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.     

Comparative analyses of Nd (4f) energy level structures in various crystalline hosts

We have performed an in-depth analysis of the energy-level structures in seven Nd³⁺ (4f³) crystal systems: [Nd(H₂O)₉]·3CF₃SO₃, Nd³⁺: Cs₂NaGdCl₈, four Nd³⁺-doped garnets (Nd³⁺: A₃B₅O₁₂), and Nd³⁺: CsCdBr₃. A model Hamiltonian employing 20 free-ion operators and the appropriate one-electron crystal-field interaction operators (plus selected two-particle correlation crystal-field (CFF) operators) was diagonalized within the full 364 SLJM, basis of the f³ electronic configuration. Ample spectroscopic experimental data allowed us to use least-squares fitting routines to produce a crystal-field energy-level structure for Nd³⁺ in each host. Particular attention is given in this report to some differences in the values of the free-ion parameters among the seven hosts and to the strong effects of three CCF operators on crystal-field level ordering in specific multiplets. The effects of CCF operators (originally proposed by Judd and later studies by Reid) are shown to be a small perturbation on the standard one-particle (B_q^k) formalism.     

Crystal fields in ROF:Tb (R = La, Gd)

Laser excited luminescence and site selective excitation spectra of trivalent terbium (Tb³⁺) in two rare earth oxyfluoride (ROF) matrices, LaOF and GdOF, were detected at temperatures down to liquid helium. From the analyses of spectra, the crystal field (c.f.) fine structure of the ⁷F_J (J=0–6) ground multiplet was obtained for the 4f⁸ electron configuration. The energy level schemes comprising 26 c.f. sublevels out of 33 allowed for the C_3v point symmetry of the R³⁺ site were simulated according to a phenomenological model taking into account only the c.f. effect.

The simulation including six c.f. parameters (B₀², B₀⁴, B₃⁴, B₀⁶, B₃⁶, and B₆⁶) reproduced the experimental energy level schemes in a satisfactory manner with rms deviations of 5 and 6 cm⁻¹ for the LaOF and GdOF hosts, respectively. The c.f. parameter sets are characterized by B₀² values close to zero while the fourth and sixth rank parameters assumed rather high values. The O_h to C_3v descending symmetry method revealed a slight trigonal distortion from the cubic fluorite CaF₂-type structure. The c.f. parameters vary only slightly as a function of the host matrix. The ⁷F₆ level was not simulated correctly probably because of the electron–phonon interaction.     

Optical properties and crystal field calculations of europium tellurium oxide

Absorption and fluorescence measurements have been performed on pure Eu₂Te₄O₁₁ tellurium oxide and on Eu³⁺-doped Gd₂Te₄O₁₁ samples. The spectra were recorded at liquid helium, liquid nitrogen and room temperatures. In this monoclinic host matrix, the rare earth occupies a single crystallographic point site, with the very low symmetry C₁. The energy level scheme is derived, from the experimental data, for the ⁷F_j (J= 0,1,2,3,4) ground multiplet of the 4f⁶ configuration of Eu³⁺. Despite the low symmetry for the rare earth, parametrization of the crystal field effects was carried out with very good r.m.s. standard deviations and residues.     

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.     

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.     

Correlation-crystal-field ‘δ-function' analysis of Pr(4f) energy-level structure

Inclusion of two-electron correlation-crystal-field (CCF) contributions to the model (energy-level) Hamiltonian has been shown to resolve disparities between calculated and observed energy-level splittings within problematic multiplet manifolds of Nd³⁺ and Er³⁺ systems. Application of these CCF terms to other problematic multiplets, such as the ¹G₄ and ¹D₂ multiplets of Pr³⁺ has not been feasible, owing to the fact that the number of applicable CCF terms far exceed the number of observed levels in these multiplets. However, this problem may be obviated by use of a simplified ‘δ-function' CCF model that assumes the dominant contributions to the CCF interactions come from paired electrons within the same orbital. In the present study, we examine the 4f²(Pr³⁺) electronic energy-level structures reported for Pr³⁺:LaCl₃, Pr³⁺:GdCl₃, Pr³⁺:Cs₂NaYCl₆ and Cs₂NaPrCl₆ systems and show that inclusion of δ-function CCF interaction parameters resolves the disparities between observed and calculated energy-level splittings for the problematic ¹G₄ and ¹D₂ multiplets of these systems.     

First direct calculation of 4f‘ 4f oscillator strengths for dipolar electric transitions in PrCl3

In two previous papers, we interpreted completely the crystal field splitting and Zeeman data of trivalent praseodymium ions in PrCl₃ taking into account configuration interaction between 4f², 4f5d and 4f6p. The obtained eigenvectors are now used to calculate directly the oscillator strengths originating from dipolar electric processes between opposite parity configurations, giving rise to apparent 4f ‘4f transitions. The calculation reproduces in a satisfactory way the absorption from the ground levels toward the ³P₂ and ¹D₂ multiplets but not toward ³P₀ and ³P₁.     

Simulation of the crystal field effect on the Pr ion in K2La1−xPrxCl5 ternary chlorides

The high resolution absorption, luminescence and excitation spectra of the orthorhombic potassium lanthanum praseodymium ternary chloride, K₂La_1−xPr_xCl₅, (0.02 ≤ x ≤ 0.15) single crystals were recorded at 4, 77 and 293 K with different excitation sources. The experimental 4f² energy level scheme of the Pr³⁺ ion in K₂LaCl₅ derived from the absorption and emission spectra consisted of 86 (out of 91) Stark components. This energy level scheme was simulated by using a phenomenological crystal field (cf) model which included eight free ion and nine cf parameters according to the C_2v symmetry. Despite the approximate C_2v point symmetry instead of the real C_s one, the simulation yielded a very satisfactory rms deviation of 17 cm⁻¹ between the experimental and calculated energy level schemes. The results, especially the weak cf strength, are discussed taking into account the bonding characteristics in K₂LaCl₅.     

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.     

Magnetic properties of pentavalent uranium ternary oxides with fluorite structure Part II

BiUO₄, MgU₂O₆ and SrU₂O₆ have been prepared and then identified by powder X-ray diffraction analysis. All the magnetic susceptibility vs. temperature curves have a peak at around 4–7 K with a similar shape. The effective number of Bohr magnetons is 1.46, 1.23 and 1.29 and the temperature of X_max is 4.4, 7.4 and 5 K for BiUO₄, MgU₂O₆ and SrU₂O₆ respectively. These temperatures of X_max fall in the same temperature range as for ScUO₄, YUO₄, CaU₂O₆ and CdU₂O₆ with the fluorite structure reported previously. The electron paramagnetic resonance parameters of g factor and linewidth for the present three mixed oxides are 2.3, 2.1 and 2.2 (which is slightly anisotropic) and 0.09, 0.069 and 0.03 T respectively. These g values are explained by crystal field theory for U⁵⁺ with [Rn]5f¹ configuration in a crystal field with eightfold cubic symmetry. On the other hand, the magnetic susceptibility vs. temperature curves with the maximum are almost reproduced by scaling the curve obtained by calculation using an isotropic Heisenberg interaction model with S = 1/2.     

Crystal field analysis of EuCl3.6H2O

Absorption and magnetic circular dichroism (MCD) spectra of polycrystalline EuCl₃.6H₂O samples were recorded between 16 000 and 36 000 cm⁻¹ at ambient temperature and at 77 K. A total of 104 crystal field transitions has been observed in the spectra. The MCD spectrum resembles the first derivative of the absorption spectrum, indicating the presence of (pseudo) A-terms. The advantage of MCD is the possibility of detecting quasi-degenerate crystal field levels, which are not resolved in the absorption spectrum. The coordination polyhedron around Eu³⁺ is a distorted square antiprism (C.N.=8). In order to make the crystal field calculations more tractable, the real C₂ symmetry of the europium site is approximated by a C_2v symmetry. The energy levels of the 4f⁶ configuration of Eu³⁺ are parametrized in terms of 20 free ion parameters and 9 B_q^k crystal field parameters. J-mixing is taken into account.     

Spectroscopic and 1.06 μm laser properties of Nd-doped K–Sr–Al phosphate and fluorophosphate glasses

Optical absorption, fluorescence and decay curves for the ⁴F_3/2 level of Nd³⁺ ions in phosphate (P₂O₅–K₂O–SrO–Al₂O₃) and fluorophosphate (P₂O₅–K₂O–SrO–Al₂O₃–AlF₃ and P₂O₅–K₂O–SrO–Al₂O₃–BaF₂) glasses doped with three concentrations (0.1, 1.0 and 2.0 mol%) of Nd³⁺ ions have been investigated. The Judd–Ofelt (JO) theory has been applied to the absorption spectra of 1.0 mol% Nd³⁺-doped glasses to derive JO intensity parameters which are in turn used to calculate the radiative properties of the Nd³⁺ ion fluorescent levels. The assigned energy level data of Nd³⁺ (4f³) ions are analysed in terms of a parametrized free-ion Hamiltonian model that consists of 20 interaction parameters of atomic nature. The stimulated emission cross section and branching ratios have been calculated using the emission spectra. The relatively higher branching ratio for ⁴F_3/2 → ⁴I_11/2 transition shows the suitability of these glasses for laser application. It is interesting to note that the measured decay curves of the ⁴F_3/2 level remain nearly single exponential even for higher Nd³⁺ ion concentration but with shortening of lifetime.     

What makes the luminescence of Prdifferent in CaTiO3 and CaZrO3 ?

Although orthorhombic CaZrO₃ and CaTiO₃ perovskites are characterised by very similar structural features, they show different luminescence properties when doped with Pr³⁺ ions: the former exhibits the typical greenish-blue ³P₀→³H₄ emission, while the latter shows a single red ¹D₂→³H₄ luminescence. The explanation of this difference requires to understand the mechanism responsible for total quenching of ³P₀ emission in CaTiO₃ host. Among the different possible relaxation pathways from ³P₀ to ¹D₂, intersystem crossing through low lying 4f¹5d¹ levels has been proposed in a previous study. Owing to new spectroscopic results on CaZrO₃:Pr³⁺ and CaTiO₃:Pr³⁺, the role of the 4f5d band is reconsidered and an alternative relaxation channel is proposed via low lying intervalence charge transfer state.     

Quark-like structures in the atomic f shell

Instead of thinking of the 15 electronic configurations f^N (0N 14) as distinct entities, we can construct all 16 384 states of the f shell by coupling together four independent quarks and including two parity labels. Each quark is eight-dimensional, has the angular momentum structure s + f, and belongs to the simplest spinor representation of the Lie group SO(7). Two coupled quarks provide the spin-up states of either even N or odd N configurations; the other two perform a similar role in the spin-down space. Transformations among the eight components of a quark bring the group SO(8) into play. Just how the representations of this group collapse into representations of the three possible SO(7) subgroups and their common subgroup G₂ is illustrated by projecting the four-dimensional weight space on to the two-dimensional space of the printed page. The quarks make themselves felt by providing selection rules and unexpected proportionalities between matrix elements. Among various examples so far investigated is that provided by the relation between some matrix elements of the spin-spin type for the quintet states of f⁴ (with even L) and some crystal-field matrix elements for the singlets of f².     

Rare earth(iii) complexes with indole-derived acetylacetones II. luminescent intensity for europium(iii) and terbium(iii) complexes

The emission parameters for Eu³⁺(Tb³⁺) complexes with 1-indoleacetylacetone (HL¹), 3-indoleacetylacetone (HL³) and 1-acetylaceto-3-indoleacetic acid (H₂A) and the energy levels of excited triplet states for the three ligands were determined. It was found that L¹ strongly sensitizes Tb³⁺ strongly sensitizes Tb³⁺ luminescence in the complex TbL₃. mH₂O, while all the three ligands sensitize Eu³⁺ luminescence weakly. The energies of excited triplet state for the three ligands are 23 474 cm⁻¹ (L¹), 21 798 cm⁻¹ L³ and 23 310 cm⁻¹ (A) respectively. The facts that all the three ligands sensitize Eu³⁺ weakly and the order of emission intensity for the terbium(III) complexes with the three diketones is L¹ > A > L³ were explained by the relative energy gap between triplet energy level of the ligand and ⁶D₄ of Tb³⁺. This means that the triplet energy level of the ligands is the ligands is the chief factor which dominates Ln³⁺ luminescence.     

Near-infrared to visible photon upconversion in Mn and Yb containing materials

Near-infrared (NIR) laser excitation into the Yb^{3+ 2}F_7/2→²F_5/2 absorption transition around 10,200 cm⁻¹ at 12 K leads to red and green upconversion emission in MnCl₂:Yb³⁺ and Zn₂SiO₄:Yb³⁺; Mn²⁺, respectively. The photon upconversion (UC) emission is centered around 15,240 cm⁻¹ for MnCl₂:Yb³⁺ and around 19,050 cm⁻¹ for Zn₂SiO₄:Yb³⁺; Mn²⁺ and is ascribed to the Mn^{2+ 4}T₁→⁶A₁ transition in both compounds. The shift of the Mn²⁺ emission energy is due to the different coordination and ligand strength. Pulsed measurements indicate a sequence of ground-state absorption (GSA) and excited-state absorption (ESA) steps for the upconversion process. We conclude that the UC process in both compounds occurs by an exchange mechanism involving Yb³⁺ and Mn²⁺ ions. Zn₂SiO₄:Yb³⁺; Mn²⁺ is the first example of a Yb³⁺–Mn²⁺ upconversion system to show visible (VIS) by eye Mn²⁺ UC emission at room temperature.     

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.     

Paramagnetic susceptibility simulations from crystal field effects on rare earth antimonates R3Sb5O12

The magnetic susceptibilities of polycrystalline rare earth antimonates R₃Sb₅O₁₂ (R=rare earth), with cubic structure, space group I43m (No. 217), where the point site symmetry of R is S₄, have been measured from 4.2 to 800 K. Using the wave functions and energy levels derived from standard free ion and crystal field parameters deduced from the analysis of the optical spectra of pure and/or doped Pr, Nd, Eu and Er compounds, the calculation of the temperature dependent paramagnetic susceptibility has been carried out according to the Van Vleck formalism. The same calculation through crystal field parameters resulting from the ab initio simple overlap model over the entire R₃Sb₅O₁₂ series has been performed. Curves of both kinds were very similar for each configuration, with the exception of those for the Pr compound. Very good agreements with experimental data are found, especially below around 400 K, even when the approximate D_2d (near S₄) potential is considered. The standard crystal field treatment of 4f^N configurations is shown to explain the magnetic properties of these compounds with no need to consider any sort of magnetic interaction between the magnetic ions.     

Effect of Zn and Li codoping ions on nanosized Gd2O3:Eu phosphor

Nanosized Gd_1.92−x−yZn_xLi_yEu_0.08O_3−δ phosphor was fabricated by combustion synthesis. The effect of Zn²⁺ and Li⁺ ions on the crystallization behavior, morphology, and luminescence property of Gd₂O₃:Eu³⁺ was investigated. The results indicated that incorporation of Zn²⁺ and Li⁺ ions into Gd₂O₃:Eu³⁺ nanoparticles (NPs) could lead to a remarkable increase of photoluminescence or X-ray excited luminescence, and the intensity at 612 nm was increased by a factor of 7.1 or 21.5 in comparison with that of undoped sample. The enhanced luminescence was regarded as the results of the creation of oxygen vacancies due to the Gd³⁺ sites occupied by Li⁺ ions, the alteration of the crystal field surrounding the activator Eu³⁺ ions owing to the incorporation Zn²⁺ ions into interstitial sites, and the flux effect of Zn²⁺ and Li⁺ ions. The Zn- and Li-codoped Gd₂O₃:Eu³⁺ phosphor with highly enhanced luminescence is very encouraging for applications in high-resolution display devices.