EPR and optical absorption studies of Cu ions in alkaline earth alumino borate glasses

Electron paramagnetic resonance (EPR) and optical absorption spectra of Cu²⁺ ions in alkaline earth alumino borate glasses doped with different concentrations of CuO have been studied. The EPR spectra of all the glasses exhibit the resonance signals, characteristic of Cu²⁺ ions present in axially elongated octahedral sites. The number of spins participating in the resonance has been calculated as a function of temperature for calcium alumino borate (CaAB) glass doped with 0.1 mol% of CuO. From the EPR data, the paramagnetic susceptibility (χ) was calculated at different temperatures (T) and from the 1/χ-T graph, the Curie temperature of the glass has been evaluated. The optical absorption spectra of all the glasses show a single broad band, which has been assigned to the ²B_1g → ²B_2g transition of the Cu²⁺ ions. The variation in the intensity of optical absorption with the ionic radius of the alkaline earth ion has been explained based on the Coulombic forces. By correlating the EPR and optical absorption spectral data, the nature of the in-plane σ bonding between Cu²⁺ ion and the ligands is estimated. From the fundamental ultraviolet absorption edges of the glasses, the optical energy gap (E_opt) and the Urbach energy (ΔE) are evaluated. The variation in E_opt and ΔE is explained based on the number of defect centers in the glass.     

EPR and optical absorption studies of Cr3+ ions in alkaline earth alumino borate glasses

Electron paramagnetic resonance (EPR) and optical absorption spectra of Cr³⁺ ions in Calcium alumino borate (CaAB) glasses have been studied. The EPR spectra exhibit weak resonance signal at g ≈ 4.50 and intense resonance signal at g ≈ 1.98. A sharp resonance signal at g ≈ 1.97 was also observed at lower concentrations of chromium. The concentration dependence of the linewidth of the resonance signal at g ≈ 1.98 suggests the formation of Cr³⁺ ion clusters by magnetic superexchange interactions. The temperature dependence of the peak to peak intensity and the linewidth of the resonance signal at g ≈ 1.98 suggests that the exchange interactions between Cr³⁺ ions in the present sample were antiferromagnetic in nature with Néel temperature, T _N = 233 K. From the number of spins participating in the resonance at g ≈ 1.98, the paramagnetic susceptibility (χ) was calculated at different temperatures (233–295 K). A plot of 1/χ and T was found to obey Curie-Weiss law with negative Curie temperature. By measuring the relative intensities of the resonance signal at g ≈ 1.98, at different temperatures, the value of antiferromagnetic coupling constant (J) has been estimated. The optical absorption spectrum of chromium doped CaAB glass exhibits four bands, characteristic of Cr³⁺ ions, in nearly octahedral symmetry. From the band positions, the crystal field splitting parameter, Dq and the Racah interelectronic repulsion parameters, B and C were evaluated. The optical band gap (E_opt) and the Urbach energy (ΔE) were calculated from the ultraviolet absorption edges.     

Cu2+ containing TeO2−B2O3−PbO glasses studied by EPR

EPR absorption spectra of Cu²⁺ were investigated in vitreous matrices of the system [70TeO₂-25B₂O₃-5PbO] over a wide range of concentrations. The detected absorptions are strongly dependent on the matrix composition. Fluctuations of the ligand field were evident in the Cu²⁺ environment. The concentration of the paramagnetic species that cause the EPR absorptions decreases considerably at the upper limit of the investigated compositional range.     

ESR of Cu2+and Tl2+ in thallium borate glasses

The electron spin resonance (ESR) and optical absorption spectra of Cu²⁺ were measured in thallium borate glasses in order to investigate the effects of glass transition temperature,T _g, upon the responses of cupric ion in alkali borate glasses. The ESR of Tl²⁺ induced by-ray irradiation was also obtained. An abrupt increase in the covalency of in-plane Cu²⁺-O-bonding was observed in the Tl₂O system as well as in the Na₂O system in a similar B₂O₃ composition range althoughT _g for the Tl₂O glasses has little dependency on the composition compared with the Na₂O glasses. The trend in the variation of the S-character of the Tl²⁺ unpaired electron with composition agreed with that of the covalency of in-plane Cu²⁺-O-bonding. The structure of the anion group present in Tl₂O glasses was also examined by laser reman spectroscopy.     

Electron spin resonance and optical absorption spectra of Cu2+ ions in Na2SO4-ZnSO4 glasses

Electron spin resonance (ESR) and optical absorption spectra of Cu²⁺ ions in Na₂SO₄-ZnSO₄ glasses have been studied. The ESR spectra of Cu²⁺ ion-doped glasses exhibited a pronounced peak atg=2.07 and a shallow quadruplet atg=2.35, the latter arising from the hyperfine splitting of g. ESR spectra of Cu²⁺ ion-doped glasses were also studied by varying the concentration of Cu²⁺ions, temperature and composition of the glasses. The optical absorption spectra exhibited a broad absorption band in the near infrared region, which is attributed to²B_1g²B_2g transition. By correlating the ESR and optical absorption data, the bonding orbital coefficients ² and ₁ ² for Cu²⁺ ions have been evaluated.     

EPR and optical absorption of Cr3+ in binary Na2O-B2O3 glasses

The optical absorption and EPR spectra of Cr³⁺ in binary sodium borate glasses have been studied as functions of chromium concentration and Na₂O/B₂O₃ ratio in the glass; the ligand field and EPR parameters have been calculated and were found to be independent of Cr₃₊ concentration in any particular glass. In low-alkali borate glasses (Na₂O = 11 or 14 mol %) a single symmetrical EPR line was observed with g _¦=g =1.984±0.001 corresponding to perfect octahedral symmetry of the Cr³⁺ ion in these glasses. With increasing Na₂O content of the glass, the EPR line becomes more asymmetric (characteristic two-peaked pattern); this has been explained as being due to axial elongation of the six co-ordinated Cr³⁺-complex in these glasses.     

Ultrasonic investigation and infrared absorption of some sodium borate glasses

Ultrasonic velocities and absorption measurements in sodium borate glasses containing Na₂O have been made using the pulse echo technique at 4, 5 and 6 MHz. Results showed an increase in velocity as the sodium oxide content (mol%) increased. The ultrasonic absorption results showed the presence of very high and well-defined peak which shifts its position to lower temperatures with increasing frequency. This suggests some sort of relaxation process and the activation energy of this process increased with increasing Na₂O concentration (mol%). The infrared absorption spectra of sodium borate glasses confirmed the results obtained by ultrasonic investigations.     

Spectroscopic properties of Pr3+ and Er3+ ions in lead-free borate glasses modified by BaF2

Lead-free oxyfluoride borate glasses singly doped with Pr³⁺ and Er³⁺ were prepared and next investigated using absorption and luminescence spectroscopy. In the studied glass system, barium oxide was substituted by BaF₂. Two luminescence bands of Pr³⁺ located at visible spectral region are observed, which correspond to ³P₀–³H₄ (blue) and ¹D₂–³H₄ (reddish orange) transitions, respectively. The luminescence bands due to ¹D₂–³H₄ transition of Pr³⁺ are shifted to shorter wavelengths, when BaO was substituted by BaF₂. Near-infrared luminescence spectra of Er³⁺ ions in lead-free borate glasses modified by BaF₂ correspond to ⁴I_13/2–⁴I_15/2 transition. Their spectral linewidths increase with increasing BaF₂ concentration. The changes in measured lifetimes of rare earth ions are well correlated with the bonding parameters calculated from the optical absorption spectra.     

Electron paramagnetic resonance of Cu2+ and VO2+ ions in phosphate glasses

Electron spin resonance (ESR) spectra ofx(CuO · V₂O₅ (1 –x) (Na₂O · P₂O₅) andx(CuO · 2V₂O₅) (1 –x) (Na₂O · P₂O₅) glasses for 0 x 40 have been studied at the X-band and at 300 K. It is found that forx 5, both Cu²⁺ and VO²⁺ are present, mostly as isolated species. Forx 10, broad resonance lines atg = 2.1524 forx(CuO · V₂O₅) (1 –x) (Na₂O · P₂O₅) and atg = 2.1448 forx(CuO · 2V₂O₅) (1 –x) (Na₂O · P₂O₅) are observed which may be mainly due to dipole-dipole type interaction between transition metal (TM) ions. Spin Hamiltonian parameters of TM ions have been calculated. Optical spectra of the sodium phosphate glasses doped with single TM ions have also been studied. The theoretical optical basicity, A_th, of these doped glasses has been calculated. It is found that for VO²⁺ ionsg ,g and increase whileA ,P and g _|/g decrease with increase in A_th. However, no significant change is observed in the spin Hamiltonian parameters of Cu²⁺ with the change in A_th.     

Optical absorption and EPR studies of borate glasses with PbCrO4 and Pb2CrO5 microcrystals

The optical absorption (in the 400–1000 nm region) and electron paramagnetic resonance (EPR) spectra of 50PbO-20Cr₂O₃-(30-y)B₂O₃-yAl₂O₃ glasses and those crystallized by suitable thermal treatments have been studied. The results are applied to account for the crystallization process, which revealed the crystalline products due to PbCrO₄ and Pb₂CrO₅, and crystallization yield was found to be drastically enhanced (by as much as 85 vol%) with Al₂O₃ additives. The addition of Al₂O₃ in the present glasses and/or the thermal treatments induce the optical absorption intensity in the visible region at the expense of the near infrared region and reflect a gradual decrease in the EPR linewidth, H, at g2. The chromium in these cases most likely exists in a thermodynamic equilibrium between Cr³⁺ and Cr⁶⁺ oxidation states and the Al₂O₃ favours the Cr³⁺Cr⁶⁺ nucleation transformation to accord with the above observations. In addition, the crystalline PbCrO₄ and Pb₂CrO₅ comprise CrO ₄ ^2– groups with the Cr⁶⁺ oxidation state of chromium. This presents a phenomenological correlation for a local symmetry similarity of the CrO ₄ ^2– groups in glass and the microcrystalline products and observation of an enhanced crystallization yield due to Pb₂CrO₅ crystallites in the glasses using Al₂O₃ additives. The chromium occupying the tetragonal CrO ₄ ^2– anion sites migrates rather easily from the glass (subject to the thermal treatment) to the growing nucleation sites of PbCrO₄/Pb₂CrO₅, whereas those Cr³⁺ usually occupying the octahedral sites, which are associated with characteristically large stabilization energy, hardly play a direct role in the crystallization.     

Ultrasonic velocity in sodium borate glasses

Ultrasonic velocities in sodium borate glasses are measured as a function of composition at a frequency of 10 MHz and at a temperature of 298 K by making use of the ultrasonic pulse echo overlap method. Elastic properties of these glasses are analysed in terms of the elastic internal energy due to deformation; elastic resistances of the network-former, B₂O₃, and the modifier, Na₂O, are obtained as a function of composition from the plot ofMV ² againstx ₂, whereM is the molar mass of sodium borate glasses,V the velocity of sound andx ₂ the mole fraction of Na₂O. The elastic resistances of B₂O₃ and Na₂O are as follows: (i) forx ₂<0.33, the elastic resistance of B₂O₃ is smaller than that of Na₂O; (ii) atx ₂=0.33, the elastic resistances of B₂O₃ and Na₂O are equal; (iii) forx ₂>0.33, the elastic resistance of B₂O₃ is greater than that of Na₂O; (iv) atx ₂≈0.15, the elastic resistances of B₂O₃ and Na₂O become respectively maximal and minimal; (v) atx ₂≈0.23, the elastic resistances of B₂O₃ and Na₂O become respectively minimal and maximal; (vi) abovex ₂=0.36, the elastic resistance of Na₂O becomes negative.     

Efficient stabilization of Cu+ ions in phosphate glasses via reduction of Cu2+ by Sn2+ during ambient atmosphere melting

Glasses containing substantial amounts of well-dispersed luminescent Cu⁺ ions are attractive materials for applications in solid-state lighting, photonic waveguides, and solar cells. Thus far, coming across a simple yet effective method for the preparation of such has remained elusive given the instability of Cu⁺ relative to Cu²⁺, especially for syntheses carried out under the oxidizing air atmosphere. In this work, high concentrations of monovalent copper ions are shown to be successfully incorporated in a high-solubility phosphate glass matrix by a simple melt-quench method. The traditional Cu²⁺ spectrophotometric analysis commonly utilized for liquid solutions is proposed herein for the solid-state material to estimate the reduction efficiency of Cu²⁺ during the material preparation process. Reproducibly, the use of relatively large quantities of copper(II) oxide with equal amounts of reducing agent tin(II) oxide (up to 20 mol%), together with the use of sucrose to assist as antioxidant during melting in air atmosphere, yields high-reduction efficiencies estimated at 98 %. Along with the optical absorption analysis, photoluminescence spectroscopy is employed in evaluating the emission properties of the glasses in connection to the Cu⁺ ions. Further, solid-state ³¹P nuclear magnetic resonance spectroscopy reveals the structural features of the glasses that support the remarkable stabilization of the Cu⁺ ions.     

Redox ratio and optical absorption of polyvalent ions in industrial glasses

The changes in glass structure and redox ratio, R (reduced ion to oxidized ion) of Mn²⁺-Mn³⁺, Cu⁺-Cu²⁺, Cr³⁺-Cr⁶⁺, Ni²⁺-Ni³⁺ and Co²⁺-Co³⁺ couples and optical absorption due to Mn³⁺, Cu²⁺, Cr³⁺, Ni²⁺ and Co²⁺ ions in industrial soda-lime-silica glass were investigated as a function of Na₂O concentration in the range 11–19 mol%. With increasing Na₂O concentration in the experimental glasses, the basicity, expressed as calculated basicity, Λ_cal, increased. ²⁹Si NMR and X-ray diffraction were used to investigate the structural change in glasses. The NMR spectra showed high non-bridging oxygens (NBOs) when the basicity of glass was increased. The results were interpreted to be due to the tetrahedral networks; Q ⁴ species were depolymerized by replacing the bridging oxygens (BOs) with NBOs to Q ³ species. These results confirmed the shift of broadening peaks of XRD patterns. The redox reactions of the Mn²⁺-Mn³⁺, Cu⁺-Cu²⁺ and Cr³⁺-Cr⁶⁺ couples shifted more toward their oxidized ions due to the oxygen partial pressure, p(O₂), during melting and the oxide ion activity, a _O2−, increased with increasing glass basicity. These changes caused the redox ratio of these ion couples to decrease. The Ni²⁺-Ni³⁺ and Co²⁺-Co³⁺ couples were assumed to be present only in the Ni²⁺ and Co²⁺ ions in these glasses, respectively. The optical absorption bands due to Mn³⁺, Cu²⁺, Cr³⁺, Ni²⁺ and Co²⁺ ions were also investigated. Their spectra occurred at constant wavelengths with different optical densities or intensities as a function of glass basicity. The increase in the intensities of the absorption bands of these absorbing ions, except for Cr³⁺ ion, at the maximum wavelength, depends not only on the ion concentration but also on the increase of polarizability of oxide (−II) species, α _oxide(−II), surrounding the ions. This value affected directly the extinction coefficients of the ions, ɛ _ion. The increase of ɛ _ion caused the colour of glasses appearing in high intensity. In the case of Cr³⁺ ion, the results were reversed such that the lower the concentration, the higher the intensities of colour.     

EPR study of Cu2+ in Na2O-NaF-B2O3-Bi2O3 glasses

Electron paramagnetic resonance (EPR) studies have been performed on the glass system (30–x) NaF-xNa₂O-50B₂O₃-20Bi₂O₃ doped with CuO as a paramagnetic probe. The calculated g values indicate that Cu²⁺ is in a tetragonally elongated octahedral co-ordination, and the Jahn-Teller character gives rise to anisotropic hyperfine structure. The spin-Hamiltonian parameters and -bonding parameter, ², are calculated. Varying the fluoride ion concentration and its effect on these parameters is also discussed.     

Optical absorption and fluorescence properties of Er3+ in sodium borate glass

Spectroscopic properties of Er³⁺ ions in sodium borate glass have been studied. The indirect and direct optical band gaps (E_opt) and energy level parameters (RacahE ¹, E² and E³), spin-orbit (ξ_4f) and configurational interaction (α)) are evaluated. Spectral intensities for various absorption bands of Er₃₊ doped sodium borate glass are calculated. Using Judd-Ofelt intensity parametersΩ ₂, Ω₄, Ω₆, radiative transition probabilities (A), branching ratios (β) and integrated absorption cross sections (Σ) are reported for certain transitions. The radiative lifetimes (τ_R) for different excited states are estimated. From the fluorescence spectra, the emission cross section (σ_P) for the transition,⁴I_13/2 → ⁴I_15/2 is reported.     

Luminescence behavior of Dy ions in lead borate glasses

Dy-doped lead borate glasses were studied. The luminescence spectra showed two characteristic bands at 480 and 573 nm due to ⁴F_9/2–⁶H_15/2 (blue) and ⁴F_9/2–⁶H_13/2 (yellow) transitions of Dy³⁺. The yellow/blue luminescence of trivalent dysprosium was analyzed as a function of the B₂O₃/PbO ratios, the activator (Dy³⁺) and the PbX₂ (X = F, Cl, Br) content.     

Investigation of luminescence and spectroscopic properties of Nd3+ions in cadmium alkali borate glasses

Neodymium doped cadmium alkali borate glasses having composition 20CdO20R₂O59.5H₃BO₃0.5Nd₂O₃; (R = Li, Na and K) were prepared by conventional melt-quenching technique. The amorphous nature of the glasses was confirmed by X-ray diffraction studies. The physical properties such as density, refractive index, molar volume, rare earth ion concentration etc. were determined. Optical absorption and fluorescence spectra were recorded. The Judd-Ofelt theory was applied on the optical absorption spectra of the glasses to evaluate the three phenomenological intensity parameters Ω₂, Ω₄ and Ω₆. These parameters were in turn used to predict the radiative properties such as the radiative transition probability (A), radiative lifetime (τ_R) and branching ratio (β_R) for the fluorescent levels of Nd³⁺ ion in the present glass series. The lasing efficiency of the prepared glasses has been characterized by the spectroscopic quality factor (Ω₄/Ω₆), the value of which is in the range of 0.2–1.5, typical for Nd³⁺ in different laser hosts. The variation of Ω₂ with the change in alkali oxide has been attributed to the changes in the asymmetry of the ligand field at the rare earth ion site. The shift of the hypersensitive bands, study of the oscillator strengths and the variation of the spectral profile of the transition ⁴I_9/2 → ⁴F_7/2 + ⁴S_3/2 indicate a maximum covalency of NdO bond for glass with potassium ions. From the fluorescence spectra, peak wavelength (λ_p), effective line widths (Δλ_eff) and stimulated emission cross-section (σ_p) have been obtained for the three transitions ⁴F_3/2 → ⁴I_9/2, ⁴F_3/2 → ⁴I_{11/2 and} ⁴F_3/2 → ⁴I_13/2 of Nd³⁺ ion. The relatively high values of σ_p obtained for Nd³⁺ in present glass system suggest that these materials can be considered as suitable candidates for laser applications. The glass with potassium ions shows the highest value of the stimulated emission cross-section.