Green emission enhancement of Ba2SiO4:Eu2+ phosphor via Gd co-doping and charge compensation

A series of new green-emitting Ba_2?x?2ySiO₄:xEu²⁺, yGd³⁺, yR⁺ (R = Li, Na or K) phosphors were synthesized by the solid-reaction method. X-ray diffraction (XRD) and fluorescence spectrophotometer are utilized to characterize the crystal structure and luminescence properties of the as-synthesized phosphors, respectively. The XRD patterns reveal that the doping of Gd³⁺, Eu²⁺ and R⁺ ions have no significant influence on the Ba₂SiO₄ phase. The green emission of Eu²⁺ ion associated with 4f⁶5d¹ → 4f⁷ can be obtained by 396 nm UV excitation source, which match well with the emission wavelength of UV-LEDs chip (380–420 nm). Moreover, the effect of charge compensator ions (Li⁺, Na⁺ or K⁺) on the luminescence intensity of (Ba, Gd)₂SiO₄:Eu²⁺ phosphors were also investigated. When introducing the Li⁺ ions into the (Ba, Gd)₂SiO₄ host lattices, the as-prepared phosphors show the strongest emission. The emission intensity of Ba_1.95SiO₄:0.04Eu²⁺, 0.005Gd³⁺, 0.005Li⁺ is about 1.39 times than that of Ba_1.96SiO₄:0.04Eu²⁺. Furthermore, the mechanism of energy transfer and concentration quenching of Ba_1.982?xSiO₄:xEu²⁺, 0.009Gd³⁺, 0.009Li⁺ phosphors are also discussed.     

Investigation of energy absorption and transfer process of Tb3+ or Eu3+ excited Na3Gd(PO4)2 in the VUV region

Na₃Gd(PO₄)₂, Na₃Gd_0.94(PO₄)₂:0.06Tb³⁺ and Na₃Gd_0.94(PO₄)₂:0.06Eu³⁺ are prepared by solid-state reaction and their photoluminescence (PL) properties are investigated in the ultraviolet (UV) and vacuum ultraviolet (VUV) region. The obtained results show that Na₃Gd_0.94(PO₄)₂:0.06Tb³⁺ has an efficient emission under 147 nm excitation, but the emission efficiency of Na₃Gd_0.94(PO₄)₂:0.06Eu³⁺ is low under 147 nm excitation. We discuss the energy absorption and transfer process in the VUV region to solve the special phenomenon.     

Reaction processes in a ZnO + 1%Gd2O3 powder mixture during mechanical and laser processing

X-ray diffraction, electron paramagnetic resonance, Fourier transform infrared spectroscopy and scanning electron microscopy were used to study the defect formation and reaction processes in a ZnO + 1%Gd₂O₃ powder mixture during its mechanical and laser processing. Mechanical treatment of the ZnO + 1%Gd₂O₃ powder mixture leads to a grinding of initial ZnO particles and formation of three types of superficial paramagnetic donor defect centers. The rise of the sample temperature with increasing processing time promotes a successive annealing of ZnO defects with small activation energies and of superficial defects in Gd₂O₃. The formation of a ZnO:Gd³⁺ solid solution in the used mechanical processing regimes has not been observed. Laser surface melting of the ZnO + 1%Gd₂O₃ pellets provokes formation of a surface layer exhibiting a texture. The crystallization directions in the superficial layers of different specimens have a random character. In the superficial layers and deep sub-surface layers, processes of solid-state interactions (formation of an inhomogeneous ZnO:Gd³⁺ solid solution) take place. The surplus charges of the Gd³⁺ ions are compensated by the formation of Zn vacancies or interstitial oxygen ions which in the laser-surface-melted layers are located closer to the Gd³⁺ ions than in the case of single-crystalline samples.     

Photoluminescence and radioluminescence of pure and Ce activated Na3Gd(PO4)2

The spectroscopic properties of Na₃Gd(PO₄)₂ and Na₃Gd(PO₄)₂:Ce³⁺ phosphors in the VUV-UV spectral range were investigated. Five excitation bands of Ce³⁺ ions at Gd³⁺ sites are observed at wavelengths of 205, 246, 260, 292, and 321 nm. Doublet Ce³⁺ 5d → 4f emission bands are observed at 341 and 365 nm with a decay constant τ_1/e around 26 ns. The X-ray excited luminescence of Na₃Gd_0.99Ce_0.01(PO₄)₂ at room temperature shows a photon yield of ∼17,000 photons/MeV of absorbed X-ray energy.     

Recombination luminescence of CaI<Subscript>2</Subscript> crystals activated with Eu<Superscript>2+</Superscript>, Gd<Superscript>2+</Superscript>, Tl<Superscript>+</Superscript>, Pb<Superscript>2+</Superscript>, and Mn<Superscript>2+</Superscript>

The spectral characteristics of thermostimulated luminescence, steady-state roentgenoluminescence and photostimulated luminescence (PSL) buildup and decay kinetics, and the effect of IR irradiation on the roentgenoluminescence yield and glow curves of CaI₂:Eu²⁺, CaI₂:Gd²⁺, CaI₂:Tl⁺, CaI₂:Pb²⁺, CaI₂:Mn²⁺, and CaI₂: Pb²⁺, Mn²⁺ crystals grown by the Bridgman-Stockbarger method have been studied in the temperature range 90–295 K. Coupled with earlier data, the present results on the influence of oxygen and hydrogen impurities on the spectral characteristics of CaI₂ indicate that the activation of calcium iodide with Eu²⁺, Gd²⁺, Tl⁺, Pb²⁺, and Mn²⁺ leads to the formation of cation impurity-native defect complexes, which act as carrier traps and are responsible for the thermostimulated luminescence in the range 150–295 K. IR exposure after 90-K x-ray excitation gives rise to flash PSL and influences the thermostimulated luminescence light sum. The nature of the emission and trapping centers involved and the mechanisms of recombination luminescence excitation in the crystals are discussed.     

Thermoelectric power in Gd3+-substituted Cu-Cd ferrites

Polycrystalline Cd _x Cu_1−x Fe_2−y Gd _y O₄ ferrites fory=0·0 and 0·1 were prepared by ceramic technique. X-ray diffractograms of powder samples show cubic symmetry withx⩾0·2 fory=0·0 and 0·1, while compositions withx=0·0 fory = 0·0 and 0·1 are tetragonal. The thermopower measurements for Gd³⁺-undoped ferrites in the temperature range 300 K to 788 K shown-type conductivity forx⩾0·2. The substitution of Gd³⁺ changedn-type conductivity of the compositions top-type. The mobilities calculated show decreasing trend on Gd³⁺ substitution. The values of activation energy ΔE and drift mobilityE _d suggest polaron formation in substituted samples. The conduction mechanism is explained on the basis of localized model and formation of Gd³⁺+Fe²⁺ stable pairs at B site and Cu¹⁺+Fe³⁺ at A site.     

Investigation of Na3GdP2O8:Tb as a potential green-emitting phosphor for plasma display panels

The photoluminescent properties of a series of Tb³⁺-doped Na₃GdP₂O₈ phosphors excitable by vacuum ultraviolet and ultraviolet light are reported. The host related absorption, f-f and f-d transitions of Gd³⁺ and Tb³⁺, and charge transfer of O²⁻ → Gd³⁺ and O²⁻ → Tb³⁺ are assigned. Under 147 nm light excitation, Na₃GdP₂O₈:Tb³⁺ phosphors show efficient green emissions with a dominant peak at 545 nm. The optimal sample Na₃Gd_0.4Tb_0.6P₂O₈ shows a shorter decay time and a comparable brightness when compared with the commercial Zn₂SiO₄:Mn²⁺ green phosphor. These results demonstrate that it is a potential candidate for plasma display panels application.     

A novel strategy to synthesize Gd2O2S:Eu3+ luminescent nanobelts via inheriting the morphology of precursor

Gd₂O₃:Eu³⁺ nanobelts were fabricated by calcination of the electrospun PVP/[Gd(NO₃)₃ + Eu(NO₃)₃] composite nanobelts. For the first time, Gd₂O₂S:Eu³⁺ nanobelts were successfully prepared via inheriting the morphology and sulfurization of the as-prepared Gd₂O₃:Eu³⁺ nanobelts precursor using sulfur powders as sulfur source by a double-crucible method we newly proposed. X-ray diffraction analysis indicated that Gd₂O₂S:Eu³⁺ nanobelts were pure hexagonal in structure with space group P \( \bar{3} \) m1. Scanning electron microscope analysis results showed that the width and thickness of the Gd₂O₂S:Eu³⁺ nanobelts were ca. 2.1 μm and 129 nm, respectively. Under the excitation of 330-nm ultraviolet light, Gd₂O₂S:Eu³⁺ nanobelts emitted red emissions of predominant peaks at 628 and 618 nm which were attributed to the ⁵D₀ → ⁷F₂ energy levels transitions of the Eu³⁺ ions. It was found that the optimum doping molar concentration of Eu³⁺ ions in Gd₂O₂S:Eu³⁺ nanobelts was 5 %. Possible formation and sulfurization mechanisms of Gd₂O₂S:Eu³⁺ nanobelts were also proposed. This new sulfurization technique is of great importance, not only can inherit the morphology of rare earth oxides, but also can fabricate pure-phase rare earth oxysulfides at low temperature compared with conventional sulfurization method.     

Greatly improved photoluminescence properties of the electrodeposited Y2O3:Eu thin film phosphors by the addition of Na and K ions

In this work, Y₂O₃:Eu³⁺ thin film phosphors were prepared by electro-deposition method. The effect of Na⁺ and K⁺ ions on the photoluminescence properties of Y₂O₃:Eu³⁺ thin film phosphor was studied in details. It was found that the addition of Na⁺ and K⁺ ions could improve the photoluminescence intensity by 3 to 4 times. The highly improved photoluminescence intensity may be caused by different factors. The improved crystallinity and the increased optical volume caused by the flux effect of Na⁺ and K⁺ ions could be the major reasons for the enhanced photoluminescence intensity. It was also found that the average lifetime of Y₂O₃:Eu³⁺ thin film phosphors could be adjusted by the molar amount of Na⁺ and K⁺ ions.     

Studies on impurity incorporation in cadmium oxalate crystals grown by gel method

Aspects of gel growth of cadmium oxalate single crystals doped with monovalent (Li⁺, Na⁺, K⁺ and NH ₄ ⁺ ) and divalent (Ca²⁺, Co²⁺, Ni²⁺ and Zn²⁺) cation impurities are discussed. The growth experiments were carried out by single diffusion method. In this paper the effect of composition of the supernatant solution is considered. Rigorous characterization involves determination of composition of cations by employing atomic absorption spectrophotometer and of anions by titration method. The Pauling radius, hydration energy and distribution coefficient of the cations are tabulated and the results are described and discussed.     

Enhanced red-emitting phosphor Na<Subscript>2</Subscript>Ca<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>8</Subscript>:Eu<Superscript>3+</Superscript> by charge compensation

A series of novel red-emitting Na₂Ca_3???x Si₂O₈:xEu³⁺ phosphors were synthesized by solid state reactions. The phosphors can strongly absorb 395 nm light, and show red emission with a good color purity. The excitation and emission spectra properties of Na₂Ca₃Si₂O₈:Eu³⁺ were characterized. Na₂Ca₃Si₂O₈:Eu³⁺ with self-compensated and alkali metal ions charge compensated approaches (2Ca²⁺→Eu³⁺ + M⁺, M?=?Li⁺, Na⁺, K⁺) have investigated, which found that the red emission of luminescent intensity can be greatly enhanced, and shows superior luminescent property to the commercial Y₂0₃S:Eu³⁺. The present work implies that the efficient charge compensated phosphors are promising candidates as red-emitting phosphor for w-LEDs.     

Effect of thermal neutron irradiation on Gd ions doped in oxyfluoroborate glass: an infra-red study

Infrared spectra of oxyfluoroborate glasses of composition (70−x)H₃BO₃+20Li₂CO₃+xGd₂O₃, where x=0, 0.5, 1.0, 3.0 and 5.0 mol%, have been recorded to explore the role of Gd³⁺ ions in the structure of the glasses. We concluded that Gd³⁺ ion behaves as a glass modifier. The effect of thermal neutron irradiation on the structure of these glasses also has been explored and the changes compared with earlier results on γ-irradiation.     

Uncovering the Potential of M1-Site-Activated NASICON Cathodes for Zn-Ion Batteries

There is a long-standing consciousness that the rhombohedral NASICON-type compounds as promising cathodes for Li⁺/Na⁺ batteries should have inactive M1(6b) sites with ion (de)intercalation occurring only in the M2 (18e) sites. Of particular significance is that M1 sites active for charge/discharge are commonly considered undesirable because the ion diffusion tends to be disrupted by the irregular occupation of channels, which accelerates the deterioration of battery. However, it is found that the structural stability can be substantially improved by the mixed occupation of Na⁺/Zn²⁺ at both M1 and M2 when using NaV₂(PO₄)₃ (NVP) as a cathode for Zn-ion batteries. The results of atomic-scale scanning transmission electron microscopy, analysis of ab initio molecular dynamics simulations, and an accurate bond-valence-based structural model reveal that the improvement is due to the facile migration of Zn²⁺ in NVP, which is enabled by a concerted Na⁺/Zn²⁺ transfer mechanism. In addition, significant improvement of the electronic conductivity and mechanical properties is achieved in Zn²⁺-intercalated ZnNaV₂(PO₄)₃ in comparison with those of Na₃V₂(PO₄)₃. This work not only provides in-depth insight into Zn²⁺ intercalation and dynamics in NVP unlocked by activating the M1 sites, but also opens a new route toward design of improved NASICON cathodes.     

Phase transformation studies on pure and doped strontium carbonate

The orthorhombic-hexagonal transformation of strontium carbonate is a reversible reconstructive transformation. Cationic impurities like K⁺, Ca²⁺, Ba²⁺, Cd²⁺, Y³⁺, La³⁺, Nd³⁺, Sm³⁺, Gd³⁺, Dy³⁺, Ho³⁺, Bi³⁺, Zr⁴⁺ doped in pure strontium carbonate affect the kinetics and energetics of the transformation. Paper presented at the Annual Convention of Chemists held at Andhra University, Waltair, December 1978.     

Electron spin resonance study and improved magnetic and dielectric properties of Gd–Ti co-substituted BiFeO3 ceramics

Polycrystalline BiFeO₃ (BFO), Bi_0.90Gd_0.10FeO₃ (BGF), Bi_0.90Gd_0.10Fe_1?xTi_xO₃ (x = 0.03–0.10; BGFT_x) ceramics were prepared via solid state reaction method. X-ray diffraction studies reveal R3c symmetry for BFO and BGF samples and coexistence of R3c + Pn2 ₁ a symmetries for BGFT_x samples. The change in line width of Raman modes indicates the structural distortion and substitution of dopants ions in the BFO lattice. Magnetic studies show weak ferromagnetism in BGF and BGFT_x samples as a result of Gd³⁺–Fe³⁺, Gd³⁺–Gd³⁺ interactions and imbalance created between two antiparallel Fe³⁺ spin sublattices by Ti substitution. The maximum remnant magnetization of 0.141 emu/g is observed for BGFT_x=0.10 sample. Further, electron spin resonance study confirms the weak ferromagnetism of BGFT_x samples, associated with small grains and increase in anisotropy of particles distribution as found during SEM studies. UV–Visible absorption spectra in the spectral range from 1.6 to 3.5 eV showed one d–d crystal field transition and two charge-transfer transitions with optical band gap variation in visible region. Improved dielectric properties with very low values of dielectric loss have been observed for BGF and BGFT_x samples.     

Electron traps in Gd<Subscript>3</Subscript>Ga<Subscript>3</Subscript>Al<Subscript>2</Subscript>O<Subscript>12</Subscript>:Ce garnets doped with rare-earth ions

The curves of thermally stimulated luminescence of Gd₃Ga₃Al₂O₁₂:Ce³⁺ ceramics (a nominally pure sample and samples doped with rare-earth ions) are measured in the temperature range of 80–550 K. The depth and the frequency factor of electron traps established by Eu and Yb impurities are determined. An energy-level diagram of rare-earth ions in the bandgap of Gd₃Ga₃Al₂O₁₂ is presented.     

PbF2:Gd晶体的发光强度与发光均匀性研究

以Ｘ射线荧光光谱分析为手段,测试了用Ｂridgman方法生长的ＰbF2:Gd晶体中Ｇd离子沿生长方向的分布。发现该晶体在Ｘ射线激发下的发光强度与Ｇd在ＰbF2晶体中的含量变化均从结晶开始端至结晶结束端逐渐降低,表明ＰbF2:gD晶体的发光是不均匀的。这种不均匀性源于Ｇd离子在ＰbF2晶体中的分布不均匀性,通过在ＰbF2:Gd晶体中掺入一定量的碱金属离子,可以比较有效地改善Ｇd离子在ＰbF２晶体中的不均匀分布,使晶体的发光均匀性和透光率得到明显提高,其中以Ｎa离子的均匀化效果为最好,推测Ｎa离子的作用是消除了ＰbF2:Gd晶体中因子Ｇd离子的掺杂而造成的间隙Ｆ离子,从而使晶体中的缺陷浓度大大降低。     

Conversion of green emission into white light in Gd2O3 nanophosphors

Gd₂O₃ nanophosphors were prepared by combustion synthesis with and without doping of Dy³⁺ ions. The X-ray powder diffraction patterns indicate that as-prepared Gd₂O₃ and 0.1 mol% Dy₂O₃ doped Gd₂O₃ nanophosphors have monoclinic structures. The transmission electron microscope (TEM) studies revealed that the as-prepared phosphors had an average crystallite sizes around 37 nm. The excitation and emission properties have been investigated for Dy³⁺ doped and undoped Gd₂O₃ nanophosphors. New emission bands were observed in the visible region for Gd₂O₃ nanophosphors without any rare earth ion doping under different excitations. A tentative mechanism for the origin of luminescence from Gd₂O₃ host was discussed. Emission properties also measured for 0.1 mol% Dy³⁺ doped Gd₂O₃ nanophosphors and found the characteristic Dy³⁺ visible emissions at 489 and 580 nm due to ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions, respectively. The chromaticity coordinates were calculated based on the emission spectra of Dy³⁺ doped and undoped Gd₂O₃ nanophosphors and analyzed with Commission Internationale de l'Eclairage (CIE) chromaticity diagram. These nanophosphors exhibit green color in undoped Gd₂O₃ and white color after adding 0.1 mol% Dy₂O₃ to Gd₂O₃ nanophosphors under UV excitation. These phosphors could be a promising phosphor for applications in flat panel displays.     

Synthesis and spectral analysis of Yb/Tm/Ho-doped Na0.5Gd0.5WO4 phosphor to achieve white upconversion luminescence

Yb³⁺/Tm³⁺/Ho³⁺-doped Na_0.5Gd_0.5WO₄ phosphors were synthesized by the high-temperature solid-state method. Bright white luminescence upon 980 nm near-infrared excitation can be observed for the sample at the optimum chemical composition of Na_0.5Gd_0.5WO₄:10%Yb³⁺/1%Tm³⁺/0.4%Ho³⁺, which is produced via an upconversion (UC) process by tuning the dopant ions concentration. The measured white light consists of the blue, green, and red UC emissions which correspond to the transitions ¹G₄ → ³H₆ of Tm³⁺, ⁵F₄(⁵S₂) → ⁵I₈, and ⁵F₅ → ⁵I₈ of Ho³⁺ ions, respectively. The calculated color coordinates display that white light can be achieved in a wide range of dopant concentrations. The UC mechanisms were also proposed based on their spectral and pumping power dependence analyses.     

Na43+ clusters in sodalite

Anion-sodalites with the general formula Na₈[(AlO₂)₆(SiO₂)₆]A₂, where A = Cl, Br or I, were reacted with the vapour of alkali metals. In all cases the ionic cluster Na₄³⁺ was formed, together with a metallic cluster. The ionic cluster was investigated by electron spin resonance, electron spin echo and pulse-electron nuclear double resonance techniques. A modulation was observed in the electron spin echo envelope due to interaction with distant Na⁺ ions. These Na⁺ ions have hyperfine splitting constants between 0.24 and 1 MHz and are situated ∼ 5 Å from the centre of the sodium cluster.