Rare Earth Fluoride Research Articles

Controllable phase/morphology tailoring of REF3 and NaREF4 (RE = La-Lu, Y), and insights into the up-conversion luminescence of GdF3:Yb3+/Tm3+ spheres

Four typical rare earth fluorides of hexagonal/orthorhombic REF3 and hexagonal/cubic NaREF4 (RE = La-Lu, Y, excluding radioactive Pm), have been hydrothermally synthesized from mixed solutions of the rare earth nitrate solutions and ammonium fluoride (NH4F), in the presence of EDTA-2Na. The phase and the morphology are closely related to the ionic radius of RE3+. Large RE3+ ions, La3+, Ce3+, Pr3+, Nd3+ and Sm3+, crystallize in hexagonal REF3 (nanocrystals), and middle RE3+ ions (RE = Eu-Dy) crystallize in orthorhombic REF3 (micron-sized spheres/polyhedral micro-crystallites). For small RE3+ ions, Ho3+, Er3+, Tm3+, Yb3+ and Y3+, they tend to being incorporated into the host lattice of cubic NaREF4 (submicron spheres). However, the smallest Lu3+ tends to crystallizing in orthorhombic LuF3. Varying the F-/RE3+ molar ratio (R) and the reaction temperature can efficiently induce the phase/morphology evolution of rare earth fluorides. Increasing R up to 10–30 yielded hexagonal NaREF4 (RE = Dy, Tb, Gd, Eu, Sm, micron prisms), although the original products were orthorhombic REF3 (RE = Dy, Tb, Gd, Eu) and hexagonal SmF3 at R = 5. However, increasing R is inefficient to the phase transition for large RE elements of La, Ce, Pr, Nd. Lowering the reaction temperature from 180 °C to 80 °C gave rise to the phase transition from orthorhombic LuF3 to cubic NaLuF4. GdF3:Yb3+,Tm3+ spheres exhibited intense blue emission at ~476 nm (1G4 → 3H6 transition of Tm3+), deep-red emission at ~646 nm (1G4→3F4 transition of Tm3+), and near-infrared emission at ~695 nm (3F2,3 → 3H6 transitions of Tm3+) under the 980 nm infrared excitation. The intense near-infrared emission under the infrared excitation suggested that they might hold great potential in bio imaging application.

Comparative Life Cycle Assessment of Neodymium Oxide Electrolysis in Molten Salt

Rare earth elements are used in renewable energy generation techniques like wind turbines as well as in various high‐tech applications in the automobile industry, electrical engineering, optics, and catalyzers. Due to the environmentally harmful production of rare earths, they have been subject of life cycle assessment investigations in the past years. Most of these studies focus on rare earth oxide production. The subsequent reduction of rare earth oxides to the final metal in a molten salt electrolysis has significant environmental impacts especially on human toxicity. The main drivers are rare earth fluoride production and molten salt electrolysis. In this study, exemplarily a comparative life cycle assessment of neodymium oxide electrolysis in molten salt as well as various neodymium fluoride production processes is conducted. The different assumptions regarding inputs and outputs of the electrolysis process are discussed. Then, the impacts of the electrolysis processes modeled in different ways are analyzed in relation to the entire process chain to produce neodymium. The results show a share of the electrolysis process on the entire process chain varying from 9% to 82% depending on different assumptions. Based on this analysis, improvements for the electrolysis process are proposed.

Diamond–Rare Earth Composites with Embedded NaGdF4:Eu Nanoparticles as Robust Photo- and X-ray-Luminescent Materials for Radiation Monitoring Screens

Thin-film diamond–rare earth fluoride nanocomposites have been produced by chemical vapor deposition to be used as a source of intensive photo- and X-ray luminescence. The composites with embedded ...

Design and high-temperature tribological properties of CoCrW with rare earth fluoride composites

CoCrW matrix containing rare earth LaF3 self-lubricating composites were fabricated by using hot-pressing method. The microstructure, phases and high-temperature tribological properties of obtained composites were systematically studied. Friction and wear behaviors were evaluated by using ball-on-disc tribo-tester from RT (24 °C) to 1000 °C. Metal matrix consisted of two allotropes: ε (hcp) and γ (fcc). Mo had a solid solution strengthening effect and lubricating effect when it changed into compounds. Rare earth LaF3 and its compounds showed lubricating effect at elevated temperatures. The friction coefficients and wear resistance of reinforced composites were greatly enhanced with little hardness dropping. It was ascribed to the synergistic effect of in-situ formed and extra added solid lubricants (silver, LaCrO3, Ag2MoO4, AgF3 and metal oxides) as well as oxides film which changed the wear model of tribo-couples during sliding. The wear rate of CoCrW-10LaF3-8Mo-9Ag was about 10 times lower than those of reported Ni and ZrO2 matrix composites. The composites showed the slight abrasive wear and oxidative wear at elevated temperatures.

Na3Sc2(PO4)2F3: rational design and synthesis of an alkali rare-earth phosphate fluoride as an ultraviolet nonlinear optical crystal with an enlarged birefringence

An alkali rare-earth phosphate fluoride nonlinear optical crystal, Na3Sc2(PO4)2F3, was synthesized, showing an enlarged birefringence and a short phase matching wavelength.

Helix-constructed polar rare-earth iodate fluoride as a laser nonlinear optical multifunctional material.

The first trivalent rare-earth iodate fluoride nonlinear optical (NLO) crystal, Y(IO3)2F (YIF), was successfully designed and synthesized, featuring polarization-favorable helical chains constructed from trans-YO6F2 polyhedra and IO3 groups. It exhibited a suitable balance of a wide transparency range of 0.26-10.0 μm, high laser damage threshold (LDT) of 39.6 × AgGaS2, and moderate second harmonic generation (SHG) effect of 2 × KDP. A series of doped RE:YIF (RE = Pr, Nd, Dy, Ho, Er, Tm, and Yb) crystals were easily synthesized benefiting from the spring-shaped helix structure, which possess wide absorption and emission peaks as well as long lifetime, especially in the visible and near-infrared regions. Particularly, the remarkable fluorescence properties of Nd and Yb doped YIF crystals are comparable to and even better than those of traditional self-frequency doubling (SFD) crystals such as YAB, YCOB, and GdCOB. Thus, these RE-doped YIF crystals are promising laser SFD crystals. This work also indicated that constructing helical chains should be an effective strategy for the design of inorganic polar materials.

UV-Vis-NIR absorption spectra of lanthanide oxides and fluorides.

The absorption characteristics of lanthanide-based functional materials are of key importance for many scientists and engineers, e.g. in luminescence studies, bioimaging, optical heating/cooling, Raman spectroscopy, and industrial applications such as new light sources, optical sensors, labeling and tracing techniques, etc. Here we show the absorption spectra of solid, optically active lanthanide fluorides (CeF3, PrF3, NdF3, SmF3, EuF3, GdF3, TbF3, DyF3, HoF3, ErF3, TmF3, and YbF3) and oxides (CeO2, Pr6O11, Nd2O3, Sm2O3, Eu2O3, Gd2O3, Tb4O7, Dy2O3, Ho2O3, Er2O3, Tm2O3, and Yb2O3), measured in the UV-Vis-NIR range, from 200 to 2500 nm. The spectra were measured in diffused-reflectance mode using a spherical integrator. We assigned energy levels (2S+1LJ) of lanthanide ions(iii), i.e. intraconfigurational 4f-4f transitions to the observed absorption bands. In order to clearly distinguish the 4f → 4f transitions, we also pointed out other absorption bands commonly observed in the measured spectra, such as intrinsic absorption of the matrices, interconfigurational 4f → 5d and charge transfer transitions, artificial bands from absorbed water (present in most materials) and a quartz holder.

Wet-chemical synthesis and luminescence of KCeF4

Complex rare earth fluoride KCeF4 have been prepared by Wet-Chemical synthesis. Though, synthesis of such type of complexes are rather tricky. Fluorides are more susceptible to hydrolysis which is detrimental to the luminescence properties. X-Ray Diffraction pattern of our sample was compared to that of the standard ICDD pattern of KCeF4. Further, photoluminescence study was done. As prepared sample showed photoluminescence. Hence, need of any type of heat treatment was ruled out in the following method. Also, no separate doping process was done since Ce is present as an impurity itself in the host compound. KCeF4 exists in orthorhombic form at room temperature.

Synthesis and Upconversion Luminescence in LaF3:Yb3+, Ho3+, GdF3: Yb3+, Tm3+ and YF3:Yb3+, Er3+ obtained from Sulfide Precursors

Rare earth fluorides are mainly obtained from aqueous solutions of oxygen‐containing precursors. Probably, this method is simple and efficient, however, oxygen may partially be retained in the fluoride structure. We offer an alternative method: obtaining fluorides and solid solutions based on them from an oxygen‐free precursor. As starting materials, we choose sulfides of rare‐earth elements and solid solutions based on them. The fluorination is carried out by exposure to hydrofluoric acid of various concentrations. The transmission electron microscopy images revealed the different morphologies of the products, which depend on the concentration of the fluorinating component (HF) and the host element. The solid solution particle size varied from 30–35 nm in the case of GdF3:Yb3+, Tm3+ (4 % HF) to larger structures with dimensions exceeding 200 nm, such as that for LaF3:Yb3+, Ho3+ (40 % HF). The thermal characteristics, such as the temperatures of the transitions and melting and enthalpies, were determined for the solid solutions and simple fluorides. Applicability of the materials obtained as biological luminescent markers was tested on the example of upconversion luminescence, and good upconversion properties were detected.

Palygorskite supported rare earth fluoride for photocatalytic nitrogen fixation under full spectrum

Nitrogen reduction to ammonia under mild conditions using cost-effective and full spectrum solar-driven photocatalyst is promising from an industrial perspective. Herein, sensitizer Yb3+ and activator Tm3+ co-doping LaF3 supported by palygorskite (Pal) nanocomposites (LaF3:Yb3+, Tm3+/Pal) are successfully prepared by the microwave hydrothermal method. The photocatalytic nitrogen fixation is performed using LaF3:Yb3+, Tm3+/Pal as photocatalyst. The impact of Tm3+ doping ratio and loading amount on the nitrogen fixation are explored. Results demonstrate that modified natural Pal nanorods have large surface area facilitating the immobilization of rare earth fluoride nanoparticles. The total amount of ammonia can reach the highest of 43.2 mg/L under solar light irradiation, while 5.7 mg/L can be achieved even under near infrared (NIR) light irradiation. The enhanced photocatalytic nitrogen fixation ability is ascribed to the upconversion capability of LaF3:Yb3+, Tm3+ which converts NIR into visible and UV light, improving the utilization of full solar spectrum. Meanwhile, LaF3:Yb3+, Tm3+ and modified Pal form an indirect Z-scheme heterostructure mediated by fluorine vacancy (FV), favoring the separation of the photogenerated electron-holes and preservation of high reduction–oxidation potentials. Additionally, the defective FV efficiently provides active sites beneficial for the adsorption and dissociation of N2.

Anti-escaping of incident laser in rare-earth doped fluoride ceramics with glass forming layer

Adaptive fluoride ceramic with glass forming layer (GCZBL-Er) used in laser anti-escaping has been prepared by one-step synthesis, and the thickness of glass layer is identified as ~0.41 mm. Blue, green and red emissions of Er3+/Yb3+ codoped fluoride ceramic (CZBL-Er) and glass layer (GZBL-Er) have been investigated under ~980 nm laser pumping. With the forming of thin glass layer on ceramic surface, the absorption intensities on diffuse reflection of GCZBL-Er at 974 nm and 1.53 μm increase by 48% and 53% than those of CZBL-Er. Excited by a 979 nm laser, the presence of the glass layer increases the absolute absorption rate in spectral power from 75% in CZBL-Er to 83% in GCZBL-Er, which is consistent with the improvement in the absorbed photon number. In addition, the quantum yield of GCZBL-Er complex is raised by 28.4% compared to the case of ceramic substrate by photon quantification. Intense absorption-conversion ability and efficient macroscopical anti-escaping effect confirm the superiority of ingenious structure in the fluoride ceramics with glass forming layer, which provides a new approach for developing the absorption-conversion materials of anti-NIR laser detection.

Determination of the Faraday rotation perpendicular to the optical axis in uniaxial CeF3 crystal by using the Generalized-High Accuracy Universal Polarimeter

Many single crystals have been developed and commercialized for optical isolators. However, optical isolator materials have been limited to isotropic crystals or to the isotropic direction (optic axis) of anisotropic crystals. This study investigates the wavelength dependences of linear birefringence, linear dichroism, Faraday rotation and magnetic-circular dichroism in a single crystal rare-earth fluoride, namely CeF3. Measurements were made in the direction parallel and perpendicular to the optic axis under an applied magnetic field. The magnetic field was generated by Nd-Fe-B magnets installed in the generalized-high accuracy universal polarimeter (G-HAUP). The first application of G-HAUP to a magneto-optical material is presented. In the CeF3 crystal, the Verdet constants along directions parallel and perpendicular to the optic axis were positive over the measured wavelength region (300–680 nm), and their magnitudes were nearly equal. The success in the accurate measurement on Faraday rotation along anisotropic directions has opened the way to study on optical isolators along the direction other than optic axis.

Electrochemical separation of uranium from lanthanide (La, Eu, Gd) fluorides in molten LiCl-KCl

The electrochemical behaviors of a series of lanthanide fluorides in molten LiCl-KCl were investigated, and the results on the separation of uranium from molten LiCl-KCl-UF4-LaF3-GdF3-EuF3 by electrolysis were provided. On the basis of the cyclic voltammetry and square wave voltammetry measurements, LaF3 and GdF3 undergo a three-electron reduction to metal in molten LiCl-KCl while the fluoride of europium (III) can only be reduced to europium (II) via single electron transfer. Compared with the relatively simple electrochemical behaviors of individual LaF3, GdF3 or EuF3 in molten LiCl-KCl, the cyclic voltammograms of LaF3, UF4, GdF3 and EuF3 coexisting in the LiCl-KCl melt are complicated by the presence of F- ions which cause the appearance of new peaks especially in the oxidation processes. Potentiostatic electrolysis of the LiCl-KCl-UF4-LaF3-GdF3-EuF3 melt using the tungsten sheet electrode resulted in the formation of metallic deposits that are composed of uranium as well as small amount of UO2 according to the X-ray diffraction and scanning electron microscope analysis, suggesting that uranium can be successfully separated from lanthanide fluorides in the LiCl-KCl molten salt.

Electrical conductivity of REF3-LiF (RE=La and Nd) molten salts

The electrical conductivity of REF3-LiF (RE = La and Nd) molten salts (xREF3 = 5 mol%–40 mol%) was systematically measured over the temperature range from 1223 to 1423 K by using continuously varying cell constant (CVCC) method. Electrical conductivity values of LaF3-LiF and NdF3-LiF molten salts within the studied ranges are 4.11–9.39 and 3.62–9.51 S/cm, respectively. The composition and temperature dependences of electrical conductivity and the factor of RE2O3 on electrical conductivity were investigated. The electrical conductivity nonlinearly decreases with the increasing mole percent of REF3 for the changing of structural ion in molten salts. A good linear relationship between the natural logarithm of electrical conductivity (lnκ) and the reciprocal of the absolute temperature (T−1) can be interpreted by the trend that electrical conductivity linearly increases with increasing temperature. Although it is an important factor of RE2O3 on electrical conductivity, the influence of RE2O3 on conductivity is small for the very limited solubility of RE2O3 in REF3-LiF molten salts. The results of this work supplement the present electrical conductivity data of rare earth fluoride electrolyte.

Two-dimensional Acetate-based Light Lanthanide Fluoride Nanomaterials (F-Ln, Ln = La, Ce, Pr, and Nd): Morphology, Structure, Growth Mechanism, and Stability.

Discovery of novel two-dimensional (2D) materials is of fundamental importance but remains challenging. In this work, we design a simple and facile bottom-up approach to fabricate a new family of 2D acetate-based light lanthanide fluoride nanomaterials (F-Ln, Ln = La, Ce, Pr, Nd) at room temperature and atmosphere pressure, for the first time. Various characterization techniques confirm that as-synthesized F-Ln exhibit an ultrathin morphology with thickness up to 1.45 nm and lateral dimensions up to several hundred nanometers. Microstructure analysis demonstrates that F-Ln are a series of defect-rich 2D nanomaterials, which consist of nanocrystals with sub-10 nm domains. Structure characterization of F-Ce, a typical example, infers that BN-like F-Ce one-atom-layers sandwiched by intercalated acetate anions stack alternately along [001] direction to form nanocrystal building blocks of F-Ce. The study of growth mechanism suggests that three procedures are involved in the formation of F-Ce: hydrolysis reaction of cerium(III) acetate, structure transformation induced by fluorine ions, and assembly process guided by acetate anions. The as-prepared nanosheets show excellent stability with respect to environment stimuli such as air, heat, solvent, and high-energy electron beam. This study enriches the library of 2D materials and paves the way for future application of such 2D materials in areas such as catalysis, adsorption, separation, and energy storage/conversion.

Synthesis of multifunctional rare-earth fluoride/Ag nanowire nanocomposite for efficient therapy of cancer.

Well-dispersed Ag nanowires and PVP-modified BaGdF5: Yb3+, Er3+ spherical nanoparticles were prepared by simple solvothermal and hydrothermal method, and they were further combined to obtain photo-thermal-magnetic multifunctional Ag/BaGdF5: Yb3+, Er3+ nanocomposites. Under NIR laser irradiation, monodispersed rare-earth fluoride BaGdF5: Yb3+, Er3+ in nanocomposite exhibit good upconversion fluorescent. Meanwhile, under the action of an external magnetic field, the nanocomposite also exhibits good magnetic properties and excellent contrast efficiency by CT/MR imaging. The NCs possess good structure and photothermal stability at multiple cycles due to that Ag nanowires are modified by polyvinyl pyrrolidone and sodium citrate. The biocompatibility and low toxicity of NCs are also remarkable. Importantly, the unique linear morphology of Ag particles can achieve high efficiency conversion between light and heat. Furthermore, in vitro tests also confirm the high efficiency of photothermal therapy for cancer cells.

Anisotropy of Ionic Conductivity of TbF3 Crystals

Temperature measurements (in the range of 375–830 K) of the ionic conductivity of terbium trifluoride single crystals (structure type β-YF3) along three unit-cell crystallographic axes (a, b, and c) have been performed. It is found that TbF3 crystals (orthorhombic system, sp. gr. Pnma) have a weak anisotropy of electrical conductivity: σ||b/σ||a = σ||b/σ||c ≈ 2. The conductivity along the b axis is σ||b = 8 × 10–6 S/cm at 500 K. The anisotropy of fluorine-ion conductivity in rare-earth fluorides with β-YF3, LaF3 (tysonite), and β-BaTm2F8 structures is discussed in the contest of the specific features of their atomic structure.

Rare earth nanofluorides: synthesis using ionic liquids

AbstractThis review presents a comprehensive summary of the research progress on the synthesis of rare earth fluoride nanomaterials using the most common methods of synthesis. Special focus is on syntheses utilising ionic liquids, which is a new and promising way of preparing nanomaterials without the use of dangerous organic solvents (toxic, flammable, or combustive). Rare earth fluoride nanoparticles can be obtained with a high yield, purity, and crystallinity, and with different morphologies and luminescent properties depending on the selected method of synthesis.

Review on the structural analysis of fluoride-phosphate and fluoro-phosphate glasses

Fluoride phosphate/fluorophosphate (FP) glasses form an intriguing material's basis for diverse applications in optics, photonics and energy storage. The optimization of the materials' properties with regard to the application envisioned demands an understanding of the key concepts describing their local structures on different distance regimes. Because of the inevitable losses of fluoride during melting or annealing, the synthesis and elemental analytical characterization present important and critical challenges. In this contribution we summarize the current state of the literature regarding the structural characterization of these glasses. Foci are the detailed analysis of vibrational spectra with special emphasis on Raman scattering results and multinuclear solid-state NMR spectroscopy. While vibrational spectra give important information about the local environments (short range order) advanced multi-dimensional and multi-resonance methodology can extend the informational content of solid-state NMR into the sub-nanometer distance range, including second and third coordination spheres. Exemplary results will be presented for (a) aluminum fluoride based simple model systems, (b) fluoride phosphate glasses containing heavy transition metals or post-transition metal elements, and (c) rare-earth doped fluoride phosphate glasses for laser applications. The latter application field documents the utility of correlated spectroscopic studies using both methods in understanding the structural foundations of photophysical properties.

Small Alkaline-Earth-based Core/Shell Nanoparticles for Efficient Upconversion.

The optical efficiency of lanthanide-based upconversion is intricately related to the crystalline host lattice. Different crystal fields interacting with the electron clouds of the lanthanides can significantly affect transition probabilities between the energy levels. Here, we investigate six distinct alkaline-earth rare-earth fluoride host materials (M1- xLn xF2+x, MLnF) for infrared-to-visible upconversion, focusing on nanoparticles of CaYF, CaLuF, SrYF, SrLuF, BaYF, and BaLuF doped with Yb3+ and Er3+. We first synthesize ∼5 nm upconverting cores of each material via a thermal decomposition method. Then we introduce a dropwise hot-injection method to grow optically inert MYF shell layers around the active cores. Five distinct shell thicknesses are considered for each host material, resulting in 36 unique, monodisperse upconverting nanomaterials each with size below ∼15 nm. The upconversion quantum yield (UCQY) is measured for all core/shell nanoparticles as a function of shell thickness and compared with hexagonal (β-phase) NaGdF4, a traditional upconverting host lattice. While the UCQY of core nanoparticles is below the detection limit (<10-5%), it increases by 4 to 5 orders of magnitude as the shell thickness approaches 4-6 nm. The UCQY values of our cubic MLnF nanoparticles meet or exceed the β-NaGdF4 reference sample. Across all core/shell samples, SrLuF nanoparticles are the most efficient, with UCQY values of 0.53% at 80 W/cm2 for cubic nanoparticles with ∼11 nm edge length. This efficiency is 5 times higher than our β-NaGdF4 reference material with comparable core size and shell thickness. Our work demonstrates efficient and bright upconversion in ultrasmall alkaline-earth-based nanoparticles, with applications spanning biological imaging and optical sensing.