The influence on intrinsic light emission of calcium tungstate and molybdate powders by multivalence Pr codoping

For trivalent praseodymium (Pr³⁺) and quadrivalent praseodymium (Pr⁴⁺) codoped CaMO₄ (M = W, Mo) powders, the luminescence propriety of matrix is obviously influenced by carrier concentration. The light emission intensity of CaWO₄ matrix decreases exponentially with increasing of Pr concentration because oxygen-deficient (WO₃·V_O^··\mathrm{WO}_{3}\cdot V_{\mathrm{O}}^{\bullet \bullet}) obtains an electron supplied by Pr³⁺ (5d). However, the light emission intensity of CaMoO₄ is enhanced by Pr codoping because the quasi-free electrons increase the probability of radiative combination. The difference of photoluminescence properties in the two materials are attributed to the bonding character of M and O in the CaMO₄ structure.     

XAFS studies of the local structure and charge state of the Pr impurity in SrTiO3

Solid solutions of (Sr_{1 ? x} Pr _x )TiO₃ have been studied using X-ray methods. It has been shown that, with an increase in the praseodymium concentration, the temperature of the structural phase transition to the phase with space group I4/mcm increases and, at x ?? 0.15, the structure at 300 K is tetragonal. X-ray absorption fine structure (XAFS) spectroscopy studies have revealed that Pr ions are predominantly in the charge state 3+ and occupy the Sr sites. No indications of the off-centering of Pr atoms at the Sr sites have been revealed. The local environment of Pr atoms is characterized by a strong relaxation of the oxygen atoms, the value of which corresponds to the difference between the ionic radii of Pr³⁺ and Sr²⁺. It has been found that, in the second shell, there occurs a significant repulsion of the Pr³⁺ and Ti⁴⁺ ions, which is responsible for the weak dependence of the lattice parameter in the solid solution on the praseodymium concentration.     

On spectroscopic properties of the KYb(WO4)2:Pr3+ crystal

Spectroscopic properties of Pr³⁺ doped KYb(WO₄)₂ single crystals were investigated. The crystal lattice parameters were determined. Energy levels of Pr³⁺ in KYb(WO₄)₂ were assigned. The absorption, emission, excitation, time-resolved emission and excitation spectra were measured at low (10 K) and at room temperature. Decay times of the praseodymium emissions are non-exponential and unusually short. Site selection spectroscopy evidences several different Pr³⁺ sites. The Judd-Ofelt intensity model was used to analyse the experimental data. The Ω λ parameters, branching ratio and electric dipole transition probabilities were determined.     

Praseodymium luminescence in fluorides

This work is a continuation of studies of the Pr³⁺ cascade emission in various matrices. The effect of the environment of the luminescence center on the mutual position of the lowest 5d and the 4f level ¹S₀ of Pr³⁺ is considered. PrF₃ clustering in BaF₂ is observed at a high praseodymium concentration. The promising potential of magnesium as a charge compensator for praseodymium in SrAlF₅ is demonstrated.     

Spectroscopic characteristics of praseodymium-doped cubic double sodium-yttrium fluoride crystals Na0.4Y0.6F2.2:Pr3+. Intensities of optical transitions and luminescence kinetics

Using the Bridgman-Stockbarger technique, we have grown a series of cubic crystals Na_0.4Y_0.6F_2.2:Pr³⁺ (NYF:Pr³⁺) with a content of praseodymium in the range of 0.04–9 at %. We have determined the composition of crystals, evaluated their optical quality, and found the incorporation coefficient of Pr³⁺ ions into the Na_0.4Y_0.6F_2.2 matrix (K _Pr ～ 0.9). We have examined optical spectra of NaYF:Pr³⁺ crystals at room and low (7 K) temperatures in the range of 200–2500 nm. The low-temperature absorption spectra of NYF:Pr³⁺ crystals have been shown to consist of broad weakly structured bands. Based on the analysis of low-temperature absorption spectra, the structure of the Stark splitting of praseodymium levels has been represented in terms of a model of “quasi-centers,” which are characterized by an inhomogeneous broadening of Stark components. From experimental absorption cross-section spectra at T = 300 K, we have calculated oscillator strengths for transitions from the ground state ³ H ₄ to excited multiplets ³ H ₅, ³ H ₆, ³ F _j (j = 2, 3, 4), ¹ G ₄, ¹ D ₂, and (³ P _j ,¹ I ₆) (j = 0, 1, 2). Using the Judd-Ofelt method, we have determined intensity parameters Ω _t and found that Ω₂ = 0, Ω₄ = 4.4 × 10^?20, and Ω₆ = 2.28 × 10^?20 cm². With these values, we have calculated the probabilities of radiative transitions, the branching coefficients, and the lifetimes of the radiative levels ¹ D ₂ and ³ P ₀. The probabilities of multiphonon nonradiative transitions in NYF:Pr³⁺ crystals have been estimated. Using the method of kinetic spectroscopy with selective excitation, we have investigated the luminescence decay kinetics of praseodymium from the ³ P ₀ and ¹ D ₂ levels upon their selective resonant excitation by nanosecond laser pulses. The inference has been made that Na_0.4Y_0.6F_2.2:Pr³⁺ crystals are processable; admit doping by praseodymium in high concentrations; and, with respect to all their radiative characteristics, can be potentially considered as active media for converters of optical radiation and solid-state continuously tunable lasers in the visible range.     

Enhancement of red spectral emission intensity of Y3Al5O12:Ce phosphor via Pr co-doping and Tb substitution for the application to white LEDs

We have enhanced color-rendering property of a blue light emitting diode (LED) pumped white LED with yellow emitting Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce) phosphor using addition of Pr and Tb as a co-activator and host lattice element, respectively. Pr³⁺ addition to YAG:Ce phosphor resulted in sharp emission peak at about 610 nm through ¹D₂→³H₄ transition. And when Tb³⁺ substituted Y³⁺ sites, Ce³⁺ emission band shifted to a longer wavelength due to larger crystal field splitting. Y₃Al₅O₁₂:Ce³⁺, Pr³⁺ and (Y_1−xTb_x)₃Al₅O₁₂:Ce³⁺ phosphors were coated on blue LEDs to fabricate white LEDs, respectively, and their color-rendering indices (CRIs, R_a) were measured. As a consequence of the addition of Pr³⁺ or Tb³⁺, CRI of the white LEDs improved to be R_a=83 and 80, respectively. Especially, blue LED pumped (Y_0.2Tb_0.8)₃Al₅O₁₂:Ce³⁺ white LED showed both strong luminescence and high color-rendering property.     

Doping effects of Nb5+ on red long afterglow phosphor CaTiO3:Pr3+

A series of Nb5+ codoped red long afterglow phosphors CaTi1 xNbxO3:Pr03.+002 (0 ≤ x ≤ 0.05) is prepared by a solid state reaction method. Their photoluminescence, phosphorescence and thermoluminescence are investigated. The results indicate that codoping Nb5+ can improve the photoluminescence and phosphorescence property of CaTiO3:Pr3+ significantly. When 3-mol% Nb5+ is codoped, the emission intensity of CaTiO3:Pr3+ is enhanced twice, while the afterglow time is extended from 10 min to about 40 min. Thermoluminescence results reveal that the trapping level of CaTiO3:Pr3+ is reduced from 0.82 eV to 0.62 eV by codoping Nb5+. The effect of Nb5+ doping on enhancing the photoluminescence intensity and afterglow time of CaTiO3:Pr3+ is discussed.     

Energy transfer in Y3Al5O12:Ce, Pr and CaMoO4:Sm, Eu phosphors

Non-radiative energy transfers (ET) from Ce³⁺ to Pr³⁺ in Y₃Al₅O₁₂:Ce³⁺, Pr³⁺ and from Sm³⁺ to Eu³⁺ in CaMoO₄:Sm³⁺, Eu³⁺ are studied based on photoluminescence spectroscopy and fluorescence decay patterns. The result indicates an electric dipole-dipole interaction that governs ET in the LED phosphors. For Ce³⁺ concentration of 0.01 in YAG:Ce³⁺, Pr³⁺, the rate constant and critical distance are evaluated to be 4.5×10⁻³⁶ cm⁶ s⁻¹ and 0.81 nm, respectively. An increase in the red emission line of Pr³⁺ relative to the yellow emission band of Ce³⁺, on increasing Ce³⁺ concentration is observed. This behavior is attributed to the increase of spectral overlap integrals between Ce³⁺ emission and Pr³⁺ excitation due to the fact that the yellow band shifts to the red spectral side with increasing Ce³⁺ concentration. In CaMoO₄:Sm³⁺, Eu³⁺, Sm³⁺-Eu³⁺ transfer occurs from ⁴G_5/2 of Sm³⁺ to ⁵D₀ of Eu³⁺. The rate constant of 8.5×10⁻⁴⁰ cm⁶ s⁻¹ and the critical transfer distance of 0.89 nm are evaluated.     

Optical absorption and emission properties of Pr and Er in lithium cesium mixed alkali borate glasses

This article presents the optical absorption and emission properties of Pr³⁺ and Er³⁺ in mixed alkali borate glasses of the type 68B₂O₃·xLi₂O·(32-x)Cs₂O (where x=8, 12, 16, 20 and 24). The variation of Judd-Ofelt intensity parameters (Ω_λ), the peak wavelength of the hypersensitive transitions, radiative transition probabilities (A_rad) and peak emission cross-sections (σ_p) with x in the glass matrix have been discussed in detail. The changes in position of hypersensitive transition and intensity parameters with x are correlated to the structural changes in the host matrix. The estimated radiative lifetimes (τ_R) of certain excited states of both Pr³⁺ and Er³⁺ in lithium cesium mixed alkali borate glasses are reported. Branching ratios (β) and integrated absorption cross-sections (Σ) for certain important transitions are presented. Peak stimulated emission cross-sections (σ_p) are calculated for the observed emission peaks of Pr³⁺ and Er³⁺ ions in this glass matrix.     

The study of two-color excitation upconversion of Pr(0.5)Yb(3):ZBLAN

The excited state absorption upconversion of Pr(0.5)Yb(3):ZBLAN glass material, under two-color excitation of the 960 nm semiconductor laser and the Xe lamp light simultaneously, is reported in this article. It was found that the upconversion emission spectra of 480.1, 519.0, 601.9 and 631.8 nm coincide with the common emission spectra. Meanwhile, the upconversion-excitation spectrum has three obvious peaks under two-color excitation, and they respectively correspond to the 856.0 nm upconversion excitation transition [¹G₄(Pr³⁺)→¹I₆(Pr³⁺) and ¹G₄(Pr³⁺)→³P₁(Pr³⁺)], the 789.0 nm upconversion excitation transition ¹G₄(Pr³⁺)→³P₂(Pr³⁺), and the 803.7 nm upconversion excitation transition ³H₆(Pr³⁺)→¹D₂(Pr³⁺). The upconversion excitation transition ¹G₄(Pr³⁺)→¹I₆(Pr³⁺) is strong because its oscillator strength f = 23.040×10⁻⁶ is large, which results in a large peak appearing in the upconversion excitation spectrum. That is just the new interesting two-color excitation upconversion luminescence phenomenon of Pr(0.5)Yb(3):ZBLAN induced by one laser and one continuous normal light simultaneously.     

Quantum efficiency of non-radiative energy transfer from Eu3+ → Pr3+ in calibo glass

Study of energy transfer from optically excited Eu³⁺ to Pr³⁺ has been carried out in calibo glass. Probabilities (P_da) and efficiencies (η_T) of energy transfer from Eu³⁺ to Pr³⁺ have been calculated from the life time and emission intensity of Eu³⁺ + Pr³⁺. At low acceptor concentrations, P_da varies linearly with C_a showing migration of energy among donors. At high acceptor concentrations, P_da depends linearly on (C_a + C_d)², which is consistent with Fong and Diestler theory of dipole-dipole mechanism of energy transfer. At low temperatures the probability and the efficiency decrease due to increase in the emission intensity and life time, which suggests that at room temperature the energy is transferred to lattice by donor lowering life time and intensity. At high temperatures no emission from higher levels of donor is obtained which suggests blurring out of energy levels in glassy matrix.     

Upconversion excitations in Pr<Superscript>3+</Superscript>-doped BaY<Subscript>2</Subscript>F<Subscript>8</Subscript> crystal

We report the orange-to-blue and infrared-(IR)-to-blue wavelengths upconversion luminescence in Pr³⁺:BaY₂F₈ crystals. Mechanism of the orange light upconversion into blue ³P₀ state emission was confirmed to be energy transfer between two Pr³⁺ ions in the ¹D₂ state. IR-to-blue upconversion has only been observed under two different color IR pumping. The first resonant step was the ³H₄→¹G₄ ground state absorption transition, and the second resonant transition was the excited state absorption from the ¹G₄ to ¹I₆ and ³P_J levels. A comparison of the efficiency of the IR-to-blue upconversion in several praseodymium activated host is presented and discussed. A model of the IR pumped upconversion praseodymium blue laser is presented and the population inversion conditions are calculated.     

Energy transfer in LaMgB5O10:Pr,Mn under VUV excitation

Luminescent properties of Pr³⁺ or Mn²⁺ singly doped and Pr³⁺, Mn²⁺ co-doped LaMgB₅O₁₀ are investigated by synchrotron radiation VUV light. When LaMgB₅O₁₀:Pr³⁺ is excited at185 nm, the photon cascade emission between 4f levels of Pr³⁺ is observed. In the excitation spectra of LaMgB₅O₁₀:Mn²⁺ monitoring the 615 nm emission of Mn²⁺, several excitation bands in a spectral range from 330 to 580 nm are recorded, among which the most intense band is centered at 412 nm (⁶A_1g→⁴E_g-⁴A_1g). This band has considerable spectra overlap with the 410 nm emission (¹S₀→¹I₆) of Pr³⁺, which is favorable for energy transfer from Pr³⁺ to Mn²⁺. Such energy transfer is observed in the co-doped sample, converting the violet emission (410 nm) of Pr³⁺ into the red emission (615 nm) of Mn²⁺. The concentration dependence of transfer efficiency is also investigated.     

Pr-substituted W-type barium ferrite: Preparation and electromagnetic properties

The W-type ferrites doped with Pr³⁺, BaCoNiPr_xFe_16−xO₂₇ (x=0-0.20), were prepared by a sol-gel method. The structure and electromagnetic properties of the samples are studied using powder X-ray diffraction, field emission scanning electron microscope, vibrating sample magnetometer and vector network analyzer. All the samples are hexagonal platelet-like W-type barium ferrite. These synthesized samples exhibit paramagnetism and strong magnetism. The saturation magnetization (Ms) increases with the increase of Pr³⁺ content. The real part of complex permittivity (ε′) decreases and the imaginary part (ε″) increases with Fe³⁺ replaced by Pr³⁺. The imaginary part of complex permittivity (μ″) increases and the real part (μ′) decreases after Pr³⁺ is doped. Furthermore, the doped Pr³⁺ improves the microwave absorbency.     

Effect of interface states associated with transitional nanophase precipitates in the enhancement of red emission from SrAl12O19:Pr by Ti incorporation

SrAl₁₂O₁₉:Pr³⁺, Ti⁴⁺ phosphor suitable for field emission displays is prepared by the wet chemical gel-carbonate method and the mechanism of enhancement in red photoluminescence (PL) intensity with Ti⁴⁺ therein has been investigated. The PL spectra of Pr³⁺ show both ¹D₂-³H₄ and ³P₀-³H₆ emission in the red region with very weak intensity when excited at 355 nm. The emission intensity has increased by about 100 times at room temperature in the compositional range SrAl_12−xTi_xO_19+x/2:Pr³⁺, with 0.1≤x≤0.3 in comparison to Ti-free SrAl₁₂O₁₉:Pr³⁺. TEM investigations show the presence of exsolved nanophase of SrAl₈Ti₃O₁₉, the precipitation of which is preceded by the presence of defect centers at the interfacial regions between the semicoherent transient phase and the parent SrAl₁₂O₁₉ matrix. The presence of transitional nanophase and the associated defects modify the excitation-emission process by way of formation of electronic sub-levels at lower energy (3.5 eV) than the band gap of SrAl₁₂O₁₉ (∼7 eV) followed by non-resonance energy transfer to Pr³⁺ level, leading to magnetic-dipole related red emission with enhanced intensity. The PL intensity of Pr³⁺ decreases at high Ti⁴⁺ concentrations (x>0.3) due to higher extent of segregation of non-emissive SrAl₈Ti₃O₁₉:Pr³⁺ phase.     

Luminescent properties of trivalent praseodymium-doped lanthanum aluminum germanate LaAlGe2O7

The luminescent characteristics of Pr³⁺-activated LaAlGe₂O₇ were investigated. In response to excitement using 448 nm blue light, the emission spectra involved most of the ³P₀→³H_J transitions. The dominant emission came from the ³P₀→³H₄ transition at 487 nm. ¹D₂ fluorescence quenching was observed in highly doped samples and is related to the cross-relaxation processes among neighboring Pr³⁺ ions. In contrast with conventional Pr³⁺-activated phosphors, the extraordinary excitation spectra showed only intense f-f transition of Pr³⁺ ions, while the 4f-5d transition was eliminated. This is ascribed to photoionization. By analyzing absorption and excitation spectra, it is recognized that no efficient energy transfer occurs between Pr³⁺ and the host lattice in LaAlGe₂O₇.     

Self-assembled CaMoO4 and CaMoO4:Eu hierarchical superstructures: Facile sonochemical route synthesis and tunable luminescent properties

Tetragonal CaMoO₄ and CaMoO₄:Eu³⁺ with various novel three-dimensional (3D) hierarchical architectures were successfully synthesized via a facile, efficient sonochemistry process in the absence of any surfactant or template. XRD, EDS, FE-SEM, and photoluminescence (PL) were employed to characterize the as-obtained products. It was found that morphology modulation could be easily realized by changing pH value of the precursor. The pH value of the precursor not only affected the substructures of the hierarchical structures, but also determined the size distributions of the final products. The formation mechanism for different hierarchical architectures was proposed on the basis of time-dependent experiments. The luminescence spectra showed that CaMoO₄:Eu³⁺ phosphors can be effectively excited by the near ultraviolet (UV) (396 nm) and blue (466 nm) light, and exhibited strong red emission around 615 nm, which was attributed to the Eu³⁺⁵D₀→⁷F₂ transition. Compared with Y₂O₃:Eu³⁺ phosphor, CaMoO₄:Eu³⁺ is much more stable, efficient and suitable, therefore, this phosphors could be a promising red component for possible applications in the field of LEDs.     

Broad yellow orange emission from SrAl2O4:Pr phosphor with blue excitation for application to white LEDs

Photoluminescence excitation to intermediate atomic levels of rare earth activator ion (praseodymium) situated intragap in alkaline earth aluminate (AEA) SrAl₂O₄ has been tailored. This lead to blue excitation (2.7 eV) of large band gap AEA possible. Photoluminescence (PL) emission in the visible region extends from 525 to 650 nm corresponding to transition from ³P₀ and ¹D₂ excited states to different ³H_J and ³F_J states of Pr³⁺, broadened by crystal field effect of SrAl₂O₄. Thus SrAl₂O₄:Pr³⁺ promise to be a good candidate for solid state lighting in conjunction with blue LED.     

Compact Two-Color Pr3+-Doped ZBLAN Fiber Lasers

Two-color laser simultaneous oscillation in compact trivalent praseodymium ion (Pr³⁺)-doped ZrF₄-BaF₂-LaF₃-AlF₃-NaF (ZBLAN) fiber lasers is investigated. Fiber pigtails by coating high-reflective visible dielectric films are utilized as fiber mirrors (FMs) to construct compact all-fiber laser cavity. Reflectance/transmittance of FMs is intentionally designed to balance net gains and reduce gain competition of different lasable wavelengths in the constructed fiber lasers. By using two selected FMs with high reflectivity at green-light and low reflectivity at red-light, simultaneous green and red laser emission in all-fiber Pr³⁺-doped ZBLAN fiber laser is obtained. Moreover, simultaneous orange and red laser emission is also achieved with another selected FM of high reflectivity at orange-light and low reflectivity at red-light. Our work will provide an effective and easy method to construct compact special fiber (e.g.,ZBLAN fiber, chalcogenide fiber) lasers.     

Downconversion for Solar Cells in YF3:Pr3+, Yb3+

ABSTRACT

Energy losses in solar cells caused by the spectral mismatch can be reduced by adapting the solar spectrum using a downconversion material where one higher energy visible photon is ‘cut' into two lower energy near-infrared photons that both can be absorbed by the solar cell. Downconversion with the (Pr³⁺, Yb³⁺) couple in YF₃ is investigated. Based on analysis of luminescence and diffuse reflectance spectra it is evident that two-step energy transfer takes place from the ³P₀ level of Pr³⁺ (around 490 nm) exciting two Yb³⁺ to the ²F_5/2 level giving emission around 980 nm. The transfer efficiency increases with Yb³⁺ concentration and is 86% for YF₃ doped with 0.5% Pr³⁺ and 30% Yb³⁺. Due to concentration quenching the intensity of emission from Yb³⁺ is strongly reduced and the ²F_5/2 emission intensity reaches a maximum for the sample with 0.5% Pr³⁺ and 2–5% Yb³⁺ at 300 K. Temperature dependent measurements reveal the role of the Pr^{3+ 1}G₄ level in the energy transfer between Pr³⁺ and Yb³⁺. Back-transfer of excitation energy from the Yb^{3+ 2}F_5/2 level to the ¹G₄ level of Pr³⁺ occurs and quenches the Yb³⁺ emission. The quenching is shown to become more efficient between 4 and 50 K due to faster phonon-assisted energy transfer between the Yb³⁺ donors. Upon raising the temperature from 50 to 300 K, the luminescence life time of the Yb³⁺ emission increases again because the small energy difference between the Pr³⁺ (¹G₄) level and the Yb³⁺ (²F_5/2) level (～300 cm^?1) which makes the ¹G₄ less efficient as a trap for the excitation energy. The present results give insight into factors involved in the concentration quenching in downconversion materials based on the (Pr³⁺, Yb³⁺) couple.