Eu3+ doped ferroelectric CaBi2Ta2O9 based glass-ceramic nanocomposites: Crystallization kinetics,optical and dielectric properties

We report Eu³⁺ doped transparent glass-ceramics (GCs) containing bismuth layer-structured ferroelectric (BLSF) CaBi₂Ta₂O₉ (CBT) as the major crystal phase. The CBT crystal phase was generated in a silica rich glass matrix of SiO₂-K₂O-CaO-Bi₂O₃-Ta₂O₅ glass system synthesized by melt quenching technique followed by controlled crystallization through ceramming heat-treatment. Non-isothermal DSC study was conducted to analyze crystallization kinetics of the glass in order to understand the crystallization mechanism. The optimum heat-treatment protocol for ceramization of precursor glass that has been determined through crystallization kinetics analysis was employed to fabricate transparent GCs containing CBT nanocrystals, which was otherwise difficult. Structural analysis of the GCs was carried out using XRD, TEM, FESEM and Raman spectroscopy and results confirmed the existence of CBT nanocrystals. The transmittance and optical band gap energies of the GCs were found to be less when compared to the precursor glass. The refractive indices of the GCs were increased monotonically with increase in heat-treatment time, signaling densification of samples upon heat-treatment. The dielectric constants (ε_r) of the GCs were progressively increased with increase in heat-treatment duration indicating evolution of ferroelectric CBT crystals phase upon heat-treatment.     

Enhanced electrical properties and strong red light‐emitting in Eu3+‐doped Sr1.90Ca0.15Na0.9Nb5O15 ceramics

The luminescent‐ferroelectic materials based on Sr_1.90Ca_0.15Na_0.9Nb₅O₁₅ (SCNN) matrix doping with Eu³⁺ were synthesized by the conventional solid‐state reaction method. The crystal structure, photoluminescence, thermal stability, dielectric, ferroelectric, and piezoelectric behaviors were systematically investigated. XRD results revealed that Eu³⁺ introduction could induce the tungsten bronze phase transition from orthorhombic to tetragonal structures. The dielectric spectra of all specimens showed two broad dielectric anomalies: a high‐temperature ferroelectric phase transition (T_c) and a low‐temperature ferroelastic phase transition (T_s), both of which were suppressed at higher Eu³⁺ concentrations. The enhanced electrical properties were obtained in a proper Eu³⁺ concentration range of 0.03‐0.05. For all SCNN:xEu³⁺ samples, the strong red emission peak at 617 nm originating from the electric dipole transition of ⁵D₀ →⁷F₂ was excited by different light excitations of 395 or 463 nm. Our results demonstrated that Eu³⁺‐doped SCNN materials might have promising potential in advanced multifunctional optoelectronic applications.     

Eu2+‐Doped Yellow‐Emitting CsBaB3O6 Glass‐Ceramic Prepared by Melt Quenching and Re‐Crystallization Method

Eu³⁺‐doped cesium barium borate glass with the composition of Cs₂O·2BaO·3B₂O₃ was prepared by the conventional melt quenching method. The glass‐ceramic sample was obtained from the re‐crystallization of the as‐made glass to change the amorphous glass into a crystalline host. This reduces the Eu³⁺ in glass to Eu²⁺ ions resulting in a yellow‐emitting phosphor of Eu²⁺‐activated CsBaB₃O₆. The samples were investigated by the XRD patterns and SEM micrograph, the optical absorption, the photoluminescence spectra, and decay curves. The as‐made glass has only Eu³⁺ centers. Under the excitation of blue or near‐UV light, Eu²⁺‐doped CsBaB₃O₆ presents yellow‐emitting color from the allowed inter‐configurational 4f–5d transition in the Eu²⁺ ions. The maximum absolute luminescence quantum efficiencies of Eu²⁺‐doped CsBaB₃O₆ phosphor was measured to be 47% excited at 430 nm light at 300 K. By taking into account the efficient excitation in blue wavelength region, this new phosphor could be a potential yellow‐emitting phosphor for an application in white light‐emitting diodes fabricated with blue chips.     

Synthesis and Characterization of Eu3+‐Doped Transparent Glass–ceramics Containing Nanocrystalline SrIINbIVO3

The glass–ceramics containing a rarely achievable nanocrystalline Sr^IINb^IVO₃ phase in the 53.75SiO₂–18.25K₂O–9Bi₂O₃–9SrO–9Nb₂O₅–0.5CeO₂–0.5Eu₂O₃ (mol%) glass system were prepared by the melt‐quench technique followed by a two‐stage controlled heat treatment. The unusual oxidation state of Nb in Sr^IINb^IVO₃ crystal is 4+ and upon heat treatment of the samples at lower temperature of 500°C for several hours, the glass composition and chemical environment around Nb ions played a key role for the formation of Sr^IINb^IVO₃ in the glass–ceramics. The microstructure of the glass–ceramics was studied using TEM and FESEM. The TEM images advocate 10–40 nm crystallite size of Sr^IINb^IVO₃. FTIR study confirms that all the samples consist of SiO₄, BiO₃, BiO₆, and NbO₆ structural units. The refractive index at different wavelengths was found to vary in the range 1.7105–1.7905 and increase with increase in heat‐treatment time. The luminescence spectra of Eu³⁺‐doped glass and glass–ceramics were recorded at 465 nm excitation wavelength and the luminescence intensity is found to be increased with heat‐treatment time due to increase in crystallinity. The high intensity ratio of ⁵D₀→⁷F₂ to ⁵D₀→⁷F₁ indicates that the Eu³⁺‐doped nanocrystalline Sr^IINb^IVO₃ glass–ceramics are promising candidate materials as red‐light source.     

Thermal and optical properties of La2O3–Ga2O3– (Nb2O5 or Ta2O5) ternary glasses

La₂O₃–Ga₂O₃–M₂O₅ (M = Nb or Ta) ternary glasses were fabricated using an aerodynamic levitation technique, and their glass‐forming regions and thermal and optical properties were investigated. Incorporation of adequate amounts of Nb₂O₅ and Ta₂O₅ drastically improved the thermal stabilities of the glasses against crystallization. Optical transmittance measurements revealed that all the glasses were transparent over a wide wavelength range from the ultraviolet to the mid‐infrared. The refractive indices of the glasses increased and the Abbe number decreased upon substituting Ga₂O₃ with Nb₂O₅, and the decrease in the Abbe number was significantly suppressed when Ta₂O₅ was incorporated into the glass. As a result, excellent compatibility between high refractive index and lower wavelength dispersion was realized in La₂O₃–Ga₂O₃–Ta₂O₅ glasses. Analysis based on the single‐oscillator Drude–Voigt model provided more systematical information and revealed that this compatibility was due to an increase in the electron density of the glass.     

Thermal,Structural, and Enhanced Photoluminescence Properties of Eu3+‐doped Transparent Willemite Glass–Ceramic Nanocomposites

The precursor glass in the ZnO–Al₂O₃–B₂O₃–SiO₂ (ZABS) system doped with Eu₂O₃ was prepared by the melt‐quench technique. The transparent willemite, Zn₂SiO₄ (ZS) glass–ceramic nanocomposites were derived from this precursor glass by a controlled crystallization process. The formation of willemite crystal phase, size, and morphology with increase in heat‐treatment time was examined by X‐ray diffraction (XRD) and field‐emission scanning electron microscopy (FESEM) techniques. The average calculated crystallite size obtained from XRD is found to be in the range 18–70 nm whereas the grain size observed in FESEM is 50–250 nm. The refractive index value is decreased with increase in heat‐treatment time which is caused by the partial replacement of ZnO₄ units of ZS nanocrystals by AlO₄ units due to generation of vacancies. Fourier transform infrared (FTIR) reflection spectroscopy was used to evaluate its structural evolution. Vickers hardness study indicates marked improvement of hardness in the resultant glass‐ceramics compared with its precursor glass. The photoluminescence spectra of Eu³⁺ ions exhibit emission transitions of ⁵D₀→⁷F_j (j = 0, 1, 2, 3, and 4) and its excitation spectra show an intense absorption band at 395 nm. These spectra reveal that the luminescence performance of the glass–ceramic nanocomposites is enhanced up to 17‐fold with the process of heat treatment. This enhancement is caused by partitioning of Eu³⁺ ions into glassy phase instead of into the willemite crystals with progress of heat treatment. Such luminescent glass–ceramic nanocomposites are expected to find potential applications in solid‐state red lasers, phosphors, and optical display systems.     

Crystallization,mechanical, and optical properties of transparent,nanocrystalline gahnite glass‐ceramics

This paper describes the preparation of a transparent glass‐ceramic from the SiO₂‐K₂O‐ZnO‐Al₂O₃‐TiO₂ system containing a single crystalline phase, gahnite (ZnAl₂O₄). TiO₂ was used as a nucleating agent for the heat‐induced precipitation of gahnite crystals of 5‐10 nm. The evolution of the ZnAl₂O₄ spinel structure through the gradual formation of Al‐O bonds was examined by infrared spectroscopy. The dark brown color of the transparent precursor glass and glass‐ceramic was eliminated using CeO₂. The increase in transparency of the CeO₂‐doped glass and glass‐ceramics was demonstrated by UV‐visible absorption spectroscopy. EPR measurements confirmed the presence of Ce³⁺ ions, indicating that CeO₂ was effective in eliminating the brown color introduced by Ti³⁺ ions via oxidation to Ti⁺⁴. The hardness of the glass‐ceramic was 30% higher than that of the as‐prepared glasses. This work offers key guidelines to produce hard, transparent glass‐ceramics which may be potential candidates for a variety of technological applications, such as armor and display panels.     

Processing and Properties of Eu3+‐Doped Barium Bismuth Titanate (BaBi4Ti4O15) Glass–Ceramic Nanocomposites

Precursor glasses for the ferroelectric barium bismuth titanate (BaBi₄Ti₄O₁₅) (BBiT) have been prepared by the melt‐quench technique in the SiO₂–K₂O–BaO–Bi₂O₃–TiO₂ (SKBBT) glass system with and without Eu₂O₃ doping. BBiT glass–ceramic (GC) nanocomposites have been derived from these glasses by controlled heat treatment. The structural properties of the GCs have been investigated using X‐ray diffraction (XRD), electron microscopy (FE‐SEM, TEM), and FT‐IR reflectance spectroscopy. FE‐SEM images show the formation of randomly oriented hexagonal rod‐shaped crystals of 200–400 nm and TEM images show 10–20 nm crystallites. FT‐IR spectra exhibit the characteristic bands of BBiT at 480, 585, and 680 cm^?1. The activation energy of crystallization (E_c) varies from 295 to 307 kJ/mol. The dielectric constants (ε_r) of glass and GC nanocomposites increase with an increase in frequency up to 3.0 MHz and then decrease up to 5.0 MHz. Heat‐treated GCs show higher ε_r values, in the range 25–55, compared to the precursor glasses (20–37). Dielectric losses (tan δ) for all the samples increase from 0.005 to 1.0 with an increase in frequency from 100 Hz to 5.0 MHz. Excitation spectra were recorded by monitoring emission at 613 nm corresponding to the ⁵D₀ → ⁷F₂ transition. An intense 466 nm excitation band corresponding to the ⁷F₀ → ⁵D₂ transition was observed. Emission spectra were then recorded by exciting the glass samples at 466 nm. Longer heat‐treatment times led to a 15‐fold increase in the intensity of the red emission at 612 nm, attributed to the segregation of Eu³⁺ ions into the low phonon energy BBiT crystallites. The hardness (3.8–5.1 GPa) and fracture toughness (1.8–3.5 MPam^0.5) values obtained in the GCs are high and suitable for structural applications.     

Luminescence of Eu‐Doped Transparent LuPO4 Glass‐Ceramics

Dual valence Eu‐doped transparent glass‐ceramics containing LuPO₄ nanocrystals were fabricated by melt‐quenching technique in air atmosphere. Their luminescent properties were systematically investigated by excitation, emission spectra, and decay lifetime measurements. The prominent Stark splitting, low forced electric‐dipole ⁵D₀ – ⁷F₂ transition and long decay lifetimes of Eu^{3 +} emission for glass‐ceramics reveal the incorporation of Eu^{3 +} into LuPO₄ nanocrystals. The enhanced Eu^{2 +} emission and reduction mechanism of Eu^{3 +} to Eu^{2 +} after crystallization are discussed briefly. Our results indicate that transparent LuPO₄ glass‐ceramics may find applications in photonics.     

Thermal,Structural, and Crystallization Properties of New Tantalum Alkali‐Germanate Glasses

In this work, new glass compositions were prepared in the ternary system GeO₂–K₂O–Ta₂O₅. Potassium oxide was added to reach the complete melt of the starting mixture and two composition series were investigated: the first one with a constant K₂O molar content of 10% in the ternary system (90–x)GeO₂–10K₂O–xTa₂O₅ and the second one with the same molar content of K₂O and Ta₂O₅ in the ternary system (100–2x)GeO₂–xK₂O–xTa₂O₅. Homogeneous and transparent glasses could be obtained between x = 0 and 20. X‐ray diffraction analyzes of samples with x = 25 identified orthorhombic Ta₂O₅ in the first series and an isostructure of K_3.8Ge₃Nb₅O_20.4 in the second series where it is assumed that Ta⁵⁺ ions are inserted in the Nb⁵⁺ sites. As one of our goal with these materials is related with the preparation of glass‐ceramics containing Ta₂O₅ nanocrystals, the first series has been selected for further characterizations. An increase in glass‐transition temperatures with increasing Ta₂O₅ content as well as an increase of the thermal stability from x = 0 to 10 has been identified by differential scanning calorimetry. For higher contents, crystallization events were identified. Fourier transform infrared and Raman spectroscopic characterizations allowed to point out the intermediary behavior of Ta₂O₅ in the vitreous network where TaO₆ octahedra are inserted inside the germanate network with TaO₆ clusters identified at higher Ta₂O₅ contents. Heat‐treated samples with high tantalum contents (x = 15 and 20) exhibit preferential precipitation of orthorhombic Ta₂O₅ with nanometric size, suggesting the possibility of obtaining transparent glass‐ceramics for optical applications.     

Eu3+-doped transparent potassium lanthanum silicate (KLaSiO4) glass-ceramic nanocomposites: Synthesis,properties and application

The preparation of Eu³⁺-doped novel K₂O-La₂O₃-Al₂O₃-SiO₂ (KLAS)-based glass and transparent KLS (KLaSiO₄) glass–ceramic (GC) nanocomposites is reported. Nanostructures of the transparent GCs were analyzed by FE-SEM, H(R)-TEM and SAED techniques. The average size of the crystallites is calculated using XRD data and found to be in the range 13–19?nm which is matched well with the average particle size observed from TEM images in the range 5–18?nm. Photoluminescence spectra of Eu³⁺ ions exhibit emission transitions of ⁵D₀?→?⁷F_j (j?=?0 and 1–4) under the excitation at 394?nm. The emission spectra reveals up to 3-fold enhancement of luminescence performance of the KLS GC nanocomposites compared to as-prepared glass. This enhancement is caused due to entering of Eu³⁺ ions into the KLS crystal sites by replacing the La³⁺ ions. Such luminescence properties of KLS glass-ceramic nanocomposites could be a promising candidate as laser host for many laser devices.     

Molecular‐Like Ag Clusters and Eu3+ Co‐Sensitized Efficient Broadband Spectral Modification with Enhanced Yb3+ Emission

AgNO₃/EuF₃/YbF₃ tri‐doped oxyfluoride glass was prepared by a melt‐quenching method, in which a high‐efficient broadband spectral modification can be realized due to the simultaneous energy‐transfer processes of Eu³⁺→Yb³⁺, molecular‐like Ag (ML‐Ag) clusters→Yb³⁺, and ML‐Ag clusters→Eu³⁺→Yb³⁺. The spectral measurements indicated that besides the F‐center brought by the fluorides, the formation of the ML‐Ag clusters and the evolution of silver species within the glass matrix were also closely related to the introduction of Eu³⁺ and Yb³⁺ ions and which in return greatly affected the luminescence properties of these rare‐earth ions. As the UV‐visible irradiation in the wavelength region of 250–600 nm can be efficiently converted into near‐infrared emission around 1000 nm in the AgNO₃/EuF₃/YbF₃ tri‐doped glass, which thus has promising application in enhancing the photovoltaic conversion efficiency of the silicon solar cell.     

Eu3+‐Activated Borogermanate Scintillating Glass with a High Gd2O3 Content

Eu³⁺‐activated borogermanate scintillating glasses with compositions of 25B₂O₃–40GeO₂–25Gd₂O₃–(10?x)La₂O₃–xEu₂O₃ were prepared by melt‐quenching method. Their optical properties were studied by transmittance, photoluminescence, Fourier transform infrared (FTIR), Raman and X‐ray excited luminescence (XEL) spectra in detail. The results suggest that the role of Gd₂O₃ is of significance for designing dense glass. Furthermore, energy‐transfer efficiency from Gd³⁺ to Eu³⁺ ions can be near 100% when the content of Eu₂O₃ exceeds x = 4, the corresponding critical distance for Gd³⁺–Eu³⁺ ion pairs is estimated to be 4.57 Å. The strongest emission intensities of Eu³⁺ ions under both 276 and 394 nm excitation are simultaneously at the content of 8 mol% Eu₂O₃. The degree of Eu–O covalency and the local environment of Eu³⁺ ions are evaluated by the value of Ω_t parameters from Judd–Ofelt analysis. The calculated results imply that the covalency of Eu–O bond increases with the increasing concentration of Eu³⁺ ions in the investigated borogermanate glass. As a potential scintillating application, the strongest XEL intensity under X‐ray excitation is found to be in the case of 6 mol% Eu₂O₃, which is slightly different from the photoluminescence results. The possible reason may be attributed to the discrepancy of the excitation mechanism between the ultraviolet and X‐ray energy.     

Phosphor‐in‐fluorescent‐glasses for high color rendering white light emitting diodes

Fluorescent glass frits were prepared and used to synthesize phosphor‐in‐fluorescent glass composites (PiFGs) to realize stable white light emitting diodes with high color‐rendering properties. Commercial red, green, and blue phosphors were co‐sintered and red phosphors were partially replaced by Eu³⁺ in glass frits. Phosphor‐in‐glass composites were placed on UV‐light emitting diodes (UV‐LEDs) to generate white light. Pure white light with a luminous efficacy=58.4 lm/W, general color rendering index R_a=87 and special color rendering index for strong red R₉=73 was realized with glass frits containing 7 mol% Eu₂O₃ and RGB ratio of 35:20:15. Luminous efficacy, R_a and R₉ increased as red phosphors were replaced by red‐fluorescent glass frits.     

Sol–Gel‐Derived Amorphous‐MgNb2O6 Thin Films for Transparent Microelectronics

In this work, transparent amorphous‐MgNb₂O₆ thin films were fabricated on ITO/glass substrates using the sol–gel method. The change in the chemical states, as well as the optical and dielectric properties of MgNb₂O₆ films at various annealing temperatures is investigated. In this study, MgNb₂O₆ films exhibited the amorphous phase when the annealing temperature was below 600°C. From X‐ray photoelectron spectroscopy, the major parts of the films' chemical states can be indexed as Mg²⁺, Nb⁵⁺, Nb⁴⁺, and O^2?. Furthermore, the Nb⁴⁺ element can be reduced at higher annealing temperatures. The average transmission percentage in the visible range (λ = 400–800 nm) is over 80% for all MgNb₂O₆/ITO/glass samples, whereas the optical band gap (E_g) for all samples is estimated at ~4 eV. In addition, the dielectric constant was calculated to be higher than 20 under a 1 MHz AC electric field, with a leakage current density below 2 × 10^?7 A/cm² at 1 V. In this study, the fabrication procedure and experiment results of MgNb₂O₆ films are introduced for transparent microelectronics.     

Effects of Tm3+ Additions on the Crystallization of LaF3 Nanocrystals in Oxyfluoride Glasses: Optical Characterization and Up‐Conversion

The influence of the addition of 1 mol% Tm₂O₃ on the nanocrystallization of LaF₃ in a glass of composition 55SiO₂–20Al₂O₃–15Na₂O–10LaF₃ (mol%) has been studied. Tm₂O₃ affects the phase separation in the glass and delays the onset of crystallization with respect to the undoped glass. Additionally, the maximum LaF₃ crystal size is slightly greater than that in the undoped glass–ceramics. The microstructural and compositional changes in the glass matrix have been studied using several techniques, including viscosity, dilatometry, X‐ray and neutron diffraction (XRD, ND), quantitative Rietveld refinement, transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and Raman spectroscopy. Photoluminescence measurements indicate that the Tm³⁺ ions are distributed between the glassy matrix and LaF₃ crystals. Eu₂O₃ has been used as structure probe and part of the Eu³⁺ ions are reduced to Eu²⁺ when incorporated in the LaF₃ nano‐crystals. Up‐conversion spectra under IR‐excitation show a higher intensity of the blue emission in the Tm‐doped glass–ceramic compared with that in the glass.     

Elaboration,Structure, and Luminescence of Eu3+‐Doped BaLuF5‐Based Transparent Glass‐Ceramics

Novel Eu³⁺‐doped transparent oxyfluoride glass‐ceramics containing BaLuF₅ nanocrystals were successfully fabricated by melt‐quenching technique for the first time. Analyses of XRD patterns prove that the new precipitated glass‐ceramics are crystallized in cubic BaLuF₅ based on isostructural BaGdF₅. Intense red emissions observed in glass ceramics are attributed to the enrichment of Eu³⁺ ions into BaLuF₅ nanocrystals. Besides, obvious stark splitting emissions, low forced electric dipole ⁵D₀→⁷F₂ transition, and long decay lifetimes of Eu³⁺ ions also evidence the partition of Eu³⁺ ions into BaLuF₅ nanocrystals with low phonon energy. Such transparent material may find applications in photonics.     

Effect of Ta2O5 addition on the structure,crystallization mechanism,and properties of CaO–B2O3–SiO2 glasses for LTCC applications

CaO–B₂O₃–SiO₂–Ta₂O₅ (CBST) glass-ceramics, with different Ta₂O₅ content, (up to 6 mol%), have been prepared by using glass melt quenching followed by heat treatment between 800 and 880 °C. The Fourier Transform Infrared (FTIR) results showed that the stronger the attraction of Ta⁵⁺ to the oxygens in the BO₃^3? and SiO₃^2? structures, the more easily the B–O and Si–O bonds will be destroyed. The underlying reason is most probably the high field strength of Ta⁵⁺, which results in a weakening of the vibration intensities of the [BO₃] and [SiO₄] units. Moreover, the Differential Scanning Calorimetry (DSC) results showed that the softening point (T_g), crystallization starting temperature (T_c1), and exothermic crystallization peak temperature (T_p1), of the CaSiO₃ phase, shifted to higher values with the addition of Ta₂O₅. Also, the crystallization activation energy (E_a) and the glass stability factor (ΔT) of the CaSiO₃ phase increased, which indicated that the CaSiO₃ phase of the glass became inhibited by the addition of Ta₂O_5. It was, thus, obvious that there was a need of glass characterization. The results of the crystallization kinetics showed that the critical cooling rate decreased with the addition of Ta₂O₅, which indicated that the viscosity of the system had increased. The CBST glass-ceramics, containing 1 mol% Ta₂O_5, that were sintered at 875 °C for 15 min showed excellent dielectric properties: ε_r = 6.22 and tanδ = 1.19 × 10^?3 (1 MHz). To sum up, CaO–B₂O₃–SiO₂–Ta₂O₅ glass-ceramics are potential low temperature co-fired ceramic substrate materials.     

Crystal structure and luminescence properties of a single‐component white‐light‐emitting phosphor Ca8ZnLa(PO4)7:Eu2+,Mn2+

A series of novel green emission Whitlockite‐type Ca₈ZnLa(PO₄)₇:Eu²⁺ and color tunable Ca₈ZnLa(PO₄)₇:Eu²⁺,Mn²⁺ phosphors were prepared by the solid‐state reaction method in a reducing atmosphere. Its crystal structure and phase composition were identified by high‐resolution transmission electron microscopy, selected area electronic diffraction, X‐ray photoelectron spectroscopy, and X‐ray powder diffraction Rietveld refinement, and it was found to be trigonal, belonging to R‐3c(161) space group. The luminescence properties of Eu²⁺ singly doped and Eu²⁺/Mn²⁺ codoped Ca₈ZnLa(PO₄)₇ phosphors were revealed in detail. Ca₈ZnLa(PO₄)₇:Eu²⁺ is excitable over a broad range from 200 to 450 nm with a prominent green emitting. With varied Eu²⁺/Mn²⁺ ratios, fine‐tune emission under 365 nm excitation can be achieved from green (0.221, 0.468) to magenta (0.391, 0.276), especially the warm white light (0.392, 0.352), and CCT 3500 K can be obtained by the process of energy transfer between Eu²⁺ and Mn²⁺. The ET mechanism in this system is managed via the dipole‐dipole interaction with the maximum energy‐transfer efficiency 82.8% based on the decay lifetime data. These results suggest that as‐prepared phosphors can serve as promising candidates of UV‐pumped w‐LEDs.     

Insulating characteristics of zinc niobium borate glass‐ceramics

Zinc borate glasses with different concentrations of Nb₂O₅ were prepared and later were heat treated for prolonged times. Prepared samples were characterized by XRD, SEM, DSC, IR and optical transmission spectroscopy techniques. Later, dielectric properties viz., dielectric constant, loss tangent, electric modulii, electrical impedance and a.c. conductivity over wide ranges of frequency and temperature, were investigated as a function of Nb₂O₅ concentration. Finally, the dielectric breakdown strength was measured in air medium at ambient temperature. The results of characterization techniques viz., XRD, SEM and DSC indicated that multiple crystal grains (with sizes varying from 0.1 to 1 μm) are dispersed in the residual glass phase. The concentration of crystal grains found to increase with increase in Nb₂O₅ content. The XRD studies have further revealed that the bulk samples are composed of columbite ZnNb₂O₆ crystal phases. Using generalized gradient approximation (GGA) quantitative information on these crystal phases viz., the lattice parameters, optical band gap and band structure were evaluated. The analysis of results of IR spectral studies have indicated that there is an increasing degree of polymerization of glass network with increase in Nb₂O₅ content due to the increased connectivity between various structural groups in the glass network. The optical absorption spectra indicated an increase in optical transmittance of the bulk samples with increase in Nb₂O₅ content. The dielectric parameters are observed to decrease, whereas the dielectric breakdown strength is observed to increase to a large extent due to the crystallization of the glass with the Nb₂O₅. The increase is attributed to the formation of ZnNb₂O₆ crystalline phases that contain intertwined ZnO₆ and NbO₆ structural units. As a whole, zinc borate glasses exhibited a significant increase in the electrical insulating strength due to the crystallization with Nb₂O₅ as the crystallizing agent. Further, the value of dielectric constant is also found to be the optimal with no dispersion with frequency up to 450 K. Overall, the studied glass‐ceramics meet the necessary physical conditions to be used as insulating layers in the display panels and hence may be considered for such applications.