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.     

Comparative analysis of energy-level splitting of Pr3＋ doped in LiYF4 and LiBiF4 crystals： a complete energy matrix calculation

Based on the combination of Racah's group-theoretical consideration with Slater's wavefunction, a 91 × 91 complete energy matrix is established in tetragonal ligand field D_2d for Pr³⁺ ion. Thus, the Stark energy-levels of Pr³⁺ ions doped separately in LiYF₄ and LiBiF₄ crystals are calculated, and our calculations imply that the complete energy matrix method can be used as an effective tool to calculate the energy-levels of the systems doped by rare earth ions. Besides, the influence of Pr³⁺ on energy-level splitting is investigated, and the similarities and the differences between the two doped crystals are demonstrated in detail by comparing their several pairs of curves and crystal field strength quantities. We see that the energy splitting patterns are similar and the crystal field interaction of LiYF₄:Pr³⁺ is stronger than that of LiBiF₄:Pr³⁺.     

Tetragonal distortions of the octahedral environments of Cr3+, Fe3+ and Gd3+ ions in A2CdF4 (A = Rb,Cs) and ACdF3 crystals

Abstract

The tetragonal distortions of local octahedral environments of Cr³⁺, Fe³⁺ and Gd³⁺ ions in Rb₂CdF₄, Cs₂CdF₄, RbCdF₃ and CsCdF₃ crystals have been studied by analyzing their EPR spectra. From the studies, it is found that the tetragonal distortions for Cr³⁺ and Fe³⁺ impurity ions, which substitute Cd²⁺ and have nearly the same ionic radius, are close to each other, whereas that for Gd³⁺ impurity ion, having a larger ionic radius, is larger than those for Cr³⁺ and Fe³⁺ ions in the same crystal. It appears that not only the impurity-ligand distance, but also the tetragonal distortions of impurity centres in crystals are closely related to the size of impurity.     

Multiphonon relaxation of optical excitations in CsCdBr<Subscript>3</Subscript>:Pr<Superscript>3+</Superscript> and LiYF<Subscript>4</Subscript>:Nd<Superscript>3+</Superscript> crystals

The technique of calculation of the n-phonon transition rates between electronic sublevels of impurity rare earth ions in dielectric crystals is developed in the case when n>2. The n-phonon transition probabilities are calculated according to the 1st and 2nd orders of perturbation theory. The Hamiltonian of the electron-phonon interaction is constructed in the framework of the exchange charge model and developed as series in relative displacements of the rare earth ion and ligands. The contribution of the lattice anharmonicity on the probabilities of n-phonon transitions is taken into account. On the basis of the developed technique, the nonradiative relaxation rates of ⁴ G _7/2 multiplet of Nd³⁺ ions in LiYF₄:Nd³⁺crystal and ³P₁ multiplet of Pr³⁺ ions in CsCdBr₃:Pr³⁺ crystal were computed. The results of our calculations show that the 2nd order terms in the expressions for the probabilities studied here are comparable with, and in some cases prevail over the 1st order terms. An account of lattice anharmonicity in case of LiYF₄:Nd³⁺ crystal substantially modifies the corresponding multiphonon relaxation rates. The calculated nonradiative relaxation rates for both crystals agree well with the experimental data.     

Satellite lines in the optical absorption spectra of doped (La,Pr)Cl3-crystals

By substitution of an impurity ion, A³⁺, for a trivalent La³⁺-ion, symmetric and asymmetric distortions may be produced in the lattice of the anhydrous lanthanum trichloride. In LaCl₃ doped with Pr³⁺-ions at very low concentrations, these distortions give rise to Pr³⁺ satellite levels, the energy shift and Zeeman effect of which are calculated in a first order approximation. A survey is given of the possible types of single and double perturbations produced by substitution of one or two impurity ions in the neighbourhood of a non-Kramers ion in a hexagonal lattice of the LaCl₃-type. The results are quite general for non-Kramers rare-earth ions on hexagonal sites in any other lattices.Project of the Sonderforschungsbereich 65 Festkörperspektroskopie Darmstadt-Frankfurt, supported by the Deutsche Forschungsgemeinschaft     

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.     

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.     

The effect of codopants on spectral-kinetic characteristics of luminescence of scintillation glasses doped with terbium ions

The effect of Ce³⁺ and Pr³⁺ ions on spectral-kinetic characteristics of luminescence of lithium–phosphate–borate glasses is studied. It is shown that terbium ion luminescence caused by transitions from ⁵D₃ and ⁵D₄ multiplets to the ground 7FJ term is detected in samples containing Tb³⁺/Ce³⁺ and Tb³⁺/Pr³⁺. It has been found that an increase in the concentration of cerium ions from 0.2 to 1 wt % leads to an increase in the intensity of main luminescence bands of terbium ions. In Tb³⁺/Pr³⁺ glasses, a decrease in the relative light yield is observed with an increase in the concentration of Pr³⁺ ions. Processes of energy transfer between Tb³⁺/Ce³⁺ and Tb³⁺/Pr³⁺ ions are discussed.     

Luminescent properties and energy transfer of Y3Al5O12:Ce3+, Ln3+ (Ln=Tb,Pr) prepared by polymer-assisted sol–gel method

Y₃Al₅O₁₂:Ce³⁺, Pr³⁺ and Y₃Al₅O₁₂:Ce³⁺, Tb³⁺ nano-particles have been synthesized by polymer-assisted sol–gel method. Crystal structure, luminescent properties and energy transfer of the phosphors are analyzed. XRD study of polycrystalline powders shows that all the samples are of YAG phase without impurity. Photoluminescence (PL) emission and excitation spectra illustrate that in YAG:Ce, Pr phosphors, energy transfer occurs mutually between Ce³⁺ and Pr³⁺, while in YAG:Ce, Tb systems, only one-way path energy transfer of Tb³⁺→Ce³⁺ is observed.     

Luminescence of CaWO4:Pr3+ and CaWO4:Tb3+ at ambient and high hydrostatic pressures

Photoluminescence spectra of CaWO₄ doped with Pr³⁺ and Tb³⁺ obtained at high hydrostatic pressures up to 315 kbar applied in a diamond anvil cell (DAC) are presented. The intensities of the luminescence from the ³P₀ state of Pr³⁺ and from the ⁵D₃ state of Tb³⁺ decreased with increasing pressure. At pressures greater than 50 kbar, the ¹D₂ → ³H_J transitions in Pr³⁺ and the ⁵D₄ → ⁷F_J transitions in Tb³⁺ dominated the spectra. At pressures greater than 100 kbar, only emissions from the lower excited states were observed. At pressures greater than 150 kbar, luminescence from the ¹D₂ and ⁵D₄ states also decreased with increasing pressure, and at a pressure of 315 kbar for CaWO₄:Pr³⁺ and 190 kbar for CaWO₄:Tb³⁺, the emissions related to the Pr³⁺ and Tb³⁺ were quenched. These effects were related to the influence of impurity trapped excitons (ITEs) on the efficiency of the f–f emission in the Pr³⁺ and Tb³⁺ ions. Analysis of the emission spectra collected at different pressures allowed the energies of the ground states of the Pr³⁺ and Tb³⁺ ions with respect to the band edges of the CaWO₄ host to be estimated.     

Sensitizing effects of ZnO quantum dots on red-emitting Pr-doped SiO2 phosphor

In this study, red cathodoluminescence (CL) (λ_emission=614 nm) was observed from Pr³⁺ ions in a glassy (amorphous) SiO₂ host. This emission was enhanced considerably when ZnO quantum dots (QDs) were incorporated in the SiO₂:Pr³⁺ suggesting that the ZnO QDs transferred excitation energy to Pr³⁺ ions. That is, ZnO QDs acted to sensitize the Pr³⁺ emission. The sol–gel method was used to prepare ZnO–SiO₂:Pr³⁺ phosphors with different molar ratios of Zn to Si. The effects of the ZnO QDs concentration and the possible mechanisms of energy transfer from ZnO to Pr³⁺ are discussed. In addition, the electronic states and the chemical composition of the ZnO–SiO₂:Pr³⁺ phosphors were analyzed using X-ray photoelectron spectroscopy (XPS).     

Fundamental aspects of activated nanocrystal luminescence and possible applications

Presented results of complex study of relaxation processes and interionic interaction in Y₂SiO₅:Pr³⁺ and Lu₂SiO₅:Ce³⁺ nanocrystals clearly show two fundamental aspects: the phonon quantum confinement gives rise to the new fluorescence dynamics of doped ions; the developed surface of nanocrystals stimulates the irregular distribution of doped ions within the nanocrystal volume and could be the reason of new atomic arrangement of nanocrystal. Fluorescence spectrum of isolated Y₂SiO₅:Pr³⁺nanocrystal demonstrates the intense fluorescence from the high crystal field components of split ¹D₂ manifold of Pr³⁺ as the result of a suppression of phonon-assisted relaxation under the phonon quantum confinement. The direct comparison of the data obtained for nano- and bulk Y₂SiO₅:Pr³⁺ crystals has revealed that the concentration threshold of luminescence quenching is strikingly low for nanocrystals. This effect is caused by uphill diffusion of doped ions and preferred Pr segregation at the nanocrystal surface layer that provides the relaxation of elastic tension arising due to the difference of ionic radii of Pr³⁺ and Y³⁺. Lu₂SiO₅:Ce³⁺ nanocrystals which average size is 5 nm do not demonstrate the effect of energy storage as the result of atomic packing changing that does not permit the existence of electronic traps.     

Local lattice distortions and ligand hyperfine interactions in Eu2+-and Gd3+-doped fluorites

A computer simulation of lattice distortions around an impurity ion Eu²⁺ in MeF₂ fluorites (Me=Ca, Sr, Ba) is reported. ENDOR data on displacements of F⁻ ions distant from an impurity center were used to determine the parameters of the Eu²⁺-F⁻ short-range interaction potential. A theoretical study of the effect of hydrostatic pressure on the impurity-center local structure has been made. A comparison with experimental data permits a conclusion that the calculated ligand displacements are reliable. An experimental ENDOR investigation of the ligand hyperfine interaction (LHFI) in MeF₂:Gd³⁺ crystals (Me=Ca, Sr, Pb, Ba) has been performed. The results obtained in the simulation are used to describe the LHFI of impurity ions with the nearest-neighbor fluorine environment. The contributions to LHFI associated with ligand polarization are shown to constitute 10–50% of the experimental LHFI constants. The inclusion of such contributions results in practically linear dependences of the remaining short-range part of the LHFI on distance. Fiz. Tverd. Tela (St. Petersburg) 40, 2172–2177 (December 1998)     

Luminescence of Cs2NaScCl6:Pr3+: Effects of Changing the Elpasolite Lattice Parameter

Abstract

The luminescence spectrum of Cs₂NaScCl₆:Pr³⁺ (0.1 at.%) has been recorded at temperatures down to 10 K and assigned between 20,800 and 9900 cm^?1. Twenty‐three energy levels of the 4f² configuration Pr³⁺ ion were located and then fitted by the conventional 4f² crystal field calculation, as well as by a configuration interaction assisted crystal field (CIACF) calculation. The latter gave a much better fit. A comparison of the fit for Cs₂NaScCl₆:Pr³⁺ with fits upon the same set of energy levels in Cs₂NaYCl₆:Pr³⁺ and Cs₂NaPrCl₆, where the fifth nearest neighbor of Pr³⁺ is changed and the lattice parameter increases along this series, shows a decrease in the magnitudes of the crystal field parameters, which were also semiquantitatively simulated. Several facets of the emission spectra are interesting, including the observation of weak progressions in the totally symmetric Pr–Cl stretching vibration and the occurrences of the resonance of electronic and vibronic states. The spectra of Cs₂NaScCl₆:Pr³⁺ (1 at.%) differ considerably from those of the more dilute system and show that other species are formed rather than a complete substitution of the Sc³⁺ sites by Pr³⁺.     

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.     

Excited states dynamics under high pressure in lanthanide-doped solids

Emission related to rare earth ions in solids takes place usually due to 4fⁿ→4fⁿ and 4fⁿ⁻¹5d¹→4fⁿ internal transitions. In the case of band to band excitation the effective energy transfer from the host to optically active impurity is required. Among other processes one of the possibilities is capturing of the electron at the excited state and the hole at the ground state of impurity.The latest results on high pressure investigations of luminescence related to Pr³⁺ and Eu²⁺ in different lattices are briefly reviewed. The influence of pressure on anomalous luminescence and 4fⁿ⁻¹5d¹→4fⁿ luminescence in BaSrF₂:Eu²⁺ and LiBaF₃:Eu²⁺ systems and Pr³⁺ 4fⁿ→4fⁿ emission quenching is presented and discussed. A theoretical model describing the impurity-trapped exciton as a system where a hole is localized at the impurity and an electron is captured by Coulomb potential at Rydberg-like states is developed. The results show the importance of local lattice relaxation for the creation of stable impurity-trapped exciton states. The ligands shifts create a potential barrier that controls the effect of mixing between the Rydberg-like electron and localized electron wave functions.     

HgSe:Fe—a mixed-valency system and the problem of the ground state

The dependence of the electron mobility on the iron impurity content N _Fe and temperature is studied for three variants of the ordering of Fe³⁺ ions in crystalline HgSe:Fe, a weakly correlated gas, states with near ordering like that in a strongly correlated Coulomb liquid, and long-range ordering. The electron mobilities owing to scattering on the correlated system of Fe³⁺ ions are determined. The temperature dependence of the mobility is analyzed for electron scattering on fluctuations in the charge density in a system of Fe²⁺-Fe³⁺ iron ions with mixed valency, and the correlation length is determined. It is shown that the ordering region for the Fe³⁺ ions encompasses only the first coordination sphere, i.e., near ordering in the position of the Fe³⁺ ions is established, as in a liquid. The coupling between the ordering of the Fe³⁺ ions and the formation of a correlation gap in the density of impurity d-states and its effect on the low-temperature behavior of the electron mobility in HgSe:Fe crystals are examined. Fiz. Tverd. Tela (St. Petersburg) 40, 425–432 (March 1998)     

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-dependent fluorescence-quenching in La1−xPrxP5O14

The visible fluorescence of the Pr³⁺-ions in La_1?xPr_xP₅O₁₄ was investigated. The fluorescence starting from the level ³P₀ decays as a pure exponential with a time constant of 124 ns for all concentrations in the range of 4.2 K to 300 K, whereas the fluorescence of the level ¹D₂ shows a strongly concentration-dependent, nonexponential decay. The concentration-quenching arises from those ions which absorb in the wings of the inhomogeneous absorption lines, and can be attributed to Pr³⁺ pairs. We propose as a model of fluorescence-quenching the cross-relaxation between an excited Pr³⁺ ion and an adjacent ion in the ground state under emission of one phonon to the lattice. The frequently employed Inokuti-Hirayama continuum approximation fails when analyzing the time-resolved fluorescence of the a¹D₂ level. Only if the particular structure of the pentaphosphates is taken into account, it is possible to determine the dipole-quadrupole character of the Pr³⁺-Pr³⁺ interaction unambiguously.