Rare Earth Phosphates Research Articles

Theoretical and experimental investigation of Xenotime-type rare earth phosphate REPO4, (RE\u2009=\u2009Lu, Yb, Er, Y and Sc) for potential environmental barrier coating applications

The mechanical and thermophysical properties of Xenotime-type REPO4 (RE = Lu, Yb, Er, Y and Sc) have been theoretically and experimentally investigated for a potential environmental barrier coating (EBC) topcoat application. The results show that the current studied REPO4 exhibits a quasi-ductile property, suggesting a potential long life expectancy of its made coatings. Further, from the study of underlying parameters governing thermophysical properties of a ceramic, low thermal expansion coefficients (TECs) and low thermal conductivities cannot be achieved simultaneously, due to mutual exclusive nature of above two parameters. REPO4 has been unveiled to have rather small TECs, attributing partly to its weak lattice anharmonicity, and is thus well-matched with silicon carbide based ceramic matrix composites. Last, the current investigated REPO4 exhibits very good high-temperature water vapor corrosion resistance, excellent calcium-magnesium aluminosilicates (CMAS) resistance as well as excellent chemical compatibility with silicon bond coats at elevated temperatures. Therefore, the Xenotime-type rare earth phosphates are a promising EBC topcoat material.

Orthovanadate wakefieldite-(Ce) in symplectites replacing vanadium-bearing omphacite in the ultra-oxidized manganese deposit of Praborna (Aosta Valley, Western Italian Alps)

Abstract Because of their unique structure and properties, rare-earth (REE) orthovanadates have been extensively employed for decades in advanced ceramics, in particular in the laser industry in replacement of Nd:YAG. A Ca-bearing REE orthovanadate with the empirical formula (Ce0.279Ca0.271Y0.267Gd0.057Nd0.055Dy0.032Sm0.027La0.020Th0.027Sr0.002)(V0.9085+Cr0.0673+Fe0.0173+As0.0055+)O4⋅nH2O has been found in metacherts from Praborna (Italian Alps) as micrometer-sized euhedral crystals in clinopyroxene + plagioclase symplectites replacing eclogite-facies vanadium-bearing omphacite (Aegirine55–48Jadeite42–33Diopside10–8 with V2O3 ≤ 1.39 wt%). We applied the synchrotron radiation, single crystal, micro-diffraction technique, recently optimized at ID09A beamline (ESRF, France), to determine the crystal structure of this mineral. It is tetragonal and isostructural with zircon, with a = 7.2233(12) Å, c = 6.3949(18) Å, V = 333.66(13) Å3, Z = 4, space group I41/amd, and it has been therefore identified as Ca- and Y-bearing wakefieldite-(Ce) (ideally CeV5+O4). Cell parameters are in agreement with those of synthetic Ce0.7Ca0.3VO4. Raman spectra of the studied wakefieldite-(Ce) are comparable with natural and synthetic wakefieldite-(Ce) spectra and revealed the presence of OH groups and/or water of hydration, which is also suggested by the low totals in microprobe analyses. Mass balance indicates that wakefieldite-(Ce) is a by-product of the omphacite breakdown; omphacite and Mn-rich epidote, a minor reactant, provided vanadium and REE, respectively. Petrological observations and thermodynamic modeling suggest that the mineral, coexisting with hematite, Mn-rich epidote, and braunite, formed during retrogression to greenschistfacies conditions at ultra-oxidized conditions (DFMQ ≥ +16 log units), which are often observed in Mn-oxide ores. Wakefieldite is an effective scavenger of REE in oxidized geological environments at P-T conditions that range from sedimentary to medium-grade metamorphic settings, even where the REE bulk concentration is negligible. Its rarity reflects both the overall low abundance of vanadium and the need for ultra-oxidized conditions that are rarely achieved in metamorphic rocks, where REE phosphates (i.e., monazite, xenotime) are commonly found instead.

МИНЕРАЛОГИЯ, ГЕОХИМИЯ И ГЕНЕЗИС РЕДКОМЕТАЛЛЬНОГО ZR-NB-HF-TA-РЗЭ-GA ОРУДЕНЕНИЯ В ПЛАСТЕ XXX МИНУСИНСКОГО БАССЕЙНА

Актуальность исследования обусловлена необходимостью оценки влияния вулканогенного пирокластического материала на формирование геохимического фона редких элементов в угле. Цель: изучить минералогические и геохимические особенности углей и пород пласта XXX Минусинского угольного бассейна. Объекты: угли и породы пласта XXX Минусинского бассейна. Методы: опробование угольного пласта XXX Минусинского бассейна; химический состав определен методами масс-спектрометрии с индуктивно связанной плазмой (ICP-MS), атомно-эмиссионной спектрометрии с индуктивно связанной плазмой (ICP-AES), инструментальным нейтронно-активационным анализом (ИНАА); минеральный состав изучен методами оптической микроскопии, петрографического анализа, сканирующей электронной микроскопии, рентгеновской дифрактометрии. Результаты. Выполнены комплексные минералого-геохимические исследования редкометалльного Zr-Nb-Hf-Ta-РЗЭ-Ga оруденения в пласте XXX-XXXa Изыхского месторождения Минусинского бассейна. В золе угля максимальное содержание Zr составляет 1,4 %, Nb – 0,26 %, Hf – 164 г/т, Ta – 21,2 г/т, суммы РЗЭ – 0,8 %, Ga – 226 г/т. Рудное вещество главным образом сконцентрировано в тонкодисперсной минеральной фазе, представленной преимущественно Zr-Nb-Ti-Fe оксидами, сложными Nb-Zr-P силикатами, редкоземельными фосфатами (монацит, ксенотим). Установлена связь аномальных концентраций редких металлов с внутрипластовым породным прослоем. В минеральном составе породного прослоя преобладает каолинит (68,9 %), меньше распространены кварц (11,0 %), калиевые полевые шпаты (7,0 %) и альбит (5,6 %). Установлена вулканогенно-пирокластическая природа породного прослоя и определен его первичный состав как пантеллерит-комендитовый. Подобный по составу породный прослой, с которым связано редкометалльное оруденение в углях, известен в углях кемеровской свиты Кузбасса того же возраста. Комплексное Zr-Nb-Hf-Ta-РЗЭ-Ga оруденение в углях Кузнецкого и Минусинского бассейна, связанное с вулканогенной пирокластикой, свидетельствует о возможном проявлении активного кислого и щелочного вулканизма планетарного масштаба в период формирования углей и о возможности выявления подобного оруденения в углях пермского возраста других бассейнов.

Composition and age of monacite fragments from superior jurassic terrigenous sediments of bazhenov formation foundation (multan area in west siberia)

Bazhenov Formation is regarded as the main oil-bearing stratum mothering nearly all the fields of the Western Siberia Oil-Gas-bearing Megabasin. Presently, it is one of the most studied formations of Siberia and, probably, Eurasia as a whole. While there is an enormous amount of studies devoted to the Bazhenov Formation, there are no detailed mineralogical studies at the modern hardware level. The age and sources of the terrigenous materials of the formation have not been studied as well. We have explored the detrital monazite from the upper-Jurassic terrigenous sediments of the Multan Area at the foundation of the Bazhenov Formation in the central part of Western Siberia, Surgut District. All the detrital rare earth phosphate is of the cerium kind being a monazite-(Се). The mineral is rather dissimilar in respect of its chemical properties, especially, the content of thorium. Some fragments have been subjected to superposed secondary changes. The detrital monazite is rounded to various degrees which is indicative of the various distances from the rare earth phosphate orebody washout. As per the chemical data, most of the monazite has been washed out from the medium and basic rocks (probably subalkaline or alkaline) as well as the sialic rocks (granitoids and associated veins). According to the chemical dating, most of the monazite fragments have been washed out of the very ancient Proterozoic formations and lower-Proterozoic rocks. Terrigenous materials derives probably from the rock assemblages of the eastern and south-eastern fringes of the Western Siberian megabasin such as the Proterozoic Yenisei Ridge or Lower-Proterozoic blocks of the Altay and Sayan Faulting.

CsBaB3O6:Eu3+ red-emitting phosphors for white LED and FED: Crystal structure, electronic structure and luminescent properties

Recently, borate compounds have received much attention in the field of rare earth doped phosphors due to their excellent luminescent performance. In this work, to explore the potential in LED and FED applications, the CsBaB3O6:Eu3+ phosphor was investigated in detail by using Rietveld refinement, DFT calculations, photoluminescent and cathodoluminescent spectra. As a result, CsBaB3O6 has a planar stacked three-dimensional layered structure. Under the excitation of 395 nm n-UV light and electron beam, CsBaB3O6:Eu3+ phosphor exhibits a typical red emission of Eu3+. A good thermal stability and good resistance to saturation and degradation were observed in the CsBaB3O6:Eu3+ phosphor. The related photoluminescent and cathodoluminescent mechanisms were studied. The results indicate that CsBaB3O6:Eu3+ phosphor has potential in multifunctional applications.

Synthesis, characterization, luminescence properties of copper(I) bromide based coordination compounds

Two new copper bromide based coordination compounds 0D-Cu2Br2(3,5-dimethyl-pyridine)4 (1) and 1D-Cu2Br2(5-bromo-pyrimidine)2 (2) have been synthesized and structurally characterized. X-ray diffraction analyses reveal that the inorganic module of both compounds is Cu2Br2 rhomboid dimer coordinated by the organic ligands. Compound 1 is a zero-dimensional (0D) molecular complex while compound 2 is one-dimensional (1D) extended structure. Photoluminescence measurement results show that 1 emits green photoluminescence peaked at 520 nm, with an IQY of 82.4%. Compound 2 emits red photoluminescence peaked at 630 nm, with an IQY of 2.1%. Both compounds exhibit potential as rare-earth metal free lighting phosphor alternatives.

Library of UV-Visible Absorption Spectra of Rare Earth Orthophosphates, LnPO4 (Ln = La-Lu, except Pm)

In recent times, rare earth orthophosphates ( L n PO 4 ) have shown great potential as efficient optical materials. They possess either m o n a z i t e or x e n o t i m e –type structures. These light or heavy rare earth bearing orthophosphates also exhibit an extraordinary stability over geological time scale in nature, ∼10 9 years. In the present contribution, we measure, collect, and present a library of absorption spectra of all the L n PO 4 hosts ( L n = La–Lu, except Pm) using their single crystal samples, to conclude that the observed spectral features for wavelengths longer than 200 nm were attributable to either Ln- or defect related centers, which corroborate the fact that they have a bandgap higher than 8.0 eV. The absorption band around wavelength, 275 nm, corresponds to defect absorption related to PO 3 centers and/or oxygen vacancies. The hosts can potentially be used to study and interpret unperturbed rare earth emissions due to absence of host related absorption above 300 nm. The information presented herein is expected to serve as a library of absorption spectra for geologists, physicists, material scientists, and chemists working in the field of rare earths.

Structural study of rare earth doped fluoride phosphate glasses by solid-state nuclear magnetic resonance

A series of glasses with the compositions of 10NaF-(90-x) NaPO3-xYF3 (x = 5, 10, 15 and 20 mol%) were prepared via the conventional melt-quenching method. The structures of these glasses were characterized by multiple advanced solid-state nuclear magnetic resonance (SSNMR) techniques. Multiple phosphorus species QmnY were resolved at atomic level. Here, the value m and n denote the number of P–O–P bonds and the number of Y3+ ion bounding to phosphorus oxygen tetrahedron, respectively, which were identified by two dimensional (2D) J-resolved spectroscopy. 19F MAS spectra, 19F↔23Na and 19F↔31P rotational echo double resonance (REDOR) prove that F−ions are involved into the glass networks through Y-F•••Na and P-F•••Na linkages. The 31P {19F} J-coupling based heteronuclear multiple quantum coherence (J-HMQC) spectrum proves that F− ions selectively connect with Q1 species. F− ions show significant preference to connecting with Y3+ over P5+.

B-site Y3+ assisted charge compensation strategy to synthesize Eu3+ doped Ruddlesden-Popper Ca2SnO4 perovskite and photoluminescence properties

The rare-earth doped phosphors are widely used in luminous applications. As necessary activators, the involved rare-earth ions are quite low in amount. This may lead to the unsuccessful incorporation or existence of secondary impurity, which, in turn, causes the incorrect interpretations in photoluminescence (PL) origins and mechanisms, e.g. in Eu3+doped Ruddlesden-Popper Ca2SnO4 perovskite. In this work, we adopted a B-site Y3+ assisted charge compensation strategy to dope A-site rare earth Eu3+ cations into Ca2SnO4 in order to obtain the pure-phase red-emission phosphors. The Ca2-xEuxSn1-yYyO4 (x = y = 0.01–0.05) phosphors were synthesized via the one-step solid-state reaction. The careful and systematic XRD measurements reveal that only Eu3+ doping cannot give pure-phase Ca2-xEuxSnO4, while the combination with B-site Y3+ assistance enables achieving pure-phase Ca2-xEuxSn1-yYyO4, which crystallize in an orthorhombic Pbam structure. The STEM-EDX and XPS characterizations indicate the uniform distribution of dopants in the matrix and B-site Y3+ plays a charge compensation role and assists the A-site Eu3+ doping. The charge transfer state (CTS) presenting the energy transfer from matrix to dopants (O2--Eu3+) in excitation spectra was observed with the center at around 275 nm. This excitation wavelength can give the strongest emission signals, i.e. the overall warm-reddish emission. The comprehensive structure and PL/FT-IR/UV spectroscopy analyses indicate that Eu3+ ions substitute for Ca2+ ions at a low symmetric position, and the electric-dipole transition (5D0→7F2) is dominant, giving rise to the stronger emission at 617 nm. The optimal PL emission is achieved at doping level x = 0.03, beyond which the concentration quenching occurs, with an origin of electric dipole-dipole interaction of Eu3+ responsible.

Comparative Analysis of Novel Calcium Phosphate Based Machinable Bioceramic Composites

Calcium phosphate is widely used as an implant material in the biomedical field, mainly because they are chemically similar to the inorganic components of bones. In many applications, dimensional accuracy is critical, and implants need to be shaped, sized and machined as needed. However, as ceramic materials with strong atomic bonding forces, they have high hardness and brittleness, so the ability to shape the precise dimensions required for many critical surgical applications is limited. Therefore, these ceramic-based materials cannot compete with other metal or polymer-based systems, and are not very popular in such applications with dimensional accuracy. Therefore, novel machinable ceramic composite materials based on calcium phosphate, viz. hydroxyapatite and β-tricalcium phosphate, containing rare earth phosphates have been manufactured, and their phases, microstructure, mechanical, machining and biological activities have been studied.

Data-Driven Photoluminescence Tuning in Eu2+-Doped Phosphors.

Discovery of rare earth phosphors has generally relied on the chemical intuition and time-intensive trial-and-error synthesis; therefore, finding new materials assisted by data-driven computations is urgent. Herein, we utilize a regression model to predict the emission wavelengths of Eu2+-doped phosphors by revealing the relationships between the crystal structure and luminescence property. The emission wavelengths of [Rb(1-x)K(x)]3LuSi2O7:Eu2+ (0 ≤ x ≤ 1) phosphors, as examples for the data-driven photoluminescence tuning, are successfully predicted on the basis of the existing data of only eight systems, also consistent with the experimental results. These phosphors can be excited by blue light and exhibit broad-band red and near-infrared emission ranging from 619 to 737 nm. These findings in Eu2+-doped silicate phosphors indicate that data-driven computations through the regression mode would have bright application in discovering novel phosphors with a target emission wavelengths.

Exploring the potential use of Ca[LiAl3N4]:Eu2+ as phosphor-LED material: Ab-initio calculations

Many materials have emerged recently to replace the non rare-earth red phosphors in aim to achieve high efficiency in converting blue light into white light and replace the old incandescent light bulbs by new light-emitting diodes (LEDs). Nitridolithoaluminate, Ca[LiAl3N4]:Eu2+ has shown to be a narrow band red-emitting phosphor compound with possibility to be used as a high-power phosphor-converted LEDs (pc-LEDs). In this study we were interested in fundamental understanding of the origin of pc-LED activity by a comparative calculation of the optoelectronic properties of Ca[LiAl3N4] and Ca[LiAl3N4]:Eu2+ compounds. We applied the state of art of density functional theory (DFT) to calculate the band gap values. For the parent compound (Ca[LiAl3N4]), we used the modified Becke-Johnson (mBJ) potential with the generalized gradient approximation (GGA). For the doped compound (Ca[LiAl3N4]:Eu2+), we have considered the strong correlation of Eu-f orbitals with GGA + U approach. Throughout the work, we compared the electronic structures and the optical responses for both compounds. Doping with europium leads to a band gap reduction of the parent compound and a strong absorption in higher energy regions (blue to green), which confirm the potential use of doped Nitridolithoaluminate for pc-LEDs.

Condition evaluation and 3R guidelines for reuse of rare-earth phosphor from used fluorescent lamps in Japan

ABSTRACT Fluorescent lamps in common use utilise rare earths as key constituent elements for light emission, and these involve resource risk and are difficult to replace with other components. In Japan, the recycling of fluorescent lamps began because of environmental concerns, but is now also pursued because of resource risk, with the 3 Rs (reduce, reuse and recycle) being equally important in the case of the rare earths used in such lamps. In order to reduce costs and environmental load, it is effective to directly reuse the phosphor in such lamps as part of lamp manufacturing, rather than refining the rare-earth element component in the phosphor. Therefore, the present study conducted a performance evaluation of the deterioration condition of used phosphor from fluorescent lamps. Although the brightness of the phosphor decreased with use, the main reason was considered to be the absorption of light by coloured staining on the surface. This staining showed variation in density, depending on its positional relationship to the emitter, and the distribution of the stain was correlated to the emission intensity of the phosphor. Therefore, the proposed 3 R guidelines for the treatment of used phosphor from such lamps are designed to reflect the degree of staining.

An in-situ electron microscopy study of dual ion-beam irradiated xenotime-type ErPO4

Rare-earth phosphates adopting the xenotime (REPO4; RE = Tb − Lu & Y, Sc) structure are proposed as a potential matrix for the confinement of minor actinides. Minor actinides (e.g., Np, Am, Cm) undergo a radioactive decay process in which high-energy recoil atom (70–100 keV) and energetic alpha particles (4.5–5.8 MeV) are produced. In this study, the impact of these energetic decay products on the structure of xenotime-type ErPO4 has been investigated via high energy dual ion-beam irradiation of ErPO4 ceramics. Au2+ (1.5 MeV) and He+ (160 keV) ions were used to simulate the effects of recoil atom and α-particles, respectively. Multiple experiments were carried out in which the Au2+ and He+ ions with varying ion-fluences (ions/cm2) and ion-flux (ions/cm2/s) were implanted sequentially (Au2+ followed by He+ irradiation) and simultaneously (Au2+ + He+ irradiation) into ErPO4 ceramics. Sequential ion-irradiation experiments have shown that the xenotime structure was amorphized by Au2+ ions at a relatively lower ion-fluence (5 × 1013 ions/cm2) in comparison to the monazite structure. Upon irradiation of the amorphous ErPO4 with He+ ions, recrystallization of the amorphous xenotime due to α-particles was not observed. However, simultaneous ion-irradiation experiments on ErPO4 showed that the amorphization of the xenotime structure was prevented upon deposition of higher amounts of electronic energy (Eelectronic) in the lamella. Likewise monazite samples, the α-healing mechanism was also experimentally demonstrated in synthetic xenotime samples.

Photoluminescence and temperature sensing of lanthanide Eu3+ and transition metal Mn4+ dual-doped antimoniate phosphor through site-beneficial occupation

For a long time, rare-earth ion-doped phosphors have been widely used in temperature sensing because of their excellent light-emitting properties. However, most of the rare earth elements are relatively rare and expensive, so the transition group elements that are economical and easy to obtain have been favored by researchers. This paper presents a new type of phosphor doped with rare earth ion and transition metal for optical temperature measurement. In recent years, Mn4+-doped phosphors have attracted wide attention because of their strong deep red light-emitting properties. La2LiSbO6 provides a good host environment for Mn4+ and Eu3+ due to its unique crystal structure. In this paper, a series of La2LiSbO6 phosphors singly doped with Mn4+ and Eu3+, and co-doped with Eu3+/Mn4+ were synthesized. The crystal phases and optical properties of these materials were characterized and analyzed in detail. We specifically studied the temperature dependence of the fluorescence intensity of the optimized La2LiSbO6: Eu3+, Mn4+ phosphors at 303K–523K. The experimental results prove that the thermal responses of Mn4+ and Eu3+ are different. With increasing temperature, the thermal quenching of the Mn4+ fluorescence intensity is much faster than that of Eu3+, so the temperature characteristics can be explored by the fluorescence intensity ratio (FIR) of Eu3+ to Mn4+. At 523 K, its maximum relative sensitivity and maximum absolute sensitivity can reach 0.891% K−1 and 0.000264 K-1, respectively. Our experimental analysis shows that La2LiSbO6:Eu3+/Mn4+ phosphors have relatively high temperature sensitivity and have potential application prospects in the field of high temperature sensing.

Sugarcane juice clarification by lanthanum phosphate nanofibril coated ceramic ultrafiltration membrane: PPO removal in absence of lime pre-treatment, fouling and cleaning studies

The present work explores the possibility of employing rare earth phosphates deposited alumina substrates for ceramic membrane applications. Multichannel porous alumina supports are coated with Lanthanum phosphate nanofibrils of thickness 6 µm by sol-gel deposition technique. The performance of lanthanum phosphate coated ultrafiltration (UF) membrane for sugarcane juice clarification was studied by measuring polyphenol oxidase (PPO) enzyme, bacteria removal efficiency and permeate flux decline profile. The PPO enzyme activity was observed to be lower by 70% in permeate along with a less bacterial presence. The analysis of sucrose loss with time is also studied for raw sugarcane juice, clarified and limed sugarcane juice. A hybrid cleaning strategy involving an innovative brushing action on the membrane surface for physical cleaning combined with chemical cleaning was implemented in this study. The fouled membrane was analyzed by FESEM, EDX, and FTIR. Shelf-life study of feed and permeate confirmed that permeate was more stable than the feed. The experimental results conclude that raw sugarcane juice clarification using lanthanum phosphate coated UF membrane was viable with the use of innovative physical cleaning method as proposed in this study.

Synergistic adsorption of lanthanum ions and fatty acids for efficient rare-earth phosphate recovery: Surface analysis and ab initio molecular dynamics studies

Rare earth elements (REE) are indispensable in our daily life but they naturally occur in minerals, which are generally difficult to concentrate by the froth flotation method. Considering the significant affinity of lanthanum ions for phosphate species as well as for carboxylate groups, the influence of a lanthanum salt (LaCl3) on the adsorption of carboxylate species on monazite surface [(La,Ce,Nd)PO4] was investigated by bubble/particle adhesion tests, X-ray photoelectron spectroscopy (XPS), and atomistic simulations based on density functional theory (DFT). Both theoretical and experimental results demonstrated that lanthanum ions adsorb on the phosphate groups of monazite surface under the LaClCO3 form with a significant adsorption energy. Also, the hydrophobization of the monazite surface with fatty acids is considerably improved in presence of LaCl3, which is in accordance with DFT calculations: the lanthanum ions pre-adsorbed on monazite surface act as highly attractive adsorption sites for carboxylate collectors. In particular, these bridging cations significantly increase the adsorption energy of carboxylate collectors, from 344 to 420 kJ·mol−1 in absolute value. This synergistic adsorption will allow developing new methods for efficient monazite recovery from complex ores, a promising step for the recovery of critical raw materials.

Selective preparation of neodymium phosphates from iron mixed solution by two-step precipitation

Abstract Neodymium iron boron alloy, Nd2Fe14B, is used in the manufacture of magnets. Neodymium is one of the rare earth elements, so it is valuable and expensive. In order to protect this rare earth resource, it is required to collect, reuse, and recyle rare earth elements. Recently, a new method for recovering neodymium phosphate from mixed solutions has been reported, avoiding the difficulties reported by previous methods. Since rare earth phosphate is the main component of rare earth ore, this new phosphate process was proposed. Neodymium phosphate was precipitated selectively from iron-neodymium mixed aqueous solution by 2 step processes with pH adjustments in this work. With this method, it was possible to separate neodymium and iron without using high temperatures or specialized equipment. The optimum conditions were determined by evaluating precipitation by yield, Fe/Nd ratio, hue, UV–visible reflectance spectra, X-ray diffraction, and infrared spectroscopy. In step I where pH was adjusted using sodium hydroxide, iron hydroxide was precipitated. Then, phosphoric acid was added, and the pH was further adjusted to obtain a neodymium phosphate precipitate as step II. The optimum conditions of this separation method were clarified.

Stability of lanthanum in sulfate and phosphate systems and implications for selective rare earths extraction

The stability of Rare Earth Elements (REE), both sulfates and phosphates, was investigated using monazite ore, a residue from a synthetic rutile manufacturing process and a high-grade lanthanum sulfate sample. It was demonstrated that REE sulfates are converted into phosphates at temperatures within the range of 450–800 °C. This reaction depends on excess phosphorus present in compounds that are less stable than monazite. The presence of high partial pressures of P2O5 in the system will reconvert REE sulfates into insoluble REE polyphosphates – REE(PO3)3. Relatively high partial pressure may be attained by the decomposition of phosphorus compounds present in the sample, such as monocalcium phosphates and phosphoric acid generated by the addition of sulfuric acid. The direct contact with phosphoric acid leads, in turn, to the formation of monazite-like compounds. The gas–solid mechanism is believed to be the most suitable due to the easier access of the gas phase to the REE sulfate particles, thus making it possible to achieve the drastic decrease (conversion into phosphate) in REE extraction observed at 800 °C. This study’s results are expected to contribute to a better understanding of the factors favoring selectivity in REE extraction over iron from monazite ores.

High-efficient fabrication of core-shell-shell structured SiO2@GdPO4:Tb@SiO2 nanoparticles with improved luminescence.

SiO2@GdPO4:Tb@SiO2 nanoparticles with core-shell-shell structure were successfully synthesized by a cheap silane coupling agent grafting method at room temperature. This method not only homogeneously coated rare-earth phosphate nanoparticles on the surface of silica spheres but also saved the use of rare-earth resources. The obtained nanoparticles consisted of SiO2 core with a diameter of approximately 210 nm, GdPO4:Tb intermediate shell with thickness of approximately 7 nm, and SiO2 outer shell with thickness of approximately 20 nm. This unique core-shell-shell structured nanoparticles exhibited strong luminescence properties compared with GdPO4:Tb nanoparticles. The core-shell-shell structured nanoparticles can effectively quench the intrinsic fluorescence of bovine serum albumin through a static quenching mode. The as-synthesized nanoparticles show great potential in biological cell imaging and cancer treatment.