The spectroscopic properties, structure and interconversions of optically active oxygen-deficiency-related point defects in vitreous silica are reviewed. These defects, the E′-centers (oxygen vacancies with a trapped hole or 3-fold-coordinated silicons), different variants of diamagnetic `ODCs' (oxygen-deficiency centers), and their Ge-related analogs play a key role in the fiber-optic Bragg grating writing processes. The controversy surrounding the structural models for the Si- and Ge-related ODCs is discussed and the similarity between the bulk and surface point defects in silica is emphasized. The possible interconversion mechanisms between 2-fold-coordinated Si, neutral oxygen vacancies and E′-centers are discussed. The effects of glassy disorder have a profound effect on defect properties and interconversion processes in silica.

Optically active oxygen-deficiency-related centers in amorphous silicon dioxide

This paper is a comprehensive review of the state-of-knowledge in the field of radiation effects in glasses that are to be used for the immobilization of high-level nuclear waste and plutonium disposition. The current status and issues in the area of radiation damage processes, defect generation, microstructure development, theoretical methods and experimental methods are reviewed. Questions of fundamental and technological interest that offer opportunities for research are identified.

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Radiation effects in glasses used for immobilization of high-level waste and plutonium disposition

Luminescence centers causing optical absorption bands in the 5 eV region in oxygen-deficient pure (‘Si-doped’), as well as in Ge- and Sn-doped glassy SiO2 were studied. The spectral and particularly kinetic data show that in all cases luminescence centers with closely similar properties exist which constitute an isoelectronic series. They are attributed to defects, formed by electrically neutral twofold coordinated Si, Ge, and Sn atoms (T20 centers) in glassy SiO2. Twofold coordinated Si causes the characteristic ‘B2’ optical absorption band at 5.03 eV. Upon reaction with atomic hydrogen, T20 centers convert to the well known ‘H(I)’, ‘H(II)’ and ‘H(III)’ ESR-active defects. The alternative models for these defects are discussed.

Isoelectronic series of twofold coordinated Si, Ge, and Sn atoms in glassy SiO2: a luminescence study

Overview of radiation induced point defects in silica-based optical fibers

The disposal of radioactive “waste” generated by the nuclear fuel cycle is among the most pressing and potentially costly environmental problems of the 21st century, a heritage from the Atomic Age of the 20th century Proposed disposal strategies are complicated, not only because of the large volumes and activities of waste, but also because of the political and public-policy issues associated with the long times required for containment and disposal (104 to 106 years) The development and use of highly durable waste forms, materials that have a high chemical durability and resistance to radiation damage effects, can simplify the disposal strategy (Ewing 2001)

An interest in phosphate-based waste forms has developed because the high-level waste generated by reprocessing of spent nuclear fuel can contain substantial amounts of phosphates (up to 15 wt % P2O5) that result from processing technologies that utilized either a bismuth phosphate or tributylphosphate process (Bunker et al 1995) In addition to the high phosphate content, other metal oxides may achieve significant proportions (up to 15 wt % Fe2O3; up to 30 wt % Bi2O3; up to 30 wt % UO2) (Lambert and Kim 1994)

These complex compositions have presented special challenges in developing crystalline ceramics that can accommodate the full compositional range of the waste streams The early work on phosphate glasses led to the idea that crystalline phosphates might make extremely durable waste forms, particularly for actinides The earliest suggestion was for the use of monazite (Boatner 1978, Boatner et al 1980, McCarthy et al 1978, 1980) The attractive qualities of monazite as a nuclear waste form are: (1) a high solubility for actinides and rare earths (10 to 20 wt %); (2) evidence from natural …

Phosphates as Nuclear Waste Forms

The effects of ArF excimer laser irradiation on dehydrated high-purity silica glass were investigated on both the optical-absorption bands due to oxygen-deficient centers (ODC) and the formation of ${E}^{\ensuremath{'}}$ centers. With an intense uv flux from an excimer laser, an ${E}^{\ensuremath{'}}$-center density of the order of ${10}^{15}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ was created. The 7.6-eV absorption band remains at the original level, while the 5.0-eV absorption band having the 4.3-eV emission band decreased. Both bands were reduced by heat treatment in an ${\mathrm{O}}_{2}$ atmosphere. These results suggest that there exist two types of ODC: ODC(I), which is responsible for the 7.6-eV band, and ODC(II), for the 5.0-eV band. The concentrations of ODC(I) and ODC(II) were evaluated to be 1 \ifmmode\times\else\texttimes\fi{} ${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ through an analysis of the gas treatment data and of the order of ${10}^{14}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ through the growth curve of the ${E}^{\ensuremath{'}}$ centers, respectively. The structural origin of ODC(I) has been attributed to the Si-Si bond, judging from the fact that the peak energy and cross section of the absorption were in close agreement with those of the ${\mathrm{Si}}_{2}$${\mathrm{H}}_{6}$ molecule. Theoretical calculations on defect energy levels by O'Relly and Robertson supported the structural model for IDC(I) and suggested an unrelaxed oxygen vacancy for ODC(II). These assignments were also consistent with the results of a quenching experiment in which the fictive temperature of samples was changed; the equilibrium between concentrations of ODC(I) and ODC(II) shifted in a reasonable manner.

Two types of oxygen-deficient centers in synthetic silica glass

We have found a decrease in the intensity of the 7.6-eV band and the appearance of the Si-H band upon heating ${\mathrm{SiO}}_{2}$ glasses, which were prepared by a method involving processes of dehydration with ${\mathrm{Cl}}_{2}$ gas, in ${\mathrm{H}}_{2}$ gas. The absorption cross section of the 7.6-eV band evaluated from these changes is 7.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}17}$ ${\mathrm{cm}}^{2}$, which is close to that of a band centered at 7.56 eV of the ${\mathrm{Si}}_{2}$${\mathrm{H}}_{6}$ molecule containing a Si-Si bond. These results give experimental evidence for the Si-Si bond model of the 7.6-eV band in unirradiated ${\mathrm{SiO}}_{2}$ glass.

Experimental evidence for the Si-Si bond model of the 7.6-eV band in SiO2 glass.

Dependence of defects induced by excimer laser on intrinsic structural defects in synthetic silica glasses.

Generation of typical paramagnetic centers by \ensuremath{\gamma} irradiation was studied for various kinds of synthetic silica glasses. Growth behavior of E' centers and nonbridging oxygen hole centers (NBOHC's) with \ensuremath{\gamma}-ray dose depends on contained preexisting point defects. By irradiation at room temperature, E' centers grow linearly and show a saturating tendency in silicas that contain precursors for E' centers, such as oxygen-deficient centers (ODC's), Si-H bonds or Si-Cl bonds. In silicas that contain very few precursors for E' centers, the growth of E' centers with the dose substantially equals that of NBOHC's and both follow a sublinear growth law in which the concentration of the defects is proportional to the square root of the dose. These results lead to the conclusion that \ensuremath{\gamma} irradiation fundamentally creates defect pairs of an E' center and an oxygen hole center from an intrinsic Si-O network of amorphous silica; this is in addition to a large amount of E' centers induced from preexisting point defects as precursors. By irradiation at 77 K, E' centers and self-trapped holes are suggested to be formed as the defect pair from the Si-O network, while the conversion of the precursors into E' centers is probably lessened. Concentration of E' centers generated by \ensuremath{\gamma} irradiation is almost the same as that of irradiation of ultraviolet lasers and x rays if they are compared based on the absorbed energy. Thus, defect generation with \ensuremath{\gamma} rays is suggested to involve similar fundamental processes as those with photons, which have much smaller energy than \ensuremath{\gamma} rays.

Generation of E' centers and oxygen hole centers in synthetic silica glasses by γ irradiation

The properties and concentrations of oxygen-deficient type structural defects in type III SiO 2 glasses implanted with Ti + , Cr + , Mn + , Fe + , or Cu + to doses from 0.5×10 16 to 6×100 16 ions/cm 2 at an energy of 160 keV have been measured by using vacuum UV and EPR spectroscopies. An intense absorption band centered around 7.5 eV is observed in all the samples except for Cu-implanted ones and is attributed primarily to SiSi homo-bonds with the bond distance close to that of the Si 2 H 6 molecule. The homo-bond and implanted ion concentrations are of the same order of magnitude in the implanted layers. An E′ type center associated with the homo-bond is observed in all the samples except for Cu-implanted ones. Anomalous behaviors of the Cu-implanted samples are attributed to the formation of Cu-colloids. An enhanced formation of metallic particles or colloids is suggested for the samples implanted with Cr, Mn or Fe to doses higher than ∼3×10 16 ions/cm 2 .

Hiroshi Imagawa

Papers

Two types of oxygen-deficient centers in synthetic silica glass

Experimental evidence for the Si-Si bond model of the 7.6-eV band in SiO2 glass.

Dependence of defects induced by excimer laser on intrinsic structural defects in synthetic silica glasses.

Generation of E' centers and oxygen hole centers in synthetic silica glasses by γ irradiation

Formation of oxygen-deficient type structural defects and state of ions in SiO2 glasses implanted with transition metal ions