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UV light induced photoreduction in phosphate and fluoridephosphate glasses doped with Ni2+, Ta5+, Pb2+, and Ag+ compounds
The photoreduction of polyvalent ions was studied in high purity fluoride-phosphate and metaphosphate glasses doped with Ni2+ (3d8), Ta5+ (5d0), Pb2+ (6d2), and Ag+ (3d10). Compared to the undoped base glasses all doped samples display different electronic transitions in the UV at the irradiation wavelength. Glass samples containing 50 to 5000 ppm dopants were irradiated with excimer lasers at 193 and 248 nm, respectively. The subsequent defect centers, formed at ppm levels, were characterized by EPR and optical UV-VIS spectroscopy. The observed laser induced transmission losses in the UV and visible range increased in the order Ni, Ta, Pb to Ag. Extrinsic electron centers are formed by photoreduction of the dopants. (Ni2+)- is characterized by an optical transition with a maximum at 355 nm and an EPR signal around g ≈ 2.07. The maxima of the optical transitions of the (Pb2+)- -EC are positioned at 395 and 500 nm, of the (Ta5+)- -EC at 465 nm. The photoionization products of silver depend strongly on the silver concentration. At a silver content of 50 ppm only the (Ag+)- -EC is formed, visible in the optical spectra with a maximum around 450 nm. Α second silver species, (Ag+)2 -, which absorbs at 305 nm, is additionally observed in the sample doped with a silver concentration of 500 ppm. In the sample doped with 5000 ppm silver a third defect, the photooxidized (Ag+)+ -HC, with an optical band maximum at 405 nm and an EPR signal around g ≈ 2.3 is observed as well. The formation of extrinsic electron centers causes in all glasses an increase in the formation of intrinsic hole centers and often a decrease in the formation of intrinsic electron centers. Defect generation curves show that a very rapid darkening in the glasses is initiated by the addition of any of these dopants. The recovery rates of the defeets formed depend strongly on the dopant, not on the glass matrix.
Estimation of deep-uv and uv absorption coefficients of selected trace impurities in glasses
Glasses of the fluoride phosphate type were selected as suitable host materials for studying the uv absorption behavior of several transidon metal ions in different valence states (Fe²⁺/³⁺, Cu⁺/²⁺, Pb²⁺, Ni²⁺, Cr³⁺). Based on a large glass sample collecdon with series of transition metal-ion-doped fluoride phosphate glasses together with a carefully analytically determined real impurity content, the specific uv absorption spectra of the mentioned ions could be estimated using nonlinear deconvolution of bands.
Voltammetric investigations of the redox behaviour of Fe, Ni, Co and Sn doped glass melts of AR® and BK7® type
The redox behaviour of iron, nickel, cobah, and tin ions was studied in glass melts with compositions near AR® and BK7® type by means of square-wave voltammetry (SWV). From the linear dependence of the peak potentials on the temperature, the standard enthalpies, ΔH°, and the standard entropies, ΔS° of the transitions Fe3+/Fe2+, Co2+/Co0, Ni2+/Ni0, Sn4+/Sn2+ and Sn2+/Sn0 were calculated. The reduced states are formed in the AR glass melt already at less reducing conditions than in the BK7 melt. The dependence of the redox ratios upon temperature is larger in the AR melt. As colouring ion Co2+ is more advantageous than Ni2+ because of its higher stability against reduction. It is possible to reduce the Fe3+/Fe2+ ratio without formation of Co0 in a disturbing concentration.
Formation of radiation defects in Silicate and borosilicate glasses caused by UV lamp and excimer laser Irradiation
Solarization of colourless and coloured glasses of the type AR®, BK7® and DURAN® with high transmission in the UV-B region (280 to 320 nm) was investigated for their use as Container glasses in the solar disinfection of drinking water. The samples were coloured by doping with NiO or CoO. The influence of the Fe³⁺/Fe²⁺ content and of the reducing component Sn²⁺ was investigated on both the initial transmission of the glasses and the solarizadon. The samples were irradiated with a 1 kW Xe lamp and a XeCl excimer laser. The solarization depends on the glass matrix and is enhanced by colouring ions. The influence of Fe³⁺/Fe²⁺ and Sn²⁺ on the defect generation is different for glasses coloured with NiO than for those coloured with CoO. For the samples investigated it was possible to simulate the loss of transmission in the UV-B region due to long-time irradiation with sunlight by a laser experiment.
Α multi-method characterization of borosilicate glasses doped with 1 up to 10 mol% of Fe, Ti and Sb
The model glasses NBS1 and NBS2 are sodium borosilicate glasses of high intrinsic UV transmission. Although both glasses have an SiO2 content of 74 mol%, they possess different matrix structures due to varied Na2O / B2O3 ratios. Nonbridging oxygens occur in the NBS1 but not in the NBS2 glass. Fe, Ti and Sb oxides were added at concentrations of 1 and 10mol% to study valence, coordination and site distribution. XANES, Mössbauer, optical absorption, photoluminescence and EPR spectroscopy provided an insight into the structures and near range environments of the dopants. Large differences were found for the two Fe doped glasses. The presence of nonbridging oxygens in NBS1 glass leads to a higher solubility of the Fe ions and a higher ratio of tetrahedral over octahedral Fe3+ coordination while a clustering of Fe ions resulting in a lowered UV and VIS transmission is observed in NBS2 glass. In Ti doped glasses EPR and XANES spectroscopy shows that most Ti occurs as Ti4+, in four-, five- and sixfold coordination. Optical spectra of Ti4+ display high intensity charge transfer transitions in the UV region. Ti4+ photoluminescence in the visible range is of low intensity. Optical absorption, XANES and Mössbauer spectroscopy could only detect Sb3+ in the antimony doped glasses. The photoluminescence of Sb3+ is much stronger in the NBS1 than in the NBS2 glass.
Carbon crucible technology for optical glass melting
Fluoro crown and phosphate crown glasses are attractive candidates for application in the deep ultraviolet range. The calculated values for the ultraviolet resonance wavelenghts are comparable with those of CaF₂, BeF₂ and SiO₂. However, the experimental ultraviolet transmission of glass is frequently limited by extrinsic absorption due to trace impurities of metal ions, mainly iron, copper and lead. The influence of different technological parameters on the ultraviolet transmission and the glass quality was investigated, especially the effect of reducing melting conditions, by using a carbon crucible in a glove box. The ultraviolet absorption of the glass is strongly dependent on the redox conditions of the melt, the reducing temperature and time. Glasses melted under reducing conditions have a much higher transmission at 250 nm due to the shift of redox equilibria of polyvalent transition metal ions to the lower redox state and the fact that the molar absorption coefficients of the lower redox states are more than one order of magnitude lower than those of the higher redox state in the ultraviolet range. Undesirable side reactions that may occur under strong reducing melting conditions are described. Using carbon crucible technology for fluoro and phosphate crown glass melting it is possible to achieve high UV transmission values and a good inner glass quaUty.
Time resolved fluorescence measurements on Tb3+ and Mn2+ doped glasses
Tb3+ (4f8) and Mn2+ (3d5) ions, known as active luminescent centres for blue, green and red fluorescence, were doped in various fluoride, phosphate and Silicate glasses with well known structure. Narrow bands of f-f transitions with strong emission of Tb3+ in the blue, green and red and broad bands of d-d transitions of Mn2+ were measured with green emission in high optical basicity glasses with tetrahedrally coordinated Mn2+. Orange to red Mn2+ emission was found in glasses with low optical basicity where Mn2+ is octahedrally coordinated. Lifetimes, τe, in the range of milliseconds were recorded in dependence of glass composition and dopant concentration for both Tb3+ and Mn2+ doped glasses. Fluorescence lifetimes are as well shortened by higher basicity of the glasses as by increasing dopant concentration.
Influence of modifier cations on the radiation-induced effects of metaphosphate glasses
The influence of different modifying cations on the radiation-induced defect generation in metaphosphate glasses was studied by optical absorption and electron paramagnetic resonance spectroscopy. For this task, several model glasses were prepared. These were simple metaphosphates of alkaline earths, aluminum and zinc. The synthesized glass samples were irradiated with X-rays and with UV light from an Xe/Hg lamp. The cation-dependent differences in the structure of the phosphate glasses were found to have a great influence on the concentration of some radiation-induced defects and thus on the optical absorption. The influence of the different cations became especially distinct under UV light Irradiation due to the selective interaction of the UV light with the glass samples. Some additional phenomena were observed for the irradiated zinc and magnesium metaphosphate glasses which were not found by the previously undertaken study of complex phosphate glasses.
Optical spectroscopy of rare earth ions in glasses
The relationships between host glass composition and optical properties of rare earth ions were studied by means of absorption and emission spectroscopy. Eu³⁺ and Tb³⁺ were found to be appropriate indicator ions for determining the properties of the local environment around rare earth ions. Er³⁺ and Nd³⁺ ions are widely used in lasers and amplifiers. The knowledge of the compositional influence on the spectroscopic parameters of rare earth ions enabled the modeling of the emission properties of important Er³⁺ and Nd³⁺ transitions in order to figure out the Optimum host glasses. Fluoride phosphate and phosphate glasses are attractive candidates for lasers and amplifiers. In these glasses, the degree of covalency between the rare earth ions and surrounding ligands mainly affects the spectroscopic parameters of rare earth ions such as Judd-Ofelt parameters and electron-phonon coupling strength. The increase of the electron-phonon coupling strength with the phosphate content is responsible for the decrease of the Er³⁺ emission intensity at 540 nm. Otherwise, it increases the Er³⁺ emission intensity at 1.5 µm in the fluoride phosphate glasses. The lower microparameters of Nd³⁺ cross relaxation in phosphate glasses cause the higher lifetimes of the Nd³⁺ ⁴F₃/₂ laser State at higher Nd³⁺ concentrations with respect to fluoride phosphate glasses. The energy transfer to OH groups in phosphate glasses decreases the hfetime and emission intensity of the laser State of both Er³⁺ and Nd³⁺ ions.
Radiation-induced defects in CoO- and NiO-doped fluoride, phosphate, silicate and borosilicate glasses
The influence of cobalt and nickel on the formation of irradiation-induced defects was studied in fluoride, phosphate, silicate and borosilicate glasses. Sample plates of high-purity glasses, undoped and doped with 0.3 mol% CoO and NiO, respectively, were irradiated with UV lamps and with X-rays. The subsequent defect centers, formed at ppm levels, were characterized by EPR and optical UV-VIS spectroscopy. X-ray irradiation caused stronger solarization (excitation of inner electrons) than UV lamp irradiation (selective excitation of valence electrons). More defects were formed in doped than in undoped glasses, generally stronger for Co2+ - than for Ni2+ -doped glasses and especially strong in glasses of high optical basicity where Co2+ and Ni2+ were tetrahedrally coordinated. Co2+ was photooxidized to (Co2+)+ in all glasses, replacing some of the intrinsic hole centers (HC), with (Co2+)+ in tetrahedral coordination: charge transfer band < 400 nm, and (Co2+)+ in octahedral coordination: two bands between 300 and 450 nm. Ni2+ was photooxidized in the (boro-)silicate glasses, which all had a higher basicity, but was photoreduced in the fluoride-phosphate glasses of low basicity. Photoreduced (Ni2+)- was found in the phosphate glass of medium basicity only after X-ray irradiation. The photoionized nickel species also displayed distinct EPR signals, with (Ni2+)+: several bands from 700 to 200 nm, g=2.10; and (Ni2+)-: 330 nm, g1=2.08 and g2=2.26.