Effect of sintering atmosphere and lithium ion co-doping on photoluminescence properties of NaCaPO4:Eu phosphor

This paper reports on the photoluminescence properties of Na_1−yLi_yCa_1−xPO₄:_xEu²⁺ phosphors synthesized by a solid state reaction method. The prepared phosphors have been thoroughly characterized by means of X-ray diffraction (XRD), Field-emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectrum (FTIR), Raman spectrum, Thermo gravimetric and differential thermal analysis (TG–DTA) and photoluminescent spectral measurements. The structure of Na_1−yLi_yCa_1−xPO_4:xEu²⁺ phosphors were found to be orthorhombic in nature with a sphere-like morphology and having the particle size in micrometer range. The excitation spectra of NaCaPO₄:Eu²⁺ phosphors revealed a broad excitation band having its maximum intensity at 373 nm and ranging from 250 m to 450 nm. Incidentally, it matches well with the ultraviolet (UV) radiation of light-emitting diodes (LEDs). Upon 373 nm excitation, these phosphors exhibited intense bluish-green emission band centered at 505 nm. The effect of sintering atmospheres and co-doping of lithium ions on the photoluminescence properties of the NaCaPO₄:Eu²⁺ phosphors were studied and explained suitably. The obtained results indicate that the prepared NaCaPO₄:Eu²⁺ phosphors are promising bluish-green candidates for the phosphor-converted white LED applications.     

Enhanced luminescence of novel Ca3B2O6:Dy phosphors by Li-codoping for LED applications

The Ca_3−xB₂O₆:xDy³⁺ (0.0 ≤ x ≤ 0.105) and Ca_2.95−yDy_0.05B₂O₆:yLi⁺ (0 ≤ y ≤ 0.34) phosphors were synthesized at 1100 °C in air by solid-state reaction route. The as-synthesized phosphors were characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM), photoluminescence excitation (PLE) and photoluminescence (PL) spectra. The PLE spectra show the excitation peaks from 300 to 400 nm is due to the 4f-4f transitions of Dy³⁺. This mercury-free excitation is useful for solid state lighting and light-emitting diodes (LEDs). The emission of Dy³⁺ ions upon 350 nm excitation is observed at 480 nm (blue) due to the ⁴F_9/2 → ⁶H_15/2 transitions, 575 nm (yellow) due to ⁴F_9/2 → ⁶H_13/2 transitions and a weak 660 nm (red) due to ⁴F_9/2 → ⁶H_11/2 emissions, respectively. The optimal PL intensity of the Ca_3−xB₂O₆:xDy³⁺ phosphors is found to be x = 0.05. Moreover, the PL results from Ca_2.95−yDy_0.05B₂O₆:yLi⁺ phosphors show that Dy³⁺ emissions can be enhanced with the increasing codopant Li⁺ content till y = 0.22. By simulation of white light, the CIE of the investigated phosphors can be tuned by varying the content of Li⁺ ions, and the optimal CIE value (0.300, 0.298) is realized when the content of Li⁺ ions is y = 0.22. All the results imply that the Ca_2.95−yDy_0.05B₂O₆:yLi⁺ phosphors could be potentially used as white LEDs.     

Tunable luminescence properties and efficient energy transfer in Eu,Mn co-doped Ba2MgP4O13

A series of Ba₂Mg_1−xMn_xP₄O₁₃ (x = 0-1.0) and Ba_1.94Eu_0.06Mg_1−xMn_xP₄O₁₃ (x = 0-0.15) phosphors were prepared by conventional solid-state reaction. X-ray powder diffraction (XRD), the photoluminescence spectra, and the decay curves are investigated. XRD analysis shows that the maximum tolerable substitution of Mn²⁺ for Mg is about 50 mol% in Ba₂MgP₄O₁₃. Mn²⁺-singly doped Ba₂MgP₄O₁₃ shows weak red-luminescence peaked at about 615 nm. The Eu²⁺/Mn²⁺ co-doped phosphor emits two distinctive luminescence bands: a blue one centered at 430 nm originating from Eu²⁺ and a broad red-emitting one peaked at 615 nm from Mn²⁺ ions. The luminescence of Mn²⁺ ions can be greatly enhanced with the co-doping of Eu²⁺ in Ba₂MgP₄O₁₃. The efficient energy transfer from Eu²⁺ to Mn²⁺ is verified by the excitation and emission spectra together with the luminescence decay curves. The emission colors could be tuned from the blue to the red-purple and eventually to the deep red. The resonance-type energy transfer via a dipole-quadrupole interaction mechanism is supported by the decay lifetime data. The energy transfer efficiency and the critical distance are calculated and discussed. The temperature dependent luminescence spectra of the Eu²⁺/Mn²⁺ co-doped phosphor show a good thermal stability on quenching effect.     

The doping concentration dependent tunable yellow luminescence of Sr5(PO4)2(SiO4):Eu

Color tunable yellow-emitting phosphors of Sr_5−5xEu_5x(PO₄)₂SiO₄ (x = 0.05-0.15) were prepared by conventional solid-state reaction method. The X-ray powder diffraction patterns, the photoluminescence excitation and emission spectra were measured. The main excitation bands of the phosphors locate at a broad band extending from 300 to 500 nm, which can match the emission of ultraviolet- and blue-emitting diode chips. The tunable luminescence color was realized by the changing Eu²⁺ doping in Sr₅(PO₄)₂SiO₄. The structure and luminescence properties were investigated. Sr_5−5x(PO₄)₂SiO₄:Eu_5x displays two typical luminescence centers, which originate from two different Sr²⁺ (Eu²⁺) sites in the host. The site-occupation, the luminescence intensity and energy transfer between the Eu²⁺ ions occupying two different crystallographic Sr²⁺ sites were discussed on the base of the luminescence spectra and crystal structure. This is helpful to improve this phosphor for a potential application as a white light emitting diode phosphor.     

VUV photoluminescence properties of Y1−xGdxVO4:Eu phosphors prepared by ultrasonic spray pyrolysis

The red-emitting (Y_1−xGd_x)_0.94Eu_0.06VO₄ (0 ≤ x ≤ 1.0) phosphors were synthesized by ultrasonic spray pyrolysis. The (Y_1−xGd_x)_0.94Eu_0.06VO₄ (0 ≤ x ≤ 1.0) phosphors had the tetragonal xenotime structure with a space group of I4₁/amd (1 4 1). The calculated crystallite sizes of the annealed phosphors ranged from 58 to 68 nm. In this study, we discussed the photoluminescence properties of the (Y_1−xGd_x)_0.94Eu_0.06VO₄ phosphors under VUV excitation, depending on Gd content. The emission intensity of the (Y_1−xGd_x)_0.94Eu_0.06VO₄ phosphors increased with increasing Gd content up to x = 0.5, and then decreased with a further increase in Gd content. The purest red color was obtained for the (Y_0.5Gd_0.5)_0.94Eu_0.06VO₄ phosphors.     

Solid state reaction preparation and enhanced red luminescence of S-doped La2Mo2O9:Prphosphors

Red-light-emitting phosphors of La₂Mo₂O₉:Pr³⁺ and S-doped La₂Mo₂O₉:Pr³⁺ were prepared by high temperature solid state reaction. Under the excitation of 450 nm blue light, all samples produced a red emission peak at 650 nm corresponding to the characteristic transition of Pr³⁺ (³P₀→³F₂). The dependence of Pr³⁺ doping content (x) on the luminescent intensity was analyzed, and the optimal doping content of Pr³⁺ was x=0.07. After a small quantity of sulfur was introduced into the system, the luminescence intensity of phosphors was obviously enhanced. The reasons for the enhancement of luminescence are due to improved crystallization after S doping and the relatively large electronegativity difference between S and Mo. Additionally, the coincidence of the excitation wavelength with the emission of GaN chips may recommend this phosphor system as a potential candidate for use in white light-emitting diodes.     

Synthesis and negative thermal expansion property of Y2−xLaxW3O12 (0≤x≤2)

Y_2−xLa_xW₃O₁₂ solid solutions were successfully synthesized by the solid state reaction method. The microstructure, hygroscopicity and thermal expansion property of the resulting samples were investigated by X-ray diffraction (XRD), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM) and thermal mechanical analysis (TMA). Results indicate that the structural phase transition of the Y_2−xLa_xW₃O₁₂ changes from orthorhombic to monoclinic with increasing substituted content of lanthanum. The pure phase can form for 0≤x≤0.4 with orthorhombic structure and for 1.5≤x≤2 with monoclinic one. High lanthanum content leads to a low relative density of Y_2−xLa_xW₃O₁₂ ceramic. Thermal expansion coefficients of the Y_2−xLa_xW₃O₁₂ (0≤x≤2) ceramics also vary from −9.59×10⁻⁶ K⁻¹ to 2.06×10⁻⁶ K⁻¹ with increasing substituted content of lanthanum. The obtained Y_0.25La_1.75W₃O₁₂ ceramic shows almost zero thermal expansion and its average linear thermal expansion coefficient is −0.66×10⁻⁶ K⁻¹ from 103 °C to 700 °C.     

Preparation and characterization of novel spinel Li4Ti5O12−xBrx anode materials

Br-doped Li₄Ti₅O₁₂ in the form of Li₄Ti₅O_12−xBr_x (0 ≤ x ≤ 0.3) compounds were successfully synthesized via solid state reaction. The structure and electrochemical properties of the spinel Li₄Ti₅O_12−xBr_x (0 ≤ x ≤ 0.3) materials were investigated. The Li₄Ti₅O_12−xBr_x (x = 0.2) presents the best discharge capacity among all the samples, and shows better reversibility and higher cyclic stability compared with pristine Li₄Ti₅O₁₂, especially at high current rates. When the discharge rate was 0.5 C, the Li₄Ti₅O_12−xBr_x (x = 0.2) sample presented the excellent discharge capacity of 172 mAh g⁻¹, which was very close to its theoretical capacity (175 mAh g⁻¹), while that of the pristine Li₄Ti₅O₁₂ was 123.2 mAh g⁻¹ only.     

Synthesis and thermal expansion properties of Y2−xLaxMo3O12 (x=0, 0.5, 2)

Y_2−xLa_xMo₃O₁₂ (x=0, 0.5, 2) ceramics were successfully synthesized by the solid state reaction method. The microstructure, composition and thermal expansion property of the resulting samples were investigated by X-ray diffraction (XRD), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS) and dilatometry. Results indicate that the Y_1.5La_0.5Mo₃O₁₂ crystallizes in monoclinic Tb₂Mo₃O₁₂-type structure and it is non-hygroscopic. The Y_1.5La_0.5Mo₃O₁₂ ceramic is denser than the Y₂Mo₃O₁₂ and La₂Mo₃O₁₂ ceramics, and its relative density can reach 94.12% of the theoretical value. Most importantly, it shows almost zero thermal expansion and its thermal coefficient is 0.87×10⁻⁶ K⁻¹ from 178 °C to 600 °C. Y₂Mo₃O₁₂ ceramic shows negative thermal expansion whereas La₂Mo₃O₁₂ ceramic shows positive thermal expansion, their thermal expansion coefficients being−12.06×10⁻⁶ K⁻¹ and 8.88×10⁻⁶×10⁻⁶ K⁻¹, respectively.     

Fluorescence improvement of Ba1.3Ca0.7SiO4:Eu,Mn phosphors via Dy addition and their color-tunable properties

A series of novel single-phase white phosphors Ba_1.3Ca_0.69−x−ySiO₄:0.01Eu²⁺,xMn²⁺, yDy³⁺ were synthesized by the solid-state method. The excitation spectra of these phosphors exhibit a broad band in the range of 260–410 nm, which can meet the application requirements for near-UV LED chips (excited at 350–410 nm). The emission spectra consist of two broad bands positioned around 455 nm and 596 nm, which are assigned to 5d→4f transition of Eu²⁺, and ⁴T₁→⁶A₁ transition of Mn²⁺, respectively. The luminescence intensity of phosphors enhances obviously by doping Dy³⁺ ions, and the intensity of two bands reaches an optimum when Dy³⁺ amounts to 2 mol%. In addition, thermoluminescence investigation of phosphor was conducted, getting two shallow trap defects with activation energy of 0.43 eV and 0.45 eV, which demonstrates the energy transfer mechanism of Dy–Eu through the process of hole and electron traps. By precisely tuning the Mn²⁺ content, an optimized white light with color rendering index (CRI) of R_a=84.3%, correlated color temperature (CCT) of T_c=8416 K and CIE chromaticity coordinates of (0.2941, 0.2937) is generated. The phosphor could be a potential white phosphors for near-UV light emitting diodes.     

Relationship between glass network structure and conductivity of Li2O-B2O3-P2O5 solid electrolyte

Solid state glass electrolyte, xLi₂O-(1 − x)(yB₂O₃-(1 − y)P₂O₅) glasses were prepared with wide range of composition, i.e. x = 0.35 - 0.5 and y = 0.17 - 0.67. This material system is one of the parent compositions for chemically and electrochemically stable solid-state electrolyte applicable to thin film battery. Lithium ion conductivity of Li₂O-B₂O₃-P₂O₅ glasses was studied in the correlation to the structural variation of glass network by using FTIR and Raman spectroscopy. The measured ionic conductivity of the electrolyte at room temperature increased with x and y. The maximum conductivity of this glass system was 1.6 × 10⁻⁷ Ω⁻¹ cm⁻¹ for 0.45Li₂O-0.275B₂O₃-0.275P₂O₅ at room temperature. It was shown that the addition of P₂O₅ reduces the tendency of devitrification and increases the maximum amount of Li₂O added into glass former without devitrification. As Li₂O and B₂O₃ contents increased, the conductivity of glass electrolyte increased due to the increase of three-coordinated [BO₃] with a non-bridging oxygen (NBO).     

A reddish orange stoichiometric phosphor LiEu(PO3)4 for white light-emitting diodes

Stoichiometric phosphors LiGd_1−xEu_x(PO₃)₄(x=0, 0.2, 0.4, 0.6, 0.8, 1.0) were synthesized via traditional solid state reactions. The X-ray powder diffraction measurements show that all prepared samples are isostructural with LiNd(PO₃)₄. Eu³⁺ doped phosphors can emit intense reddish orange light under the excitation of near ultraviolet light from 370 to 410 nm. The strongest two at 591 and 613 nm can be attributed to the transitions from excited state ⁵D₀ to ground states ⁷F₁ and ⁷F₂, respectively. The typical chromaticity coordinates (x=0.620, y=0.368) of Eu³⁺ doped phosphors are in red area. The recorded absorbance spectra indicate that there is effective absorbance in the near UV region for all Eu³⁺ doped samples. Present research indicates that LiGd_1–xEu_x(PO₃)₄ is a promising phosphor for white light-emitting diodes.     

The influence of pentavalent Nb substitution for Zr on electrical property of oxide-ion conductor Gd2Zr2O7

Gd₂(Zr_1−xNb_x)₂O_7+x (0 ≤ x ≤ 0.2) ceramics are prepared via the solid state reaction process at 1973 K for 10 h in air. Gd₂(Zr_1−xNb_x)₂O_7+x (x = 0.1, 0.2) ceramics exhibit an ordered pyrochlore-type structure, whereas Gd₂Zr₂O₇ has a defective fluorite-type structure. The electrical property of Gd₂(Zr_1−xNb_x)₂O_7+x ceramics is investigated by electrochemical impedance spectroscopy over a frequency range of 10 Hz to 8 MHz from 623 to 923 K. The electrical conductivity obeys the Arrhenius equation. The grain conductivity of Gd₂(Zr_1−xNb_x)₂O_7+x ceramics varies with doping different Nb contents, and exhibits a maximum at the Nb content of x = 0.1 in the temperature range of 623-923 K. The conductivity in hydrogen atmosphere is a little bit higher than in air in the temperature range of 723-923 K, which indicates that the doping of Zr⁴⁺ by Nb⁵⁺ can increase the proton-type conduction and reduce the oxide-ionic conduction. The conduction of Gd₂(Zr_1−xNb_x)₂O_7+x is not a pure oxide-ionic conductor.     

Potential red-emitting NaGd(MO4)2:R (M=W,Mo, R=Eu,Sm, Bi) phosphors for white light emitting diodes applications

NaGd(MO₄)₂:R (M=W, Mo, R=Eu³⁺, Sm³⁺, Bi³⁺) phosphors were synthesized by solid-state reaction. The structure and photoluminescence properties of the samples were characterized using X-ray powder diffraction and fluorescence spectrophotometry. The ⁵D₀→⁷F₂ transition of Eu³⁺, which led to a red emission of the phosphors, was dominantly observed in the photoluminescence spectra. The doped Bi³⁺ and Sm³⁺ efficiently sensitized the emission of Eu³⁺ and effectively extended and strengthened the absorption of near-UV light with wavelengths ranging from 395 to 405 nm. In addition, energy transfers from Bi³⁺ to Eu³⁺ and from Sm³⁺ to Eu³⁺ occurred. The chromaticity coordinates of the obtained phosphors were close to the standard values of the National Television Standard Committee (x=0.670, y=0.330). The results suggest that NaGd(WO₄)_2−y(MoO₄)_y:Eu³⁺, Sm³⁺, Bi³⁺ is an efficient red-emitting phosphor for light-emitting diode applications.     

White emitting Dy3+ activated perovskite titanates and energy transfer by Eu3+ codoping

Perovskite type titanate phosphors Sr_0.97−xDy_0.03Li_xTi_1−xNb_xO₃, Sr_0.9−xDy_xLi_0.1Ti_0.9Nb_0.1O₃ and Sr_0.87−yDy_0.03Eu_yLi_0.1Ti_0.9Nb_0.1O₃ were prepared by conventional solid state method. Herein, white light emission from Sr_0.9−xDy_xLi_0.1Ti_0.9Nb_0.1O₃ phosphors and the lowering of its color temperature through codoping with Eu³⁺ ions are reported. Raman measurements have shown that the incorporation of dopants alters the vibrational properties of these phosphors significantly, indicating the reduction of the local symmetry in the crystal lattice. The addition of LiNbO₃ in SrTiO₃:Dy³⁺ phosphor enhances the luminescence intensity and the yellow to blue ratio resulting in emission of high quality white light with color coordinates corresponding to that of standard white. Life time measurements and data fits of Sr_0.9−xDy_xLi_0.1Ti_0.9Nb_0.1O₃ phosphors revealed the biexponential behaviour of luminescence decay profiles. From Judd-Ofelt analysis it is found that the intensity parameter Ω₂ increases with Dy³⁺ concentration and a quantum efficiency of 90.4% was obtained for optimum concentration. In the case of Dy³⁺ and Eu³⁺ codoped phosphors, the color coordinates are found to be sensitive to the Eu³⁺ concentration and the highest energy transfer efficiency of 92% was obtained for the phosphor doped with 10 mol% Eu³⁺. The emission color changes from cold white to reddish orange when the wavelength of excitation alters from 452 to 388 nm, since the energy transfer mechanism alone take place under 452 nm excitation and both direct absorption and the energy transfer mechanism occurs under 388 nm excitation.     

Sintering and electrical properties of La-modified (Na0.52K0.45Li0.03)1−3xLax(Nb0.88Sb0.09Ta0.03)O3 lead-free ceramics

(Na_0.52K_0.45Li_0.03)_1−3xLa_x(Nb_0.88Sb_0.09Ta_0.03)O₃ (NKLL_xNST) lead-free ceramics were prepared by normal sintering and their dielectric and piezoelectric properties were investigated. The X-ray methods indicate that the NKLL_xNST ceramics with x≤0.003 present a pure perovskite phase at room temperature. The bulk density of NKLL_xNST ceramics increases with proper amount of La₂O₃ contents, and reaches its highest value of 4.544 g/cm³ with the addition of 0.3 mol% La₂O₃. At x=0.003, remnant polarization P_r, piezoelectric constant d₃₃ and planar mode electromechanical coupling factor k_p of NKLL_xNST ceramics reach the highest values of 37.80 μC/cm², 346 pC/N and 40%, respectively, exhibiting excellent “soft” piezoelectric characteristics, demonstrating a tremendous potential of the compositions studied for device applications.     

Electrochemical and thermal studies of Li[NixLi(1/3−2x/3)Mn(2/3−x/3)]O2 (x = 1/12, 1/4, 5/12, and 1/2)

Samples of the layered cathode materials, Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ (x = 1/12, 1/4, 5/12, and 1/2), were synthesized at 900 °C. Electrodes of these samples were charged in Li-ion coin cells to remove lithium. The charged electrode materials were rinsed to remove the electrolyte salt and then added, along with EC/DEC solvent or 1 M LiPF₆ EC/DEC, to stainless steel accelerating rate calorimetry (ARC) sample holders that were then welded closed. The reactivity of the samples with electrolyte was probed at two states of charge. First, for samples charged to near 4.45 V and second, for samples charged to 4.8 V, corresponding to removal of all mobile lithium from the samples and also concomitant release of oxygen in a plateau near 4.5 V. Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples with x = 1/4, 5/12 and 1/2 charged to 4.45 V do not react appreciably till 190 °C in EC/DEC. Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples charged to 4.8 V versus Li, across the oxygen release plateau, start to significantly react with EC/DEC at about 130 °C. However, their high reactivity is similar to that of Li_0.5CoO₂ (4.2 V) with 1 μm particle size. Therefore, Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples showing specific capacity of up to 225 mAh/g may be acceptable for replacing LiCoO₂ (145 mAh/g to 4.2 V) from a safety point of view, if their particle size is increased.     

Crystalline morphology and photoluminescent properties of YInGe2O7:Eu phosphors prepared from microwave and conventional sintering

This paper describes an investigation of the crystalline morphology and photoluminescent properties of YInGe₂O₇:Eu³⁺ powders using microwave assisted sintering. For comparison, the properties of YInGe₂O₇:Eu³⁺ powders sintered at 1200 °C in conventional furnace for 10 h were also investigated. X-ray powder diffraction analysis confirmed the formation of monoclinic YInGe₂O₇ without second phase or phases of starting materials as YInGe₂O₇:50 mol% Eu powders sintered at 1200 °C in microwave furnace for 1 h. Scanning electron microscopy showed smaller particle size and more uniform grain size distributions are obtained by microwave assisted sintering. In the PL studies, both microwave sintered and conventionally sintered powders emitted a maximum luminescence centered at 620 nm under excitation of 393 nm with similar luminescent intensity. The results show that microwave processing has the potential to reduce the time and required energy input for the production of YInGe₂O₇:Eu³⁺ phosphors without sacrificing the photoluminescence.     

Synthesis and photoluminescence properties of La1−xAlO3:xTb green phosphors for white LEDs

Tb³⁺-doped La_1−xAlO₃ phosphor powders are successfully synthesized by the solution combustion method, using citric acid as the combustion fuel. The crystal structure and photoluminescence properties of La_1−xAlO₃:xTb³⁺ phosphors are studied, depending on Tb³⁺ content. The strongest emission peak is found at 543 nm, which originates from the ⁵D₄→⁷F₅ transition of Tb³⁺ ions, indicating green emission. Among the fabricated phosphors, the La_0.9AlO₃:0.1Tb³⁺ phosphor emits the strongest green light. The excellent luminescent properties make it a possible candidate for white light-emitting diodes and various photonic applications.     

Crystal structure and luminescence properties of green-emitting Sr1−xAl12O19:xEu2+ phosphors

In this paper, a series of novel luminescent Sr_1−xAl₁₂O₁₉:xEu²⁺ phosphors were synthesized by a high temperature solid-state reaction. The phase structure, photoluminescence (PL) properties, as well as the decay curves were investigated. The quenching concentration of Eu²⁺ in SrAl₁₂O₁₉ was about 0.15 (mol). Upon excitation at 378 nm, the composition-optimized Sr_0.85Al₁₂O₁₉:0.15Eu²⁺ exhibited strong broad-band green emission at 530 nm with the CIE chromaticity (0.2917, 0.5736). The results indicate that Sr_1−xAl₁₂O₁₉:xEu²⁺ phosphors have potential applications as green-emitting phosphors for UV-pumped white-light LEDs.