Spectral-kinetic characteristics of Pr luminescence in LiLuF4 host upon excitation in the UV-VUV range

Spectral-kinetic study of Pr³⁺ luminescence has been performed for LiLuF₄:Pr(0.1 mol%) single crystal upon the excitation within 5-12 eV range at T=8 K. The fine-structure of Pr³⁺ 4f ²→4f 5d excitation spectra is shown for LiLuF₄:Pr(0.1 mol%) to be affected by the efficient absorption transitions of Pr³⁺ ions into 4f 5d involving 4f ¹ core in the ground state. Favourable conditions have been revealed in LiLuF₄:Pr(0.1 mol%) for the transformation of UV-VUV excitation quanta into the visible range. Lightly doped LiLuF₄:Pr crystals are considered as the promising luminescent materials possessing the efficient Pr³⁺³P₀ visible emission upon UV-VUV excitation. The mechanism of energy transfer between Lu³⁺ host ion and Pr³⁺ impurity is discussed.     

Ce and Pr5d-energy levels in the (pseudo) perovskites KMgF3 and NaMgF3

The location of the 5d-energy levels of Ce³⁺ and Pr³⁺ in the cubic perovskite KMgF₃ and in the distorted perovskite NaMgF₃ was determined from spectroscopic studies in the vacuum ultraviolet. It is established that Ce³⁺ and Pr³⁺ ions both occupy the same site in each host: K⁺ sites for KMgF₃ and Na⁺ sites for NaMgF₃. The small crystal field splitting and the small value of the centroid shift of the 4fⁿ⁻¹5d-configuration yield a relatively high energy for the lowest 5d state of both Ce³⁺ and Pr³⁺. The lowest 5d state of Pr³⁺ in both hosts is found at energy higher than the 4f² state, enabling the photon cascade emission to occur.     

Low-temperature wet chemical synthesis and photoluminescence properties of YVO4: Bi, Eu nanophosphors

YVO₄: Bi³⁺, Eu³⁺nanophosphors are prepared by the citrate-assisted low-temperature wet chemical synthesis. When the colloidal solution is aged at 60 °C, the crystalline YVO₄: Bi³⁺, Eu³⁺ nanorods are formed from the amorphous gel precursors, as confirmed by transmission electron microscopy and X-ray diffractometry (XRD). YVO₄: Bi³⁺, Eu³⁺ nanophosphors emit red through energy transfer from Bi³⁺ to Eu³⁺ under near-UV-light excitation. The emission intensity increases with increasing the fraction of the crystalline phase during aging. The excitation peak corresponding to Bi³⁺-V⁵⁺ charge transfer relative to those of O²⁻-V⁵⁺ and O²⁻-Eu³⁺ charge transfers gradually becomes strong until the completion of the crystallization, although the contents of individual Bi³⁺ and Eu³⁺ ions incorporated into YVO₄ keep constant. When the aging is continued after the completion of the crystallization, the content of incorporated Bi³⁺ gradually increases, and hence the emission intensity decreases as a result of the energy migration among Bi³⁺ ions. These results suggest that in addition to the fraction of the crystalline phase and the contents of incorporated Bi³⁺ and Eu³⁺ ions, the local chemical states around Bi³⁺ play significant roles in photoluminescence properties.     

Reduction effect on the optical properties of Sm doped in SrB4O7 and SrB6O10 crystals

Reduction effects on the optical properties of Sm²⁺ ions doped in SrB₄O₇ and SrB₆O₁₀ crystals were studied by measurements of luminescence intensity decay as a function of time, X-ray irradiation and laser power effects on the photoluminescence. The fluorescence intensity of Sm²⁺ doped in SrB₄O₇ and SrB₆O₁₀ crystals decreased upon excitation at 488 nm of Ar⁺ laser and this so-called photo-bleaching effect was highly dependent on the sample preparation conditions. The fluorescence intensity of Sm²⁺ doped in SrB₄O₇ decreased about 13%, while it decreased about 55% in the SrB₆O₁₀ crystal irradiated with X-ray for 10 h. Differences of photo-beaching effect and other optical properties of Sm²⁺ doped in SrB₄O₇ and SrB₆O₁₀ are discussed.     

4-benzyl pyridinium dihydrogenmonophosphate C6H5CH2C5H4NHH2PO4 : Single-crystal structure and its conductivity in polycrystalline form

The salt 4-benzyl pyridinium dihydrogenmonophosphate is monoclinic P2₁/c with the following unit cell dimensions: ; ; ; and β=97.328(11). Also, , D_x=1.403, , F(000)=560; ; and R=0.0495 and R_w=0.0964 for 3733 independent reflections. The structure consists of infinite parallel two-dimensional planes built of H₂PO₄⁻ anions and C₆H₅CH₂C₅H₄NH⁺ cations mutually connected by strong O-H ?O and N-H ?O hydrogen bonding. There are no contacts other than the normal Van der Waals interactions between the layers. The conductivity relaxation parameters associated with some H⁺ conduction have been determined from an analysis of the spectrum measured in a wide temperature range.     

Synthesis, structure and luminescence of LaSi3N5:Ce phosphor

In this work, new LaSi₃N₅:Ce³⁺ phosphors have been synthesized by solid-state reaction. Rietveld refinement of the crystal structure of La_1−xCe_xSi₃N₅ reveals that Ce atoms substituted for La atoms occupy 4a crystallographic positions. Broad emission and excitation bands observed were attributed to the transitions between the doublet ground state of the 4f¹ configuration and the crystal field components of the 5d¹ excited state. At 77 K, the centroid and crystal field splitting ε_cfs of the 5d levels of Ce³⁺ in LaSi₃N₅:Ce³⁺ compounds were valuated at 33.4×10³ and 11.3×10³ cm⁻¹, respectively. The zero-phonon line and the Stokes shift were measured to be 26.0×10³ and 5.0×10³ cm⁻¹, respectively.     

Charge transfer transitions in the excitation spectra of PrX3:Ce (X=Cl, Br) scintillators

The Cl⁻→Pr³⁺ charge transfer transition is identified to occur in the excitation spectrum of PrCl₃:Ce³⁺ at 211 nm (47,393 cm⁻¹). A model based on the dissociation of the charge transfer state is proposed to explain the presence of the ligand-to-Pr³⁺ charge transfer transition band in the excitation spectrum of PrX₃:Ce³⁺ (X=Cl, Br) when the Ce³⁺ emission is monitored.     

Luminescence and Tb-Ce-Eu ion energy transfer in single-crystalline films of Tb3Al5O12:Ce,Eu garnet

The paper is devoted to investigation of the processes of excitation energy transfer between the host cations (Tb³⁺ ions) and the activators (Ce³⁺ and Eu³⁺ ions) in single-crystalline films of Tb₃Al₅O₁₂:Ce,Eu (TbAG:Ce,Eu) garnet which is considered as a promising luminescent material for the conversion of LED's radiation. The cascade process of excitation energy transfer is shown to be realized in TbAG:Ce,Eu: (i) from Tb³⁺ ions to Ce³⁺ and Eu³⁺ ions; (ii) from Ce³⁺ ions to Eu³⁺ ions by means of dipole-dipole interaction and through Tb³⁺ ion sublattice.     

Luminescence spectra of Eu ions and interstitial oxygen in PbWO4 crystal

The site-selective excitation and emission spectroscopy, and luminescence decay have been investigated under a pulsed, tunable, narrowband dye laser of the ⁵D₀→⁷F₀ region in the europium ions-doped lead tungstate PbWO₄ (PWO) in single crystal. In as-grown sample, the experimental results show that there is only one ⁷F₀→⁵D₀ excitation transition indicating the only one Eu³⁺ site in PbWO₄ lattices. The sequential annealing treatments were conducted to investigate the effects of oxygen components on the microstructure environments of Eu³⁺ in the lattices. The site distribution of Eu³⁺ was changed by the annealing in air atmosphere, which could create new sites in PWO lattices. Confirmation of interstitial oxygen and interpretations of charge compensation mechanism for the observed new sites were discussed in the context of site-selective excitation and emission spectra. The main Eu³⁺ site is related to the charge compensation by the [(Eu_Pb³⁺)-V″_Pb-(Eu_Pb³⁺)] complex; the other minor new sites after annealing are originated from [(Eu_Pb³⁺)-O″_i-(Eu_Pb³⁺)] defects. Emission spectra excited by 355-laser and RT-Raman spectra were also measured.     

Luminescence characteristics of Pb centres in undoped and Ce-doped Lu3Al5O12 single-crystalline films and Pb→Ce energy transfer processes

At 4.2-350 K, the steady-state and time-resolved emission and excitation spectra and luminescence decay kinetics were studied under excitation in the 2.5-15 eV energy range for the undoped and Ce³⁺-doped Lu₃Al₅O₁₂ (LuAG) single-crystalline films grown by liquid phase epitaxy method from the PbO-based flux. The spectral bands arising from the single Pb²⁺-based centres were identified. The processes of energy transfer from the host lattice to Pb²⁺ and Ce³⁺ ions and from Pb²⁺ to Ce³⁺ ions were investigated. Competition between Pb²⁺ and Ce³⁺ ions in the processes of energy transfer from the LuAG crystal lattice was evidenced especially in the exciton absorption region. Due to overlap of the 3.61 eV emission band of Pb²⁺ centres with the 3.6 eV absorption band of Ce³⁺ centres, an effective nonradiative energy transfer from Pb²⁺ ions to Ce³⁺ ions takes place, resulting in the appearance of slower component in the luminescence decay kinetics of Ce³⁺ centres and decrease of the Ce³⁺-related luminescence intensity.     

Luminescence from the Pr 4f5d and S0 states in LiLaP4O12

The photoluminescence and excitation spectra of Pr³⁺ activated LiLaP₄O₁₂ has been investigated in the 10-300 K temperature region. At all temperatures, the luminescence consists of optical transitions emanating from both the Pr³⁺ 4f¹5d¹ and the ¹S₀ states. However, at low temperatures the emission spectrum is dominated by the intraconfiguration emission transitions emanating from the Pr³⁺¹S₀ state. With increasing temperature, there is an exchange of intensity between the two emitting states; emission transitions from the ¹S₀ state exhibit strong intensity quenching while the 4f¹5d¹→4f² emission transitions reveal intensity gain. These results are explained on the basis of thermal population of the 4f¹5d¹ state by the ¹S₀ state. The energy barrier of 0.05 eV (403 cm⁻¹) for the nonradiative process is determined from the temperature dependence of the ¹S₀ lifetime.     

Luminescence and energy transfer processes in Lu2SiO5:Ce scintillator

The energy transfer processes in Lu₂SiO₅:Ce³⁺ luminescence was investigated through the temperature dependent luminescence under excitation with VUV-UV. Ce1 center emission peaking at 393 and 422 nm and Ce2 center emission peaking at 462 nm were observed. Ce2 center emission is enhanced with the temperature, which can be explained by the rate of energy transfer from Ce1 center increases when the temperature rises. The Ce1 emission shows the thermal quenching effect under the direct excitation of Ce³⁺ at 262 nm. However, under the interband excitation of 183 nm, the Ce1 center emission exhibits undulating temperature dependence. This is because the emission is governed by thermal quenching and possible thermal enhancement of the transport of free carriers with the rising temperature.     

On photoluminescence in HgGa2S4

In this paper, we investigate the kinetics of photoluminescence in excited crystals of HgGa₂S₄ which have recently been proposed for implementing tunable luminescent devices. From photoluminescence experiments, performed at various temperatures and excitation powers, it appears that two kinds of radiative recombination processes take place during crystal excitation. These originate two bands in emission spectra which were resolved by means of a fitting procedure. The dependencies of these bands on temperature and excitation power density are explained by means of a specific kinetic model. A broad band, peaking at about 1.8 eV, is ascribed to electron-hole tunnel recombinations occurring in associated donor-acceptor pairs, according to a Prener-Williams scheme. The second narrow band, peaking at about 2.3 eV, is ascribed to electron-hole recombinations occurring in centres presenting short () and long-life () excited states. At room temperature, owing to thermally activated relaxation from short- to long-life states, these centres saturate under relatively low excitation powers. The tunability of photoluminescence is a consequence of competition between monomolecular and bimolecular recombination processes.     

Photoluminescence investigations of Zn2SiO4 co-doped with Eu and Tb ions

Zinc silicate phosphors co-doped with Eu³⁺ ions and also with both Eu³⁺ and Tb³⁺ ions were prepared by high temperature solid state reaction in air or reducing atmosphere. The luminescence characteristics of the prepared phosphors were investigated. While in the samples prepared in air, Eu³⁺ emission was found to be dominant over Tb³⁺ emission, in the samples prepared in reducing atmosphere, intense Eu²⁺ emission at 448 nm was found to be predominant over narrow Tb³⁺ emission. Luminescence studies showed that Eu³⁺ ions occupy asymmetric sites in Zn₂SiO₄ lattice. The intense f-f absorption peak of Eu³⁺ at 395 nm observed in these phosphors suggests their potential as red emitting phosphors for near ultra-violet light emitting diodes.     

Unusual luminescence of octahedrally coordinated divalent europium ion in Cs2MP2O7 (M=Ca, Sr)

The excitation and emission spectra of octahedrally coordinated europium ion (Eu²⁺) ions in Cs₂M²⁺P₂O₇ (M²⁺=Ca, Sr) are reported and discussed. The remarkable features of the Eu²⁺ luminescence in these phosphate materials include (a) very large Stokes shift of emission (∼1 eV), (b) high luminescence quenching temperature, and (c) unusually low energy of the emitted photons for Eu²⁺ luminescence in phosphate-based materials. The broad emission bands of Eu²⁺ in Cs₂CaP₂O₇ and Cs₂SrP₂O₇ peak at 607 and 563 nm, respectively. The Stokes shift, crystal field splitting, centroid shift and the red shift of the Eu²⁺ 4f⁶5d¹ electronic configuration have been estimated from the relevant optical data. The radiative lifetime of the Eu²⁺ emission in Cs₂M²⁺P₂O₇ is ∼1.2 μs. The nature of the Eu²⁺ emission in Cs₂M²⁺P₂O₇ is discussed and arguments are presented to associate the luminescence with an extreme case of normal 4f⁶5d¹→4f⁷[⁸S_7/2] emission.     

Site-selective luminescence of Bi in the YBO3 host under vacuum ultraviolet excitation at low temperature

(Y_0.99Bi_0.01)BO₃ phosphor was prepared and its luminescent properties were investigated using the synchrotron radiation instrument in this paper. Site-selective luminescence of Bi³⁺ in the YBO₃ host was demonstrated by exciting/monitoring with different wavelengths at low temperature in this research. At the same time, the energy transfer between the centers of Bi³⁺ ions occupying different symmetric sites was observed. The excited energy levels of Bi³⁺ were identified through assigning the emission and excitation spectra. The paths and mechanism of energy transfer between the centers of Bi³⁺ occupying different symmetric sites were discussed finally.     

Cr-related centers in Gd3Ga5O12 polycrystals

The luminescence excitation spectra, emission spectra under photo- and X-ray excitation, luminescence decay kinetics and thermostimulated luminescence (TSL) of Gd₃Ga₅O₁₂ garnet (GGG) polycrystalline samples have been investigated. It was established that the spectrum of Cr³⁺ ion emission were present in all TSL peaks. The activation energies of traps that are responsible for appearance of TSL in the region 295-600 K were estimated. It is shown that delocalization of electrons from the Cr³⁺e traps leads to the appearance of thermoluminescence (TL) glow peak at 390 K. The nature of other TSL peaks is discussed. The influence of visible light on the TSL intensity of the preliminary X-ray-irradiated samples is shown.     

Remarkable improvement of brightness for the green emissions in Ce and Tb co-activated LaPO4 nanowires

Ce³⁺ and Tb³⁺ co-activated LaPO₄ nanowires (NWs) were synthesized by the hydrothermal method and studied in contrast to corresponding micrometer rods (MRs). The results indicate that electronic transition rate of Ce³⁺ and Tb³⁺ in NWs had only a little variation in comparison with that in MRs, and energy transfer (ET) rate and efficiency of Ce³⁺→Tb³⁺ in NWs reduced. It is interesting to observe that the brightness for ⁵D₄-⁷F₅ of Tb³⁺ via ET of Ce³⁺→Tb³⁺ in NWs increased several times than that in MRs. This was attributed to the decreased energy loss in excited states being higher than ⁵D₄ of Tb³⁺ ions due to hindrance of boundary.     

VUV excited luminescence of MGdF4:Eu (M=Na, K, NH4)

Eu³⁺-doped NaGdF₄, KGdF₄ and NH₄GdF₄ phosphors with little oxygen contamination have been synthesized by hydrothermal technique. The emission spectra show that the doped Eu³⁺ ions are located in noncentrosymmetric sites in the three compounds. The two-photon emission has been observed in NaGdF₄:Eu³⁺ and KGdF₄:Eu³⁺ compounds under VUV excitation from the ground states to higher ⁶G_J excited states of Gd³⁺ ions, while in Eu³⁺-doped NH₄GdF₄, emissions from ⁵D_1,2,3 excited states of Eu³⁺ cannot be detected in the luminescence spectra.