A covalent heterodinuclear magnetic exchange interaction formula has been established and the antiferromagneticcoupling phenomena of both heterodinuclear MnIIIFeIII and homodinuclear FeIIFeII spin-clusters in complex moleculeshave been studied. It is demonstrated that the antiferromagnetic phenomenon of transition-metal ion-pairs in moleculesshould be attributed to the combined effect of the direct-exchange interaction, the kinetic exchange interaction and thecovalent effect; the strong antiferromagnetic characteristic of MnIIIFeIII cluster in [L0MnIII

A theoretical method for studying the inter-relation between electronic and molecular structure has been proposed by diagonalizing the complete energy matrices for a d5 configuration ion in a trigonal ligand fieldand considering the second-order and fourth-order EPR parameters D and (a - F) simultaneously. As forZnSiF6

A theoretical method for studying the inter-relationships between electronic and molecular structure has beenproposed on the basis of the complete energy matrices of electron-electron repulsion, the ligand field, andthe spin-orbit coupling for the d5 configuration ion in a trigonal ligand field. As an application, the localdistortion structure and temperature dependence of zero-field splitting for Fe3+ ions in the Al2O3:Fe3+ systemhave been investigated. Our results indicate that the local lattice structure of the (FeO6)9- octahedron in theAl2O3: Fe3+ system has an elongated distortion and the value of distortion is associated with the temperature.The elongated distortion may be attributed to the facts that the Fe3+ ion has an obviously larger ionic radiusthan the Al3+ ion and the Fe3+ ion will push the two oxygen triangles upward and downward, respectively,along the 3-fold axis. By diagonalizing the complete energy matrices, we found that the theoretical results ofelectronic transition energies and EPR spectra for Fe3+ ions in the Al2O3: Fe3+system are in good agreementwith the experimental findings. Moreover, to understand the detailed physical and chemical properties of theAl2O3, the theoretical values of the zero-field splitting parameters and the corresponding distortion parametersin the range 50 Ke Te 250 Kare reported first.

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.

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