Monazite, rhabdophane, xenotime & churchite: Vibrational spectroscopy of gadolinium phosphate polymorphs

Raman spectroscopic study of anhydrous and hydrous REE phosphates, oxides, and hydroxides

Rare earth elements (REE) include the lanthanides (La-Lu), Y, and Sc which are critical elements for the green energy transition. The REE show a decrease in ionic radii with increased atomic numbers, which results in a so-called lanthanide contraction systematically affecting crystal structures and mineral properties. Here we present a compilation of reference Raman spectra of ten REE sesquioxides (A-, B- and C-type), five REE hydroxides, eight xenotime-structured REE phosphate endmembers and two solid solutions, seven monazite-structured REE phosphate endmembers and two solid solutions and seven rhabdophane endmembers with up to five Ce1-xLREEx rhabdophane solid solutions (LREE = La-Gd). Raman mode assignment is based on a detailed literature review summarizing existing analytical work and theoretical calculations and systematic trends observed in this study by analyzing different REE-bearing solids. The wavenumbers of the main REE-O Raman band systematically increase with decreasing ionic radii forming discrete linear trends within isostructural mineral groups, that can be used to estimate the REE-O mode in other solids with known REE-O coordination numbers. Photoluminescence using 266 nm, 532 nm and 633 nm excitation laser wavelengths for REE-bearing oxides, hydroxides, anhydrous and hydrous phosphates is also presented providing a new framework for identifying REE-phases in phosphate-bearing natural mineral deposits.

A Comparative Study of Cerium(III) and Cerium(IV) Phosphates for Sunscreens

Crystalline cerium(III) phosphate (CePO4), cerium(IV) phosphates, and nanocrystalline ceria are considered to be promising components of sunscreen cosmetics. This paper reports on a study in which, for the first time, a quantitative comparative analysis was performed of the UV-shielding properties of CePO4, Ce(PO4)(HPO4)0.5(H2O)0.5, and CePO4/CeO2 composites. Both the sun protection factor and protection factor against UV-A radiation of the materials were determined. Ce(PO4)(HPO4)0.5(H2O)0.5 was shown to have a sun protection factor of 2.9, which is comparable with that of nanocrystalline ceria and three times higher than the sun protection factor of CePO4. Composites containing both cerium dioxide and CePO4 demonstrated higher sun protection factors (up to 1.8) than individual CePO4. When compared with the TiO2 Aeroxide P25 reference sample, cerium(III) and cerium(IV) phosphates demonstrated negligible photocatalytic activity. A cytotoxicity analysis performed using two mammalian cell lines, hMSc and NCTC L929, showed that CePO4, Ce(PO4)(HPO4)0.5(H2O)0.5, and nanocrystalline ceria were all non-toxic. The results of this comparative study indicate that cerium(IV) phosphate Ce(PO4)(HPO4)0.5(H2O)0.5 is more advantageous for use in sunscreens than either cerium(III) phosphate or CePO4/CeO2 composites, due to its improved UV-shielding properties and low photocatalytic activity.

Incorporation of U(IV) in monazite-cheralite ceramics under oxidizing and inert atmospheres

This work is the first attempt to prepare Nd1-xCaxUxPO4 monazite-cheralite with 0 < x ≤ 0.1 by a wet chemistry method. This method relies on the precipitation under hydrothermal conditions (T = 110 °C for four days) of the Nd1-xCaxUxPO4·nH2O rhabdophane precursor, followed by its thermal conversion for 6 h at 1100 °C in air or Ar atmosphere. The optimized synthesis protocol led to the incorporation of U and Ca in the rhabdophane structure. After heating at 1100 °C for 6 h in air, single-phase monazite-cheralite samples were obtained. However, α-UP2O7 was identified as a secondary minor phase in the samples heated under Ar atmosphere. The U speciation in the samples converted in an oxidising atmosphere was carefully characterized using synchrotron radiation by combining HERFD-XANES and XRD. These results showed the presence of a minor secondary phase containing hexavalent uranium and phosphate with a stoichiometry of U : P = 0.78. This highly labile uranyl phosphate phase incorporated 21 mol% of the uranium initially precipitated with the rhabdophane precursor. This phase was completely removed by a washing protocol. Thus, single-phase monazite-cheralite was obtained through the wet chemistry route described in this work with a maximum U loading of x = 0.08.

Composite Anion Exchange Membranes Based on Quaternary Ammonium-Functionalized Polystyrene and Cerium(IV) Phosphate with Improved Monovalent-Ion Selectivity and Antifouling Properties

The possibility of targeted change of the properties of ion exchange membranes by incorporation of various nanoparticles into the membranes is attracting the attention of many research groups. Here we studied for the first time the influence of cerium phosphate nanoparticles on the physicochemical and transport properties of commercial anion exchange membranes based on quaternary ammonium-functionalized polystyrenes, such as heterogeneous Ralex^® AM and pseudo-homogeneous Neosepta^® AMX. The incorporation of cerium phosphate on one side of the membrane was performed by precipitation from absorbed cerium ammonium nitrate (CAN) anionic complex with ammonium dihydrogen phosphate or phosphoric acid. The structures of the obtained hybrid membranes and separately synthesized cerium phosphate were investigated using FTIR, P31 MAS NMR, EDX mapping, and scanning electron microscopy. The modification increased the membrane selectivity to monovalent ions in the ED desalination of an equimolar mixture of NaCl and Na₂SO₄. The highest selectivities of Ralex^® AM and Neosepta^® AMX-based hybrid membranes were 4.9 and 7.7, respectively. In addition, the modification of Neosepta^® membranes also increased the resistance to a typical anionic surfactant, sodium dodecylbenzenesulfonate.

FTIR spectral signatures of amazon inorganic phosphates: Igneous, weathering, and biogenetic origin

The characterization of phosphates is generally hampered by the variability of their sources, the complexity of the mineralogical assemblies and/or the thermochemical transformations undergone. Fourier transform infrared (FTIR) spectroscopy can characterize and differentiate phosphates in a practical and efficient way. In this sense, in order to differentiate phosphates from different Amazonian deposits and establish a spectral database, initially small because it is starting, six samples of phosphate rocks were analyzed by FTIR spectroscopy in the near-IR and middle-IR regions using the transmittance, attenuated reflectance, and diffuse reflectance methods. X-ray diffraction and X-ray fluorescence spectroscopy were also used as complementary analyses. The IR results revealed that the transmittance and diffuse reflectance methods are the most suitable for the analysis of phosphate materials, and they should be used together whenever possible. The identification of the PO₄ bands, as well as of the (CO₃)^2-, Al₂OH, and NH₄ bands, allowed the differentiation of the phosphate materials according to their geological source and the establishment of a database of the studied materials by both the transmittance and diffuse reflectance methods.

Mineral Physicochemistry Underlying Feature-Based Extraction of Mineral Abundance and Composition from Shortwave, Mid and Thermal Infrared Reflectance Spectra

Reflectance spectroscopy allows cost-effective and rapid mineral characterisation, addressing mineral exploration and mining challenges. Shortwave (SWIR), mid (MIR) and thermal (TIR) infrared reflectance spectra are collected in a wide range of environments and scales, with instrumentation ranging from spaceborne, airborne, field and drill core sensors to IR microscopy. However, interpretation of reflectance spectra is, due to the abundance of potential vibrational modes in mineral assemblages, non-trivial and requires a thorough understanding of the potential factors contributing to the reflectance spectra. In order to close the gap between understanding mineral-diagnostic absorption features and efficient interpretation of reflectance spectra, an up-to-date overview of major vibrational modes of rock-forming minerals in the SWIR, MIR and TIR is provided. A series of scripts are proposed that allow the extraction of the relative intensity or wavelength position of single absorption and other mineral-diagnostic features. Binary discrimination diagrams can assist in rapidly evaluating mineral assemblages, and relative abundance and chemical composition of key vector minerals, in hydrothermal ore deposits. The aim of this contribution is to make geologically relevant information more easily extractable from reflectance spectra, enabling the mineral resources and geoscience communities to realise the full potential of hyperspectral sensing technologies.

Crystallization, Luminescence and Cytocompatibility of Hexagonal Calcium Doped Terbium Phosphate Hydrate Nanoparticles

Luminescent lanthanide-containing biocompatible nanosystems represent promising candidates as nanoplatforms for bioimaging applications. Herein, citrate-functionalized calcium-doped terbium phosphate hydrate nanophosphors of the rhabdophane type were prepared at different synthesis times and different Ca2+/Tb3+ ratios by a bioinspired crystallization method consisting of thermal decomplexing of Ca2+/Tb3+/citrate/phosphate/carbonate solutions. Nanoparticles were characterized by XRD, TEM, SEM, HR-TEM, FTIR, Raman, Thermogravimetry, inductively coupled plasma spectroscopy, thermoanalysis, dynamic light scattering, electrophoretic mobility, and fluorescence spectroscopy. They displayed ill-defined isometric morphologies with sizes ≤50 nm, hydration number n ~ 0.9, tailored Ca2+ content (0.42-8.11 wt%), and long luminescent lifetimes (800-2600 µs). Their relative luminescence intensities in solid state are neither affected by Ca2+, citrate content, nor by maturation time for Ca2+ doping concentration in solution below 0.07 M Ca2+. Only at this doping concentration does the maturation time strongly affect this property, decreasing it. In aqueous suspensions, neither pH nor ionic strength nor temperature affect their luminescence properties. All the nanoparticles displayed high cytocompatibility on two human carcinoma cell lines and cell viability correlated positively with the amount of doping Ca2+. Thus, these nanocrystals represent promising new luminescent nanoprobes for potential biomedical applications and, if coupled with targeting and therapeutic moieties, they could be effective tools for theranostics.

Micro-Raman—A Tool for the Heavy Mineral Analysis of Gold Placer-Type Deposits (Pianu Valley, Romania)

In the current study, different heavy minerals typical of gold placer deposits were identified by means of micro-Raman spectroscopy, and their chemical composition analyzed and discussed (garnet, kyanite, staurolite, zircon, allanite, monazite, xenotime, rutile, anatase, cassiterite, titanite, barite). Even complex solid solution series, such as those of garnets, can be deciphered with the aid of systematic trends observed in Raman line frequencies. The ν1 mode in garnets will shift from high to low frequencies as a function of the ionic radius of the X2+ cation, from Mg2+, to Fe2+ and Mn2+, while the presence of Ca2+ will make the band to be shifted strongly to even lower wavenumbers. This approach has successfully been taken to differentiate between polymorph triplets such as kyanite-sillimanite-andalusite and rutile-anatase-brookite. Minerals under consideration with high contents of REE, U and Th are affected by intensive metamictization, particularly zircon and titanite. Raman peak features, such as shape, symmetry and intensity, respond to this radiation damage of the lattice and enable fine-tuning of these heavy minerals, such as in the case of fluorite (fetid fluorite).

Synthesis, characterization and optical spectroscopy of GdPO4 :Er3

A series of Er³⁺ -doped GdPO₄ phosphors was synthesized using a conventional solid-state reaction. The monazite structure (space group P12₁ /n₁ ) of the obtained materials was confirmed using X-ray diffraction and Fourier transform infrared spectroscopy. Their optical spectra (excitation, emission, absorption, decay curves) were measured at room temperature in the visible and near-infrared (NIR) regions. The UV-visible-NIR optical absorption spectrum of GdPO₄ :7% Er³⁺ was analyzed based on Judd-Ofelt (J-O) theory and the J-O intensity parameter (Ω₂ , Ω₄ , Ω₆ ) was calculated. J-O intensity parameters were used to evaluate spontaneous emission properties such as branching ratios, transition probabilities, and radiative lifetime. The calculated quantum efficiency of the 1.5 μm emission (⁴ I_13/2 -⁴ I_15/2 ) was calculated to be 89%. This result proved that GdPO₄ :Er³⁺ is suitable for use in optical amplifiers and is a potential host for laser applications. The most interesting transitions, located at about 540 nm, and 1.0 and 1.5 μm were investigated as a function of doping level and of temperature, to assess the conditions needed for the highest emission performance and to explore the range of application, in particular in the fields of lighting, thermal sensing, and of phosphors for bio-imaging.

The Petyayan-Vara Carbonatite-Hosted Rare Earth Deposit (Vuoriyarvi, NW Russia): Mineralogy and Geochemistry

The Vuoriyarvi Devonian carbonatite–ijolite–pyroxenite–olivinite complex comprises several carbonatite fields: Neske Vara, Tukhta-Vara, and Petyayan-Vara. The most common carbonatites in the Tukhta-Vara and Neske-Vara fields are calciocarbonatites, which host several P, Fe, Nb, and Ta deposits. This paper focuses on the Petyayan-Vara field, in which the primary magmatic carbonatites are magnesian. The least altered magnesiocarbonatites are composed of dolomite with burbankite and are rich in REE (up to 2.0 wt. %), Sr (up to 1.2 wt. %), and Ba (up to 0.8 wt. %). These carbonatites underwent several stages of metasomatism. Each metasomatic event produced a new rock type with specific mineralization. The introduction of K, Si, Al, Fe, Ti, and Nb by a F-rich fluid (or fluid-saturated melt) resulted in the formation of high-Ti magnesiocarbonatites and silicocarbonatites, composed of dolomite, microcline, Ti-rich phlogopite, and Fe–Ti oxides. Alteration by a phosphate–fluoride fluid caused the crystallization of apatite in the carbonatites. A sulfate-rich Ba–Sr–rare-earth elements (REE) fluid (probably brine-melt) promoted the massive precipitation of ancylite and baryte and, to a lesser extent, strontianite, bastnäsite, and synchysite. Varieties of carbonatite that contain the highest concentrations of REE are ancylite-dominant. The influence of sulfate-rich Ba-Sr-REE fluid on the apatite-bearing rocks resulted in the dissolution and reprecipitation of apatite in situ. The newly formed apatite generation is rich in HREE, Sr, and S. During late-stage transformations, breccias of magnesiocarbonatites with quartz-bastnäsite matrixes were formed. Simultaneously, strontianite, quartz, calcite, monazite, HREE-rich thorite, and Fe-hydroxides were deposited. Breccias with quartz-bastnäsite matrix are poorer in REE (up to 4.5 wt. % total REE) than the ancylite-dominant rocks (up to 11 wt. % total REE).

Surface features on aged 238Pu-doped Eu-monazite

Several 238Pu-doped Eu monazite single crystals stored at ambient conditions are monitored for 15 years using Scanning and Transmission electron microscopy, spectroscopy, diffraction and optical microscopy. Despite preservation of high crystalline quality, mechanical cracking and formation of small flakes is observed. After several month of aging, a new phase appeared on surfaces of the crystals, which later formed a continuous shell of most crystallographic faces. Electron diffraction indicated that the shell consists of submicron Pu-containing rhabdophanes. Its formation likely occurs due to combined action of atmospheric moisture and recrystallisation of radiation damage in monazite domains adjacent to external and internal surfaces. Extent of the rhabdophane formation appears to be influenced by crystallography and Pu content of corresponding growth sectors of the parent monazite. Whereas macroscopic rhabdophanes and monazites are relatively stable against irradiation, formation of sub-microscopic particles is a point of concern for development of monazite-based ceramic forms for actinide immobilization.

Pressure-Induced Hexagonal to Monoclinic Phase Transition of Partially Hydrated CePO4

We present a study of the pressure dependence of the structure of partially hydrated hexagonal CePO₄ up to 21 GPa using synchrotron powder X-ray diffraction. At a pressure of 10 GPa, a second-order structural phase transition is observed, associated with a novel polymorph. The previously unknown high-pressure phase has a monoclinic structure with a similar atomic arrangement as the low-pressure phase, but with reduced symmetry, belonging to space group C2. Group-subgroup relations hold for the space symmetry groups of both structures. There is no detectable volume discontinuity at the phase transition. Here we provide structural information on the new phase and determine the axial compressibility and bulk modulus for both phases. They are found to have an anisotropic behavior and to be much more compressible than the denser monazite-like polymorph of CePO₄. In addition, the isothermal compressibility tensor for the high-pressure structure is reported at 10 GPa and the direction of maximum compressibility described. Finally, the possible role of water and the pressure medium in the high-pressure behavior is discussed. The results are compared with those from other rare-earth orthophosphates.

Effect of Ca2+ ion co-doping on radiative properties via tuning the local symmetry around the Eu3+ ions in orange red light emitting GdPO4:Eu3+ phosphors

Photo-luminescent properties of Ca²⁺ substituted GdPO₄:Eu³⁺ novel phosphors.