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2 pages, 146 KiB

Open AccessEditorial

Rare Earths Doped Materials

by Maria Milanova and Martin Tsvetkov

Crystals 2021, 11(3), 231; https://doi.org/10.3390/cryst11030231 - 26 Feb 2021

Cited by 4 | Viewed by 1857

Abstract

Тhe properties of the Rare Earth Elements allow a wide range of applications in optoelectronics, fiber amplifiers, solid-state lasers, telecommunications, biosensing, and photocatalysis, just to mention a few [...] Full article

(This article belongs to the Special Issue Rare Earths-Doped Materials)

Research

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20 pages, 4342 KiB

Open AccessArticle

Features of the Phase Preferences, Long- and Short-Range Order in Ln₂(WO₄)₃ (Ln = Gd, Dy, Ho, Yb) with Their Relation to Hydration Behavior

by Victor V. Popov, Yan V. Zubavichus, Alexey P. Menushenkov, Alexey A. Yastrebtsev, Bulat R. Gaynanov, Sergey G. Rudakov, Andrey A. Ivanov, Fyodor E. Dubyago, Roman D. Svetogorov, Evgeny V. Khramov, Nadezhda A. Tsarenko, Nataliya V. Ognevskaya and Igor V. Shchetinin

Crystals 2022, 12(7), 892; https://doi.org/10.3390/cryst12070892 - 23 Jun 2022

Viewed by 1453

Abstract

The effect of synthesis conditions on the features of the long- and short-range order of Ln

_{2}

(WO

_{4}

)

_{3}

(Ln = Gd, Dy, Ho, Yb) powders synthesized via coprecipitation of salts has been studied by a complex of physico-chemical techniques [...] Read more.

The effect of synthesis conditions on the features of the long- and short-range order of Ln

_{2}

(WO

_{4}

)

_{3}

(Ln = Gd, Dy, Ho, Yb) powders synthesized via coprecipitation of salts has been studied by a complex of physico-chemical techniques including synchrotron X-ray powder diffraction, X-ray absorption spectroscopy, Raman and infrared spectroscopy, and simultaneous thermal analysis. It was found that crystallization of amorphous precursors begins at 600 °C/3 h and leads to the formation of the monoclinic structure with sp. gr. C12/c1(15) for Ln

_{2}

(WO

_{4}

)

_{3}

(Ln = Gd, Dy) and with sp. gr. P12

_{1}

/a1(14) for Ln = Yb, whereas crystallization of Ho precursor requires even higher temperature. After annealing at 1000 °C, the P12

_{1}

/a1(14) phase becomes the dominant phase component for all heavy lanthanoid types except for Ln = Gd. It was shown that the Ln (Ln = Dy, Ho, and Yb) tungstates with the P12

_{1}

/a1(14) monoclinic structure correspond to trihydrates Ln

_{2}

(WO

_{4}

)

_{3}

·3H

_{2}

O formed due to a rapid spontaneous hydration under ambient conditions. It was concluded that the proneness to hydration is due to a specific structure of the P12

_{1}

/a1(14) phase with large voids available to water molecules. Modifications in the local structure of Ln-O coordination shell accompanying the structure type change and hydration are monitored using EXAFS spectroscopy. Full article

(This article belongs to the Special Issue Rare Earths-Doped Materials)

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15 pages, 9696 KiB

Open AccessArticle

Characterization of Tungstates of the Type Hf_1−xLn_xW₂O_8−x/2 (Ln = Eu, Tm, Lu) Synthesized Using the Hydrothermal Method

by Martin Tsvetkov, Martin Nedyalkov, Evgenia Valcheva and Maria Milanova

Crystals 2022, 12(3), 327; https://doi.org/10.3390/cryst12030327 - 26 Feb 2022

Viewed by 2008

Abstract

Pure HfW₂O₈- and Ln³⁺-containing solid solutions, Hf_1−xLn_xW₂O_8−x/2 (Ln = Eu, Tm, Lu), were synthesized using the hydrothermal method. The lanthanide ions were selected based on the differences between their ionic [...] Read more.

Pure HfW₂O₈- and Ln³⁺-containing solid solutions, Hf_1−xLn_xW₂O_8−x/2 (Ln = Eu, Tm, Lu), were synthesized using the hydrothermal method. The lanthanide ions were selected based on the differences between their ionic radii. A content of the Ln³⁺ ions in the range of 0.01–0.15 mol with a step of 0.02 was used for Hf_1−xLn_xW₂O_8−x/2 preparation, although the main research was performed on x = 0.01 and 0.05 samples because of an inhomogeneity detected by powder X-ray diffraction (XRD) when the content of Ln³⁺ was above 0.07–0.09 mol. X-ray diffraction measurements were supported by Raman and infrared spectroscopy. A new band in the Raman spectra of the samples with 0.05 mol Ln³⁺, as well as a red shift of the most intensive band (assigned to valence stretching of W-O-W bonds) as a result of the Ln³⁺ presence, was detected. The Scanning Electron Microscopy and Transmission Electron Microscopy micrographs revealed well-crystalized microcrystals with lengths in the range of 2–5 μm, with larger interplanar distances, measured in the solid solutions of the same crystal plain. The alpha-HfW₂O₈ → beta-HfW₂O₈ order-to-disorder phase transition was followed by high temperature XRD, and its reversibility was evident. The influence of the Ln³⁺ both on the unit cell parameters of the solid solutions and on the temperature of phase transition and on the coefficient of thermal expansion, CTE, was observed. A band gap energy in the range of 2.8–3.1 eV for pure HfW₂O₈ and for the solid solutions Hf_1−xLnxW₂O_8−x/2 (x = 0.01 and 0.05) was determined. Full article

(This article belongs to the Special Issue Rare Earths-Doped Materials)

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12 pages, 21287 KiB

Open AccessArticle

Luminescence Properties of Gd₂(MoO₄)₃ Modified with Sm(III) and Tb(III) for Potential LED Applications

by Martin Tsvetkov, Denitsa Elenkova and Maria Milanova

Crystals 2022, 12(1), 120; https://doi.org/10.3390/cryst12010120 - 17 Jan 2022

Cited by 6 | Viewed by 1769

Abstract

Light-emitting phosphors, doped with lanthanide ions of Tb(III) and Sm(III) of the type Gd_1.97−y Sm_yTb_0.03(MoO₄)₃ (y = 0.01–0.11, step 0.02) and Gd_1.95−xSm_0.05Tb_x(MoO₄)₃ (x = 0.01–0.09, step [...] Read more.

Light-emitting phosphors, doped with lanthanide ions of Tb(III) and Sm(III) of the type Gd_1.97−y Sm_yTb_0.03(MoO₄)₃ (y = 0.01–0.11, step 0.02) and Gd_1.95−xSm_0.05Tb_x(MoO₄)₃ (x = 0.01–0.09, step 0.02), were synthesized and characterized by X-ray diffraction, UV-Vis spectroscopy, scanning and transmitting electron microscopy (SEM, TEM) as well as photoluminescence spectroscopy. The effect of the doping content of Tb/Sm was followed. The unit cell parameters for Gd_1.97−ySm_yTb_0.03(MoO₄)₃ and Gd_1.95−xSm_0.05Tb_x(MoO₄)₃ changed with the increase in the Tb/Sm content. The microstrain values also increased, proposing an increased concentration of defects. The mean particle size was estimated to be approximately 0.6 µm. Based on a Williamson–Hall plot, the size of the crystallites was determined to be in the range of 42–60 nm for modified and pure Gd₂(MoO₄)₃ samples, respectively. The samples excited at 406 nm exhibited characteristic emission lines of Sm (485, 555, 646 nm). The host material Gd₂(MoO₄)₃ emission in visible light was explained by the crystal structure defects, namely, oxygen vacancies. The CIE x/y color coordinates of the phosphors were determined and the related points were located in the green-yellow/pale yellow region of the visible light. The excited state lifetimes were determined for both groups of the samples, showing values in the millisecond range and indicating the samples as promising phosphors. Full article

(This article belongs to the Special Issue Rare Earths-Doped Materials)

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23 pages, 8308 KiB

Open AccessArticle

Synthesis and Crystal Structure of the Short LnSb₂O₄Br Series (Ln = Eu–Tb) and Luminescence Properties of Eu³⁺-Doped Samples

by Felix C. Goerigk, Veronica Paterlini, Katharina V. Dorn, Anja-Verena Mudring and Thomas Schleid

Crystals 2020, 10(12), 1089; https://doi.org/10.3390/cryst10121089 - 27 Nov 2020

Cited by 12 | Viewed by 3097

Abstract

Pale yellow crystals of LnSb₂O₄Br (Ln = Eu–Tb) were synthesized via high temperature solid-state reactions from antimony sesquioxide, the respective lanthanoid sesquioxides and tribromides. Single-crystal X-ray diffraction studies revealed a layered structure in the monoclinic space group [...] Read more.

Pale yellow crystals of LnSb₂O₄Br (Ln = Eu–Tb) were synthesized via high temperature solid-state reactions from antimony sesquioxide, the respective lanthanoid sesquioxides and tribromides. Single-crystal X-ray diffraction studies revealed a layered structure in the monoclinic space group P2₁/c. In contrast to hitherto reported quaternary lanthanoid(III) halide oxoantimonates(III), in LnSb₂O₄Br the lanthanoid(III) cations are exclusively coordinated by oxygen atoms in the form of square hemiprisms. These [LnO₈]¹³⁻ polyhedra form layers parallel to (100) by sharing common edges. All antimony(III) cations are coordinated by three oxygen atoms forming ψ¹-tetrahedral [SbO₃]³⁻ units, which have oxygen atoms in common building up meandering strands along [001] according to

{{[{SbO}_{2 / 2}^{v} O_{1 / 1}^{t}]}^{-}}_{\infty}^{1}

(v = vertex-sharing, t = terminal). The bromide anions are located between two layers of these parallel running oxoantimonate(III) strands and have no bonding contacts with the Ln³⁺ cations. Since Sb³⁺ is known to be an efficient sensitizer for Ln³⁺ emission, photoluminescence studies were carried out to characterize the optical properties and assess their suitability as light phosphors. Indeed, for both, GdSb₂O₄Br and TbSb₂O₄Br doped with about 1.0–1.5 at-% Eu³⁺ efficient sensitization of the Eu³⁺ emission could be detected. For TbSb₂O₄Br, in addition, a remarkably high energy transfer from Tb³⁺ to Eu³⁺ could be detected that leads to a substantially increased Eu³⁺ emission intensity, rendering it an efficient red light emitting material. Full article

(This article belongs to the Special Issue Rare Earths-Doped Materials)

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19 pages, 4377 KiB

Open AccessArticle

Influence of Stabilizing Ion Content on the Structure, Photoluminescence and Biological Properties of Zr_1–xEu_xO_2–0.5x Nanoparticles

by Alexander N. Bugrov, Ruslan Yu. Smyslov, Anastasia Yu. Zavialova, Gennady P. Kopitsa, Tamara V. Khamova, Demid A. Kirilenko, Ilya E. Kolesnikov, Dmitrii V. Pankin, Vadim A. Baigildin and Christophe Licitra

Crystals 2020, 10(11), 1038; https://doi.org/10.3390/cryst10111038 - 15 Nov 2020

Cited by 5 | Viewed by 2841

Abstract

Quasi-spherical nanoparticles of ZrO₂ containing EuO_1.5 from 2 to 15 mol.% were synthesized from the chlorides of the corresponding metals under hydrothermal conditions. The structural changes of Zr_1–xEu_xO_2–0.5x (x = 0.02 ÷ 0.15) nanoparticles depending on [...] Read more.

Quasi-spherical nanoparticles of ZrO₂ containing EuO_1.5 from 2 to 15 mol.% were synthesized from the chlorides of the corresponding metals under hydrothermal conditions. The structural changes of Zr_1–xEu_xO_2–0.5x (x = 0.02 ÷ 0.15) nanoparticles depending on the content of europium (III) ions were studied using the complementary methods (X-ray diffraction, electron microdiffraction, Raman and photoluminescence spectroscopy). It was shown that increasing the Eu³⁺ concentration in the Zr_1–xEu_xO_2–0.5x nanoparticles leads to a transition from the equilibrium monoclinic zirconia phase to metastable tetragonal and cubic polymorphic modifications. In this case, the size of the nanoparticles decreases from 11.5 nm to 9 nm; the specific surface area grows from 80.2 to 111.3 m²/g, and the electrokinetic potential increases monotonously from −8.7 to 16.3 mV. The evolution of the phase composition of Zr_1–xEu_xO_2-0.5x nanoparticles from monoclinic to tetragonal/cubic allomorphs with an increase in the molar fraction of stabilizer ions was correlated with changes in the sublevel structure of ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₄ optical transitions for Eu³⁺ in the luminescence spectra. Besides, for the nanoparticles obtained by hydrothermal synthesis from chlorides, the quantum efficiency does not exceed 3%. According to the M.T.T. assay, as a result of three-day human fibroblast cultivation in the aqueous dispersion of Zr_1–xEu_xO_2–0.5x (x = 0.02 ÷ 0.15) nanoparticles, the proliferation activity of the cells is maintained, indicating that they do not have cytotoxic properties. Such nanoparticles can be used in organic–inorganic composites for medical applications in order to strengthen the polymer scaffolds and visualize changes in their structure within time. Full article

(This article belongs to the Special Issue Rare Earths-Doped Materials)

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16 pages, 5532 KiB

Open AccessArticle

Symmetry of the Optical Phonons in LuVO₄: A Raman Study

by Peter Rafailov, Dimitre Dimitrov, Yen-Fu Chen, Chi-Shen Lee and Jenh-Yih Juang

Crystals 2020, 10(5), 341; https://doi.org/10.3390/cryst10050341 - 26 Apr 2020

Cited by 5 | Viewed by 3077

Abstract

A thorough analysis of the first-order vibrational spectrum of LuVO₄ is presented by using polarized micro-Raman spectroscopy with special focus on the phonon modes with the weakest intensity and occasional controversial assignment. Group-theory analysis is carried out to demonstrate the determination of [...] Read more.

A thorough analysis of the first-order vibrational spectrum of LuVO₄ is presented by using polarized micro-Raman spectroscopy with special focus on the phonon modes with the weakest intensity and occasional controversial assignment. Group-theory analysis is carried out to demonstrate the determination of numbers and symmetries of the Raman active modes. Crystal- and correlation-field splitting effects in the vibrational spectrum of LuVO₄ are discussed. Under conditions adjusted to minimize the birefringence effects we recorded, in each main scattering configuration, a series of Raman spectra in different sample orientations achieved by rotating the sample around the incident laser beam. The dependence of the Raman intensity on the rotational angle allowed us to identify the correct symmetry of the phonons with exceptionally weak scattering cross-section. A complete assignment of all twelve first-order Raman active phonons of LuVO₄ is thus obtained. Full article

(This article belongs to the Special Issue Rare Earths-Doped Materials)

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14 pages, 3796 KiB

Open AccessArticle

Optical Investigation of Eu³⁺ Doped Bi₁₂GeO₂₀ (BGO) Crystals

by M. Kowalczyk, T.F. Ramazanova, V.D. Grigoryeva, V.N. Shlegel, M. Kaczkan, B. Fetliński and M. Malinowski

Crystals 2020, 10(4), 285; https://doi.org/10.3390/cryst10040285 - 9 Apr 2020

Cited by 13 | Viewed by 7866

Abstract

The spectroscopic properties of Eu³⁺ doped Bi₁₂GeO₂₀ (BGO) sillenite bulk crystals that were grown by the low-thermal-gradient Czochralski technique (LTG Cz) were investigated. The absorption spectra and the emission properties have been measured at room temperature (300 K) and [...] Read more.

The spectroscopic properties of Eu³⁺ doped Bi₁₂GeO₂₀ (BGO) sillenite bulk crystals that were grown by the low-thermal-gradient Czochralski technique (LTG Cz) were investigated. The absorption spectra and the emission properties have been measured at room temperature (300 K) and at 10 K. Luminescence was observed both due to the direct Eu³⁺ ion excitation, as well as under UV excitation due to the energy transfer between Bi³⁺ and Eu³⁺ ions. Bi³⁺ → Eu³⁺ energy transfer mechanisms in Eu³⁺:BGO doped host were investigated. The Ω_λ parameters, as well as radiative lifetimes, were calculated based upon the Judd-Ofelt formalism. The branching ratios and electric dipole transition probabilities were also determined, based upon the obtained experimental results. Luminescence has been observed from the ⁵D_0,1,2 levels of Eu³⁺, with emissions from the ⁵D₀ level being the strongest. The strongest observed luminescence band corresponds to the ⁵D₀ → ⁷F₀ transition at 578.7 nm. Reasons for the strong presence of the theoretically forbidden ⁵D₀ → ⁷F₀ emission were investigated. Full article

(This article belongs to the Special Issue Rare Earths-Doped Materials)

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12 pages, 17272 KiB

Open AccessArticle

Effect of the Fluorine Substitution for –OH Group on the Luminescence Property of Eu³⁺ Doped Hydroxyapatite

by Xiaojun Zhang, Qingguo Xing, Lixuan Liao and Yingchao Han

Crystals 2020, 10(3), 191; https://doi.org/10.3390/cryst10030191 - 10 Mar 2020

Cited by 14 | Viewed by 3379

Abstract

In this study, different fluoridated hydroxyapatite doped with Eu³⁺ ion nanoparticles were prepared by the hydrothermal method. The relationship between luminescence enhancement of Eu³⁺ ions and a fluorine substitution ratio for hydroxyl group in hydroxyapatite was discussed. Moreover, the effect of [...] Read more.

In this study, different fluoridated hydroxyapatite doped with Eu³⁺ ion nanoparticles were prepared by the hydrothermal method. The relationship between luminescence enhancement of Eu³⁺ ions and a fluorine substitution ratio for hydroxyl group in hydroxyapatite was discussed. Moreover, the effect of fluorine substitution for a hydroxyl group on phase composition, crystallinity, and crystal size was studied. Phase composition and chemical structures were identified by X-ray diffraction (XRD) and Fourier Transform Infrared (FT-IR) Spectroscopy analyses. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) patterns were performed to analyze the morphology and particle size. X-ray Photoelectron Spectroscopy (XPS) patterns were observed to analyze fluorine substitution for the hydroxyl group and chemical state of Eu³⁺ ions in fluoridated hydroxyapatite. The results of these experiments indicated that the samples with a different fluorine substitution ratio were prepared successfully by maintaining the apatite structure. With an increasing fluorine substitution ratio, the morphology maintained a rod-like structure but the aspect ratio tended to decrease. XPS patterns displayed that the fluorine replaced the hydroxyl group and brought environmental variation. The fluorine ions could affect the crystal field environment and promote luminescence conversion. There was a linear relationship between the fluorine substitution ratio and luminescence enhancement. Full article

(This article belongs to the Special Issue Rare Earths-Doped Materials)

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