Point Defects on Silica Surface

A new view on this problem arose after the diamagnetic centers with two-coordinate silicon atom were observed at the surface of mechanically activated silica [70]. The subsequent studies of the optical properties of these centers [71] have shown that they are close to those centers that were observed earlier in the bulk of quartz glass but referred to the oxygen vacancies. It has become evident that, apart from the combination reaction of two radicals (=Si-0)3Si and (=Si-0)3Si resulting in the oxygen vacancy, an alternative reaction of their disproportionation with the formation of a center containing two-coordinate silicon and the restoration of one of the siloxane bonds is possible  

The succeeding section is devoted to the structure and reactivity of these diamagnetic groups. 

The comparable number of diamagnetic groupings of another type, namely (sSi-0-)2Si 02 Si(-0-Si=)2 (SRs) (see Section 2.1 and Table 7.1), are also stabilized on the surfaces of RSi samples in addition to SCs. The absorption bands at 888 and 908 cm-1 [3,4] in a silica IR transparency window are connected with these groupings. Such centers were registered for the first time on the dehydroxylated surface of silica. Defects of the type considered are characterized by a high reactivity with respect to H20, CH3OH, and NH3 molecules [3,72]. At the same time, reliable information about the reactivity of SRs with respect to another molecule (e.g., H2(D2)) is actually lacking. The first part of this section is concerned with a discussion about physico-chemical properties of SRs. 

Warming up of a hydrated sample at temperatures above 1100K was accompanied by the release of molecular deuterium and the restoration of SR absorption bands at 888 and 908 cm-1. Measuring the number of desorbed molecules it was also possible to foresee the number of SRs in the sample. 

F2Si -0-( 0)SiF2 (3A ) [F2Si 02 SiF2...Fl20] (complex) 

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