Electron paramagnetic resonance of Fe3+ ions in borate glass: computer simulations

Computer simulations of Fe3+ electron paramagnetic resonance spectra at X (9.5 GHz) and Q (34 GHz) bands in the alkali borate glass Li2O-2B2O3 doped with Fe2O3 have been carried out using an approach based on the eigenfield method applied to the 'rhombic' spin Hamiltonian, which contains only the Zeeman and quadrupole fine-structure terms. In order to account for the structural disorder in the glass, two different distribution densities of fine-structure parameters D and E have been tried: a two-dimensional Gaussian function of D and lambda = mod E/D mod , and the 'Czjzek function', analogous to the one used in Mossbauer-effect studies. In simulating the experimental spectra, care has been taken to fit not only to the most prominent features arising at gef approximately=4.3 (at X and Q bands) and gef approximately=2.0 (at Q band), but also to an obvious plateau of the derivative of the absorption, which extends down to the magnetic field corresponding to gef approximately=9.7 (at both bands). As a result, the Czjzek function can be ruled out. The agreement between the experimental and computer-simulated spectra found with the Gaussian distribution density suggests the existence, besides orthorhombic symmetry sites (with lambda approximately=1/3), of a considerable number of Fe3+ sites with axial or feebly rhombic distortions ( lambda