Perturbations of the luminescence spectra of europium-edta salts caused by selected monovalent counterions

High resolution emission studies of europium-EDTA salts of the form M(I)Eu(EDTA), where M≡Li, Na, K, Rb, Cs, Ag and Tl, have been carried out. Each europium-EDTA salt produced a distinct luminescence spectrum recorded over the spectral range which included the transitions in the europium(III) ion from the ⁶D₀ level to the ⁷F_0–4 levels. Spectral changes included slight shifts in peak position and varying line splitting patterns. Evaluations of the splitting patterns suggest that the site symmetries of the europium(III) ion in all compounds are low. Tentative site symmetry assignments have been made to fit best the observed luminescence spectra.     

Subject Index

Luminescence, luminescence excitation and electronic absorption spectra of europium salts of isomeric lutidine N-oxide derivatives were studied at 293 and 77 K. The role of radiative and non-radiative processes in emission intensity is discussed. The absorption transitions to the ³F_J, ⁵D_J, ⁵L_J and ⁵G_J levels and emissions from the ⁵D₀ and ⁵D₂ levels were detected. The role of CT transition inside the ligand and its influence on the LMCT transition is analysed. The co-ordinating function of the lutidine N-oxide group is discussed using IR spectroscopy.     

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.     

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.     

Electronic absorption and vibrational IR and Raman studies of binary phosphate β-K4Ce2P4O15

The binary phosphate K₄Ce₂P₄O₁₅ was prepared in the polycrystalline state in the solid state reaction of cerium oxide and potassium phosphate KPO₃. The phosphate fragment of this compound appears in the form of two PO₄³⁻ and one P₂O₇⁴⁻ anions occupying the sites of low symmetry. Electronic absorption, emission as well as infrared and Raman spectroscopic methods have been applied to characterise the properties and structure of the compound studied. Its electronic spectra agree with the Ce³⁺ ion spectroscopic characteristics. The ²F_5/2→²F_7/2 transition appears in the typical for this ion region: about 2000 cm⁻¹. The multiplet structure of the spectrum suggests the existence of at least two crystallographic different sites of this ion in the unit cell. The absorption bands in the range 25000–45000 cm⁻¹ have been assigned to the 4f¹→5d¹ transitions of the cerium ion and CT transition of the phosphate ligands. The vibrational spectra were discussed on the basis of correlation diagrams and factor group analysis.

The radiation-less mechanism of the return from the excited state to the ground state via CT states in the system studied is proposed.     

Spectroscopy of related series of Eu and Tb carboxylates

A critical review of the recent works of author and his colleagues on spectroscopy of related series of europium and terbium pyridine-carboxylates and pyridine-dicarboxylates is undertaken. Luminescence, luminescence excitation, IR, Raman, and vibronic spectra of these compounds were discussed. Studies of spectra of the complex lanthanide nitrates with sulfoxides were reviewed also. Intraligand charge transfer (CT) bands in the luminescence excitation spectra were demonstrated by comparison of the spectra of europium and terbium salts. f–d transitions in spectra of some terbium salts were revealed also. A series of complex terbium salts with increasing donor–acceptor interaction of the metal cation and ligand’s nitrogen was forecasted. Comparison of the spectra of pyridine-carboxylate and pyridine-dicarboxylate shed light on the coordination function of the carboxylate groups and on the nature of the ligand-to-metal-ion charge transfer (LMCT) in the lanthanide pyridine-dicarboxylates. The influence of the introduction of spacers in the system of conjugated π-orbitals of the ligands on the rate of the energy transfer to the lanthanide cation was demonstrated also.     

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.     

Spectroscopy, magnetism and non-radiative processes in heteronuclear Cu:Pr squarate; [Pr2Cu(C4O4)4(H2O)16]·2H2O

In the present work, the spectroscopic and magnetic properties of heteronuclear Cu:Pr squarate are reported. Single crystals of [Pr₂Cu(C₄O₄)₄(H₂O)₁₆]·2H₂O were obtained by reaction of squaric acid, praseodymium chloride and copper chloride in water solution according to the procedure described earlier. The crystals of title compound are isomorphic with [La₂Cu(C₄O₄)₄(H₂O)₁₆]·2H₂O crystal, where squarate anions participate as bridging ligands between metal ions.

The UV region of absorption spectra of the title compound is dominated by C–T band of Cu(II), f–d transition of Pr(III) and internal π–π*(A_1g→E_u) and π–π*(A_1g→E_g) ligand transitions. In visible and IR regions, t_2g–e_g of copper Cu(II) as well as ³H₄→³P_J, ¹D₂, ¹G₄, ³F_J, ³H₆ Pr(III) transitions at 293 and 4 K were recorded. At low temperature splitting given by Jahn–Teller effect can be observed. Significant anisotropy of d–d transitions intensities confirms well the Jahn–Teller effect, too. Unexpectedly high intensity of ³H₄→¹G₄ transition is probably due to the intensity borrowing from the Cu (II) d–d transition.

The ³P₀ and ¹D₂ emission of Pr(III) in the [Pr₂Cu(C₄O₄)₄(H₂O)₁₆]·2H₂O crystals is quenched even at 77 K. Whereas emission of appropriate polynuclear europium squarate was detected. The pathways of excited state quenching by e_g levels of Cu(II), multhiphonon relaxation and concentration quenching can be considered in the system under studies. Magnetic susceptibility measurements were carried out in 300–1.7 K temperature range and are discussed in relation to the structure.

Effect of the polymeric structure on spectroscopic behaviour is presented. Selectivity of polymeric europium squarate in vitro test for different tumor cells is shown.     

Optical study of praseodymium dicarboxylate [Pr(H2O)]2[O2C(CH2)3CO2]3.4H2O

Praseodymium dicarboxylate, [Pr(H₂O)]₂[O₂C(CH₂)₃CO₂]₃.4H₂O]–glutarate, Pr[glut], is synthesized by hydrothermal techniques. The title compound crystallizes in the monoclinic space group C2/c (No. 15). The rare earth cation is coordinated by nine oxygen atoms, eight oxygen atoms from the carboxylate groups and one from the water molecule. The local symmetry of Pr site is low, C_s. The absorption spectra of Pr[glut] are recorded from the visible to the far IR domain at 300, 77 and 9 K. Under various Ar⁺ laser excitations no emission is detected from ³P₀ and ¹D₂ excited levels of Pr³⁺ ion. In the low temperature absorption spectra only one electronic line is recorded for ³H₄→³P₀ transition. It confirms a unique local environment for the rare earth ion in Pr[glut]. The utility of the ‘barycenter curves’ in the attribution of electronic lines is demonstrated. Energy level scheme of 36 Stark components is deduced from the absorption spectra. The parametric calculation was performed on the whole 4f² (Pr³⁺) configuration with the starting set of crystal field parameters obtained previously for the Eu³⁺ ion in the isostructural compound. Eight free ion and nine phenomenological crystal field parameters in C_2v symmetry reproduce quite well several electronic levels of Pr³⁺ ion experimentally observed in Pr[glut]. A good r.m.s. standard deviation of 14.8 cm⁻¹ is obtained.     

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.     

Optical intensities of Pr ions in transparent oxyfluoride glass and glass–ceramic. Applications of the standard and modified Judd–Ofelt theories

The optical characterisation of Pr³⁺ ions in transparent SiO₂–Al₂O₃–CdF₂–PbF₂–YF₃ based glass and glass–ceramic have been performed. From absorption and emission spectra the oscillator strengths of the 4f²–4f² electronic transitions have been obtained. The intensity parameters have been calculated using both the Judd–Ofelt theory and the modified theory developed by Kornienko, Kaminskii and Dunina. A comparison of the experimental oscillator strengths, the spontaneous emission probabilities and the lifetimes of the ³P₀ level and those calculated using the above theoretical procedures has been performed for both samples. The root mean square deviation found using the standard Judd–Ofelt theory is larger than the value obtained with the modified treatment.     

Upconversion dynamics in Er doped nanocrystalline YAlO3

We report the luminescence and upconversion spectra of nanocrystalline YAlO₃ doped with trivalent erbium at concentrations of 5.0, 1.0 and 0.1 mol.%. The powder samples were prepared using a solution combustion reaction method, and the resulting YAlO₃ nanocrystals show, under wide-angle X-ray diffraction, a size in the range 20–40 nm. Efficient green and red emissions are observed at room temperature under continuous-wave pumping at 980 nm. A weak emission can also be detected in the blue at 410 nm. The upconversion dynamics were studied measuring the decay times and the pump-power dependence of the transitions to the ⁴I_15/2 ground state starting from the ²H_11/2, ⁴S_3/2 and ⁴F_9/2 excited states. Excited-state absorption (ESA) is found to be responsible for the higher energy (²H_11/2, ⁴S_3/2→⁴I_15/2) green transitions. On the other hand, for the ⁴F_9/2→⁴I_15/2 red transition a competing energy-transfer upconversion (ETU) mechanism is found, which accounts for the more than 100-fold increase in intensity of the red emission on passing from the lowest (0.1 mol.%) to the highest (5 mol.%) erbium concentration.     

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.     

Spectroscopic properties of praseodymium-doped YVO4 crystal grown by the Czochralski technique

Optical absorption and emission spectra of YVO₄ crystal containing 1 at.% of praseodymium have been measured at 4.2 and 300 K. Absorption spectra recorded with polarised light at 4.2 K are found to be strongly inhomogeneously broadened but generally consistent with selection rules for D_2d local symmetry except for the ³H₄(1)−³F₂ transition.

Extensive vibronic side-bands of electronic lines corresponding to transitions from the ground ³H₄(1) state to the ³P₀ and ³P₁ states have been observed. Luminescence decay curves have been recorded at several temperatures between 4.2 and 300 K. Strong dependence of the ¹D₂ lifetime on sample temperature and lack of the ³P₀ emission at any temperature between 4.2 and 300 K has been discussed.     

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.     

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.     

Spectral properties of Yb ions in LiNbO3 single crystals: influences of other rare-earth ions, OH ions, and γ-irradiation

Absorption spectra have been studied in 190–3100 nm region at various temperatures from 16 to 292 K for Yb³⁺-doped and Yb³⁺/Nd³⁺-, Yb³⁺/Er³⁺- and Yb³⁺/Pr³⁺-co-doped LiNbO₃ single crystals before and after γ-irradiation with a dose of 10⁵–10⁷ Gy. Intense 400 and 500 nm absorption bands were observed after γ-irradiation, which are due to the creation of oxygen vacancy (F-type color center) and Nb⁴⁺ polaron, respectively. Different change was observed in the 2870 nm OH⁻ absorption band intensity among the various rare-earth doped crystals. These are interpreted by discrepancy of ionic radii between substituting rare earth dopant ion and Li⁺ or Nb⁵⁺ host ion. The observed temperature dependence of the hot bands is understood by electronic transition from the thermally populated ²F_7/2 Stark levels to the excited ²F_5/2 level. The position of the Yb^{3+ 2}F_7/2→²F_5/2 first resonant line was observed to be slightly different among the co-doped crystals. This is due to the perturbation of Yb³⁺ by co-doped rare earth ion which is located at the neighborhood of theYb³⁺.     

-NMRC-NMR study on κ-(BEDT-TTF)2X

The electronic state of the κ phase family of BEDT-TTF compounds, κ-(BEDT-TTF)₂X [X = Cu(NCS)₂, Cu[N(CN)₂]Br and Cu[N(CN)₂]Cl], has been investigated by -NMR¹³C-NMR as well as H-NMR.¹-NMR. The ¹³C isotope was substituted selectively into the central double bonded carbon sites of BEDT-TTF. The ¹³C nuclear spin-lattice relaxation rate and line spectra have been measured for the Cu(NCS)₂ and Cu[N(CN)₂]Br salts. The Knight shift evaluated from the the width of the spectra of the powdered samples is temperature-insensitive and scaled to the uniform susceptibility. However, the relaxation rate, T₁⁻¹, does not obey the Korringa law but exhibits anomalous temperature dependence with a peak formation around 50 K. The absolute value of T₁⁻¹ is in excess of the Korringa relation. These behaviors are considered to come from strong aniferromagnetic spin fluctuation with finite wave vector, which is suppressed below 50 K. On the other hand, the H¹ NMR and magnetization measurements gave an unambiguous evidence for antiferromagnetic transition with spin canting at 25 K in κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl. The κ phase family, situated in the vicinity of metal-insulator and magnetic transitions, is characterized by strong electron correlations.     

Absorption and magnetic circular dichroism spectra of praseodymium doped fluorozirconate (ZBLAN) glass

Optical absorption and magnetic circular dichroism (MCD) spectra of 1% Pr³⁺ doped ZBLAN fluorozirconate glass have been recorded at several temperatures between 4.2 K and room-temperature. The dipole strengths of the 4f–4f transitions are parametrized in terms of three phenomenological Judd–Ofelt intensity parameters Ω_λ (λ=2, 4 and 6). The method of moments was used to extract the Faraday parameters from the MCD spectra. While the room-temperature MCD spectra are dominated by A-terms (which have the shape of the first derivative of an absorption curve), C-terms become more important at lower temperature. The temperature dependence of the ³P₀←³H₄ transition is a typical example. The temperature dependent change of the MCD spectrum can be explained by the differential thermal population of the Zeeman levels from which absorption of left and right circularly polarized light takes place.     

Spectroscopy of europium and terbium pyridine-carboxylates

Luminescence, luminescence excitation, IR, and Raman spectra of europium and terbium isomeric pyridine-carboxylates: isonicotinates, nicotinates and picolinates were studied. The ways of the excitation energy transfer from the ligand to Ln³⁺ were analysed in dependence on the structure and the type of co-ordination of ligand. Intraligand charge transfer bands and Tb³⁺ f–d transition bands were identified in the luminescence excitation spectra of terbium and europium compounds. Co-ordination of nitrogen atom by terbium ion in picolinates (and dipicolinates) causes low-frequency shift of the f–d bands and promotes the excitation of luminescence through d-states of Tb³⁺.