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

Computer simulations of Fe 3+ electron paramagnetic resonance spectra at X (9.5 GHz) and Q (34 GHz) bands in the alkali borate glass Li 2 O-2B 2 O 3 doped with Fe 2 O 3 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 g ef approximately=4.3 (at X and Q bands) and g ef 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 g ef 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 Fe 3+ sites with axial or feebly rhombic distortions ( lambda <or=0.08). The relatively high mean value of the axial fine-structure parameter D is consistent with a highly distorted environment of Fe 3+ ions in the glass.