Multifrequency ESR analysis of theEδ′defect ina-SiO2

Abstract

A multifrequency (X, K, Q band) electron spin resonance (ESR) study has been carried out on the nature of the E{sub {delta}}{sup '} defect in vitreous (v) SiO{sub 2}, both at cryogenic and room temperature (T), using two detection modes on six types of commercial v-silica subjected to three kinds of irradiation [UV, vacuum ultraviolet (VUV), and {sup 60}Co {gamma} photons]. The E{sub {delta}}{sup '} signal could be generated only in three types of v-SiO{sub 2}, and only, yet always, by two types of irradiation, VUV and {sup 60}Co {gamma} rays--not UV--suggesting that the E{sub {delta}}{sup '} activation starts predominantly from preexisting sites through ionization processes. The inferred intensity ratio of the resolved {sup 29}Si hyperfine (hf) structure to the central Zeeman signal is found to reassuringly coincide over the observational frequencies and T's studied. The as-inferred average value ({approx_equal}20.3%) alone would point to hf interaction of the unpaired spin with n=4 equivalent Si sites, confirming the center's delocalized nature as initially proposed. Yet, with the inclusion of previous experimental experience, the n=5 case cannot be excluded, and in fact, should be given full consideration as well. In line with the previous conclusion, the result decisively refutes the recurrent theoreticallymore » propagated positively charged Si dimer (Si{sub 2}) model, thus urging discontinuation of the use of the Si{sub 2} label for the E{sub {delta}}{sup '} center. In fact, none of the thus-far advanced atomic models may apply, as all are theoretically projected to essentially correspond to n=1(2) cases due to limited spin delocalization. Extrinsic Al-related defects ([AlO{sub 4}]{sup 0}, Al E{sup '}) are co-observed, where a noteworthy finding is that the E{sub {delta}}{sup '} is only observed in those (3) v-silica also showing the Al E{sup '} center, possibly Na{sup +} compensated, in the initial {gamma}-irradiated state. This may signify some indirect role of charge compensators in activating/stabilizing E{sub {delta}}{sup '} centers, which may need incorporation to further theoretical modeling. The exposed T independence of the ESR parameters, including hf, refers to an electronically rigid structure with no dynamical readjustment process occurring in the range T{>=}4.2 K. The experimental absence of the theoretical Si{sub 2} defect is discussed within the context of its projected closely isospectral ESR appearance to the E{sub {delta}}{sup '} signal.« less