Spectroscopic study of europium doped RCOB host lattices: evidence of local perturbations

The results of the high-resolution absorption, selective excitation, time-resolved emission and emission kinetics of Eu³⁺ in GdCOB and EuCOB single crystals are presented. The Eu³⁺ spectra show at least three non-equivalent centers whose static spectral characteristics were used to assign structural models. The crystal field parameters were estimated for prevailing Eu³⁺ center in EuCOB. The inhomogeneous broadening and shifts of the ⁵D₀→⁷F₀ and ⁵D₀→⁷F₁ Eu³⁺ lines in GdCOB under selective excitation in main lines were explained by J mixing of ⁷F₂ states into ⁷F₁ and ⁷F₀ by second order crystal field terms and connected to disordered structure in cationic spheres.     

Crystal fields in (La1−xGdx)OCl:Eu solid solutions

The formation of cation solid solution in the (La_1−xGd_x)OCl:Eu³⁺ (0≤x_Gd≤1; Δx_Gd=0.1) series was studied by photoluminescence spectroscopy. The luminescence from the ⁵D_0–2 to the ⁷F_0–4 levels of the Eu³⁺ ion in the (La_1−xGd_x)OCl series was recorded at 77 K by using argon ion laser excitation (457.9 nm). The interpretation of the spectra according to the C_4v site symmetry of the Eu³⁺ ion in the tetragonal PbFCl-type structure yielded nearly complete sets (18 to 19 levels) for the ⁷F_0–4 levels. Simulations of the Stark level schemes were carried out with the aid of a phenomenological c.f. theory utilizing the five non-zero c.f. B_q^k parameters (B₀², B₀⁴, B₄⁴, B₀⁶ and B₄⁶) allowed for the C_4v site symmetry. By using the calculated c.f. parameter sets a quantitative measure was obtained to monitor the formation of cation solid solutions. The strength of the c.f. effect was estimated with the c.f. strength parameters S and S^k (k=2, 4 and 6). The c.f. parameter sets reproduced the experimental ⁷F_J (J=0–4) energy level schemes with the rms deviations between 4 and 11 cm⁻¹. The individual parameters as well as the c.f. strength parameters were found to evolve in a smooth manner indicating complete solid solubility in the (La_1−xGd_x)OCl series. Some local distortions from the C_4v symmetry — probably of long range—leading to the splitting of the ⁷F₁ doubly degenerate E level were observed, however.     

Photophysical properties of Eu supported on silica gel functionalized with propyl β-diketonates

New luminescent materials consisting of Eu³⁺ supported on silica gel functionalized with acetylacetone (EuAPG) and benzoyltrifiuoroacetone (EuBPG) showed a much more intense emission than Eu³⁺ supported either on silica gel (EuSG) or on silica gel functionalized with imidazole (EuIPG). The lifetimes of the Eu³⁺⁵D₀→⁷F₂ transition in the samples were determined. All the silica contained about 0.2–0.3 wt.% of Eu³⁺.     

Use of fluorescent probes for the investigation of solutions: the case of europium—carboxymethoxysuccinate complexes

The investigation of Eu-cmos solutions frozen in liquid nitrogen performed under siteselective excitation and with time-resolved detection shows the existence of three different species, each characterized by one particular set of spectroscopic features (⁶D₂ sublevels energy scheme+⁶D_o → ⁷F_o emission position+⁶D₀ lifetime). The relative occurrence of the three species varies with the Eu:cmos ratio. In the liquid state only two 0→0 lines attributed respectively to Eu(cmos) · nH₂O and Eu(cmos)₂ · nH₂O are observed. We have checked by comparison with frozen EuCl₃ aqueous solutions that Eu³⁺(aquo) is not the third species existing at LNT. We discuss its attribution to oligomeric species yet detected in Tb-cmos solutions and the influence of the ice structure on the spectroscopic characteristics of Eu³⁺-ligand frozen solutions.     

High pressure spectroscopy of Pr in LiNbO3

Absorption, emission, and luminescence excitation spectra of the LiNbO₃ crystal doped with 0.5% Pr³⁺ and 0.8% Yb³⁺ are presented. Additionally the photoluminescence spectra at high pressure have been measured. Hydrostatic pressures up to 135 kbar were applied with a diamond anvil cell. Absorption of the Pr³⁺:LiNbO₃ crystal is characterized by the strong threshold at about 400 nm, related to the band-to band-transitions and the sharp structure in the visible region attributed to the transitions to ³P_J and ¹D₂ levels of Pr³⁺ ion. After the 488 nm excitation the yellow emission related to the ¹D₂→³H_J transition of Pr³⁺ have been observed when the ³P₀ emission has not been detected. The excitation spectra of the ¹D₂ luminescence consist of the sharp lines related to the ³H₄→³P_J (J=0, 1, 2) transitions and two broad bands peaked at 340 and 400 nm related probably to the bound exciton. The ¹D₂→³H_J emission shifts with pressure toward the lower energies with the rate of −2.4 cm⁻¹ kbar⁻¹. Additionally, for higher pressures the ¹D₂ emission is considerably quenched. This is explained as being due to the decrease of the energy of the bound exciton with pressure which results in the higher nonradiative depopulation rate of the ¹D₂ state.     

Crystal structure transformation and luminescent behavior of the red phosphor for plasma display panels

To improve the chroma of red phosphor for plasma display panels, Eu³⁺ activated phosphors were prepared by combustion method, and were investigated in detail by XRD and PL spectra. With the decreasing of the amount of H₃BO₃, the phosphor crystal structures transform from hexagonal ortho-borate to monoclinic RE₃BO₆, and then to cubic oxide. The dominating emissions of the phosphors also change from ⁵D₀–⁷F₁ transition to ⁵D₀–⁷F₂ transition. Accordingly, CIE coordinates of the phosphors show that x values are increasing while y values are decreasing. Thus, phosphor with higher color purity could be achieved by adjusting the mole ratio of H₃BO₃.     

Spectral properties of Eu(III) complexes with heteropolyanions immobilised in xerogel matrices

Eu³⁺ complexes with two heteropolymetalates (HPMs) were entrapped in various xerogel matrices by the sol-gel process. The spectral behaviour of these complexes in the rigid xerogel was studied by means of absorption and emission spectra. The luminescence emission experiments prove that photoexcitation of the HPM lattices (tungstate and molybdate) induced the energy transfer from the oxygen-to-metal charge transfer {O→M (= W, Mo)} triplet states to Eu³⁺ (⁷F_0,1,2→⁵D_0,1). Among the studied matrices with the Eu(III) complexes the best results, i.e. highest relative intensity and longest lifetime, were observed for methylated silicate (SiO₂-polydimethylsiloxane) and silica. In both the cases intensity of the emission could be improved by drying at elevated temperature. The studied materials consisting of complexes Eu³⁺ with the HPMs exhibit in practice stable emission intensity during UV irradiation.     

Chiroptical activity of holmium pyrogermanate: tetragonal Ho2Ge2O7

Chiroptical luminescence and circular dichroism measurements are reported for single crystals of Ho₂Ge₂O₇. These crystals belong to the tetragonal space group P4,2,2 (or P4₃2₁2) with Z=4. Each Ho³⁺ ion in the crystal structure is coordinated to seven oxygens to form a distorted pentagonal bipyramid. The Ho³⁺ ions exhibit luminescence from several excited multiplet levels; chiroptical luminescence spectra are reported for ⁶I₈ → ⁶F₆, ⁶S₂, and ⁶F₃ at a sample temperature of 10 K. Room-temperature absorption and circular dichroism measurements are reported for the ⁶I₈ → ⁶F₆, ⁵S₂, ⁶F₄, and ⁶F₃ transition regions.     

Calculation of the ligand–lanthanide ion energy transfer rate in coordination compounds: contributions of exchange interactions

Recently, one of us (O.L.M.) developed a theoretical approach to the energy transfer process between a ligand and a lanthanide ion in luminescent compounds. This approach takes into account the direct Coulomb interaction only. Here we present further calculations which also include the exchange Coulomb interaction (isotropic and anisotropic). The selection rules on the total angular momentum, J, of the lanthanide 4f states, derived from each interaction are different. They are in fact complementary. We discuss the case of energy transfer to the ⁵D₁ manifold of the Eu³⁺ ion, which is forbidden when only the direct Coulomb interaction is considered but becomes allowed through the exchange interaction. In this case numerical estimates show that, depending on resonance conditions and the nature of the ligand donor state, the transfer rate may assume very high values. It is also emphasised that, according to the present approach, energy transfer to the ⁵D₀ level of the Eu³⁺ ion is forbidden by both mechanisms, provided J-mixing between the ⁷F₀ and ⁷F₂ manifolds is neglected.     

Optical spectroscopy of yttrium double phosphates doped by cerium and praseodymium ions

This paper reports the spectroscopic properties of cerium- and praseodymium-doped alkali metal yttrium double phosphates, M₃Y(PO₄)₂:Pr³⁺, Ce³⁺; M = Na, Rb. These phosphates were obtained by a solid state reaction between lanthanide phosphate hydrates and M₃PO₄. The absorption, reflection, emission and excitation spectra were measured at room temperature, 77 and 4 K in the IR-vis-ultraviolet (UV) range. For both the Ce³⁺- and Pr³⁺-doped double phosphates, the 4f^N↔4f^N−15d transitions were detected. For the Pr³⁺-doped double phosphates, the 4f–4f transitions from the ³H₄ ground manifold were analyzed. The low temperature ³H₄→³P₀ absorption spectra were used to characterize the structural modifications between the sodium and rubidium salts. For the Ce³⁺-doped double phosphate, the strong blue 5d¹→4f¹ emission band splits into two components due to the ²F_5/2−²F_7/2 splitting of the 4f¹ configuration. Intense emission occurs mainly from the ³P₀ level at high dopant concentrations, since the ¹D₂ emission is strongly quenched but was detected at the 2 mol% doping level. In spite of the forbidden 4f–4f character, the ³P₀ transitions have very short decay times, of the order of one μs. Dynamics of the excited states will be discussed based on the decay times and selective excited emission.     

Upconversion-induced ultraviolet emission in Ho doped SrLaGa3O7 and SrLaGaO4 crystals

An analysis of the red to ultraviolet wavelength upconversion in Ho³⁺ in SrLaGa₃O₇ and SrLaGaO₄ crystals is given. Upconverted, ultraviolet emission from the ³D₃ level under cw 647 nm excitation at room temperature was observed. Excitation of the ⁵F₅ level, corresponding to the ⁵I₈→⁵F₅ transition, leads to intense emission from the ⁵I₇, ⁵I₆, ⁵F₅, ⁵S₂, ⁵F₃, ³G₅ and ³D₃ levels. Based on the energy level diagram of Ho³⁺, the pump intensity dependencies and experimental time dependencies of the observed emissions, an excitation scheme is proposed.     

Luminescence properties of isomeric and tautomeric lanthanide pyridinedicarboxylates

The luminescence and PMR spectra of europium salts of six isomers of 2,3-, 2,4-, 2,5-, 2,6-, 3,4-, and 3,5-pyridinedicarboxylic acids (PDA) had been studied. The distribution of the effective charge in the nearest surroundings of the Eu³⁺ ion in these salts was evaluated from Stark splittings of electronic transitions. The values of relative integral intensities of electronic transitions ⁵D₀–⁷F_J (J=0–4) in the luminescence spectra were reported. Compounds investigated were divided into three subgroups taking into account the details of the structure of the ligands and details of the luminescence spectra. The ligand coordination manners as well as the strength of interaction between lanthanide ion and ligands were confirmed by data of the PMR and IR spectroscopy.     

Red photoluminescence and morphology of Eu doped Ca3La3(BO3)5 phosphors

Europium doped phosphors Ca₃La₃(BO₃)₅ were first synthesized by a sol–gel process technique. The reaction temperature of the sol–gel process was 300 °C lower than that of the solid-state reaction and the reaction time of the sol–gel process was shorter. The photoluminescence properties of Eu³⁺ doped Ca₃La₃(BO₃)₅ indicated that the phosphors exhibited a strong luminescence of ⁵D₀–⁷F₂ transition at 612 nm under the excitation at 237 nm. The emission intensity of the phosphors prepared by the sol–gel process was higher than those prepared by the solid-state reaction. The relationship between optical properties and morphologies were studied. In particular, Li⁺ ion doping effectively enhanced the luminescent properties of the Eu³⁺ doped Ca₃La₃(BO₃)₅ phosphors. The highest brightness was observed in the phosphor Ca₃La_2.82Eu_0.1Li_0.08B₅O_15−δ prepared by the sol–gel process.     

Characterization of crystal field and nuclear quadrupole interactions in the D1 state of Am in LaCl3 and CaWO4

The crystal field splitting and hyperfine energy level structure in the first ⁵D₁ excited state of ²⁴³Am³⁺ in LaCl₃ and CaWO₄ have been studied using selective laser excitation, Zeeman and spectral hole burning experiments. Two crystal field levels and associated vibronic structures were observed in the electric dipole forbidden ⁷F₀↔⁵D₁ transition, that has a very weak magnetic dipole moment. The hyperfine energy level structure, including nuclear quadrupole splitting, was resolved in spectral hole burning studies. The existing free ion and crystal field interaction model provides a consistent interpretation of the observed ordering of crystal field levels and the magnitude of the nuclear quadrupole splitting of ²⁴³Am³⁺ in CaWO₄, whereas a discrepancy between the model and experimental results is encountered for ²⁴³Am³⁺ in LaCl₃. A discussion on the relationship between nuclear quadrupole interaction and free ion modeling is given.     

Properties of ytterbium and neodymium doped alkali metal yttrium double phosphates of the M3Y1−xLnx(PO4)2 type

The spectroscopic behaviour of the Nd³⁺ and Yb³⁺ doped alkaline metal yttrium double phosphates, M₃Y_1−xLn_x(PO₄)₂ (M=Na, Rb; x=0.01–0.3) were studied for both powder and single crystal samples. The high resolution absorption and emission spectra were measured in the visible and IR regions. Spectral changes with the Nd³⁺ and Yb³⁺ concentration were interpreted. The absorption strengths of the 4f–4f transitions were analysed and used to assess the structural modifications of the two double phosphates. Based on the 4 K absorption spectra the number of metal sites occupied by the dopants was investigated.

Strong emission from Na₃Y_1−xNd_x(PO₄)₂ involving the ⁴F_3/2→⁴I_9/2,⁴I_11/2,⁴I_13/2,⁴I_15/2 transitions were observed whereas the corresponding emission from the rubidium phosphate was presumably quenched by multiphonon processes due to the water molecules absorbed in the channel-like structure.

The IR spectra were used to assign the vibronic components of the electronic transitions. The Yb³⁺ emission bands were broadened depending on the Yb³⁺ concentration (1–10 mol%). The tentative energy level scheme of the ground and excited ²F_J (J=7/2, 5/2) levels was described.     

Synthesis and characterization of lanthanide trifluoroacetate complexes with 3-picoline-N-oxide

Complexes with composition Ln(F₃C-COO)₃·2(3-picNO)·ξH₂O were synthesized. According to X-ray powder patterns and IR data five series of isomorphous compounds were detected: La---Nd; Sm---Eu; Gd>Tm, Y; Ho, Er, Tb; Dy, Yb, Lu. They behave as nonelectrolytes in methanol. IR spectra show a shift of νNO to lower frequencies, as compared with the free ligand, as a result of coordination through the oxygen. The bands attributed to the anions indicate two modes of coordination. The neodymium spectrum at 77 K shows that the central ion is not involved in a cubic site. Electrostatic bonds between Nd³⁺ and ligands were evidenced by the nephelauxetic, Sinha's parameters and covalent factor from the room temperature spectrum. Using the electronic emission spectrum, at 77 K, it was possible to conclude that europium ions present two different symmetry sites, since a splitting of the ⁵D₀ → ⁷F₉ band was observed. The complex species are probably polymeric or at least dimeric, and europium ions are involved in two different sites, but with the same C_3ν symmetry.     

The reduction of Eu to Eu in air and luminescence properties of Eu activated ZnO–B2O3–P2O5 glasses

A valence change from Eu³⁺ to Eu²⁺ was observed in the europium ion-doped ZnO–B₂O₃–P₂O₅ glasses prepared at high temperature in air. The fluorescence emission spectrum of the sample consists of a broad emission band ascribed to the 5d–4f transition of Eu²⁺ ion and sharp emission peaks assigned to the transitions of ⁵D_o–⁷F_J (J = 0, 1, 2, 3, and 4) of Eu³⁺ ion, indicating that part of Eu³⁺ can be reduced into Eu²⁺ in the glass. A charge compensation model is proposed. The rigid tetrahedral network structure of glasses plays an important role in stability of Eu²⁺. The fabrication conditions are also studied.     

Infrared tuneable up-conversion phosphor based on Er-doped nano-glass–ceramics

Up-conversion luminescence has been studied in the Er³⁺-doped oxyfluoride glass, its daughter nano-glass–ceramics and the polycrystalline ErF₃ when excited at infrared wavelengths of either 800 or 980 nm. The mechanism of the up-conversion luminescence is shown to differ for excitations at 800 and 980 nm since the different excited levels of the Er³⁺ are involved, respectively. An order of magnitude increase of red-to-green up-conversion luminescence ratio has been observed with nano-ceramming of the precursor glass when excited at 800 nm and a full dominance of the red up-conversion luminescence has been observed in the ErF₃ at the both excitation wavelengths. Observed changes in the spectra of the up-conversion luminescence from the precursor glass to its daughter nano-glass–ceramics provide a tool for tuning the colour of the up-conversion luminescence by ceramming of the precursor glass. These changes are shown to be due to decrease of the vibration energy of phonons coupled to the Er³⁺ ions embedded to the PbF₂ nano-crystals with nano-ceramming. Hence we report for the first time that the red up-conversion luminescence from the Er³⁺-doped material can be due to the radiative transition ⁴F_5/2, ⁴F_3/2 → ⁴I_13/2 of the Er³⁺ ion, which is principally allowed only in such a low-phonon energy host as the PbF₂ nano-crystals, while such red up-conversion luminescence is almost completely non-radiatively quenched in the oxyfluoride precursor glass. The up-conversion emission spectrum of the ErF₃ is dominated by a cross-relaxation mechanism typical of hosts with a very high doping level of the Er³⁺ ions.     

Synthesis, luminescence and quantum yields of Eu(III) mixed complexes with 4,4,4-trifluoro-1-phenyl-1,3-butanedione and 1,10-phenanthroline-N-oxide

The new complex tris(4,4,4-trifluoro-1-phenyl-1,3-butanedionate)(1,10-phenanthroline-N-oxide)europium(III) has been synthesized, characterized, and its photophysical properties (excitation and luminescence spectra, and quantum yields) investigated down to 4.2 K. A similar complex containing H₂O instead of phenNO molecules was also investigated for comparison. The more pronounced temperature dependence of the quantum yield q and the larger difference between the q values upon ligand and direct Eu³⁺ excitation for the hydrated compound show that there are other quenching processes operative, besides the expected multiphonon relaxation via the H₂O vibrations. The results clearly show that the substitution of the water molecules by phenNO leads to greatly enhanced quantum yields (i.e. 30% vs. 66%, upon ligand excitation at 300 K) and longer ⁵D₀ lifetimes (380 μs vs. 670 μs respectively). This can be ascribed to a more efficient ligand-to-metal energy transfer and to less efficient non-radiative ⁵D₀ relaxation processes.     

Synthesis, characterization and microwave dielectric properties of nanocrystalline CaZrO3 ceramics

The energy-transfer mechanisms and frequency upconversion emissions in 0.5Er³⁺/xHo³⁺ co-doped tellurite glasses by exciting at 980 nm have been investigated. Three intense upconversion luminescence emissions are observed at around 525, 548, and 660 nm, which correspond to Er³⁺:²H_11/2 → ⁴I_15/2, Er³⁺:⁴S_3/2 → ⁴I_15/2 + Ho³⁺:⁵S₂(⁵F₄) → ⁵I₈, and Er³⁺:⁴F_9/2 → ⁴I_15/2 + Ho³⁺:⁵F₅ → ⁵I₈ transitions, respectively. The upconversion emissions reach the maximum values when Ho₂O₃ is 0.5 mol%, and the intensities of the green and red light emissions were about 4.5 and 6 times stronger than those un-doped Ho₂O₃, respectively. The possible upconversion mechanisms and energy transfer between Er³⁺ and Ho³⁺ were also estimated and evaluated. All the three emissions are based on two photon absorption processes.