ESR Studies of Lithium Borate Glasses and Compounds γ Irradiated at 77°K: Evidence for a New Interpretation of the Trapped-Hole Centers Associated with Boron

Five glasses and four polycrystalline compounds of the lithium borate system have been subjected to γ irradiation at 77°K and subsequently examined by ESR techniques at 9.1 and 35 GHz while maintaining this low temperature. The familiar “five-line-plus-a-shoulder” spectrum is observed at X band for both the Li2O·4B2O3 compound and the glasses containing ≲25 mole % Li2O. However, previously unreported structure is noted on the low-field shoulder of the spectrum for the compound. This new structure, like the “five-line” part, is due to a hyperfine interaction with boron. An entirely new spin Hamiltonian having orthorhombic symmetry is proposed to account for the over-all spectrum and is substantiated by successful computer simulations of the experimental line shapes. The spectra observed in the glasses of both low and high lithium oxide contents are also simulated by computer, under the assumption that the ensembles of centers induced in the glasses are characterized by substantial spreads or distributions in certain spin-Hamiltonian parameters. Aside from these distributions, the centers in the low-alkali-oxide glasses are qualitatively the same as those encountered in crystalline Li2O·4B2O3. There are at least two inequivalent paramagnetic sites in the latter material: for site I the parameters are found to be A1 = 12.2 G, A2 = 14.4 G, A3 ≈ 7 G, g1 = 2.002, g2 = 2.010, and g3 ≈ 2.035, with the other site(s) differing principally in the values of g3 and A3. Analyses of the g and hyperfine tensors for site I show the unpaired spin to be located in a boron–oxygen π system; the hyperfine interaction is with a three-coordinated boron and the oxygen is one that is shared between the latter boron and a boron in four coordination. Most of the centers exhibiting boron hyperfine structure which are observed in alkali borate materials are probably of this same fundamental nature.