ESR Studies of an Intrinsic Trapped-Electron Center in X-Irradiated Alkali Borate Glasses

Electron spin resonance studies have been carried out at X‐band frequencies on a large number of glasses spanning the lithium, sodium, and potassium borate glass systems, following x irradiation at cryogenic temperatures $\text{(T\hspace{0.17em}∼\hspace{0.17em}25}\mathrm{or}\text{∼\hspace{0.17em}77°}\mathrm{K})$ . A new, intrinsic paramagnetic defect center characterized by a sizeable hyperfine interaction with boron ${\mathrm{[A\; (}}^{11}\mathrm{B}\text{)\hspace{0.17em}∼\hspace{0.17em}100}\mathrm{G}]$ was observed, in addition to the more familiar boron–oxygen hole centers. The new center, which is readily destroyed by visible light or thermal annealing above ∼ 100°K, is shown to be of the trapped‐electron type. By means of computer simulations of the spectra, a reasonably complete set of spin–Hamiltonian parameters is derived in spite of the obscuring effects of a substantial distribution in hyperfine coupling constants. Possible models for the new defect are considered and its potential usefulness as a “probe” of the structure of alkali borate glasses is discussed.