically high-order Zeeman effect of the electronic states between the high-spin 6A1 and the low-spin 2T2 states.

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.

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.

The electronic structure of trigonal fivefold-coordinated Fe(III) molecules has been studied. Theligand-field model that we use takes into account the covalency effect which modifies the ligand-field pa-rameters. The complete matrices for electron-electron repulsion, ligand-field, and spin-orbit coupling, have been diagonalized, including the Zeeman interaction in the case of magnetic-property calculations. The energy levels position can be expressed as a function of the ratio A/C, A being the energy gap 6.4 1-4E, and the spin-orbit-coupling parameter. It is shown that, by varying this ratio, the intermediate spin level 4E(aT1) can be stabilized as the ground state, instead of the 4A2(aT1) level in the case of a tetragonal distortion. Near the crossover point (6Al-aE transition), strong quantum admixtures and large zero-field splittings occur: the ground Kramers doublet contains 41% high-spin state and 59% 5 intermediate-spin state; the corresponding ground-state wave functions are associated with MJ=

The relationship between the EPR cubic-field parameter 10g(a), the covalence factor N, and the crystal-field strength Dq has been established by diagonalizing the complete matrices for ligand-field and spin-orbit coupling. A general method for calculating the local thermal-expansion coefficient of the im-purity ion has been proposed. It is shown that the experiment of Rubio and Medrano is in agreement with the infrared and Raman spectra.