DILUTED AND NON-DILUTED FERRIC IONS IN BORATE GLASSES STUDIED BY ELECTRON-PARAMAGNETIC-RESONANCE

Electron paramagnetic resonance (EPR) spectra of lithium berate glass (1-x)(0.63B(2)O(3).0.37Li(2)O).xFe(2)O(3), with x varying from 0.001 t o 0.1, were measured at different microwave frequencies and temperatur es. For low Fe3+ concentrations (Fe2O3 molar contents from 0.001 to 0. 01), X-band EPR spectra, consisting of a g(ef) = 4.3 peak accompanied by a shoulder continuing down to g(ef) = 9.7, are computer simulated o n the basis of a 'rhombic' spin-Hamiltonian with Zeeman and fine-struc ture terms. A good fit to the experimental spectra for various Fe2O3 c ontents is observed with the same values of the spin-Hamiltonian param eters and assuming a Lorentzian lineshape and a linewidth increasing l inearly with the concentration of Fe3+ ions. It is concluded that the spectrum is due to diluted Fe3+ ions in a relatively strong crystal fi eld of orthorhombic symmetry, with largely distributed fine-structure parameters. From the concentration dependence of the line width, by ex tending to glasses a theoretical EPR linewidth expression derived for polycrystalline systems, the minimum distance between diluted Fe3+ ion s is estimated as 4.9 Angstrom. A diluted state of Fe3+ ions in the gl ass network in this range is also confirmed by the temperature depende nce of the g(ef) = 4.3 resonance which follows a Curie law. For interm ediate concentrations of Fe3+ ions (Fe2O3 molar contents from 0.01 to 0.1), the width of the g(ef) = 4.3 line is proportional to the square root of concentration, still indicating dipolar interactions. On the o ther hand, the microwave frequency dependence of a broad g(ef) approxi mate to 2 line, which coexists at these concentrations with the g(ef) = 4.3 line, shows that the former line is due to pairs or small cluste rs of exchange-coupled Fe3+ ions. The temperature dependence of the g( ef) approximate to 2 line intensity in 0.1 mol Fe2O3 glass is consiste nt with a more antiferromagnetic character by comparison with the 0.05 mel Fe2O3 glass, which is attributed to an appearance, at higher Fe2O 3 contents, of iron-containing microclusters not incorporated in the r andom glass network, with smaller distances between the paramagnetic i ons. These microclusters are probably the origin of a new narrow line superposed with the broad g(ef) approximate to 2 line in the low-tempe rature EPR spectra.