Electron Spin Resonance and Optical Absorption in GeO2

Abstract

Paramagnetic states of GeO2 in the glass and in the powdered crystalline hexagonal and tetragonal phases have been investigated by the means of electron spin resonance after irradiation with γ rays and electrons. Three of the resonances observed in the glass were identified as intrinsic paramagnetic states on the basis of concentration, g values, and the shapes of the resonances. The g values of these lines were 1.9957, 2.0060, and 2.0080. The g=1.9957 line had a shape which was invariant with temperature (300°, 78°, and 2°K) and with magnetic field. The shape was a characteristic powder pattern for a center with S=½ and an approximately axially symmetric g tensor, where g‖=2.0016 and g⊥=1.9957. This line was characteristic of all glass and hexagonal crystal-phase specimens for all irradiations at temperatures∼300°K. The spin concentration after an irradiation of ∼1017 (1.5 meV) electrons cm−2 was ∼3×1018 cm−3 with no indication of saturation. Tentative correlation of this paramagnetic center with optical absorption in the region of 4.5 eV was established by a series of irradiations and subsequent heat treatments. The data on the g=1.9957 line was sufficient for the proposal of a model in which an electron is trapped at an oxygen vacancy. The g=2.0080 line was observed in all electron-irradiated glass specimens and the g=2.0060 line was found in all γ-ray irradiated glass specimens along with the g=2.0080 line. Other lines were observed at lower g values in β and γ irradiated glass and were tentatively assumed to be due to impurities. Irradiation of the glass with 60Co γ rays at 78°K and measurements without intervening warmup showed only one resonance with g=1.9957 but with a shape quite different from the g=1.9957 line found after irradiation with the same dose of 60Co γ rays at approximately 40°C. This line was assumed to be another intrinsic center. Resonances observed in the tetragonal-phase powder bore no relation to the resonances in the glass and hexagonal-phase powder.