Structure and charge transfer dynamics of uranyl ions in boron oxide and borosilicate glasses

Laser spectroscopic experiments, molecular dynamics simulation, and charge transfer-lattice interaction modeling have been conducted for studying the electronic and structural properties of the uranyl ion UO ₂ ²⁺ in boron oxide and borosilicate glasses. The charge transfer electronic and vibrational energy levels for uranyl ions in the glass matrices were obtained from laser excitation and fluorescence spectra of UO ₂ ²⁺ . A model structure for uranyl ions in the glass matrices was established using the method of molecular dynamics (MD) simulation in comparison with the results of extended x-ray absorption fine structure (EXAFS) for U⁶⁺ ions in the glasses we studied. The formation and stabilization of uranyl clusters in glass matrices are interpreted in terms of charge transfer-lattice interactions on the basis of self-consistent charge transfer accompanied by lattice distortion. The latter is in the framework of the simultaneous action of pseudo-Jahn-Teller and pseudo-Jahn-Teller analog effects on charge transfers between oxygen and uranium ions.     

237Np Mössbauer spectroscopy on neptunium doped borosilicate glasses

Mössbauer and magnetization measurements have been carried out on borosilicate glasses containing neptunium. Different melting and annealing times, in various crucibles have been investigated. Using the 59.5 keV Mössbauer resonance in²³⁷Np we have characterized valency states of neptunium ions in glasses and studied hyperfine interactions. Roughly, absorption spectra show two sites easily differenciated above 50 K by two lines. Isomer shifts (relative to NpAl₂) are respectively: – 10.0 (1.0) and + 32.5(.5) mm/s. These resonance lines can be assigned, the first at Np⁴⁺, the second at Np³⁺ species. The intensity ratio Np⁴⁺/Np³⁺ varies with elaboration conditions. At 4.2 K, Np⁴⁺ ions exhibit a hyperfine splitting arising from relaxation phenomenon. Magnetization measurements do not show any magnetic ordering.     

Ultrasonic loss in sodium borosilicate glasses

A broad absorption peak dominates the ultrasonic loss in sodium borosilicate glasses below room temperature; a subsidiary peak occurs at liquid helium temperatures. The broad peak and its counterpart in vitreous silica are shown to be consistent with a structural relaxation process rather than an Akhieser type phonon-phonon interaction.     

Microstructural changes induced in borosilicate glasses by 1 MeV electrons

Radiation damage to borosilicate glass has been studied using a high voltage electron microscope to simultaneously generate and image structural changes At low doses and dose rates (φ ? 5 × 10²⁴ electrons m^-2, F ? 5 × 10²² el m^-2 s^-1), ionic depletion generates a new crystalline phase rich in S₁O₂ For incubation doses exceeding 5 × 10²⁴ electrons m^-2, gas bubbles are observed High damage rates are necessary for bubble nucleation though not for their subsequent growth The critical nucleation flux increases rapidly with irradiation temperature, whereas the gas precipitation efficiency remains constant above 300 K.     

Diffusion of sodium ions in borosilicate glasses by molecular dynamics method

Molecular dynamics simulations of borosilicate glasses with different sodium oxide content has been carried out. The generated structure indicates that the major number of oxygen atoms introduced as Na₂O consumes in the borate network to convert the BO₃ triangles to BO₄ tetrahedra. The formation of nonbridging oxygen ions in the silica site occurs during the formation of BO₄ tetrahedra with low rate. At higher concentrations of Na₂O, the NBO's increases abruptly. The self diffusion of sodium ions depends essentially on the temperature, composition (number of Na⁺ ions) and the rate of cooling. The phase separation mechanism in borosilicate glasses can be detected by MD simulation since the environment of the B–O, Si–O and B–O–Si can be easily reached.     

Optical properties of II-VI semiconductor nanocrystals produced by sol-gel synthesis in sodium borosilicate glasses

Size quantization effects on excitonic optical transitions are studied by optical absorption experiments on sodium borosilicate glasses doped with CdS, CdSe or CdTe nanocrystals. The original sol-gel synthesis procedure involves the preparation of complex cadmium-based molecules in an aqueous solution to which other inorganic precursors are added, in order to start the gelation of the vitreous matrix. The formation of semiconductor crystallites is achieved by a redox reaction, during the thermal degradation of the gel, which is then melted and quenched to provide the glass. The effects of thermal annealing, at temperatures around and above the T_g of the glass, are investigated by spectroscopic measurements. The latter are analysed by using a calculation of excitonic energies and oscillator strengths, based on the envelope-function approximation, including the confinement-induced mixing of valence-band states. Our studies show that disorganised clusters with a rather narrow size-distribution, controlled by initial conditions, are present before the annealing, which rapidly improves the crystalline quality. Meanwhile, the width of the size-distribution function is reduced down to an optimum, after a few hours of treatment.     

Raman analysis of perrhenate and pertechnetate in alkali salts and borosilicate glasses

Sodium borosilicate glasses containing rhenium or technetium were fabricated and their vibrational spectra studied using confocal Raman microscopy. Glass spectra were interpreted relative to new high‐resolution spectra of pure crystalline NaReO₄, KReO₄, NaTcO₄, and KTcO₄ salts. Spectra of perrhenate and pertechnetate glasses exhibited sharp Raman bands, characteristic of crystalline salt species, superimposed on spectral features of the borosilicate matrix. At low concentrations of added KReO₄ or KTcO_4, the characteristic pertechnetate and perrhenate features are weak, whereas at high additions, sharp peaks from crystal field‐splitting and C_4h symmetry dominate glass spectra, clearly indicating ReO₄⁻ or TcO₄⁻ is locally coordinated with K and/or Na. Peaks indicative of both K and Na salts are evident in many Raman spectra, with the Na form being favored at high concentrations of the source chemicals, where more K⁺ is available for ion exchange with Na⁺ from the base glass. The observed ion exchange likely occurred within depolymerized channels where nonbridging oxygens create segregation from the glass network in regions containing anions such as ReO₄⁻ and TcO₄⁻ as well as excess alkali cations. Although this anion exchange provides evidence of chemical mixing in the glass, it does not prove the added salts were homogeneously incorporated in the glass. The susceptibility to ion exchange from the base glass indicates that long‐term immobilization of Tc in borosilicate glass must account for excess charge compensating alkali cations in melt glass formulations. Published 2014. This article is a U. S. Government work and is in the public domain in the USA.     

Piezooptical effects in the tellurite glasses doped by europium and gold

We present the temperature dependence of piezooptical coefficients for three samples of TeO₂-GeO₂-PbO glasses doped with 0.5% of Eu₂O₃, 0.5% and 1% of Au₂O₃, after different thermoannealing times. We have established that there exist two temperatures singularities - minima in the range 655-695 K and maxima - at 850 K. It is crucial that for the glasses annealed during 61 h, at temperatures about 850 K, the anomaly of piezooptical coefficient disappears. Simultaneously the minima within the range 655-695 K changed depending on the duration of the thermoannealing which leads to low temperature shift of the minima. Towards lower temperature the piezooptical maxima occurs around 850 K and disappears after the increase of the annealing time. It is also crucial that the values of the piezooptical coefficients decrease with the enhancement of the thermoannealing. The observed temperature dependence with the piezooptical coefficients has a good correlation with the temperature dependences of the DSC. We have found that the pure glasses and glasses doped only by Au₂O₃ and Eu₂O₃ possess the piezooptical coefficients one order less with respect to the samples possessing simultaneously Au₂O₃ and Eu₂O₃.     

二维胶体玻璃中玻色峰与结构无序度的关联

本文从实验上研究了胶体玻璃在相同面密度下随着体系结构无序程度的增加, 振动态密度和玻色峰的变化规律. 通过调制两种不同粒径的温敏性水凝胶的数量比来改变体系的无序程度. 通过分析无序体系的声子模式得到体系的振动特性. 研究发现, 随着无序度的增加, 态密度在低频区域增强、玻色峰增高、玻色峰的峰值向低频区域移动. 不同无序程度的样品引起玻色峰的低频声子模式都表现出准局域的特点, 且低频准局域声子模式与样品中无序结构存在关联.     

Exciton photoluminescence of ZnSe and CdS quantum dots in borosilicate glasses prepared by the sol-gel method

The spectra of exciton photoluminescence (PL) of ZnSe and CdS quantum dots (QDs) synthesized in borosilicate glasses by the sol-gel method are measured and analyzed. It is shown that the positions of the PL bands in the spectra of both systems are related to the spatial quantization of the energy of excitons in QDs. Significant differences in the conditions of this process in ZnSe and CdS QDs are revealed. It is ascertained that, at some critical concentration of the semiconductors (x _cr), exciton percolation levels are formed in both systems, which manifest themselves in an abrupt change in the shape and spectral position of the emission bands of both systems and the constancy of the noted band parameters with a further increase in the concentration. The values of the critical and mean radii of QDs in the stage of their Ostwald ripening are obtained for several concentrations of the doping semiconductors.     

Development of a system for measuring the complex impedance of borosilicate glasses at high pressures and temperatures: Application to the study of Li- and Na-doped borosilicate glasses

An apparatus for measuring the complex impedance of samples with high impedances is described. Complex impedance spectra were collected from a range of borosilicate glasses of composition (B₂O₃)₄(Li₂O)(LiBr)_x(NaBr)_1−x at pressures and temperatures ranging from 1 to 5 GPa and 350 to 450 °C, respectively. These data were used to determine AC conductivities and activation energies in order to test the Modified Random Network model of glass structure. Our results are in line with the predictions of this theory.     

Transient electrical transport in pure and doped chalcogenide glasses

Transient hole transport measurements in undoped As₂Se₃ and Se and in doped As₂Se₃ are explained without recourse to the presence of deep traps. We suggest that the conduction process involves small polarons as originally proposed by Emin. Correlations between the measured activation energies and the dispersion in the Se and As₂Se₃ systems as well as the dependences of the drift mobilities and the dc mobilities on doping in As₂Se₃ are explained.     

Mode-locking of neodymium lasers by glasses doped with PbS nanocrystals

Nonlinear optical absorption properties of silicate glasses doped by the PbS nanocrystals were investigated by means of the picosecond pump-probe absorption spectroscopy and absorption bleaching technique. The glass samples were used as saturable absorbers for passive mode-locking of Nd:YAG and Nd:glass lasers. Application of the PbS doped glass together with the active mode-locking and laser cavity quality control enabled the stable of generation of 28 ps and 5 ps duration pulses in Nd:YAG and Nd:glass lasers respectively. PACS 42.60.Fc; 42.70.Hj; 78.47.+p     

Lateral transport in boron doped SiGe quantum wells

The temperature dependence of lateral conductivity and hole mobility in boron doped Si/SiGe/Si quantum well structures were studied. The conductivity at the temperatures below 20 K is shown to be due to hopping over B centers while at higher temperatures it is due to two-stage excitation consisting of thermal activation of holes from the ground to strain-split B states and the next hole tunneling into the valence band.     

Ac conductivity in boron doped amorphous carbon films

Experimental results on frequency and temperature dependence of ac conduction in boron doped amorphous carbon films are analyzed in the framework of available microscopic models. Depending on the response, the conductivity plot is divided into three regimes (low frequency high temperature; moderate frequency intermediate temperature; high frequency low temperature) and the data in the respective regimes are corroborated with the various theoretical models accordingly. The conductivity data at high frequency and low temperature suggests that relaxation via quantum mechanical tunneling might be the dominant conduction mechanism. At intermediate temperatures and moderate frequencies, the conductivity data is in good agreement with extended pair approximation model with interaction correction. Signature of enhanced interaction effect is observed at low temperature.     

Probing of the spectral diffusion in doped glasses by fluctuations of dichroism

A method of studying spectral diffusion using temporal fluctuations of the dichroism of an isotropic sample is described. Unlike conventional methods of absorption spectroscopy of single molecules, this method is easier to realize, provides direct information about the distribution of two-level systems over the relaxation rate, and does not require that the absorption lines be Stark modulated in frequency. The latter fact means that this method is applicable to conducting matrices (e.g., for studying current-induced relaxations in polymer materials). In combination with the probing of spectral diffusion using fluorescence intensity fluctuations, this method allows one to determine the degree of correlation between the distributions of impurity centers over the relaxation rate and over the fluorescence quantum yield.