Microstructure, mechanical, EPR and optical properties of lithium disilicate glasses and glass-ceramics doped with Mn ions

The microstructure, mechanical, EPR and optical properties of transparent MnO₂ doped lithium disilicate (LDS) glass-ceramics prepared by melt quenching and controlled crystallization, have been studied. The microstructure of the glass-ceramics has been characterized using FE-SEM, TEM, FT-IR and XRD techniques. FE-SEM micrographs show elongated, highly interlocked, dense (∼80 vol.%) nanocrystals of LDS with an average size ∼100 nm. XRD and FT-IR studies reveal that the only crystalline phase formed after heat-treatment at 700 °C for 1 h is LDS. A good combination of average microhardness ∼5.6 GPa, high fracture toughness ∼2.8 MPa m^1/2, 3-point flexural strength ∼250 MPa and moderate elastic modulus 65 GPa has been obtained. The EPR spectra of both LDS glasses and glass-ceramics exhibit resonance signals with effective g values at g = 4.73, g = 4.10, g = 3.3, and g = 1.98. The resonance signal at g = 1.98 is found to be more intense than the other signals and exhibits hyperfine structure at lower concentration of manganese. From the observed spectrum, the spin-Hamiltonian parameters have been evaluated. In glass samples the optical absorption spectrum exhibits a broad band around ∼20,320 cm⁻¹ which has been assigned to the transition ⁶A_1g(S) → ⁴A_1g(G) ⁴E_g(G)-of Mn²⁺ ions. The cerammed samples upon 394 nm excitation emit a green luminescence (565 nm, ⁴T_1g → ⁶A_1g(G) transition of Mn²⁺ ions), and a weak red emission (710 nm). From the ultraviolet absorption edges, the optical bandgap energies (E_opt) were evaluated and are discussed.     

EPR and photoluminescence studies on lithium-potassium borophosphate glasses doped with Mn ions

The mixed alkali borophosphate xLi₂O–(30 − x)K₂O–35B₂O₃–34.5P₂O₅ (5 ≤ x ≤ 25) glasses doped with 0.5 mol% of manganese ions have been studied using EPR and photoluminescence techniques. The EPR spectra of all the investigated samples exhibit resonance signals which are characteristic of Mn²⁺ ions. The resonance signal with effective g value at g = 2.02 exhibits a six line hyperfine structure. The zero-field splitting parameter (D) at room temperature has been evaluated from the intensities of allowed hyperfine lines. The EPR spectra at different temperatures (123–295 K) have also been studied. It is found that the spin concentration (N) exhibits non-linear behaviour with composition showing the mixed alkali effect (MAE). From the EPR data, the paramagnetic susceptibility (χ) has been calculated at various temperatures. The Curie constant (C) and paramagnetic Curie temperature (θ_p) have been evaluated from the 1/χ versus T graph. The photoluminescence (PL) spectrum exhibits a green emission band at 582 nm and this band has been assigned to a transition from the upper ⁴T_1g → ⁶A_1g ground state of Mn²⁺ ions.     

Structural studies on iron-tellurite glasses prepared by sol-gel method

In this study, we report structural properties of the iron-tellurite glasses obtained using the sol-gel synthesis. The samples were characterized by X-ray diffraction, FTIR, UV-vis and EPR spectroscopy. Our results indicate dominant presence of iron ions in the trivalent state and the existence some Fe²⁺ ions. The analysis of the IR spectra indicates a gradual transformation of iron ions from tetrahedral into octahedral sites when the concentration of Fe(NO₃)₃ is increased beyond 0.64 mol%. EPR studies show that the increase of Fe(NO₃)₃ content in the host matrix induces the growth of the number of effective g values. This can be explained considering that the orbitals of O²⁻ ion with a large spin-orbit interaction constant will interact with the 3d orbital of Fe³⁺ ion bonded to this O²⁻ ion, thus leading to appearance of an orbital angular momentum which contributes to the magnetic moment of Fe³⁺ ion. A strong dipolar interaction, which is more predominant in a glass with higher content of Fe(NO₃)₃, causing a localized magnetic field along the site of the Fe³⁺ ions and the increase the effective g values.     

Dual role of the six-coordinated lead and copper ions in structure of the copper-lead-tellurate glasses

Glasses from xCuO·(100 − x)[4TeO₂·PbO₂] system where x = 0-40 mol% were studied by density measurements, FTIR, UV-vis and EPR spectroscopy in order to obtain information about the changes that appear in the structure of matrix glass with the doping of copper ions.The FTIR data analysis shows that the copper ions will be distributed in the six-coordinated interstices ([PbO₆] structural units) and some [TeO₄] structural units will be converted to [TeO₃] structural units because the lead ions have a strong affinity towards these groups containing non-bridging oxygens, with negative charge.From the EPR studies, we can conclude that Cu⁺² ions have an environment elongated along the z-axis and the ground state of the Cu⁺² is dx²−y² orbital (²B_1g state). When the concentration of CuO is increased beyond 5 mol%, the intensity and width of both the parallel and perpendicular hyperfine components are observed to increase.     

Synthesis, thermal stability and properties of [(Fe1−xCox)72Mo4B24]94Dy6 bulk metallic glasses

A series of [(Fe_1−xCo_x)₇₂Mo₄B₂₄]₉₄Dy₆ (x = 0.1, 0.2, 0.3, 0.4 and 0.5 at.%) bulk metallic glasses (BMGs) in rod geometries with critical diameter up to 3 mm were fabricated by copper mold casting method. This alloy system exhibited good thermal stability with high glass transition temperature (T_g) 860 K and crystallization temperature (T_x) 945 K. The addition of Co was found to be effective in adjusting the alloy composition deeper to eutectic, leading to lower liquidus temperature (T_l). The [(Fe_0.8Co_0.2)₇₂Mo₄B₂₄]₉₄Dy₆ alloy showed the largest supercooled liquid region (ΔT_x = T_x − T_g = 92 K), reduced glass transition temperature (T_rg = T_g/T_l = 0.622) and gamma parameter (γ = T_x/(T_g + T_l) = 0.424) among the present system. Maximum compressive fracture strength of 3540 MPa and micro-Vickers hardness of 1185 kg/mm² was achieved, resulting from the strong bonding structure among the alloy constituents. The alloy system possessed soft magnetic properties with high saturation magnetization of 56.61-61.78 A m²/kg and coercivity in the range of 222-264.2 A/m, which might be suitable for application in power electronics devices.     

Effect of CuO on the sintering temperature and piezoelectric properties of MnO2-doped 0.75Pb(Zr0.47Ti0.53)O3-0.25Pb(Zn1/3Nb2/3)O3 ceramics

The sintering temperature of 0.75Pb(Zr_0.47Ti_0.53)O₃-0.25Pb(Zn_1/3Nb_2/3)O₃ ceramics containing 1.5 mol% MnO₂ was decreased from 930 to 850 °C with the addition of CuO. The CuO reacted with the PbO and formed a liquid phase during the sintering, which assisted the densification of the specimens. Most of the Cu²⁺ ions existed in the CuO second phase, thereby preventing any possible hardening effect from the Cu²⁺ ions. Variations of the k_p, Q_m, ?₃^T/?₀ and d₃₃ values with CuO were similar to that of the relative density. The specimen containing 0.5 mol% CuO sintered at 850 °C showed the good piezoelectric properties of k_p = 0.5, Q_m = 1000, ?₃^T/?₀ = 1750 and d₃₃ = 300 pC/N.     

Physical study of Sm doped borochromate glass system

Sm³⁺ doped borochromate glasses of composition [xSm₂O₃-74.5B₂O₃-25Li₂O-(0.5 − x)Cr₂O₃] with x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5 mol% have been prepared by conventional quenching melting method. The XRD profile confirm the amorphous nature of the glass samples. The density measurements were made using Archimedes’ principle of the prepared samples. Molar volume V_M, rare-earth ion concentration N and ionic radius r_p as well as field strength F have been estimated from the density measurements. The average optical basicity, Λ_th, electronegativity χ_2av and electronic polarizability α²⁻ were calculated for the prepared glass samples. Influence of rare earth ions on structural behavior in borochromate glasses have been investigated using infrared spectroscopy (IR). The structural changes have been analyzed with increasing rare earth concentration. Partial BO₃ ↔ BO₄ conversion as a function of rare earth concentration was observed. From the relative peak areas of BO₃ and BO₄ in structural groups the ratio N₄ has been calculated. ESR spectra were recorded at room temperature. The obtain ESR signals were used to estimate the paramagnetic susceptibility (χ).     

Absorption and emission spectral studies of Sm-doped lead tungstate tellurite glasses

The present work reports the effect of concentration on photoluminescence properties of Sm³⁺ ions doped lead tungstate tellurite (LTTSm) glasses by using the absorption, emission and decay measurements. The Judd-Ofelt theory has been used to evaluate the three Judd-Ofelt intensity parameters (Ω_2,4,6) and calculated oscillator strengths (f_c). LTTSm glasses exhibited intense reddish-orange emission when excited with 477 nm wavelength. Concentration quenching has been noticed beyond 1.0 mol% of Sm³⁺ ion concentration. The decay curves of ⁴G_5/2 level exhibited single exponential behavior for all the concentrations and the measured lifetimes are found to depend strongly on Sm³⁺ concentration. From the emission characteristic parameters of ⁴G_5/2 level, it is concluded that the LTTSm glasses could be useful for photonic devices like visible lasers, fluorescent display devices and optical amplifiers.     

Compositional dependence of thermal stability, glass-forming ability and fragility index in some Se-Te-Sn glasses

Non-isothermal DSC thermograms were obtained for the ternary Se₉₀Te_10−xSn_x (x = 2, 4, 6 and 8) chalcogenide glasses in order to determine the melting temperature T_m, glass transition temperature T_g, onset T_c and peak T_p temperatures of crystallization. These temperatures were utilized to investigate the thermal stability through the calculations of temperature difference (T_c − T_g), the glass transition activation energy E_t, the parameter S and the average value of crystallization rate factor 〈K_p〉. In addition, the glass forming ability was estimated by the criteria of reduced glass transition temperature, T_rg and Hruby parameter H_R. The fragility index m for the present glasses was determined in order to see whether these materials are obtained from strong or fragile glass-forming liquid. Results reveal that, both thermal stability and glass forming ability exhibit a maximum at x = 4 at.% of Sn. Meanwhile, the prepared glasses were obtained from strong glass-forming liquid as evident from the fragility index calculations. The compositional dependence of the above parameters was discussed on the basis of Philips and Thorpe topological model and the critical composition occurs at an average coordination number 〈r〉 = 2.16 but not 2.40. This is due to the formation of iono-covallent bonds when the glass doped with heavy elements like Sn.     

Structural impact of tungsten ions on ZnO-Sb2O3-As2O3 glass system

Five glasses in the quaternary system 5 ZnO-(50 − x) As₂O₃-45 Sb₂O₃: x WO₃ with the values of x ranging from 0 to 20 mol% (in steps of 5 mol%) are prepared. The samples are characterized by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy (EDS) and differential thermal analysis (DTA) techniques. The DTA studies have indicated that the glass forming ability decreases with the increasing content of WO₃. A number of studies, like, spectroscopic (optical absorption, IR, Raman, ESR spectra) and dielectric studies (dielectric constant ?, loss tan δ, a.c. conductivity σ_a.c.) over a wide range of frequency and temperature and dielectric break down strength at room temperature, have been carried out and are analysed in the light of different oxidation states and environment of tungsten ions in these glasses. These glasses have potential photonic applications.     

Resonant and non-resonant microwave absorption in the magnetoelectric YCrO3 through ferro-paraelectric transition

An electron paramagnetic resonance (EPR) study in the polycrystalline magnetoelectric YCrO₃ is carried out at X-band (8.8-9.8 GHz) in the 300-510 K temperature range. For all the temperatures, the EPR spectra show a single broad line attributable to Cr³⁺ (S = 3/2) ions. The onset of the ferro-paraelectric transition has been determined from the temperature dependence of three main parameters deduced from the EPR spectra: the peak-to-peak linewidth (ΔH_pp), the integrated intensity (I_EPR) and the g-factor; these parameters indicate a behavior in agreement with a diffuse phase transition. Low-field microwave absorption (LFMA) is used to give a further knowledge on this material; where this technique also gives evidence of the ferro-paraelectric transition.     

Effect of high pressure on photoluminescence properties of Eu: K-Ba-Al-fluorophosphate glasses

The effect of hydrostatic pressure and active ion concentration on the luminescence spectra and lifetimes of Eu³⁺-doped P₂O₅-K₂O-BaO-Al₂O₃-KF glasses have been studied up to 44.8 GPa at room temperature. The ⁵D₀ → ⁷F_0-3 transitions exhibit pressure induced red-shift with different magnitudes. The crystal-field (CF) strengths at ambient condition are 338, 332 and 350 cm⁻¹ for 0.05 mol%, 2.0 mol% and 6.0 mol% Eu₂O₃-doped glasses, respectively. These values slowly increase with increasing pressure and finally reach 518, 410 and 381 cm⁻¹ at the highest pressure. The observed increase in the CF strength parameter is found to have an almost cubic dependence on pressure. The luminescence intensity ratio, ⁵D₀ → ⁷F₂/⁵D₀ → ⁷F₁, of the Eu³⁺ ions is found to decrease with increasing pressure. The decay curves follow single exponential nature for all the three concentrations of Eu₂O₃-doped glasses for the entire pressure range studied.     

Investigation on the synthesis and quantum confinement effects of pure and Mn added Zn(1−x)CdxS nanocrystals

Zn_(1−x)Cd_xS and Zn_(1−x)Cd_xS:Mn²⁺ semiconductor quantum dots (2-4 nm) have been prepared by a novel solvothermal route assisted microwave heating method. The growth parameters governing the smaller size and higher yield have been optimized. The synthesized QDs exhibit a significant blue shift as compared to their corresponding bulk counterpart in the UV-vis optical absorption spectrum. The dielectric constant value varies from 2.79 to 6.17 (at 40 °C, 1 kHz) depending upon the composition of the alloy; lower value corresponds to Zn_0.75Cd_0.25S:Mn²⁺ and the higher value corresponds to Zn_0.25Cd_0.75S:Mn²⁺. The crystallite size to exciton bohr radius ratio being <1 indicates a strong quantum confinement effect in both CdS and ZnS QDs. The quantum confinement effect exists in the sequence of ZnS:Mn²⁺ < Zn_(1−x)Cd_xS:Mn²⁺ (x < 0.5) < ZnS < Zn_(1−x)Cd_xS < CdS < CdS:Mn²⁺.     

Magnetic properties of a new iron lead vanadate Pb2FeV3O11

Temperature dependence of dc/ac magnetization and electron paramagnetic resonance (EPR) spectra of Pb₂FeV₃O₁₁ iron lead vanadate has been investigated. The dc magnetic measurements have shown the presence of antiferromagnetic interactions with Curie-Weiss temperature, T_CW = −15.2 K, in the high temperatures range while the field cooled (FC) magnetization revealed a maximum at T_N = 2.5 K which coincides with a long range magnetic ordering. Temperature dependence of χ′ has shown a maximum at the same temperature. EPR spectrum of Pb₂FeV₃O₁₁ at room temperature is dominated by nearly symmetrical, very intense and broad resonance line centered at g_eff ∼ 2.0 that could be attributed to the correlated system of iron ions. The temperature dependence of magnetic resonance parameters (amplitude, g-factor, linewidth, integrated intensity) has been determined in the 4-300 K range and it suggests the existence of short range correlated spin system up to high temperatures. The temperature dependence of the amplitude of the resonance line has shown a pronounced maximum at 12.5 K that indicates on the existence of two subsystems of weakly and strongly coupled iron pairs. Comparison of dc magnetic susceptibility and EPR integrated intensity points to the presence of correlated spin agglomerates that play an important role in determination of the magnetic response of Pb₂FeV₃O₁₁.     

Structural, optical and EPR studies on ZnO:Cu nanopowders prepared via low temperature solution combustion synthesis

Cu (0.1 mol%) doped ZnO nanopowders have been successfully synthesized by a wet chemical method at a relatively low temperature (300 °C). Powder X-ray diffraction (PXRD) analysis, scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Fourier transformed infrared (FTIR) spectroscopy, UV-Visible spectroscopy, Photoluminescence (PL) and Electron Paramagnetic Resonance (EPR) measurements were used for characterization. PXRD results confirm that the nanopowders exhibit hexagonal wurtzite structure of ZnO without any secondary phase. The particle size of as-formed product has been calculated by Williamson-Hall (W-H) plots and Scherrer's formula is found to be in the range of ∼40 nm. TEM image confirms the nano size crystalline nature of Cu doped ZnO. SEM micrographs of undoped and Cu doped ZnO show highly porous with large voids. UV-Vis spectrum showed a red shift in the absorption edge in Cu doped ZnO. PL spectra show prominent peaks corresponding to near band edge UV emission and defect related green emission in the visible region at room temperature and their possible mechanisms have been discussed. The EPR spectrum exhibits a broad resonance signal at g ∼ 2.049, and two narrow resonances one at g ∼ 1.990 and other at g ∼ 1.950. The broad resonance signal at g ∼ 2.049 is a characteristic of Cu²⁺ ion whereas the signal at g ∼ 1.990 and g ∼ 1.950 can be attributed to ionized oxygen vacancies and shallow donors respectively. The spin concentration (N) and paramagnetic susceptibility (χ) have been evaluated and discussed.     

Optical properties of Yb-doped phosphate laser glasses

Ytterbium-doped phosphate glasses have been prepared and studied their spectroscopic properties through absorption, emission and Fourier transform infrared (FTIR) spectral studies and time-resolved luminescence decay curves. The absorption cross-section has been found to vary with the variation of Yb₂O₃ concentration. The results of the FTIR spectra show that the OH⁻ content is increasing with increase of the Yb₂O₃ concentration in these glasses. The decay curves of the ²F_5/2 level of Yb³⁺ ions exhibit a single exponential nature for all the concentrations. The lifetimes of the ²F_5/2 level of Yb³⁺ ions decreases from 1.04 to 0.27 ms when the Yb₂O₃ concentration is increased from 0.1 to 6.0 mol%. The quenching of lifetimes has been found to vary directly with the inter-ionic distance between the Yb³⁺ ions. The concentration quenching of the lifetime has been analyzed using different energy transfer processes and no evidence of cooperative luminescence of Yb³⁺ ions has been found in these glasses, which reveals that the present glasses are useful for photonic device applications. The laser performance properties have also been evaluated for these glasses and compared with those of other reported Yb³⁺-doped glass systems.     

Study of glass formation in the Sb2O3-PbO-MnO ternary system

Vitreous systems based on antimony oxide Sb₂O₃ have been investigated. The influence of MnO substitution on the mechanical and physical properties in the (80 − x)Sb₂O₃-20PbO-xMnO and (70 − x)Sb₂O₃-(30 − x)PbO-2xMnO systems has been studied. Vickers hardness, density, molar volume, Young modulus, glass temperature transition, infrared and UV transmission spectra depend on the MnO concentration. Crack analysis of the glass surface under indentor deformation shows the tenacity changes according to concentration of the MnO.     

Effects of Ce concentration, beam voltage and current on the cathodoluminescence intensity of SiO2:Pr-Ce nanophosphor

SiO₂:Pr³⁺-Ce³⁺ phosphor powders were successfully prepared using a sol-gel process. The concentration of Pr³⁺ was fixed at 0.2 mol% while that of Ce³⁺ was varied in the range of 0.2-2 mol%. High resolution transmission electron microscopy (HRTEM) clearly showed nanoclusters of Pr and Ce present in the amorphous SiO₂ matrix, field emission scanning electron microscopy (FE-SEM) indicated that SiO₂ clustered nanoparticles from 20 to 120 nm were obtained. Si-O-Si asymmetric stretching was measured with Fourier transform-IR (FT-IR) spectroscopy and it was also realized that this band increased with incorporation of the activator ions into the SiO₂ matrix. The broad blue emission from the Ce³⁺ ions attributed to the 5d¹-4f¹ transition was observed from the SiO₂:0.2 mol% Pr³⁺-1 mol% Ce³⁺ phosphor. This emission was slightly enhanced compared to that of the singly doped SiO₂:1 mol%Ce³⁺ phosphor. Further investigations were conducted where the CL intensity was measured at different beam voltages and currents from 1 to 5 kV and 8.5 to 30 μA, respectively, in order to study their effects on the CL intensity of SiO₂:0.2 mol% Pr³⁺-1 mol% Ce³⁺. The electron-beam dissociated the SiO₂ and as a result an oxygen-deficient surface dead or non-luminescent layer of SiO_x, where x < 2 on the surface, was formed.     

Structural, absorption and fluorescence spectral analysis of Pr ions doped zinc bismuth borate glasses

Glasses having composition 20ZnO·xBi₂O₃·(79.5 − x)B₂O₃·0.5Pr₆O₁₁ were prepared by melt quench technique. The amorphous nature of the prepared glass samples was confirmed by X-ray diffraction. The spectroscopic properties of these glasses were investigated by recording optical absorption and fluorescence spectra. The structural investigations of the glasses were carried out by recording the IR spectra. The optical properties of Pr³⁺ ions doped zinc borate glasses with varying concentration of bismuth oxide have been studied. The Judd-Ofelt intensity parameters Ω_λ (λ = 2, 4, 6) and other radiative properties like radiative transition probability, radiative life time, branching ratio and stimulated emission cross-section of the prepared glasses have been calculated. The variation of Ω₂ with Bi₂O₃ content has been attributed to changes in the asymmetry of the ligand field at the rare earth ion site and to the changes in their rare earth oxygen (RE-O) covalency. The branching ratio for ³P₀ → ³F₂ transition is 42% and the predicted spontaneous radiative transition probability rates are fairly high (14,347-14,607 s⁻¹). This is beneficial for lasing emission. The stimulated emission cross-section for all the emission bands has been calculated.