Modeling EPR powder spectra using numerical diagonalization of the spin Hamiltonian

A new modeling code, ZFSFIT (standing for Zero Field Splitting FITting), wr itten in FORTRAN 77 is proposed. It is designed for computing and fitting E PR powder spectra described by any spin Hamiltonian including second- and f ourth-order ZFS terms (S less than or equal to 5/2) and/or a hyperfine term (I less than or equal to 7/2). Based on numerical diagonalization of the s pin Hamiltonian, this code computes the powder spectrum, the calculated ang ular dependencies, and the energy levels at any orientation. Least-squares refinement of the spin Hamiltonian parameters is performed either by adjust ing powder line positions (EPRPLP module) or by directly fitting the powder spectra (ZFSFIT code). Especially, simultaneous fitting of EPR powder line positions recorded at distinct frequencies improves the accuracy of the re fined EPR parameters. Superhyperfine effects as wed as broadening effects d ue to site-to-site distribution of g-, A-, and ZFS parameters are treated u sing first-order perturbation theory and can also be refined. Parameters fo r several distinct centers can be fitted simultaneously, allowing quantific ation of their relative amounts in the sample. After a description of the a lgorithm, determination of second- and fourth-order ZFS parameters of Cr3+, Mn2+, and Fe3+ centers in low-symmetry sites in minerals are treated, incl uding first evidence of structural Fe3+ centers in alpha Al(OH)(3). The cod e is available without charge to academic users from the authors. (C) 1999 Academic Press.