Peroxy entity

Si and Fe3+---0 -Si centers in quartz. However, they may also form without attendant ionizing radiation. One possiblity is the redox conversion of OH or Si-OH pairs into molecular H2 plus peroxy. Such OH or Si-OH pairs derive from traces of dissolved "water . Peroxy anions, 022 , or peroxy entities, X/°° Y (X, Y=Si, Al,Fe3+...) are equivalent to self-trapped positive hole pairs. 

O dimers may appropriately be called peroxy anions, 022 . In a silicate matrix they appear to occur as peroxy links or, more generally, peroxy entities, X/00 Y where X,Y = Si4+,A13+, Fe3+... Chemically a peroxy entity represents an extra oxygen it may be called an interstitial oxygen atom though this does not correctly convey the covalent binding in the 0-0 bond. 

Peroxy entities may also dissociate thermally. In fact, 0 centers have been detected in CaO after rapid quenching from T < 550°C without application of ionizing radiation (20.) Dissociation occurs at the covalent 0-0 bond which is isoelectronic with the F-F bond in the F2 molecule. The 0-0 bond is weak because of the repulsive interaction between the two oxygen nuclei at <1.5 A distance. Dissociation leads to two 0 radicals ... 

The message of eqs. (3b/c) is If peroxy entities exist in insulating materials/ they are in fact dormant charge carriers. In the following we shall refer to 0 to mean any kind of thermally generated 0" or 03Si-0 radical state irrespective of its detailed environment in the host matrix. 

Ionizing radiation is generally believed to be a necessary ingredient to form 0" 02" + hv = 0 + e . However, since the samples under study are wide band gap insulators, heating will certainly not suffice to ionize 02" to 0 . Therefore we are complelled to invoke precursors which are capable of thermally generating 0 states. Peroxy entities which dissociate according to eqs. [3b/c] are the most likely candidates. 

This immediately leads to the question when and where did the obsidian acquire peroxy entities The parent magma was certainly not so highly oxidized as to dissolve oxygen according to the scheme 02" + 1/2 02 = 022" or X/° X + 1/2 02 X/00 X. Magmas tend to be always reduced, ranging from strongly reduced like basalts Q ) to weakly reduced like the obsidian where the Fe3+/Fe2+ ratio is reportedly of the order of 0.5 (21). ... 

In accordance with the Charge Transfer (CT) reaction mechanism outlined by eqs. [2a/b] peroxy was probably introduced into the obsidian via the dissolution of "water", e.g. via Si-OH pairs which converted to to Si/00 Si plus H2. Since the glass structure which is relatively open some or most of the H2 molecules thus generated may have diffused out over geological time, leaving the peroxy entities behind. 

Peroxy entities may indeed represent an ubiquitous type of defects in oxides and silicates, both natural and synthetic. The reason is that traces of dissolved "water" are everywhere. In optical waveguide fibers made of low-OH fused silica a dramatic increase of Si-OH and attendant loss of transmittance have been reported upon exposure to H2 probably by way of the reaction Si/°° Si + H2 2 Si-OH (15,35). ... 

The reactions of peroxysulphuric acid and of hydrogen peroxide with the thiocyanate are obviously very similar. In both reactions the reactive intermediate is the same or at least has a very similar structure, and the subsequent reactions of this entity are equally fast either with peroxy acid or with hydrogen peroxide. Accordingly it can be said that if both peroxy acid and hydrogen peroxide are present in the solution a competition between these two substances for the intermediate occurs, as a result of which not only the peroxy acid but also the hydrogen peroxide will be reduced. 

Peroxy anions or entities cannot be detected by any currently known technique unless the 0-0 bond is activated. One method is ionizing radiation. In the case of high purity fused silica irradiating with y- and x-rays led to peroxy radicals ( ) and dangling bonds at the three-fold coordinated silicon which were fully characterized using EPR (16.17) ... 

Hence, our first conclusion is The obsidian must contain peroxy linkages or similar 0 precursor entities. 

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