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,optical absorption and luminescence studies of Cr3+-doped antimony phosphate glasses

Antimony phosphate glasses (SbPO) doped with 3 and 6 mol% of Cr³⁺ were studied by Electron Paramagnetic Resonance (EPR), UV–VIS optical absorption and luminescence spectroscopy. The EPR spectra of Cr³⁺-doped glasses showed two principal resonance signals with effective g values at g = 5.11 and g = 1.97. UV–VIS optical absorption spectra of SbPO:Cr³⁺ presented four characteristics bands at 457, 641, 675, and 705 nm related to the transitions from ⁴A₂(F) to ⁴T₁(F), ⁴T₂(F), ²T₁(G), and ²E(G), respectively, of Cr³⁺ ions in octahedral symmetry. Optical absorption spectra of SbPO:Cr³⁺ allowed evaluating the crystalline field Dq, Racah parameters (B and C) and Dq/B. The calculated value of Dq/B = 2.48 indicates that Cr³⁺ ions in SbPO glasses are in strong ligand field sites. The optical band gap for SbPO and SbPO:Cr³⁺ were evaluated from the UV optical absorption edges. Luminescence measurements of pure and Cr³⁺-doped glasses excited with 350 nm revealed weak emission bands from 400 to 600 nm due to the ³P₁ → ¹S₀ electronic transition from Sb³⁺ ions. Cr³⁺-doped glasses excited with 415 nm presented Cr³⁺ characteristic luminescence spectra composed by two broad bands, one band centered at 645 nm (²E → ⁴A₂) and another intense band from 700 to 850 nm (⁴T₂ → ⁴A₂).     

Cu2+ ions in sodium fluoride-sodium borate glasses studied by EPR and optical absorption techniques

Electron paramagnetic resonance (EPR) and optical absorption spectra of Cu²⁺ ions in 80Na₂B₄O₇-(20 – x)NaF – xCuO (NFNB) glass system with 0 x 6 mol% have been studied. EPR spectra of all the glass samples exhibit resonance signals characteristic of Cu²⁺ ions. The values of spin-Hamiltonian parameters indicate that the Cu²⁺ ions in sodium fluoride-sodium borate (NFNB) glasses were present in octahedral sites with tetragonal distortion. The number of spins (N) participating in resonance was calculated as a function of temperature for NFNB glass sample containing 1 mol% of Cu²⁺ ions and the activation energy was calculated. From the EPR data, the paramagnetic susceptibility () was calculated at various temperatures and the Curie constant was calculated from the 1/ – T graph. The optical absorption spectra of these samples show a broad absorption band centered at 13280 cm^–1 which is assigned to the ² B _1g ² B _2g transition of Cu²⁺ ions in distorted octahedral sites. The optical band gap energy (E _opt) and Urbach energy (E) are calculated from their ultraviolet edges. It is observed that as the copper ion concentration increases, E _opt decreases while E increases. This has been explained as due to the creation of additional localized states by CuO, which overlap and extend in the mobility gap of the matrix. By correlating the EPR and optical data, the molecular orbital coefficients 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.     

EPR and optical absorption studies of Cr3+ doped lithium potassium sulphate single crystals

X-band electron paramagnetic resonance (EPR) studies of Cr³⁺ doped lithium potassium sulphate single crystals have been done at room temperature. The Cr³⁺ crystal field and spin Hamiltonian parameters have been evaluated by employing resonance line positions observed in the EPR spectra for different orientations of external magnetic field. The evaluated g, D and E values are: g_x = 2.0763 ± 0.0002, g_y = 1.9878 ± 0.0002, g_z = 1.8685 ± 0.0002 and D = 549 ± 2 × 10^?4 cm^?1, E = 183 ± 2 × 10^?4 cm^?1. Using EPR data the site symmetry of Cr³⁺ ion in the crystal is discussed. Cr³⁺ ion enters the lattice substitutionally replacing K⁺ site. The optical absorption study of the single crystal is also done in 195–925 nm wavelength range at room temperature. By correlating optical and EPR data the nature of bonding in the crystal is discussed. The calculated values of Racah parameters (B and C), crystal field parameter (D_q) and nephelauxetic parameters (h and k) are obtained as: B = 697, C = 3247, D_q = 2050 cm^?1, h = 1.146 and k = 0.21.     

Luminescence behavior of Dy3+ 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.     

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.     

Fluorescence properties of Eu3+-doped alumino silicate glasses

Alumino silicate glasses of a very broad range of molar compositions doped with 1 ⋅ 10²⁰ Eu³⁺ cm⁻³ (about 0.2 mol% Eu₂O₃) were prepared. As network modifier oxides Li₂O, Na₂O, K₂O, MgO, CaO, SrO, BaO, ZnO, PbO, Y₂O₃ and La₂O₃ have been used. All glasses show relatively broad fluorescence excitation and emission spectra. For most glasses only a weak effect of the glass composition on the excitation and emission spectra is observed. Although the glasses should be structurally similar, notable differences are found for the fluorescence lifetimes. These increase steadily with decreasing mean atomic weight, decreasing refractive index and decreasing optical basicity of the glasses, which may be explained by local field effects. An exception from this rule are the strontium, barium and potassium containing glasses, which show significantly increased fluorescence lifetimes despite of their high refractive index, optical basicity and molecular weight. The non mono-exponential fluorescence decay curves as well as the fluorescence spectra indicate a massive change in the local surroundings of the doped rare earth ions for these glasses.     

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.     

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.     

Electron Paramagnetic Resonance and optical absorption spectra of Cr(III) ions in alkali cadmium borosulphate glasses

The Electron Paramagnetic Resonance (EPR) spectrum at 9.205 GHz and at 300 K has been examined for a series of Cr³⁺ ions doped alkali cadmium borosulphate [R₂SO₄-CdSO₄-B₂O₃ (R=Li,Na,K or Cs)] glasses. The low field EPR spectral resonance lines are attributed to isolated Cr³⁺ ions where as the high field spectral resonance lines are attributed to exchange coupled pairs of Cr³⁺ ions. The absorption spectra of Cr³⁺ ions ions in these glasses show two broad bands characteristic of Cr³⁺ ions in octahedral symmetry. The bands are slightly shifted to higher wavelength side from Li glass to Cs glass. From the optical spectra the crystal field parameter Dq and the Racah interelectronic repulsion parameters B and C are evaluated. From the EPR and optical data, it is concluded that, the site symmetry around the Cr³⁺ ions is octahedral and the nature of bonding is predominantly covalent in nature.     

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.     

The optical absorption edge of rare earth sesquisulfides and alkaline earth - rare earth sulfides

Optical band gaps were measured by diffuse reflectance spectroscopy on a series of Ln₂S₃ compounds of α, δ, ε and τ structure types and on a series of ternary sulfides MLn₂S₄ (M = Sr, Cd, Mg) of Th₃P₄, CaFe₂O₄ and spinel structure types. Sharp absorption edges are observed with band gaps ranging from 2.2 to 3.5 eV. Systematic changes occur with rare earth radii. Analysis of some of the absorption edges suggests direct forbidden transitions.     

Physical,optical and structural studies of copper-doped lead oxychloro borate glasses

Bluish coloured glasses are obtained from the composition PbCl\(_{2}\)–PbO–B\(_{2}\)O\(_{3}\) doped with Cu\(^{2+}\) ions. Basic physical properties and spectroscopic studies (optical absorption, electron paramagnetic resonance, Fourier transform infrared and Raman spectroscopies) were carried out on these samples. The increase in PbCl\(_{2}\) content resulted in the decrease in density and increase in molar volume. At optical frequencies, band gaps and Urbach energies were evaluated and their variation is explained. Spin-Hamiltonian parameters (SHP) obtained from the EPR spectra suggest that the ligand environment around Cu\(^{2+}\) is tetragonally distorted octahedral sites and the orbital \(d_{x^{2}-{y}^{2}} \) is the ground state. The characteristics broad bands in the optical absorption spectra are assigned to the \(^{2}\)B\(_{\mathrm{1g}}\,\rightarrow \, {}^{2}\)B\(_{\mathrm{2g}}\) transition. The bonding coefficient values were evaluated using optical data and SHP. FTIR studies suggested that the glass structure is built up of BO\(_{3}\) and BO\(_{4}\) units. The presence of diborate, pyroborate, pentaborate groups, etc. in the glass network was confirmed from Raman spectra.     

Position of the optical absorption edge of alkaline earth borates

Based on the results of luminescent measurements on MB₂O₄ (M = Ca, Sr), Ca₂B₅O₉Cl, SrB₆O₁₀, SrAl₂B₂O₇ and the literature data, the influence of the crystal structure of alkaline earth borates on the position of their optical absorption edge is discussed. It is found that the optical absorption edge tends to shift to higher energy from β-BaB₂O₄ (6.4 eV) to M₃(BO₃)₂, MB₂O₄ (6.9–7.2 eV) to SrB₆O₁₀ (7.7 eV), SrB₄O₇ (>7.8 eV). It is shown that this tendency can be interpreted as a result of (a) the removal of M²⁺ states from the top of the valence band; (b) a more efficient stabilization of the O 2p states by B–O covalent bonding with increasing condensation degree of the borate anions.     

Dispersion and thermal properties of lithium aluminum silicate glasses doped with Cr3+ ions

A series of new lithium aluminum silicate (LAS) glass systems doped with chromium ion is prepared. The reflectance and transmittance of the glass slabs are recorded. By means of an iteration procedure, the glass refractive index n and the extinction coefficient k and their dispersions are obtained. Across a wide spectral range of 0.2-1.6 microm, the dispersion curves are used to determine the atomic and quantum constants of the prepared glasses. These findings provide the average oscillator wavelength, the average oscillator strength, oscillator energy, dispersion energy, lattice energy, and material dispersion of the glass materials to be calculated. For optical waveguide applications, the wavelength for zero material dispersion is obtained. Dilatometric measurements are performed and the thermal expansion coefficient is calculated to throw some light on the thermo-optical properties of the present glasses correlating them with their structure and the presence of nonbridging oxygen ions.     

Luminescence properties of Cr3+ doped borate glasses: Influence of crystal field strength and microcrystallite formation

The luminescence spectra of a diversity of chromium doped borate glasses were analysed. In these glasses the dopant ions are subject to a high average crystal field. For a particular set of ions the field is high enough to cause the lowest excited state to be the ²E level and high quantum efficiencies could be expected. However, there is a strong destructive influence of the nearby ⁴T₂ level.In a ZnO containing glass microcrystallites were optically identified as ZnCr₂O₄ spinels. In the formation of these crystallites Al₂O₃ plays an important role.     

Electrical conductivity and optical absorption studies on NiPc-substituted borate glass matrix

Electrical conductivity and optical absorption of nickel phthalocyanine (NiPc)-substituted borate glass is reported here for the first time. We have successfully prepared NiPc-substituted lithium borate glass at 850 °C and some preliminary studies on this glass are reported. Activation energy and optical band gap were determined for the glass from the Arrhenius plot of conductivity and optical absorption spectra, respectively.     

Thermodynamics of Cr3+-Cr6+ equilibrium in borate melts

The Cr³⁺-Cr⁶⁺ equilibrium was studied in lithium and sodium aluminoborate melts in an air atmosphere at different temperatures. A mechanism of the Cr³⁺-Cr⁶⁺ redox reaction in glass melts has been suggested. Enthalpy and entropy of the reaction in molten borates were calculated and the results thus obtained have been discussed thermodynamically.