Luminescence properties of Ca4Y6(SiO4)6O:RE (RE = Eu, Tb, Dy, Sm and Tm) under vacuum ultraviolet excitation

The vacuum ultraviolet excited luminescent properties of Eu³⁺, Tb³⁺, Dy³⁺, Sm³⁺ and Tm³⁺ in the matrices of Ca₄Y₆(SiO₄)₆O were investigated. The bands at about 173 nm in the vacuum ultraviolet excited spectra were attributed to host lattice absorption of the matrix Ca₄Y₆(SiO₄)₆O. For Eu³⁺-doped samples, the O²⁻ → Eu³⁺ CTB was identified at 258 nm. Typical 4f-5d absorption bands in the region of 195-300 nm were observed in Tb³⁺-doped samples. For Dy³⁺-doped and Sm³⁺-doped samples, the broad excitation bands consisted of host absorptions, CTB and f-d transition. For Tm³⁺-doped samples, the O²⁻ → Tm³⁺ CTB was located at 191 nm. About the color purity and emission intensity, Ca₄Y₆(SiO₄)₆O:Tb³⁺ is an attractive candidate of green light PDP phosphor, and Ca₄Y₆(SiO₄)₆O:Dy³⁺ has potential application in the field of mercury-free lamps.     

Synthesis and electrochemical properties of Ti doped Li3 − xFe2 − xTix(PO4)3/C cathode materials

Li_3 − xFe_2 − xTi_x(PO₄)₃/C (x = 0-0.4) cathodes designed with Fe doped by Ti was studied. Both Li₃Fe₂(PO₄)₃/C (x = 0) and Li_2.8Fe_1.8Ti_0.2(PO₄)₃/C (x = 0.2) possess two plateau potentials of Fe³⁺/Fe²⁺ couple (around 2.8 V and 2.7 V vs. Li⁺/Li) upon discharge observed from galvanostatic charge/discharge and cyclic voltammetry. Li_2.8Fe_1.8Ti_0.2(PO₄)₃/C has higher reversibility and better capacity retention than that of the undoped Li₃Fe₂(PO₄)₃/C. A much higher specific capacity of 122.3 mAh/g was obtained at C/20 in the first cycle, approaching the theoretical capacity of 128 mAh/g, and a capacity of 100.1 mAh/g was held at C/2 after the 20th cycle.     

Monophase domain, fibers single crystals grown by the micro-pulling down technique and optical characterisation of LiGd1−xYbx(WO4)2

We have determined the single phase domain of LiGd_1−xYb_x(WO₄)₂. The lattices parameters decrease as a function of Yb³⁺ substitution in Gd³⁺ sites. Transparent LiGd_1−xYb_x(WO₄)₂ fibers single crystals were successfully grown by the micro-pulling down technique (μ-PD). The Yb³⁺-doped LiGd(WO₄)₂ fibers single crystals have been pulled under stationary stable growth conditions corresponding to flat crystallization interface with meniscus length equal to 120 μm. The fibers diameters varied from 0.5 to 1 mm depending on the capillary die diameter, pulling rate and the molten zone temperature. Fibers single crystals free of defects are observed for Ytterbium concentration in the melt up to 5 at%. Above this limit, inclusions and cracks appear and the optical quality of the fibers were deteriorated. The emission spectra of Yb³⁺-doped LiGd(WO₄)₂ were investigated.     

Luminescence properties of Eu ions doped in Y2−xGdxO3 thin films grown by pulsed laser deposition method

Luminescence properties of Y_2−xGd_xO₃:Eu³⁺ (x = 0 to 2.0) thin films are investigated by site-selective laser excitation spectroscopy. The films were grown by pulsed laser deposition method on SiO₂ (100) substrates. Cubic phase Y₂O₃ and Gd₂O₃ and monoclinic phase Gd₂O₃ are identified in the excitation spectrum of the ⁷F₀ → ⁵D₀ transition of Eu³⁺. The emission spectra of the ⁵D₀ → ⁷F_J (J = 1 and 2) transition from individual Eu³⁺ centers were obtained by tuning the laser to resonance with each excitation line. The excitation line at around 580.60 nm corresponds to the line from Eu³⁺ with C₂ site symmetry of cubic phase. New lines at 578.65 and 582.02 nm for the C_S sites of Gd₂O₃ with monoclinic phase are observed by the incorporation of Gd in Y₂O₃ lattice. Energy transfer occurs between Eu³⁺ ions at the C_S sites and from Eu³⁺ ions at the C_S sites to those at the C₂ site in Y_2−xGd_xO₃.     

Passive Q-switching performance of intracavity frequency-doubled red laser with mixed Nd:GdxY1−xVO4 crystals

Passive Q-switching laser operation of Nd:Gd_xY_1−xVO₄ mixed crystals at 671 nm has been compared in the same V-shape folded cavity. Based on our comparative analysis, the best laser characteristics - the highest pulse energy and peak power and the shortest pulse width - were demonstrated by Nd:Gd_0.63Y_0.37VO₄ crystal. At the same time, this crystal possesses the lowest thermal conductivity in comparison with the other Nd:Gd_xY_1−xVO₄ crystals, which could result in decreasing the average laser output power. To verify this assumption we also measured the focal lengths of thermal lens induced in the crystals during the laser operation.     

Chemical diffusion coefficient of lithium ion in Li3V2(PO4)3 cathode material

The kinetic properties of monoclinic lithium vanadium phosphate were investigated by potential step chronoamperometry (PSCA) and electrochemical impedance spectroscopy (EIS) method. The PSCA results show that there exists a linear relationship between the current and the square root of the time. The D?_Li values of lithium ion in Li_3-xV₂(PO₄)₃ under various initial potentials of 3.41, 3.67, 3.91 and 4.07 V (vs Li/Li⁺) obtained from PSCA are 1.26 × 10^− 9, 2.38 × 10^− 9, 2.27 × 10^− 9 and 2.22 × 10^− 9 cm²·s^− 1, respectively. Over the measuring temperature range 15-65 °C, the diffusion coefficient increased from 2.67 × 10^− 8 cm²·s^− 1 (at 15 °C) to 1.80 × 10^− 7 cm²·s^− 1 (at 65 °C) as the measuring temperature increased.     

Structure and thermal conductivity of Gd2(TixZr1 − x)2O7 ceramics

The Gd₂(Ti_xZr_1 − x)₂O₇ (x = 0, 0.25, 0.50, 0.75, 1.00) ceramics were synthesized by solid state reaction at 1650 °C for 10 h in air. The relative density and structure of Gd₂(Ti_xZr_1 − x)₂O₇ were analyzed by the Archimedes method and X-ray diffraction. The thermal diffusivity of Gd₂(Ti_xZr_1 − x)₂O₇ from room temperature to 1400 °C was measured by a laser-flash method. The Gd₂Zr₂O₇ has a defect fluorite-type structure; however, Gd₂(Ti_xZr_1 − x)₂O₇ (0.25 ≤ x ≤ 1.00) compositions exhibit an ordered pyrochlore-type structure. Gd₂Zr₂O₇ and Gd₂Ti₂O₇ are infinitely soluable. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ increases with increasing Ti content under identical temperature conditions. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ first decreases gradually with the increase of temperature below 1000 °C and then increases slightly above 1000 °C. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ is within the range of 1.33 to 2.86 W m^− 1 K^− 1 from room temperature to 1400 °C.     

Substitution site and photoluminescence spectra of Eu-substituted SrTiO3 prepared by Pechini method

Eu³⁺-substituted SrTiO₃ (Sr_1 − xEu_xTiO₃) without second phase has been successfully prepared by Pechini method. X-ray diffraction patterns have suggested that solid-solubility limit would be 1-2 mol% due to large ionic radius difference between Eu³⁺ and Sr²⁺. Photoluminescence spectra of Sr_1 − xEu_xTiO₃ have shown the intense ⁵D₀-⁷F₁ transition higher than ⁵D₀-⁷F₂ transition, which indicates that Eu³⁺ was substituted for Sr²⁺ site.     

Sr3.5Mg0.5Si3O8Cl4: Eu bluish-green-emitting phosphor for NUV-based LED

Sr₄Si₃O₈Cl₄:Eu²⁺ and Sr_3.5Mg_0.5Si₃O₈Cl₄:Eu²⁺ phosphors were prepared by a conventional solid state reaction (SS). Excited by 370 nm near-ultraviolet light, the phosphors show an efficient bluish-green wide-band emission centering at 484 nm, which originates from the 4f5d¹ → 4f⁷ transition of Eu²⁺ ion. The excitation spectra of the phosphors are a broad band extending from 250 nm to 400 nm. Mg²⁺-codoping greatly enhances the bluish-green emission of the phosphors. An LED was fabricated by coating the Sr_3.5Mg_0.5Si₃O₈Cl₄:0.08Eu²⁺ phosphor onto an ~ 370 nm-emitting InGaN chip. The LED exhibits bright bluish-green emission under a forward bias of 20 mA. The results indicate that Sr_3.5Mg_0.5Si₃O₈Cl₄:0.08Eu²⁺ is a candidate as a bluish-green component for fabrication of NUV-based white LEDs.     

Determination of the chemical diffusion coefficient of lithium in Li3V2(PO4)3

The chemical diffusion of lithium ion in Li₃V₂(PO₄)₃ were investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) methods. The CV results show that there exists a linear relationship between the peak current (i_p) and the square root of the scan rate (ν^1/2). The impedance spectrum exhibits a single semicircle and a straight line in a very low frequency region. A linear behavior was observed for every curve of the real resistance as a function of the inverse square root of the angular frequency in a very low frequency region. The obtained chemical diffusion coefficient from EIS measurements varies within 10^− 9 to 10^− 8 cm²·s^− 1, in good agreement with those from CV results.     

Er:YAl3(BO3)4 glassy thin films from polymeric precursor and sol-gel methods: Waveguides for integrated optics

This paper presents the characterization of single-mode waveguides for 980 and 1550 nm wavelengths. High quality planar waveguide structure was fabricated from Y_1 − xEr_xAl₃(BO₃)₄ multilayer thin films with x = 0.02, 0.05, 0.1, 0.3, and 0.5, prepared through the polymeric precursor and sol-gel methods using spin-coating. The propagation losses of the planar waveguides varying from 0.63 to 0.88 dB/cm were measured at 632.8 and 1550 nm. The photoluminescence spectra and radiative lifetimes of the Er^{3+ 4}I_13/2 energy level were measured in waveguiding geometry. For most samples the photoluminescence decay was single exponential with lifetimes in between 640 μs and 200 μs, depending on the erbium concentration and synthesis method. These results indicate that Er doped YAl₃(BO₃)₄ compounds are promising for low loss waveguides.     

Preparation and characterization of Sr4Al2O7:Eu, Eu phosphors

A new composition of red strontium aluminate phosphor (Sr₄Al₂O₇:Eu³⁺, Eu²⁺) is synthesized using a solid state reaction method in air and in a reducing atmosphere. The investigation of firing temperature indicates that a single phase of Sr₄Al₂O₇ is formed when the firing temperature is higher than 1300 °C and that a Sr₃Al₂O₆ phase is formed as the main peak below 1300 °C. The effects of firing temperature and doping concentration on luminescent properties are investigated. Sr₄Al₂O₇ phosphors exhibit the typical red luminescent properties of Eu³⁺ and Eu²⁺. A comparison photoluminescence study with Sr₃Al₂O₆ phosphor shows that Sr₄Al₂O₇ has higher emission intensity than Sr₃Al₂O₆ as a result of the higher optimum doping concentration of Sr₄Al₂O₇ phosphor.     

Growth and spectral properties of Cr:KAl(MoO4)2 crystal

The Cr³⁺:KAl(MoO₄)₂ single crystal was grown by top seeding solution growth method (TSSG). Based on the absorption and emission spectra, the crystal field strength Dq, the Racah parameters B and C, the effective phonon energy ?ω and the Huang-Rhys factor S were calculated: Dq = 1494.8 cm^− 1, B = 585.5 cm^− 1 and C = 3049 cm^− 1, ?ω = 373.8 cm^− 1 and the Huang-Rhys factor S = 3.74, respectively. The value Dq/B = 2.55 indicates that Cr³⁺ ion occupies the strong crystal field site in KAl(MoO₄)₂ crystal. A comparison of crystal field parameters for Cr³⁺:KAl(MoO₄)₂ with other Cr³⁺-doped crystals was presented. The results of spectral measurement show that Cr³⁺:KAl(MoO₄)₂ may be a potential candidate for broadband laser applications.     

Effect of Ca Substitution on the Superconductivity of La2.5Y0.5CaBa3(Cu0.88Fe0.12)7Oz

The structural and superconducting properties of single-phase La_2.5–y Y_0.5Ca_1+y Ba₃ (Cu_0.88Fe_0.12)₇O _z (LYCaBCuFe) (y= 0.0–1.0) compounds with triple perovskite structure are investigated using X-ray diffraction, resistivity, a.c. susceptibility, and oxygen content measurements. Increasing Ca substitution for La resulted in a decrease in unit cell axes and volume. T _c ^R=0 shows a marginal increase from 31 K to 37 K for y = 0.0–0.21 and thereafter it decreases with increasing y leading to zero T _c ^R=0 at y 0.84. This shows that the suppression of T _c from 80 K to 31 K by Fe doping at x = 0.12 La_2.5Y_0.5CaBa₃(Cu_1–x Fe _x )₇O _z cannot be compensated by appropriate hole doping with Ca in LaYCaBCuFe.     

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.     

Luminescent properties of Eu-activated (Sr1−z, Caz)(Al1−y, By)2O4 phosphors for UV LEDs

A series of (Sr_1−z, Ca_z)(Al_1−y, B_y)₂O₄:xEu²⁺ phosphors were synthesized by the sol–gel process and were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), and photoluminescence (PL) excitation and emission spectra. The experiment results revealed that the highest intensity of Sr(Al_1.98, B_0.02)O₄:Eu²⁺ phosphor with pure monoclinic SrAl₂O₄ was achieved by annealing at the temperature of 1200 °C and the Eu²⁺ content of 8 mol%. However, when the post-treatment temperature for Sr(Al_1.98, B_0.02)O₄: Eu²⁺ was over 1200 °C, the Sr₄Al₁₄O₂₅ phase appeared as a minor phase, inducing small blue-shift in the emission peak (520–509 nm). Doping higher content of B³⁺ (y = 0.02–0.40) into SrAl₂O₄:Eu²⁺ at 1200 °C resulted in the transformation of phase from SrAl₂O₄ to Sr₄Al₁₄O₂₅ as well as to SrB₂Al₂O₇, which made the emission intensity enhance and the emission shift to a much shorter wavelength region (λ_p = 467 nm). It was found that, instead of purely using Sr atoms, Ca atoms with content of 20–40% could induce the crystal structure of (Sr_1−z, Ca_z)(Al_1−y, B_y)₂O₄:xEu²⁺, which led to SrAl₂O₄ from monoclinic to hexagonal phase. As a result, SrAl₂O₄ solid solution was obtained and then SrAl₂O₄:Eu²⁺ to emit 518 nm green light. At higher Ca content (z > 40%), a new CaAl₂O₄ solid solution was formed and a blue emission of CaAl₂O₄:Eu²⁺ was obtained.     

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.     

Effects of Zn doping on the structural and luminescent properties of Sr4Si3O8Cl4:Eu phosphors

Sr₄Si₃O₈Cl₄: Eu²⁺ phosphors were synthesized by the solid-reaction at high temperature. The emission intensity reaches a maximum at 0.08 mol% of Eu²⁺ concentration. The present paper mainly focused on the effects of Zn²⁺ on the crystallization behavior and photoluminescence (PL) properties of Sr₄Si₃O₈Cl₄:0.08Eu²⁺. Results suggested that no new phase is introduced by co-doping with a small amount of Zn²⁺ ions, but when co-doped with excessive amount of Zn²⁺ ions, Sr₂ZnSi₂O₇ appears. We find that the co-doping of a small amount of Zn²⁺ could remarkably improve the PL intensity of Sr₄Si₃O₈Cl₄:0.08Eu²⁺. When x = 0.05, the intensity of Sr₄Si₃O₈Cl₄:0.08Eu²⁺,xZn²⁺ was increased up to 2.3 times that of pure Sr₄Si₃O₈Cl₄:0.08Eu²⁺, which could be attributed to the flux effect of Zn²⁺ ions, and the Zn²⁺ doping reduces the opportunities of the energy transfer between Eu²⁺.     

Influence of excitation wavelengths on luminescent properties of Sr3Al2O6:Eu, Dy phosphors prepared by sol-gel-combustion processing

Eu²⁺ and Dy³⁺ ion co-doped Sr₃Al₂O₆ red-emitting long afterglow phosphor was synthesized by sol-gel-combustion methods using Sr(NO₃)₂, Al(NO₃)₃·9H₂O, Eu₂O₃, Dy₂O₃, H₃BO₃ and C₆H₈O₇·H₂O as raw materials. The crystalline structure of the phosphors were characterized by X-ray diffraction, luminescent properties of phosphors were analyzed by fluorescence spectrophotometer. The effect of excitation wavelengths on the luminescent properties of Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphors was discussed. The emission peak of Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphor lays at 516 nm under the excitation of 360 nm, and at 612 nm under the excitation of 468 nm. The results reveal that the Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphor will emit a yellow-green light upon UV illumination, and a bright red light upon visible light illumination. The emission mechanism was discussed according to the effect of nephelauxetic and crystal field on the 4f⁶5d¹ → 4f⁷ transition of the Eu²⁺ ions in Sr₃Al₂O₆. The afterglow time of (Sr_0.94Eu_0.03Dy_0.03)₃ Al₂O₆ phosphors lasts for over 600s after the excited source was cut off.