Oxygen Exchange at the Internal Surface of AmorphousSiO2Studied by Photoluminescence of Isotopically Labeled Oxygen Molecules

Abstract

The exchange between lattice and interstitial oxygen species in an oxide was studied by the 16O-18O isotope shift of the a1Deltag(v=0)-->X3Sigmag-(v=1) infrared photoluminescence band of the oxygen molecules (O2) incorporated into the interstitial voids of amorphous SiO2 (a-SiO2) by thermal annealing in 18O2 gas. A large site to site variation of the oxygen exchange rate, originating from structural disorder of a-SiO2, is found. The average exchange rate has an activation energy of approximately 2 eV, which is much larger than that for the diffusion of interstitial O2 (approximately 0.8-1.2 eV). The average exchange-free diffusion length of interstitial O2 exceeds approximately 1 microm below 900 degrees C, providing definite evidence that oxygen diffuses as interstitial molecules in a-SiO2.

Oxygen-excess amorphous SiO2 with 18O-labeled interstitial oxygen molecules

“…The subscript i reflects possible variation in k among different network oxygen sites, which can originate from the structural disorder in a-SiO 2 . For numerical handling, the network oxygen sites were separated into 2I+1 groups (|i|≦I and I=15 in this study) characterized by discrete k i values where logk i was equally spaced [15]. The variation of N i +N i ≡N i T with logk i was postulated to follow a Gaussian function peaked at i=0 [15].…”

“…To analyze the spatial distribution of isotopologues of interstitial O 2 in detail, the VSB spectrum was simulated by solving onedimensional differential equations for the diffusion and oxygen exchange of interstitial O 2 [15]. It was assumed that the exchange reactions are reversible second-order processes, where only one of two oxygen atoms in an interstitial O 2 molecule is replaced in each exchange event [11,21],…”

“…For numerical handling, the network oxygen sites were separated into 2I+1 groups (|i|≦I and I=15 in this study) characterized by discrete k i values where logk i was equally spaced [15]. The variation of N i +N i ≡N i T with logk i was postulated to follow a Gaussian function peaked at i=0 [15]. The full width at half maximum (FWHM) of the distribution in logk i (w=7.6) was determined from separate experiments performed for this type of glass at higher temperatures (≳700°C) at which the oxygen exchange is much more significant [22].…”

“…The diffusion of interstitial O 2 in a-SiO 2 can be regarded basically as permeation [3][4][5][6][7][8][9][10], however, exchange between interstitial O 2 and oxygen atoms in the a-SiO 2 network also takes place, particularly at high temperatures [11][12][13][14][15]. The activation energy for the oxygen exchange is~2 eV [13][14][15] and is much larger than that of the permeation (diffusion) of interstitial O 2 (~0.8-1.2 eV [3,8,9,[16][17][18]).…”

“…The activation energy for the oxygen exchange is~2 eV [13][14][15] and is much larger than that of the permeation (diffusion) of interstitial O 2 (~0.8-1.2 eV [3,8,9,[16][17][18]). Thus, the contribution of the oxygen exchange increases with an increase in temperature.…”