A quantum-admixture model for the d6 configuration ferrous complex molecules with the high-spin T lowspintransition has been established by using the unified crystal-field-coupling (UCFC) scheme. A generalstudy has been made on the spin transition of octahedrally coordinated d6 complexes, and a special applicationhas been given to an Fe(II) compound FeII(TRIM)2(PhCO2)(ClO4). The results show the following: (i) Thequantum picture of the spin transition of a d6 system, such as Fe(II), is much more complex than a simpletransition between the pure 5T2g and 1A1g states as usually understood. In practice, owing to spin-orbit coupling,spin is no longer a good quantum number and there is no longer a pure 5T2g or 1A1g state. Each of them splitsinto substates and each substate is a linear combination of various multiplets. The high-spin f low-spintransition of an octahedrally coordinated d6 ion is practically the crossover of the two lowest substates of 5T2gat the critical point. (ii) At the spin-transition critical point the magnetic moment íeff 5.22íB, which isobviously different from the simple average of the íeff values of high-spin and low-spin states but near thesaturation value. (iii) The calculation of the effective molecular magnetic momenteff for an octahedrallycoordinated Fe(II) ion shows that the íeff-T curve is in good agreement with Lemercier et al.’s experimentand both the low-spin value íeff ) 0.51íB and the high-spin value íeff ) 5.4íB are comparable with theexperimental values 0.76íB and 5.4íB, respectively. (iv) The T dependence of the crystal field parameter Dqin the spin-transition region is approximately linear.

A quantum admixture model for the d5 configuration of the high-spin T low-spin (HS T LS) transition (6A1T 2T2) Fe(III) complex molecules has been established by using the UCFC scheme. The analysis shows thatthere exists an indirect spin-orbital coupling interaction between the high-spin and low-spin states via theintermediate-spin states. This interaction may be described approximately by two small matrices (the orderis 25 and 14, respectively) of the Hamiltonian, which involves the electron-electron repulsion, the ligandfield,the spin-orbit coupling, and the Zeeman interactions. By diagonalizing these matrices, the electronicand magnetic structures of the Fe(III) molecules in the spin-transition region can be derived. The resultsshow a sensitive dependence on the ratio

The optical-absorption and EPR spectra of octahedral Fe3+ center in yttrium aluminum garnet have been studied by diagonalizing the complete energy matrices for a d5 configuration ion in a trigonal ligand field. It is shown that the local lattice structure around an octahedrally coordinated Fe3+ center has an expansion distortion. The expansion distortion may be attributed to the fact that the radius of Fe3+ ion is larger than that of Al3+ion, and the Fe3+ ion will push the oxygen ligands outwards. Simultaneously, the local lattice structure distortion parametersR=0.0907

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.

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.