DOE OSTI2011
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 theoretically 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.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗