Investigations of the EPR parameters for the tetrahedral [FeX_4]^-   clusters in AgX (X=Cl, Br)
  The EPR parameters g factors and the superhyperfine parameters for the tetrahedral [FeX_4]^- clusters in AgX (X=Cl, Br) are theoretically investigated from the perturbation formulas of these parameters for a 3d^5 ion under tetrahedra by considering both the crystal-field and charge transfer contributions. The related model parameters are quantitatively determined from the cluster approach in a uniform way. The g-shift \Delta g (=g-g_s, where g_s\approx 2.0023 is the spin only value) from the charge transfer contribution is opposite (positive) in sign and much larger in magnitude as compared with that from the crystal-field one. The importance of the charge transfer contribution increases rapidly with increasing the covalency and the spin-orbit coupling coefficient of the ligand and thus exhibits the order of AgCl<AgBr. The unpaired spin densities of the halogen ns, np\sigma\ and np\pi\ orbitals are quantitatively determined from the related molecular orbital coefficients based on the cluster approach. 
B.-T. Song, S.-Y. Wu, M.-Q. Kuang, Z.-H. Zhang
cond-mat.mtrl-sci