Photoluminescence of CaGa<Subscript>2</Subscript>S<Subscript>4</Subscript>:Pr Polycrystals

The photoluminescence (PL) and PL excitation spectra of CaGa₂S₄ polycrystals doped with praseodymium are studied in the regions of the activator absorption and the fundamental absorption of the host. It is found that the PL excitation spectrum consists of two regions: broadband absorption in the range of 200-380 nm corresponding to the fundamental absorption of the host and the narrow-band absorption of the dopant in the range of 430–515 nm. The luminescence spectra are different for different excitation wave-lengths, which occurs because Pr³⁺ ions substitute divalent cations occupying different crystallographic positions in the host crystal lattice.     

Double Fluorescence Conversion in Ultraviolet and Visible Region for Some Praseodymium Complexes of Aromatic Carboxyates

Four praseodymium complexes of aromatic carboxylates (benzoate, 4-tert-butylbenzoate, 2-benzoylbe-noate, and benzimidazole-5-carboxylate) have been synthesized and characterized, whose photophysical properties have been studied with ultraviolet spectra, phosphorescence spectra, and fluorescence spectra. The fluorescent emission spectra of all praseodymium complexes show two emission peaks under the excitation band of 245 nm at about 395 and 595 nm respectively, while one peak under 415 nm at about 595 nm, which attributed to be ¹S₀ → ¹I₆ (395 nm) transition and the characteristic emission ¹D₂ → ³H₄ (595 nm) transition of Pr³⁺ ion. The ¹S₀ → ¹I₆ transition can be ascribed as the transition of charge transfer state, and the ¹D₂ → ³H₄ can be further proved that there exists an antenna effect in the fluorescence of praseodymium with aromatic carboxylic acids. In conclusion, the praseodymium complexes systems can realize the double fluorescent conversion in both ultraviolet and visible region and can be further studied the application of this conversion.     

Visible upconversion emissions in Pr3+-doped TeO2–ZnO glass

The Pr³⁺-doped tellurite zinc oxide (TZO) glasses by conventional melt and quenching technique have been prepared. The absorption spectra of samples doped with different concentrations of triply ionized praseodymium ions have been analyzed. Several upconversion emission bands of the Pr³⁺ ions doped in tellurite zinc oxide glasses under 980?nm excitation have been observed. The possible excitation mechanisms responsible for upconversion emissions spanning from blue to near infrared region have been discussed in detail.     

Donor doping process and white light generation in CaMoO4 powders with multivalence Pr codoping

Both trivalent praseodymium (Pr³⁺) and quadrivalent praseodymium (Pr⁴⁺) were doped in molybdate powders. Visible emission from matrix was enhanced by multivalent Pr codoping. It was proposed that Pr³⁺ ions was donor and supplied quasi-free electron when Pr³⁺ took place the Pr⁴⁺ sites. The result showed that multivalence codoping would be an effective way to enhance emission of CaMoO₄. White light can be generated from Ca_0.98Pr_0.02MoO₄ powder via combination of broadband emissions originated from CaMoO₄ matrix and radiative transition of Pr³⁺. It showed warm white light with T_c of 3450 K that implies promising application in white light emitting diodes (LEDs).     

Ground-state measurement of Pr<Superscript>3+</Superscript> in Y<Subscript>2</Subscript>O<Subscript>3</Subscript> by photoconductivity

1 at % Pr³⁺-doped Y₂O₃ single-crystal fibers were prepared using a laser-heated pedestal growth method. The emission and excitation spectra of the fibers were measured. The emissions of 4f-4f transitions from ¹ D ₂ to the ³ H ₄ and ³ H ₅ states are found at 620 and 720 nm, respectively. The ³ P ₂, ³ P ₁, ¹ I ₆, and ³ P ₀ 4f-4f absorptions are observed at 456, 472, 482, and 492 nm, respectively. A 4f-5d absorption band is detected at 288 nm. Photoconductivity measurements show that the 4f-5f transition of Pr³⁺ around 285 nm produces a direct photocurrent. Taking the onset of photocurrent to be at 320 nm, the ground state of Pr³⁺ is determined at 1.7 eV above the valence band of the host. The text was submitted by the authors in English.     

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.     

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.     

Site selective 4f5d spectroscopy of CaF2 : Pr

Spectroscopic investigations were performed on a single crystal of CaF₂ doped with 0.05% Pr³⁺. Three different Pr³⁺ sites with different luminescent properties were identified. The 4f² →4f¹5d¹ excitation spectrum of the first site has a sharp maximum at 221.3 nm. Excitation in the 4f5d bands of this site yields strong 4f5d emissions in the UV/VIS part of the spectrum and also weaker intraconfigurational 4f² emissions. By comparing the intraconfigurational 4f emissions and their decay times with data from the literature, these 4f5d bands are assigned to transitions on Pr³⁺ ions on a site with C_4V symmetry. The fd excitation spectrum of the second site has a zero phonon line at 223.3 nm. Upon selective excitation in this band, only 4f5d emission is observed. Probably, these 4f5d bands correspond to Pr³⁺ ions on a O_h site. The third set of 4f5d bands has a 4f5d onset at 208 nm. By comparison of the luminescence spectra of the intraconfigurational 4f² transitions with literature data, these transitions are assigned to Pr³⁺ on an L site. Excitation in these 4f5d band yields ¹S₀ emission followed by emission from the ³P₀ state. The present results clarify some contradictions reported in the literature.     

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.     

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.     

Effects of Zn impurities on the electronic properties of Pr doped CaTiO3

Microcosmic investigations of weak red-emitting materials are crucial for their further development and application. In this work, we have focused on the band structures and electronic properties of Pr mono- and (Zn, Pr) co-doped CaTiO₃ using density functional theory. Zn substitution for Ca or Ti tends to form clusters energetically with Pr substituting for Ca in CaTiO₃. In Pr mono-doped CaTiO₃, the O_2p→Ti_3d transition in CaTiO₃ host corresponds to the centered 330 nm excitation spectra. The gap states above the valence band of ∼1.30 eV and ∼2.06 eV are hybridized by Pr_4f, O_2p and Ti_3d orbitals. They are mainly due to Pr_4f orbitals in CaTiO₃:Pr. The former gap level is related to red emission at 614 nm due to ¹D₂→³H₄ transition of Pr³⁺ activator. The latter is related to the excitation spectra centered at 380 nm due to the low-lying Pr-to-mental intervalence charge transfer transitions (Pr³⁺-O²⁻-Ti⁴⁺?Pr⁴⁺-O²⁻-Ti³⁺). The band structures of (Zn, Pr) co-doped CaTiO₃ keep the similar gap levels to those in Pr mono-doped CaTiO₃. The incorporation of Zn brings out the two stronger localized gap states, which are hybridized by Pr_4f, O_2p and Ti_3d orbitals, in comparison with those in Pr mono-doped CaTiO₃. Therefore, when Zn impurities are added into Pr doped CaTiO₃, the present calculations visualize the two enhanced levels and the distorted structures around Pr.     

VUV spectroscopic properties of Ce and Pr-doped AREP2O7-type alkali rare earth diphosphates (A=Na, K, Rb, Cs; RE=Y, Lu)

Spectroscopic properties of Ce³⁺ and Pr³⁺-doped AREP₂O₇-type alkali rare earth diphosphates (A=Na, K, Rb, Cs; RE=Y, Lu) have been investigated using VUV spectroscopy technique. Ce³⁺-doped samples show typical Ce³⁺ emission in the range of 325-450 nm. The strong host absorption band starting at around 160 nm indicates that the optical band gap of AREP₂O₇ hosts is at least 7.7 eV, and the host→Ce³⁺ energy transfer process is rather efficient. However, AREP₂O₇:Pr³⁺ samples show less efficient host→Pr³⁺ energy transfer. The direct Pr³⁺ 4f²→4f¹5d¹ excitation, which are 12160±640 cm⁻¹ higher respect to that of Ce³⁺, leads to strong 4f¹5d¹→4f² emission bands in the range of 230-325 nm but no obvious 4f²→4f² emission lines.     

Pr3+ luminescence center in Lu2Si2O7 host

Pr³⁺‐doped Lu₂Si₂O₇ (LPS:Pr) microcrystalline phosphor was prepared by the sol–gel method. We study the LPS:Pr luminescence properties under UV and X‐ray excitation within 80–500 K. The emission spectrum is dominated by fast 5d–4f band peaking at 261 nm having 16 ns decay time. By purely optical contactless methods we determine the energy barrier of 300 meV for thermal ionization of the Pr³⁺ 5d₁ relaxed excited state in LPS host. The barrier is high enough to keep the room temperature quantum efficiency of the Pr³⁺ luminescence center close to unity. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

Crystal growth and luminescence properties of Pr-doped LuLiF4 single crystal

0.1, 1, and 3% Pr (with respect to Lu) doped LuLiF₄ (Pr:LuLiF₄) single crystals were grown by the micro-pulling-down (μ-PD) method. Transparency of the grown crystals was higher than 70% in the visible wavelength region with some absorption bands due to Pr³⁺ 4f-4f transitions. Intense absorption bands related with the Pr³⁺ 4f-5d transitions were observed at 190 and 215 nm. In radioluminescence spectra, Pr³⁺ 5d-4f emissions were observed at 220, 240, 340, and 405 nm. In the pulse height spectra recorded under ¹³⁷Cs γ-ray excitation, the Pr 3% doped sample showed the highest light yield of 2050 photons/MeV and the scintillation decay time of it exhibited 23 and 72 ns also excited by ¹³⁷Cs γ-ray.     

Ultraviolet excitation of rare-earth-doped YSZ crystals

Abstract

The optical properties of nominally pure and Er³⁺- or Pr³⁺ -doped yttria-stabilized zirconia single crystals were investigated under UV light excitation. In the excitation spectra of both types of doped crystals, a broad UV band is observed. Under excitation with light of different wavelengths inside this band, the luminescence features of the doped crystals are different. YSZ: Pr³⁺ samples exhibit the characteristic 4f → 4f emission of the Pr³⁺ ions. In YSZ: Er³⁺ crystals, both the Er³⁺ ion and the intrinsic luminescence are observed. Host to Er³⁺ ion radiative energy-transfer is also demonstrated. No dependence of the transfer process with the excitation wavelength was found. These results suggest that the UV band in Er³⁺ -doped crystals is associated with the lattice-dopant ion interaction rather than with the 4f5d interconfigurational band of the Er^3? ions.     

Radiation-induced defects in Pr3+-activated Liyf4 laser host

Rare earth doped fluorides have been used in laser applications. Not much is known about the effect of ionizing radiation on the lasing and other properties of fluorides. Therefore, in recent years much attention has been paid to the study of radiation-induced defects in laser materials, as they affect the optical and stimulated emission properties. The defect formation by γ-ray irradiation in Pr³⁺ activated LiYF₄, powder prepared by melt method, have been studied by thermoluminescence and electron spin resonance techniques and are reported in this paper. It is shown that LiYF₄:Pr³⁺ is sensitive to γ-ray radiation. Characterization of this laser material using ESR and photoluminescence techniques is also described.     

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.     

Emission color variation of M2SiO4:Eu (M=Ba, Sr, Ca) phosphors for light-emitting diode

The luminescent properties of alkaline earth orthosilicates M₂SiO₄ (M=Ba, Sr, Ca) doped with Eu²⁺ ions are investigated. Two emission bands are assigned to the f-d transitions of Eu²⁺ ions doped into two different cation sites in host lattices confirmed by electron paramagnetic resonance signal. Two emission bands show the different emission color variation with substituting M²⁺ cations with smaller cations. This behavior is discussed in terms of two competing factors of the crystal field strength and covalence. Also the decay times are in order of 600-1000 ns. These phosphors with maximum excitation of around 370 nm can be applied as a color-tunable phosphor for light-emitting diode based on ultraviolet chip/phosphor technology.