Strong quenching of praseodymium f-f luminescence induced by a surface of Y2SiO5:Pr nanocrystal

The direct comparison of the luminescence decay data obtained for nano- and bulk Y₂SiO₅:Pr³⁺ crystals has revealed that the concentration threshold of luminescence quenching is strikingly low for nanocrystals. Nanocrystal inhomogeneous stress field induced by a surface stimulates the segregation of the doped Pr³⁺ ions within the surface layer that provides the relaxation of elastic tension arising due to the difference of the ionic radii of Pr³⁺ and Y³⁺. The Pr³⁺ irregular distribution in the nanocrystal volume results in the Pr³⁺ local concentration increasing that facilitates the luminescence quenching.     

Concentration quenching anomalies of activated Y<Subscript>2</Subscript>SiO<Subscript>5</Subscript>:Pr<Superscript>3+</Superscript> nanocrystal luminescence

For the fist time in Y₂SiO₅:Pr³⁺ nanocrystals, the ordered stage in the ¹ D ₂ luminescence decay curves for Pr³⁺ ions has been observed at anomalously low doped ion concentration (0.5 at %). This effect is caused by preferred location of the activator ions in the near-surface layer of the nanocrystal that provides the relaxation of elastic tension arising due to the difference of ionic radii of Pr³⁺ and Y³⁺ ions. Concentration quenching of Pr³⁺ luminescence is caused by the cooperative cross-relaxation.     

Strong segregation of doped ions in Y2SiO5:Pr3+ nanocrystals

Strong temperature controlled segregation of doped ions in Y₂SiO₅:Pr³⁺ nanocrystals detected by spectroscopic techniques is reported. The elastic interactions stimulate Pr³⁺ segregation thus leading to non-uniform distribution of doped ions, pair formation and, as a consequence, to abnormal low threshold of luminescence concentration quenching for Y₂SiO₅:Pr³⁺ nanocrystals.     

Anomalies in the concentration quenching of luminescence in doped Y2SiO5:Pr3+ nanocrystals

In Y₂SiO₅:Pr³⁺ nanocrystals, an ordered phase is observed in the ¹ D ₂ luminescence decay curves of Pr³⁺ ions at their anomalously low concentration (0.5 at %). This effect is caused by the predominant accumulation of activator ions near the nanocrystal surface, which provides relaxation of the elastic strains arising as a result of the misfit between the ionic radii of Pr³⁺ and Y³⁺. The concentration quenching of Pr³⁺ luminescence is due to cooperative cross relaxation.     

Temperature-dependent segregation of Pr3+ impurity ions in Y2SiO5:Pr3+ and YPO4:Pr3+ nanocrystals

Stationary and time-resolved spectroscopic methods are used to show that the impurity ions in Y₂SiO₅:Pr³⁺ and YPO₄:Pr³⁺ nanocrystals are distributed nonuniformly. This nonuniform distribution is found to be caused by the temperature-dependent segregation of Pr³⁺ ions near the surface of a nanocrystal. The motion of the activator ions from the bulk of a nanocrystal to the near-surface layer is traced when the activator concentration and the heat-treatment parameters are varied over wide ranges, and the main parameters of this effect (impurity redistribution intensity and time, diffusion coefficient) are estimated.     

The nature of activation centers in Y2SiO5:Pr3+, Gd2SiO5:Pr3+, and Lu2SiO5:Pr3+ crystals

A complex study of the energy spectra and relaxation channels for the excitation energy of activation centers in Y₂SiO₅:Pr³⁺, Lu₂SiO₅:Pr³⁺, and Gd₂SiO₅:Pr³⁺ was performed. An analysis of the low-temperature optical spectra showed that the energy parameters and the character of field splitting of the ¹ D ₂ and ³ H ₄ activator ion terms were substantially different in crystals of different crystallographic types. The pseudosymmetry effect was observed in splitting of the ¹ D ₂ and ³ H ₄ terms of Pr³⁺ ions situated in nonequivalent crystal lattice cation sites of Y₂SiO₅ and Lu₂SiO₅. Activator ions nonuniformly populated nonequivalent cation sites of the Y₂SiO₅ crystal lattice. At high activator ion concentrations (>1 at %), luminescence decay in Y₂SiO₅ could not be described by a simple exponential time dependence. The complex luminescence decay law was caused by activator ion excitation energy migration and capture by acceptors. The role of energy acceptors was played by activator ion dimers.     

Enhanced luminescence in transparent glass ceramics containing BaYF5: Ce3+ nanocrystals

Ce³⁺ doped transparent glass ceramics containing BaYF₅ nanocrystals were prepared by controlled heat treatment of 45SiO₂–15Al₂O₃–10Na₂O–24BaF₂–6Y₂O₃–0.5Ce₂O₃ (mol%) glass. X-ray diffraction and transmission electron microscopy data have revealed the formation of BaYF₅ nanocrystals. Both photoluminescence and X-ray excited luminescence spectra have shown a blue-shift of the emission band of Ce³⁺ on ceramization, which is consistent with the Ce³⁺ environment evolving from a glassy oxide to a fluoride phase. The luminescent intensity of Ce³⁺ ions in the transparent glass ceramics is greatly enhanced compared with the precursor glass under ultraviolet and X-ray excitation. This could be attributed to the Ce³⁺ ions present in the BaYF₅ crystalline phase.     

Investigation of gain characteristics in mixed crystals LiMeF4 (Me = Y,Lu, Yb) doped by Ce3+ ions

Differential gain spectra in the range 295–335 nm were measured in crystals of scheelite structure LiY_{1 ? x} Lu _x F₄ (x = 0–1), doped by Ce³⁺ ions. It is shown that variation of Lu³⁺ and Y³⁺ ions relative content in LiY_{1 ? x} Lu _x F₄ crystals allows to manipulate the spectral width of the amplification band. Cross-sections of excited-state absorption at the wavelengths of Ce³⁺ luminescence, probability ratios of formation and thermal destruction of color centers depending on the Y³⁺ ions content in LiY_{1 ? x} Lu _x F₄ crystals were estimated. Even better gain characteristics have been demonstrated by LiLuF₄:Ce³⁺, doped by Yb³⁺ ions. The highest optical gain coefficient with a wide amplification band among studied samples was observed in LiLuF₄:Ce³⁺ crystal, codoped by Yb³⁺ ions.     

Lu2SiO5:Ce and Y2SiO5:Ce single crystals and single crystalline film scintillators: Comparison of the luminescent and scintillation properties

The paper is dedicated to development of scintillators based on the single crystalline films of Ce³⁺ doped Lu₂SiO₅ (LSO:Ce) and Y₂SiO₅ (YSO:Ce) orthosilicates grown by Liquid Phase Epitaxy method onto YSO substrates from melt-solutions based on the PbO–B₂O₃ flux. We also compare the luminescent and scintillation properties of Ce doped LSO:Ce and YSO:Ce single crystalline films with those of their single crystal counterparts, grown by the Czochralski method.     

Growth and the luminescence properties of a lutetium gadolinium garnet doped with Ce3+ and Pr3+ ions

Crystals of lutetium gadolinium garnet solid solutions (Lu_{1 − x} Gd _x )Al₅O₁₂ (0 ≤ x ≤ 0.6) doped with Ce³⁺ and Pr³⁺ ions have been prepared by the horizontal directional crystallization method, and their optical and luminescence properties have been investigated. It has been established that the introduction of gadolinium into the lutetium garnet lattice leads to a decrease in the antisite luminescence (Lu_Al centers) in the UV spectral range and to sensitization of the Ce³⁺ ion luminescence. By contrast, the presence of gadolinium results in the quenching of the Pr³⁺ luminescence due to the nonradiative excitation transfer from Pr³⁺ ions to Gd³⁺ ions.     

EPR Study of Sc<Subscript>2</Subscript>SiO<Subscript>5</Subscript>:Nd<Superscript>143</Superscript> Isotopically Pure Impurity Crystals

The Sc₂SiO₅ single crystals doped with 0.001 at.% of the ¹⁴³Nd³⁺ ion were studied by continuous-wave and pulse electron paramagnetic resonance methods. The g-tensors and hyperfine structure tensors for two magnetically non-equivalent Nd ions were obtained. The spin–spin and spin–lattice relaxation times were measured at 9.82 GHz in the temperature range from 4 to 10 K. It was established that three relaxation processes contribute to the spin–lattice relaxation processes. There are one-phonon spin–phonon interaction, two-phonon Raman interaction and two-phonon Orbach–Aminov relaxation processes. It was established that spin–spin relaxation time is of the same magnitude for neodymium ion doped in Sc₂SiO₅ and in Y₂SiO₅.     

Luminescence properties of Pr3+-doped transparent oxyfluoride glass–ceramics containing BaYF5 nanocrystals

Transparent oxyfluoride glass–ceramics containing BaYF₅ nanocrystals were successfully synthesized by appropriate heat-treatment on the SiO₂–Al₂O₃–Na₂O–BaF₂–Y₂O₃–Pr₆O₁₁ precursor glass. The structure and luminescence properties of the precursor glass and glass–ceramics were investigated by DSC, XRD, TEM, optical transmission, photoluminescence, decay time and radioluminescence spectra. The XRD results indicate that the BaYF₅ nanocrystals can percitated in the precursor glass and the sharper emission peaks of Pr³⁺ in glass ceramic suggests that Pr³⁺ ions are incorporated into the BaYF₅ nanocrystals. The higher the heat-treatment temperature is, the more the Pr³⁺ ions are centered into BaYF₅ nanocrystals, which results in the optimal concentration of Pr³⁺ in glass ceramic changes on heat-treatment temperature. It is notable that the emission intensity of both photoluminescence and radioluminescence for 0.1 mol% Pr³⁺ in the glass ceramic (GC665) are stronger than those in the precursor glass. The mechanism of enhanced luminescence is also discussed.     

Optical spectroscopy of Ca3Sc2Si3O12, Ca3Y2Si3O12 and Ca3Lu2Si3O12 doped with Pr

The silicates Ca₃Sc₂Si₃O₁₂, Ca₃Y₂Si₃O₁₂ and Ca₃Lu₂Si₃O₁₂, both undoped and doped with Pr³⁺ ions, have been synthesized by solid-state reaction at high temperature. The luminescence spectroscopy and the excited state dynamics of the materials have been studied upon VUV and X-ray excitation using synchrotron radiation. All doped samples have shown efficient 5d-4f emission upon direct VUV excitation of 5d levels, but only Ca₃Sc₂Si₃O₁₂:Pr³⁺ shows luminescence upon interband VUV or X-ray excitation. The VUV excited emission spectra of Ca₃Y₂Si₃O₁₂:Pr³⁺ and Ca₃Lu₂Si₃O₁₂:Pr³⁺ show features attributed to emission from two distinct sites accommodating the Pr³⁺ dopant. The decay kinetics of the Pr³⁺ 5d-4f emission in Ca₃Sc₂Si₃O₁₂:Pr³⁺ upon VUV excitation across the band gap are characterized by decay times in the range 25-28 ns with no significant rise after the excitation pulse. They appear to be faster upon X-ray irradiation than for VUV excitation. Weak afterglow components are attributed to defect luminescence.     

The luminescence and energy transfer in Pr3+–Ce3+ co-doped Lu0.8Sc0.2BO3 crystals

Lu_0.8Sc_0.2BO₃ crystals doped with 1 at%Ce³⁺ and co-doped 0.1 at% and 0.5 at%Pr³⁺ were grown by the Czochralski method. The concentrations of Pr³⁺ and Ce³⁺ in crystals were measured by the ICP-AES method. Absorption spectra, VUV–UV spectra, fluorescence decay time and X-ray excitation luminescence spectra were investigated at room temperature. The excitation luminescence spectra of Ce³⁺ emission and decay curves from the lower excited state levels of the 4f¹5d¹ and 5d¹ electronic configurations of the Pr³⁺ and Ce³⁺ conspicuously indicated the non-radiative energy transfer from Pr³⁺ to Ce³⁺. The detailed pathways were shown in the energy level diagram of the respective Ce³⁺ and Pr³⁺ in Lu_0.8Sc_0.2BO₃ host. In addition, the scintillation efficiency data indicated that the energy transfer effect is directly associated with the Pr³⁺ concentration.     

Luminescence and origin of lead-related centers in single crystalline films of Y2SiO5 and Lu2SiO5

In the temperature range 4.2–350 K, the steady-state and time-resolved emission and excitation spectra and luminescence decay kinetics are studied for the undoped Y₂SiO₅ and Lu₂SiO₅ single crystalline films grown by liquid phase epitaxy method from the PbO-based flux and, owing to that, containing lead ions substituting for Y³⁺ or Lu³⁺ ions. Luminescence characteristics of Pb-related centers of different types are identified. On the basis of the results obtained, we suggest that the ultraviolet emission of Pb-related centers arises from the Pb²⁺ ions substituting for Y³⁺ or Lu³⁺ ions in the Y1 and Lu1 lattice sites of the X₂ structure. Possible hypotheses on the origin of the intense complex lead-related blue emission are discussed. We propose phenomenological models describing the excited-state dynamics of the studied luminescence centers. We also determine characteristic parameters of the corresponding relaxed excited states, in particular, the energy separations between the excited states and the rates of the radiative and non-radiative transitions from these states.     

VUV 5<Emphasis Type="Italic">d</Emphasis>-4<Emphasis Type="Italic">f</Emphasis> luminescence of Gd<Superscript>3+</Superscript> and Lu<Superscript>3+</Superscript> ions in the CaF<Subscript>2</Subscript> host

The first observation and characterization of Lu³⁺ 4f ¹³5d-4f ¹⁴ luminescence from the CaF₂: Lu³⁺ crystal are reported, and the multisite structure in the spectra of Ce³⁺, Gd³⁺, and Lu³⁺ ions in the CaF₂ host is analyzed with the high-resolution VUV spectroscopy technique using synchrotron radiation. It is shown that vibronic structure in the emission and excitation spectra of interconfigurational transitions in Gd³⁺ and Lu³⁺ ions doped into CaF₂ differs from that observed for Ce³⁺ ions entering mainly at the tetragonal (C _4v ) sites. However, the exact types of sites in which the Gd³⁺ and Lu³⁺ ions reside in a CaF₂ lattice cannot be identified using only the obtained experimental spectroscopid data. The text was submitted by the authors in English.     

Echo spectroscopy of two-level systems in a Y2SiO5: Pr3+ crystal

The results of an experimental investigation of low-temperature optical spectra and phase relaxation of electronic excitations of Pr³⁺ impurity ions in a Y₂SiO₅ crystal are reported. It is established that at low temperatures spectral lines are broadened by a mechanism that is uncharacteristic for crystals and is due to the interaction of impurity ions with two-level systems. The constants characterizing the interaction of Pr³⁺ impurity ions with phonons and two-level systems are found. Zh. éksp. Teor. Fiz. 115, 704–715 (February 1999)     

5d–4f luminescence of Ce3+, Gd3+ and Lu3+ in LiCaAlF6

The emission and excitation spectra as well as decay kinetics of luminescence due to 5d–4f transitions in Ce³⁺, Gd³⁺ and Lu³⁺ ions doped into LiCaAlF₆ crystals have been analyzed with high spectral and time resolution using synchrotron radiation for excitation. The rich fine structure originating from electronic origins of transitions and their phonon replica has been well resolved and identified. Experimental data are compared with the spectra simulated in the framework of the semiphenomenological models of the crystal field and the crystal lattice dynamics.     

EPR study of Ce ions in lutetium silicate scintillators Lu2Si2O7 and Lu2SiO5

Two Ce³⁺-doped scintillator crystals, LSO (Lu₂SiO₅:Ce) and LPS (Lu₂Si₂O₇:Ce), are studied by EPR spectroscopy. The analysis indicates that Ce³⁺ substitutes for Lu³⁺ ion in a C₂-symmetry site for LPS and in two C₁-symmetry sites for LSO, with a preference for the largest one, with 6+1 oxygen neighbors. Angular dependence of the EPR spectrum shows that the electronic ground state of Ce³⁺ is different in these two matrices. It is mainly composed of |M_J|=5/2 state in LPS and |M_J|=3/2 state in LSO. The temperature dependence of the linewidth shows a noticeably long spin lattice relaxation time, especially in LPS, which is the result of a stronger crystal field in LPS than in LSO.     

Influence of nanoenvironment on luminescence of Euactivated SnO2 nanocrystals

The emission intensity of the peak at 612 nm (⁵D₀→⁷F₂) of the Eu³⁺ ions activated SnO₂ nanocrystals (doped and coated) is found to be sensitive to the nanoenvironment. We have compared the luminescence efficiencies of the nanocrystals of SnO₂ doped by Eu₂O₃ with those of SnO₂ coated by Eu₂O₃ and we found that the intensities are significantly higher in coated nanocrystals. Furthermore, it is clear from luminescence intensity measurements that Eu³⁺ ions occupy low symmetry sites in the Eu₂O₃ coated SnO₂ nanocrystal. The analysis suggests that the radiative relaxation rate is higher in Eu₂O₃ coated SnO₂ nanocrystals than Eu₂O₃ doped SnO₂ nanocrystals due to the asymmetric environment of Eu³⁺ ions in coated samples.