Electron paramagnetic resonance and thermoluminescence study of Ag ions in Li2B4O7 crystals

Electron paramagnetic resonance (EPR) is used to investigate the effects of ionizing radiation on Ag-doped lithium tetraborate (Li₂B₄O₇) crystals. Two similar, yet distinct, trapped-hole centers (Ag²⁺ ions substituting for Li⁺ ions) are produced by 60 kV x rays. One Ag²⁺ ion, labeled Center A, has no nearby defects and the other Ag²⁺ ion, labeled Center B, has a neighboring impurity which is most likely a Ag⁺ ion substituting for a Li⁺ ion. The production and thermal decay properties of the two Ag²⁺ ions are described and their g matrices and ¹⁰⁷Ag and ¹⁰⁹Ag hyperfine matrices are obtained from the EPR angular dependences. The principal values of the g matrices are similar for the two centers, but the hyperfine principal values differ significantly (Center B has smaller values than Center A). There are also differences in the directions of the principal axes for the two centers. Together, these results imply (1) that the unpaired spin is less localized for Center B and (2) that the ground-state positions of the neighboring oxygen ions are different for Centers A and B. This explains why the peaks of the Ag²⁺ charge-transfer photoluminescence bands associated with Centers A and B occur at different wavelengths (502 and 725 nm, respectively). An isochronal pulsed thermal anneal shows that these radiation-induced Ag²⁺ ions serve as the recombination site for the intense thermoluminescence peak observed near 152 °C.