Synthesis of NaLn(WO4)2 phosphors via a new phase-conversion protocol and investigation of up/down conversion photoluminescence

The two-dimensional (2D) crystallite morphology and low OH⁻/Ln³⁺ molar ratio of Ln₂(OH)₄SO₄·2H₂O (Ln = lanthanide) make it an ideal precursor for materials synthesis via phase conversion, which was manifested in this work by the direct generation of well-defined NaLn(WO₄)₂ phosphor particles via hydrothermal reaction with Na₂WO₄. Kinetics study showed that pure NaLn(WO₄)₂ can be produced by reaction at 180 °C for ~24 h or at 200 °C for ~6 h under WO₄²⁻/Ln³⁺ = 10 M ratio. Morphology analysis revealed that, though NaLn(WO₄)₂ evolved via re-precipitation, the layered crystal structure and 2D crystallite morphology of the precursor could have templated the nucleation/growth of NaLn(WO₄)₂, leading to uniform particles (~4–5 μm) of a unique microdisc-like morphology. Under 394 nm excitation, the Ln = La_0.95Eu_0.05 phosphor showed down-conversion luminescence having an absolute quantum yield of ~35.4%, a fluorescence lifetime of ~1.13 ms for its 616 nm main emission, and color coordinates of around (0.63, 0.37). Under 978 nm laser excitation, the Ln = La_0.97Yb_0.02Ho_0.01 and Ln = La_0.97Yb_0.02Er_0.01 phosphors exhibited the strongest up-conversion (UC) luminescence at ~660 nm (the ⁵F₅ → ⁵I₈ transition of Ho³⁺) and 551 nm (the ⁴S_3/2 → ⁴I_15/2 transition of Er³⁺), average fluorescence lifetimes of ~178.3 and 82.3 µs for the above emissions, and chromaticity coordinates of around (0.62, 0.38) and (0.25, 0.72), respectively. The two UC phosphors were also analyzed to exhibit UC luminescence through a two-photon mechanism.     

Facile and green synthesis of (La0.95Eu0.05)2O2S red phosphors with sulfate-ion pillared layered hydroxides as a new type of precursor: controlled hydrothermal processing,phase evolution and photoluminescence

This study presents a facile and green route for the synthesis of (La_0.95Eu_0.05)₂O₂S red phosphors of controllable morphologies, with the sulfate-type layered hydroxides of Ln₂(OH)₄SO₄·2H₂O (Ln = La and Eu) as a new type of precursor. The technique takes advantage of the fact that the precursor has had the exact Ln:S molar ratio of the targeted phosphor, thus saving the hazardous sulfurization reagents indispensable to traditional synthesis. Controlled hydrothermal processing at 120 °C yielded phase-pure Ln₂(OH)₄SO₄·2H₂O crystallites in the form of either nanoplates or microprisms, which can both be converted into Ln₂O₂S phosphor via a Ln₂O₂SO₄ intermediate upon annealing in flowing H₂ at a minimum temperature of ～ 700 °C. The nanoplates collapse into relatively rounded Ln₂O₂S particles while the microprisms retain well their initial morphologies at 1 200 °C, thus yielding two types of red phosphors. Photoluminescence excitation (PLE) studies found two distinct charge transfer (CT) excitation bands of O^2? → Eu³⁺ at ～ 270 nm and S^2? → Eu³⁺ at ～ 340 nm for the Ln₂O₂S phosphors, with the latter being stronger and both significantly stronger than the intrinsic intra-f transitions of Eu³⁺. The two types of phosphors share high similarities in the positions of PLE/PL (photoluminescence) bands and both show the strongest red emission at 627 nm (⁵D₀ → ⁷F₂ transition of Eu³⁺) under S^2? → Eu³⁺ CT excitation at 340 nm. The PLE/PL intensities show clear dependence on particle morphology and calcination temperature, which were investigated in detail. Fluorescence decay analysis reveals that the 627 nm red emission has a lifetime of ～ 0.5 ms for both types of the phosphors.     

Synthesis and lattice parameters of rare earth bismuth tungstates,BiLnWO6 and their solid solutions

The rare earth bismuth tungstates with composition BiLnWO₆ (Ln=rare earth, including La and Y) have been prepared as ceramics at 1050°C. They had a monoclinic $\text{P}\text{2}\text{a}$ structure. The precise lattice parameters are given for all BiLnWO₆ compounds. The unit cell volumes of BiLnWO₆ were proportional to the Ln³⁺ volumes. A series of Bi_2-xLn_xWO₆ solid solutions was also synthesized for some Ln³⁺ ions at 1050°C, and their solubility limits were established. The cell volumes of Bi_2-xLn_xWO₆ series — Ln concentration x plots showed good linearity, but the slopes of the straight lines depended on ionic radii of Ln³⁺. In the Bi_2-xLn_xWO₆ series, the 6s² lone-pair character of Bi³⁺ is constrained, so that the ionic radius of Bi³⁺ decreased to that of Nd³⁺.     

Synthesis and crystal structure of double Ln(III) malonates with Co(NH3) 6 3+ in the outer sphere

Double Ln(III) malonates of two different compositions crystallize from malonate solutions containing [Co(NH₃)₆]³⁺ ions. Lanthanides of the beginning of the series form compounds of the composition [Co(NH₃)₆][Ln(mal)₂]₃·6H₂O (I) (Ln = La, Ce, Pr, Nd; mal = C₃H₂O^2?), and those of the end of the series form compounds of the composition [Co(NH₃)₆]₂[Ln₃(mal)₇(Hmal)(H₂O)₄]·nH₂O (II) (Ln = Tb, Ho, Er, Tm). Structure I is based on trimeric anionic complexes [Ln₃(mal)₆]^3? linked with each other to form a branched 3D network with [Co(NH₃)₆]³⁺ cations and water molecules accommodated in large voids. The coordination mode of malonate ions in I with the coordination capacity equal to 5 was unknown previously for lanthanide malonate compounds. The Ln(1) atom has the maximum possible for malonate compounds coordination number (CN) 12, and the Ln(2) atom has CN 9. The structure of II consists of anionic chains [Ln₃(mal)₇(Hmal)(H₂O)₄] _n ^3? between which the [Co(NH₃)6]₃₊ cations and water molecules are arranged. One independent malonate ion in the structure is coordinated in the bidentate chelate fashion to the Ln(1) atom, and the other independent chelate-bridging ligand is coordinated in the bidentate fashion to the Ln(2) atom and in the monodentate fashion to the Ln(1) atom. As a result, tetrameric fragments linked in anionic chains are formed in the structure of II. The Ln(1) and Ln(2) atoms have CN 8.     

Extraction of Lanthanide(III) Nitrates from Aqueous Solutions with Mixtures n-Octanol-Decane, Tri-n-butyl Phosphate-n-Octanol, and Diisoamyl Methylphosphonate-n-Octanol

Extraction of lanthanide(III) (La-Eu) nitrates from aqueous solutions with 3.15 M solution of octanol (ROH) in n-decane (extractant 1), 2.02 M solution of diisoamyl methylphosphonate (S = DIAMP) in n-octanol (extractant 2), and 1.83 M solution of tri-n-butyl phosphate (S = TBP) in n-octanol (extractant 3) was studied at T = 298.15 K. The extraction of lanthanide(III) nitrates with extractant 1 at C(aq) > 0.6 M is described by the equation of the heterogeneous reaction Ln³⁺(aq) + 3NO ₃ ⁻ (aq) + 4ROH(o) = [Ln(NO₃)₃ ⋅ (ROH)₄](o). The extraction of Ln(III) nitrates with extractants 2 and 3 involves the reaction described above in combination with the heterogeneous reaction Ln³⁺(aq) + 3NO ₃ ⁻ (aq) + 3S(o) = [Ln(NO₃)₃(S)₃](o) and the homogeneous reaction ROH(o) + S(o) = [ROH ⋅ S](o), where S is DIAMP or TBP. The electronic absorption and IR spectra of Nd(III) and Pr(III) nitrates in n-octanol and n-octanol-TBP mixtures were analyzed.__________Translated from Radiokhimiya, Vol. 47, No. 3, 2005, pp. 245–251.Original Russian Text Copyright © 2005 by Kudrova, Keskinov, Pyartman.     

Mesogenic lanthanoid metal complexes of a non-mesogenic Schiff-base, N,N′-di-(4-hexadecyloxysalicylidene)-l′,8′-diamino-3′,6′-dioxaoctane

A non-mesogenic Schiff-base, N,N′-di-(4-hexadecyloxysalicylidene)-l′,8′-diamino-3′,6′-dioxaoctane, H₂dhdsdd (H₂L²), was synthesized, structure studied by elemental analyses and mass, NMR and IR spectra and ligated to some Ln^III metal ions that yielded mesogenic (SmA/N) Ln^III complexes of the general composition, [Ln₂(L²H₂)₃(NO₃)₄](NO₃)₂, where Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy and Ho. IR and NMR spectral data imply a bi-dentate bonding of the Schiff-base in its zwitterionic form (as L²H₂) to the Ln^III ions through two phenolate oxygens, rendering the overall geometry around Ln^III to distorted mono-capped octahedron.     

Preparation of Ln2O2S (Ln = Gd,Dy, Y,Er, Lu) in flowing hydrogen and hydrogen sulfide

The exposure of anhydrous Ln₂(SO₄)₃ (Ln = Gd, Dy, Y, Er, Lu) sulfates to flowing hydrogen in the range 500–1050°C leads to the formation of Ln₂O₂S + Ln₂O₃ materials. With increasing reaction temperature, the mole fraction of Ln₂O₂S in the samples decreases. Single-phase Ln₂O₂S (Ln = Gd, Dy, Y, Er, Lu) compounds have been obtained by exposing the rare-earth sulfates first to flowing hydrogen in the range 500–600°C and then to hydrogen sulfide in the range 850–950°C.     

Cation disorder in Bi2Ln2Ti3O12 Aurivillius phases (Ln = La, Pr, Nd and Sm)

The synthesis and structure of triple layered Bi₂Ln₂Ti₃O₁₂ Aurivillius phases (Ln=La, Pr, Nd and Sm), prepared from K₂Ln₂Ti₃O₁₀ Ruddlesden-Popper precursors, has been investigated. These materials adopt a body centred tetragonal structure (space group I4/mmm, with unit cell parameters a∼3.8 Å and c∼33 Å) comprising a regular intergrowth of [Bi₂O₂]²⁺ fluorite-type and [Ln₂Ti₃O₁₀]²⁻ perovskite-type layers. A significant degree of cation disorder is present in the Bi₂Ln₂Ti₃O₁₂ system, involving the cross-substitution of Ln/Bi cations onto the Bi/Ln sites in the fluorite- and perovskite-type layers, respectively. As the size of the lanthanide cation is reduced, Bi/Ln disorder is significantly suppressed due to the effect of bond length mismatch in the perovskite-type layer in the crystal structure of Bi₂Ln₂Ti₃O₁₂. This offers a potential strategy for the chemical control of cation disorder in the Bi₂Ln₂Ti₃O₁₂ system.     

Preparation and structural studies in the (70 − x)TeO2–20WO3–10Li2O–xLn2O3 glasses

Quaternary tellurite glass systems (70 ? x)TeO₂–20WO₃–10Li₂O–xLn₂O₃ where x = 0, 1, 3 and 5 mol% and Ln are La, Pr, Nd, Sm, Er and Yb, respectively, have been prepared by the melt quenching technique. Densities of the obtained glasses were measured and the molar volume was calculated. IR absorption spectra of the present glass systems were determined at room temperature over the range of wavenumbers from 400–1,600 cm^?1. Raman spectra of the present glass samples were measured in the range of 30–1,030 cm^?1. Density, molar volume, IR and Raman spectra of the glasses were discussed by calculating average cross-link density, packing density, theoretically calculated Poisson’s ratio and number of bonds per unit volume of the studied glasses. Also, the quantitative interpretations were based on concentration of ions per unit volume of Te, Ln and O, short distance in nanometre between ions for (Te–O) of TeO₄ and TeO₃ groups, (W–O) of WO₄, WO₆ groups and calculated wavenumber, $ \bar{\upsilon } $ , for TeO₄ and TeO₃, respectively. The average stretching force constant that present in these quaternary glasses has been calculated in order to interpret the data obtained.     

Oxygen interstitial and vacancy conduction in symmetric Ln2 ± x Zr2 ± x O7 ± x/2 (Ln = Nd,Sm) solid solutions

We have compared (Ln_{2 ? x} Zr _x )Zr₂O_{7 + x/2} (Ln = Nd, Sm) pyrochlore-like solid solutions with interstitial oxide ion conduction and Ln₂(Zr_{2 ? x} Ln _x )O_{7 ? δ} (Ln = Nd, Sm) pyrochlore-like solid solutions with vacancy-mediated oxide ion conduction in the symmetric systems Nd₂O₃-ZrO₂ (NdZrO) and Sm₂O₃-ZrO₂ (SmZrO). We have studied their structure, microstructure, and transport properties and determined the excess oxygen content of the (Sm_{2 ? x} Zr _x )Zr₂O_{7 + x/2} (x = 0.2) material using thermal analysis and mass spectrometry in a reducing atmosphere (H₂/Ar-He). The Ln_{2 ± x} Zr_{2 ± x} O_{7 ± x/2} (Ln = Nd, Sm) solid solutions have almost identical maximum oxygen vacancy and interstitial conductivities: (3–4) × 10^?3 S/cm at 750°C. The lower oxygen vacancy conductivity of the Ln₂(Zr_{2 ? x} Ln _x )O_{7 ? δ} (Ln = Nd, Sm; 0 < x ≤ 0.3) solid solutions is due to the sharp decrease in it as a result of defect association processes, whereas the interstitial oxide ion conductivity of the (Ln_{2 ? x} Zr _x )Zr₂O_{7 + x/2} (Ln = Nd, Sm; 0.2 ≤ x < 0.48) pyrochlore-like solid solutions is essentially constant in a broad range of Ln₂O₃ concentrations.     

Microwave dielectric properties of xMgTiO3-(1 − x)(Na1/2Ln1/2)TiO3 ceramics

Na_1/2Ln_1/2)TiO₃> ceramic has a high relative dielectric constant and a positive temperature coefficient of resonant frequency (_f) (where Ln represents a lanthanide: La⁺³, Pr⁺³, Nd⁺³ and Sm⁺³). On the other hand, MgTiO₃ ceramic has a high Qf value and a negative temperature coefficient. We have investigated the microwave dielectric properties of MgTiO₃-(1 – x)(Na_1/2 Ln_1/2) TiO₃. In this system, there are no indications of a solid-solution or a secondary phase. There are mixed phases only with MgTiO₃ and Na_1/2 Ln_1/2)TiO₃ phases. Its dielectric characteristics (Q ^*f, temperature coefficient and dielectric constant) are intermediate between (Na_1/2 Ln_1/2) TiO₃ and MgTiO₃ and are predictable by the logarithmic mixing rule. The temperature coefficient of dielectric ceramic compositions approximates to zero at each Ln = La, x = 0.9, Ln = Pr, x = 0.87, and Ln = Nd, x = 0.84. At this time, there are Q ^*f values in the range of 55 000 to 28 000 GHz and relative dielectric constants in the range of 22 to 25.     

Preparation and characterization of Ln2Zr2O7 microspheres by an inorganic sol-gel route

Free-flowing Ln₂Zr₂O₇ microspheres (Ln=lanthanide) were prepared by an aqueous inorganic sol-gel route without any intermediate phase formation. The gel spheres obtained at room temperature were shown by X-ray diffraction to be amorphous but calcination to 750 °C produced fully crystalline fluorite phases. On calcination to 850 °C, pyrochlore phases were formed with suitable lanthanides. The microspheres were characterized by X-ray diffraction and scanning electron microscopy with energy dispersive analysis of X-rays to give accurate determination of structure, composition and crystallite size.     

Sol–gel auto-combustion synthesis and luminescence properties of RVO4: Ln3+ (R=La,Gd; Ln=Sm,Er, Ho,Yb/Er) microcrystals

RVO₄: Ln³⁺ (R=La, Gd; Ln=Sm, Er, Ho, Yb/Er) microcrystals were successfully synthesized by a facile and rapid sol-gel method using glycine as the chelating agent. The crystalline structure, morphology and luminescence properties of obtained products were investigated in detail. The peaks of X-ray diffraction (XRD) patterns were well-indexed to the standard RVO₄ patterns, indicating that the Ln³⁺ ions were well doped into the crystalline lattices and had not changed the crystalline structure. The luminescence properties of the samples are systematically studied. The typical peaks of the emission spectra were sharp and intense, revealing energy was transferred efficiently from VO₄ ^3- to Ln³⁺ after excitation. The schematic diagram for energy transfer between the host matrix and doped lanthanide ions were also discussed. The chromaticity coordinates of RVO₄: Ln³⁺ microcrystals were calculated. This work reveals that the rare-earth vanadates are potential candidates as excellent host matrices of phosphors.     

Synthesis process and luminescence properties of Ln doped NaY(WO4)2 nanoparticles

The hydrothermal synthesis process and luminescence properties of Ln³⁺ doped NaY(WO₄)₂ nanoparticles have been investigated. Nearly spherical Eu³⁺ doped NaY(WO₄)₂ nanoparticles can be observed. The luminescence concentration quenching of Eu³⁺ in the NaY(WO₄)₂ nanoparticles was found to be similar to that in the NaY(WO₄)₂ crystals. The upconversion luminescence intensity of the Yb³⁺-Er³⁺ codoped NaY(WO₄)₂ nanoparticles was found to be much stronger than that of the Er³⁺ doped NaY(WO₄)₂ nanoparticles.     

EDTA-assisted phase conversion synthesis of (Gd0.95RE0.05)PO4 nanowires (RE = Eu,Tb) and investigation of photoluminescence

Abstract

Hexagonal (Gd_0.95RE_0.05)PO₄·nH₂O nanowires ~300 nm in length and ~10 nm in diameter have been converted from (Gd_0.95RE_0.05)₂(OH)₅NO₃·nH₂O nanosheets (RE = Eu, Tb) in the presence of monoammonium phosphate (NH₄H₂PO₄) and ethylene diamine tetraacetic acid (EDTA). They were characterized by X-ray diffraction, thermogravimetry, electron microscopy, and Fourier transform infrared and photoluminescence spectroscopies. It is shown that EDTA played an essential role in the morphology development of the nanowires. The hydrothermal products obtained up to 180 °C are of a pure hexagonal phase, while monoclinic phosphate evolved as an impurity at 200 °C. The nanowires undergo hexagonal→monoclinic phase transformation upon calcination at ≥600 °C to yield a pure monoclinic phase at ~900 °C. The effects of calcination on morphology, excitation/emission, and fluorescence decay kinetics were investigated in detail with (Gd_0.95Eu_0.05)PO₄ as example. The abnormally strong ⁵D₀→⁷F₄ electric dipole Eu³⁺ emission in the hexagonal phosphates was ascribed to site distortion. The process of energy migration was also discussed for the optically active Gd³⁺ and Eu³⁺/Tb³⁺ ions.     

Structure and electrical conductivity of Ln2+x Hf2−x O7−x/2 (Ln = Sm-Tb; x = 0, 0.096)

Data are presented on the evolution of the pyrochlore structure in the Ln_2+x Hf_2?x O_7?δ (Ln = Sm, Eu; x = 0.096) solid solutions and Ln₂Hf₂O₇ (Ln = Gd, Tb) compounds prepared from mechanically activated oxide mixtures. Sm_2.096Hf_1.904O_6.952 is shown to undergo pyrochlore-disordered pyrochlore-pyrochlore (P-P1-P) phase transformations in the temperature range 1200–1670°C. The former transformation leads to a rise in 840°C conductivity from 10^?4 to 3 × 10^?3 S/cm in the samples synthesized at 1600°C, and the latter leads to a drop in 840°C conductivity to 6 × 10^?4 S/cm in the samples synthesized at 1670°C. The reduction in the conductivity of Sm_2.096Hf_1.904O_6.952 is accompanied by the disappearance of the assumed superstructure. In the range 1300–1670°C, Eu_2+x Hf_2?x O_7?δ (x = 0.096) and Ln₂Hf₂O₇ (Ln = Gd, Tb) have a disordered pyrochlore structure. The highest 840°C conductivity is offered by Eu_2.096Hf_1.904O_6.952, Gd₂Hf₂O₇, and Tb₂Hf₂O₇ synthesized at 1670°C: 7.5 × 10^?3, 5 × 10^?3, and 2.5 × 10^?2 S/cm, respectively.     

Superconducting transition temperatures in the Ru-1232 system, RuSr2(Gd1? x Ln x Ce1.8Sr0.2)Cu2O z (Ln = Sm, Dy, and Ho)

We have investigated effects of the lanthanide element Ln and the composition changes on the superconducting transition temperatureT _c in the Ru-1232 system, RuSr₂(Gd_1−x Ln _x Ce_1.8Sr_0.2)Cu₂O _z (Ln = Sm, Dy, and Ho). At first, in the case of the samples with Ln = Sm among almost the single 1232 phase samples, the values of the superconducting onset temperatureT _co are almost the same forx=0.00−0.15, and each of the lattice parametersa andc is almost constant. While, in each of the cases of the samples with Ln = Dy and Ho, the sample withx=0.05 shows the maximum values for both the superconducting onset temperatureT _co and the zero resistivity temperatureT _cz. Especially for the sample with Ln = Dy, the values ofT _co andT _cz are 18.5 and 6.5 K, respectively. These are higher than those of the mother sample of RuSr₂(GdCe_1.8Sr_0.2)Cu₂O _z . Moreover, from variations ofT _co, lattice parameters ofa andc in the RuSr₂(Gd_1−x Dy _x Ce_1.8Sr_0.2)Cu₂O _z system as a function of Dy contentx, the relationship between the superconducting transition temperature and the lattice parameters in the present system are investigated.     

Raman spectra and phase transformations of the MLn(WO4)2 (M = Na, K; Ln = La, Gd, Y, Yb) tungstates

The 300-K crystal structure, phase transformations, and melting of the MLn(WO₄)₂ (M = Na, K; Ln = La, Gd, Y, Yb) tungstates and the structure of their melts were studied by Raman spectroscopy.     

Etude cristallochimique et dielectrique des oxyfluorures de terres rares Na4Ln(WNb2)O9F5

Six new compounds with formula Na₄Ln(WNb₂)O₉F₅ (Ln = Y, Nd, Eu, Gd, Dy, Lu) have been synthetized. The corresponding room temperature phases have a tetragonal symmetry and a chiolite-type structure. At low temperature a ferroelectric-paraelectric transition is detected for each compound. The Curie temperature increases with the size of the Ln³⁺ ion.     

Lumine scence of Ce3+ and Tb3+ in M3(PO4)2 (M = Sr,Ba)

The luminescence properties of Ce³⁺- and Tb³⁺-activated M₃(PO₄)₂ (M = Sr,Ba) are reported. It is shown that only a small part (⩽ 0.3%) of the M2+ ions can be replaced by Ln³⁺ ions. At higher concentrations a second phase, M₃Ln(PO₄)₃, is formed. The luminescence properties are due to these two different phases.