Synthesis and luminescence properties of color-tunable Ba1−ySryLa4−xTbx(WO4)7 (x = 0.02-1.2, y = 0-0.4) phosphors

Ba_1−ySr_yLa_4−xTb_x(WO₄)₇ (x = 0.02-1.2, y = 0-0.4) phosphors were prepared via a solid-state reaction and their photoluminescence properties were investigated. An analysis of the decay behavior indicates that the energy migration between Tb³⁺ ions is conspicuous in the ⁵D₃ → ⁷F₄ transition due to the cross-relaxation in BaLa₄(WO₄)₇. A partial substitution of Ba²⁺ by Sr²⁺ can not only enhance the emission intensity but also increase the solid solubility of Tb³⁺ in Ba_1−ySr_yLa_4−xTb_x(WO₄)₇. The emission intensity of the ⁵D₄ → ⁷F_J (J = 4, 5, 6) transitions can be enhanced by increasing Sr²⁺ and Tb³⁺ concentrations, with the optimal conditions being x = 1.2, y = 0.4 (Ba_0.6Sr_0.4La_2.8Tb_1.2(WO₄)₇). Under near-UV excitation at 379 nm, the CIE color coordinates of Ba_1−ySr_yLa_4−xTb_x(WO₄)₇ vary from blue (0.212, 0.181) at x = 0.04, y = 0, to green (0.245, 0.607) at x = 1.2, y = 0.4.     

Frequency up-conversion luminescence in Yb3+–Ho3+ co-doped PbxCd1−xF2 nano-crystals precipitated transparent oxyfluoride glass-ceramics

Oxyfluoride glasses were developed with composition 30SiO₂·15AlO_1.5·28PbF₂·22CdF₂·(4.9−x)GdF₃·0.1HoF₃·xYbF₃ (x=0, 0.1, 0.2, 0.5, 1, 2, 3, and 4) in mol%. Powder X-ray diffraction analysis revealed that the heat-treatments of the oxyfluoride glasses at the first crystallization temperature cause the precipitation of Yb³⁺–Ho³⁺ co-doped fluorite-type nano-crystals of about 17.8 nm in diameter in the glass matrix. These transparent glass-ceramics exhibited very strong green up-conversion luminescence due to the Ho³⁺: (⁵F₄, ⁵S₂)→⁵I₈ transition under 980 nm excitation. The intensity of the green up-conversion luminescence in the glass-ceramics was much stronger than that in the precursor oxyfluoride glass. The reasons for the highly efficient Ho³⁺ up-conversion luminescence in the oxyfluoride glass-ceramics are discussed.     

Pechini sol-gel deposition and luminescent properties of SrLa1−xRExGa3O7 (RE = Eu, Tb) phosphor films

SrLa_1−xRE_xGa₃O₇ (RE = Eu³⁺, Tb³⁺) phosphor films were deposited on quartz glass substrates by a simple Pechini sol-gel method. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), atomic force microscopy, field-emission scanning electron microscopy, photoluminescence spectra, and lifetimes were used to characterize the resulting films. The results of XRD indicated that the films began to crystallize at 700 °C and crystallized fully at 900 °C. The results of FT-IR spectra were in agreement with those of XRD. Uniform and crack-free films annealed at 900 °C were obtained with average grain size of 80 nm, root mean square roughness of 46 nm and thickness of 130 nm. The RE ions showed their characteristic emission in crystalline SrLa_1−xRE_xGa₃O₇ films, i.e., Eu³⁺⁵D₀-⁷F_J (J = 0, 1, 2, 3, 4), Tb³⁺⁵D₄-⁷F_J (J = 6, 5, 4, 3) emissions, respectively. The optimum concentrations (x) of Eu³⁺ and Tb³⁺ were determined to be 50, and 80 mol% in SrLa_1−xRE_xGa₃O₇ films, respectively.     

Structure and luminescence properties of Eu/Tb codoped oxyfluoride glass ceramics containing Sr2GdF7 nanocrystals

Eu/Tb codoped transparent oxyfluoride borosilicate glass ceramics containing Sr₂GdF₇ nanocrystals were fabricated under a reductive atmosphere and the conversion of Eu³⁺ ions to Eu²⁺ ions was observed. The Sr₂GdF₇ nanocrystals with an average size of 32 nm were homogeneously precipitated in the oxyfluoride borosilicate glass matrix, which could be evidenced by X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX) spectroscopy. The enhancement of photoluminescence emission intensity, reduction of the relative emission intensities between ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁, and long fluorescence lifetimes of Eu²⁺, Eu³⁺, and Tb³⁺ ions revealed that more rare earth ions were partitioned into the low phonon energy environment Sr₂GdF₇ nanocrystals. Under ultraviolet excitation, pure and bright white light emission was obtained in the oxyfluoride borosilicate glass ceramic, which may be a potential blue, green and red-emitting phosphor for white LEDs.     

Dependence of optical properties on the composition of (Ba1−x−ySrxEuy)Si2O2N2 phosphors for white light emitting diodes

BaSi₂O₂N₂: Eu²⁺ is an efficient phosphor because of its high quantum yield and quenching temperature. Partial substitution of Ba²⁺ by Sr²⁺ is the most promising approach to tune the color of phosphors. In this study, a series of (Ba_1−x−ySr_xEu_y)Si₂O₂N₂ (x = 0.0–0.97, y = 0.00–0.10) phosphors are synthesized via high-temperature solid-state reactions. Intense green to yellow phosphors can be obtained by the partial substitution of the host lattice cation Ba²⁺ by either Sr²⁺ or Eu²⁺. The luminescent properties and the relationships among the lowest 5d absorption bands, Stokes shifts, centroid shifts, and the splitting of Eu²⁺ are studied systematically. Then, based on (Ba_1−x−ySr_xEu_y)Si₂O₂N₂ phosphors and near-ultraviolet (∼395 nm)/blue (460 nm) InGaN chips, intense green–yellow light emitting diodes (LEDs) and white LEDs are fabricated. (Ba_0.37Sr_0.60)Si₂O₂N₂: 0.03Eu²⁺ phosphors present the highest efficiency, and the luminous efficiency of white LEDs can reach 17 lm/w. These results indicate that (Ba_1−x−ySr_xEu_y)Si₂O₂N₂ phosphors are promising candidates for solid-state lighting.     

Synthesis and characterization of Ba1−xNdxF2+x (0.00 ≤ x ≤ 1.00)

A series of mixed fluorides with general composition Ba_1−xNd_xF_2+x (0.00≤x≤1.00) was prepared by vacuum heat treatment of the mixture of starting fluorides, and analyzed by powder XRD. From the XRD analysis, the low temperature phase equilibria in BaF₂-NdF₃ system is elucidated. The initial compositions in this series, that is, up to the nominal composition Ba_0.65Nd_0.35F_2.35 (x≤0.35) exist as cubic fluorite-type solid solution. Beyond the solid solution limit, that is, x>0.35, a rhombohedral fluorite related ordered phase is observed. Further, NdF₃-rich compositions (x≥0.50) exist as a mixture of rhombohedral ordered phase and NdF₃ (tysonite)-type phase. About 10 mol% of BaF₂ could be retained in the NdF₃ lattice, forming a tysonite-type solid solution, under the short annealed and slow cooled conditions.     

Structural and electrical properties of La2−xBaxMo2O9−x/2 (x = 0-0.18)

La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) have been prepared by solid state reaction method. The lattice parameter of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) determined by XRD data refinement shows a linear dependence on the dopant Ba content x. For the specimen with a La/Ba molar ratio of 0.18-0.2, additional reflection of secondary phase exists in the XRD pattern, so the value of solubility limit for Ba in La₂Mo₂O₉ is defined in range of 0.18 < x < 0.2. As the replacement degree of La³⁺ by Ba²⁺ increases, the bulk conductivity of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) decreases initially and then increases, a minimum value at La_1.9Ba_0.1Mo₂O_8.95 exists. Hebb-Wagner studies in argon atmosphere, which use an oxide-ion blocking electrode, show that La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) are predominantly oxide-ion conducting in the temperature ranging from 773 to 1173 K. The average thermal expansion coefficient of La_1.84Ba_0.16Mo₂O_8.92 determined by high-temperature XRD was deduced as great as 17.5 × 10⁻⁶ K⁻¹ between 298 and 1173 K.     

Enhancement of green emission for Al-doped YBO3:Tb

High-quality Y_1−x−yAl_xTb_yBO₃ (0 ≤ x ≤ 0.1, 0.04 ≤ y ≤ 0.16) phosphor powders with fine size, spherical and regular morphology, and non-agglomeration were successfully prepared by ultrasonic spray pyrolysis. The blue emission from the ⁵D₃→⁷F_J (J = 4, 5, and 6) transition for the Y_0.94−xAl_xTb_0.06BO₃ phosphor was quenched. The optimal concentration of Tb³⁺ was reduced by doping Al³⁺ into the Y_1−yTb_yBO₃ phosphor, i.e., y = 0.1 and 0.12 for Y_0.975−yAl_0.025Tb_yBO₃ and Y_1−yTb_yBO₃, respectively. The Al³⁺ doping was highly effective for improving the photoluminescence characteristics. The photoluminescence emission intensity of the Al³⁺-doped Y_0.915Al_0.025Tb_0.06BO₃ phosphor at 543 nm was about three times stronger than that of the Al³⁺-free Y_0.94Tb_0.06BO₃ phosphor.     

Enhanced photoluminescence of Gd0.94(P1−xVx)O4:Eu0.06 red-emitting (0 ≤ x ≤ 1.0) phosphors

High-quality Gd_0.94(P_1−xV_x)O₄:Eu_0.06 (0 ≤ x ≤ 1.0) powders having small size, spherical morphology, smooth surface, and nonaggregation are synthesized by the ultrasonic spray pyrolysis. The complex host composition, Gd_0.94(P_1−xV_x)O₄:Eu_0.06 (x ≥ 0.5), shows a single phase with the tetragonal xenotime structure. The introduction of V⁵⁺ ions in the P⁵⁺ lattice yields the deviation from the centrosymmetry of Eu³⁺ ions. By increasing the V⁵⁺ content, the emission intensity corresponding to the ⁵D₀ → ⁷F₁ transition decreases. On the other hand, the emission intensity corresponding to the ⁵D₀ → ⁷F₂ transition increases up to x = 0.5, and then decreases upon further increasing the V⁵⁺ content. The emission intensity, peaking at 620 nm, of complex composition Gd_0.94(P_0.5V_0.5)O₄:Eu_0.06 is approximately five and two times stronger than that of the Gd_0.94PO₄:Eu_0.06 and Gd_0.94VO₄:Eu_0.06, respectively. It is believed that the introduction of the complex composition Gd_0.94(P_1−xV_x)O₄:Eu_0.06 is highly effective for improving the emission properties.     

Study on properties of BaxSmyTi7O20 ceramics with CaO-SiO2-B2O3 glass

The effect of CaO-SiO₂-B₂O₃ (CSB) glass addition on the sintering temperature and dielectric properties of Ba_xSm_yTi₇O₂₀ ceramics has been investigated using X-ray diffraction, scanning electron microscopy and differential thermal analysis. The CSB glass starts to melt at about 970 °C, and a small amount of CSB glass addition to Ba_xSm_yTi₇O₂₀ ceramics can greatly decrease the sintering temperature from about 1350 to about 1260 °C, which is attributed to the formation of liquid phase. It is found that the dielectric properties of Ba_xSm_yTi₇O₂₀ ceramics are dependent on the amount of CSB glass and the microstructures of sintered samples. The product with 5 wt% CSB glass sintered at 1260 °C is optimal in these samples based on the microstructure and the properties of sintering product, when the major phases of this material are BaSm₂Ti₄O₁₂ and BaTi₄O₉. The material possesses excellent dielectric properties: ?_r = 61, tan δ = 1.5 × 10⁻⁴ at 10 GHz, temperature coefficient of dielectric constant is −75 × 10⁻⁶ °C⁻¹.     

Different microstructure and dielectric properties of Ba1−xCaxTiO3 ceramics and pulsed-laser-ablated films

A comparative study of the microstructure and dielectric properties between Ba_1−xCa_xTiO₃ (BCT) ceramics and films were performed in the whole Ca concentration range of x = 0-1. The ceramics were prepared by conventional solid-state reaction technique and the films by the method of pulsed-laser deposition. X-ray diffraction (XRD) study of the BCT ceramics exhibited a pure tetragonal phase for x = 0-0.25, a tetragonal-orthorhombic diphase for x = 0.25-0.85 and a pure orthorhombic phase for x = 0.90-1.00. And the dielectric phase transition temperature from tetragonal to cubic was marginally affected by the Ca doping into BaTiO₃. However, BCT films deposited on Pt/Si/SiO₂/Si substrates showed a different microstructure and dielectric properties. Tetragonal-orthorhombic diphase was not found in the BCT films for x = 0.25-0.85, and a large decrease of the Curie point and diffuse phase transition were observed in the BCT films. Based on the compositional analysis, such phenomena were ascribed to the occupancy of some Ca²⁺ to the Ti⁴⁺ sites in the BCT films.     

Near-infrared quantum cutting platform in transparent oxyfluoride glass–ceramics for solar sells

This study investigated the photoluminescent properties of Er³⁺/Yb³⁺ and Ce³⁺/Er³⁺/Yb³⁺ -doped oxyfluoride glass–ceramics. The transparent oxyfluoride glass–ceramics were prepared by high temperature melting method and subsequent heat treatment. Effect of heat treatment schedules on crystallization behavior and microstructure were analyzed by differential scanning caborimetry, X-ray diffraction, infrared spectrum and scanning electron microscopy. The structure of fluoride nanocrystals indicates that the main phase in the oxyfluoride glass ceramics is CaF₂ nanocrystal sized at 25 nm at the optimal crystallization temperature 600 °C for 8 h. The Ce³⁺/Er³⁺/Yb³⁺ tri-doped oxyfluoride glass–ceramics shows wider absorption bands comparing with Er³⁺/Yb³⁺ co-doped oxyfluoride glass–ceramics. The effective energy transfer processes from Ce³⁺ to Yb³⁺, Er³⁺ to Yb³⁺ and Ce³⁺ to Er³⁺ all can take place simultaneously. The idea of using Ce³⁺ together with Er³⁺ and Yb³⁺ ions could enhance the ultraviolet visible light absorption and the 960–1040 nm near infrared emission. Results of this study demonstrate that the tri-activator Ce³⁺/Er³⁺/Yb³⁺ materials are promising for practical application to enhance the energy efficiency of crystalline Si solar cells via spectrum modification.     

Crystal structure and Eu/Tb doped luminescent properties of a new borate Ba3BiB9O18

Undoped and doped either by Eu³⁺ or Tb³⁺ bismuth borate Ba₃BiB₉O₁₈ was structurally characterized and analyzed by fluorescence spectroscopy. Belonging to synthetic borate member of the family Ba₃XB₉O₁₈, layers of planar triangular B₃O₆ groups connecting with deformed BaO₆ hexagons are interleaved by 9-coordinate Ba atoms, and 6-coordinate Bi atoms. Its crystal structure was determined and refined from powder X-ray diffraction data by the Rietveld method and the results showed that Ba₃BiB₉O₁₈ belongs to space group P6₃/m with unit cell dimensions of a = 7.1999(2) Å, c = 17.3567(6) Å, and z = 2. Curves of differential thermal analysis and thermogravimetric analysis showed that Ba₃BiB₉O₁₈ is a congruent melting compound and chemically stable above 728 °C. Ba₃Bi_1−xEu_xB₉O₁₈ and Ba₃Bi_1−xTb_xB₉O₁₈ form a continuous solid solution from x = 0.01 to x = 0.9. The ultraviolet excited photoluminescence intensity increased with both Eu³⁺ and Tb³⁺ concentration in the matrix of Ba₃BiB₉O₁₈. There may be an interesting correlation between spectroscopic properties and lattice structural features of doped Ba₃BiB₉O₁₈.     

New transparent Er-doped oxyfluoride tellurite glass ceramic with improved near infrared and up-conversion fluorescence properties

Transparent glass ceramics have been obtained by nucleation and growth of Y₂Te₆O₁₅ or Er₂Te₅O₁₃ cubic phase in a new Er³⁺-doped oxyfluoride tellurite glass. Effect of heat treatment on absorption spectra, luminescence and up-conversion properties in the oxyfluoride tellurite glass has been investigated. With heat treatment the ultraviolet absorption edge red shifted evidently for the oxyfluoride telluride glass. The near infrared emission that corresponds to Er³⁺:⁴I_13/2 → ⁴I_15/2 can be significantly enhanced after heat treatment. Under 980 nm LD pumping, red and green up-conversion intensity of Er³⁺ in the glass ceramic can be observed much stronger than that in the base glass.     

Chemical co-precipitation composition of hybrid precursors to synthesize Y0.5−xDyxLi1.5VO4 microcrystalline phosphors

Y_0.5−xDy_xLi_1.5VO₄ phosphor particles have been synthesized by a new wet-chemical method of in situ co-precipitation via assembling inorganic/organic hybrid precursors. X-ray diffraction and scanning electronic microscope show that these materials have the micrometer dimension. All the emission spectra exhibit the characteristic transitions of Dy³⁺, which is due to the ⁴F_9/2 → ⁶H_15/2 (blue) and ⁴F_9/2 → ⁶H_13/2 (yellow) and the concentration quenching phenomenon occurs in the system of Y_0.5−xDy_xLi_1.5VO₄.     

Synthesis and optical properties of erbium-doped BaF2 nanoparticles

Erbium-doped BaF₂ nanoparticles were prepared using the ultrasonic surge method. The particles were characterized by X-ray diffraction, transmission electron microscope, photoluminescence and absorption. The results show that when Er³⁺ ions were introduced into the BaF₂ lattice, cubic fluorite-type Ba_1−xEr_xF_2+x solid solution, x = 0.42 was formed. Furthermore, the crystalline phase of Ba_1−xEr_xF_2+x solid solution was largely influenced by the annealing temperature.     

Effects of GeO2 on the thermal stability and optical properties of Er/Yb-codoped oxyfluoride tellurite glasses

The aim of this article was to study the influence of GeO₂ on the thermal stability and optical properties of Er³⁺/Yb³⁺ codoped (70 − x)TeO₂–xGeO₂–PbF₂–BaF₂ (TGEYx) glasses prepared by using a melting method. The properties of Er³⁺/Yb³⁺ codoped glasses were investigated by using differential scanning calorimetry, upconversion luminescence, Raman and optical absorption spectra. The results indicated that TGEY35 glass with the germanate–tellurite mixed network showed the best thermal stability and poor crystallization tendency. With increasing the GeO₂ content, the maximum phonon energy of oxyfluoride tellurite glass network increased, while the phonon density decreased. The upconversion emission intensities enhanced obviously based on the decreasing phonon density of glass hosts, while the increasing red emission (657 nm) with the increase of GeO₂ concentration was attributed to the relative larger maximum phonon energy which matching the energy gap between the pertinent ⁴S_3/2 and ⁴F_9/2 levels.     

Synthesis and characterization of Ba1−xErxF2+x (0.00≤x≤1.00)

A series of mixed fluorides with general composition Ba_1−xEr_xF_2+x (0.00≤x≤1.00) were prepared by a vacuum heat treatment of the binary fluorides and analyzed by powder XRD, which revealed the phase equilibrium in BaF₂-ErF₃ system. The nominal compositions with x≤0.25 compositions exist as cubic fluorite type solid solution. A rhombohedral ordering is observed in the vicinity of 42 mol% of ErF₃ in BaF₂. A monoclinic phase with the composition Er₂BaF₈ dominates the ErF₃-rich compositions. The compositions for x>0.67 exist as a mixture of Er₂BaF₈ and leftover ErF₃, whereas the compositions for x<0.67 exist as a mixture of Er₂BaF₈ and rhombohedral fluorite derived phase. The evolution of the various phases will be discussed in reference to the powder XRD data.     

Tunable green-red luminescence in Ho/Yb/Ce: YF3 nanocrystals embedded transparent glass ceramics

The Ho³⁺/Yb³⁺/Ce³⁺:YF₃ nanocrystals embedded transparent oxyfluoride glass ceramics were successfully synthesized and the emission spectra have been measured. We changed the doped concentrations of ion Ce³⁺ in the glass ceramics and unexpectedly found that the different concentrations brought forth different luminescence wavelengths, a tunable luminescence wave band from green to red, which might be useful to the tunable visible laser or display applications. In addition, excitated with wavelength 975 nm LD, the transitions of Ho³⁺: ⁵G₄ → ⁵I₈ at 390 nm, ⁵G₅ → ⁵I₈ at 420 nm and ⁵F₃ → ⁵I₈ at wavelength 490 nm were observed in the glass ceramic GCCe0, while in samples GCCeJ(J=1,3,5,8), the three peaks disappeared.     

Ba3ZnTa2−xNbxO9 and Ba3MgTa2−xNbxO9 (0≤x≤1): synthesis, structure and dielectric properties

Oxides belonging to the families Ba₃ZnTa_2−xNb_xO₉ and Ba₃MgTa_2−xNb_xO₉ were synthesized by the solid state reaction route. Sintering temperatures of 1300°C led to oxides with disordered (cubic) perovskite structure. However, on sintering at 1425°C hexagonally ordered structures were obtained for Ba₃MgTa_2−xNb_xO₉ over the entire range (0≤x≤1) of composition, while for Ba₃ZnTa_2−xNb_xO₉ the ordered structure exists in a limited range (0≤x≤0.5). The dielectric constant is close to 30 for the Ba₃ZnTa_2−xNb_xO₉ family of oxides while the Mg analogues have lower dielectric constant of ∼18 in the range 50 Hz to 500 kHz. At microwave frequencies (5-7 GHz) dielectric constant increases with increase in niobium concentration (22-26) for Ba₃ZnTa_2−xNb_xO₉; for Ba₃MgTa_2−xNb_xO₉ it varies between 12 and 14. The “Zn” compounds have much higher quality factors and lower temperature coefficient of resonant frequency compared to the “Mg” analogues.