Raman and infrared spectroscopy of monazite-type ceramics used for nuclear waste conditioning

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.

Monazite-Type SmPO4 as Potential Nuclear Waste Form: Insights into Radiation Effects from Ion-Beam Irradiation and Atomistic Simulations

Single-phase monazite-type ceramics are considered as potential host matrices for the conditioning of separated plutonium and minor actinides. Sm-orthophosphates were synthesised and their behaviour under irradiation was investigated with respect to their long-term performance in the repository environment. Sintered SmPO₄ pellets and thin lamellae were irradiated with 1, 3.5, and 7 MeV Au ions, up to fluences of 5.1 × 10¹⁴ ions cm^-2 to simulate ballistic effects of recoiling nuclei resulting from α-decay of incorporated actinides. Threshold displacement energies for monazite-type SmPO₄ subsequently used in SRIM/TRIM simulations were derived from atomistic simulations. Raman spectra obtained from irradiated lamellae revealed vast amorphisation at the highest fluence used, although local annealing effects were observed. The broadened, but still discernible, band of the symmetrical stretching vibration in SmPO₄ and the negligible increase in P-O bond lengths suggest that amorphisation of monazite is mainly due to a breaking of Ln-O bonds. PO₄ groups show structural disorder in the local environment but seem to behave as tight units. Annealing effects observed during the irradiation experiment and the distinctively lower dose rates incurred in actinide bearing waste forms and potential α-radiation-induced annealing effects indicate that SmPO₄-based waste forms have a high potential for withstanding amorphisation.

Pressure driven structural phase transition in EuTaO4: experimental and first principles investigations

In this article we report the synthesis, characterization and high pressure (HP) investigation on technologically important, rare earth orthotantalate, EuTaO₄. Single phase polycrystalline sample of EuTaO₄has been synthesized by solid state reaction method adopting monoclinic M'-type fergusonite phase with space groupP2/c. Structural and vibrational properties of as synthesized compound are investigated using synchrotron based x-ray powder diffraction, and Raman spectroscopic techniques respectively. Both the techniques show presence of an isostructural, first order, reversible phase transition near 17 GPa. Bulk modulus obtained by fitting the experimental pressure volume data for low pressure and HP phase is 136.0(3) GPa and 162.8(21) GPa. HP phase is accompanied by an increase in coordination number around Ta atom from 6 to 8. First principles calculations under the frame work of density functional theory also predicts the isostructural phase transition and change in coordination around Ta atom, corroborating the experimental findings.

Phase Transformation, Photocatalytic and Photoluminescent Properties of BiPO4 Catalysts Prepared by Solid-State Reaction: Degradation of Rhodamine B

Polycrystalline bismuth phosphate BiPO4 was synthesized by solid-state reaction at different temperatures varying from 500 to 900 °C. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDS) and Raman spectroscopy. The low-temperature phase of BiPO4 has monoclinic structure with a space group P21/n, and was transformed into the monoclinic phase P21/m with a slight distortion of monoclinic lattice when it was heated above 500 °C. The effect of the transformation on the structure, morphology and photocatalytic properties was examined. The photocatalytic activity of each sample, in presence of Rhodamine B (RhB) in aqueous solution, was carried out and analyzed under UV light irradiation. Photoexperiments showed that the material prepared at 500 °C is the best catalyst with degradation efficiency of the order of 96% after 12 min of reaction time under UV light irradiation. This high photocatalytic efficiency could be due to their structural and morphological changes. The photocatalytic degradation mechanism of RhB in the presence of the best photocatalyst BiP-500 °C is proposed. The stability of the catalyst was also examined by carrying out four successive tests of the degradation in the presence of BiP-500 °C. Total organic carbon (TOC) was used to further estimate the rate of mineralization in the presence of BiP-500 °C (83% TOC removal). Photoluminescence experiments performed under UV-laser light irradiation revealed emissions in the green-orange range, with optimal intensities for the mix systems observed at 550 °C.

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).

Influence of Light and Temperature on the Extractability of Cerium(IV) as a Surrogate of Plutonium(IV) and its Effect on the Simulation of an Accidental Fire in the PUREX Process

Modeling of plutonium(IV) behavior during an accidental fire in a reprocessing plant was considered using various non-radioactive metallic surrogates. Among those elements, cerium(IV) was supposed to be a suitable candidate due to possible formation of a complex with TPB, but its extractability and stability have not been studied previously under representative plutonium uranium reduction extraction (PUREX) conditions. In this work, we investigated the chemical analogy between cerium(IV) and plutonium(IV) in this extractive process and combustion thereof. Distribution ratios are reported for acidities of 1-4 mol L^-1 in equal volumes of nitric acid and a 30:70 mixture of tributylphosphate and hydrogenated tetrapropylene. The influences of light, temperature, and extraction time were studied by UV-vis spectroscopy. The results showed that cerium(IV) is extracted quantitatively but is reduced over time to cerium(III) in the organic mixture. Spectrophotometric investigations of this reaction kinetics revealed an apparent rate constant k of 0.021 ± 0.002 mol^0.5 L^0.5 min^-1 at 298 K and an apparent fractional reaction order of 0.5. The activation energy of this reduction was found to be around 82 ± 2 kJ mol^-1 by the Arrhenius plot method. The combustion of mono- and biphasic solutions prepared with a cerium(IV) concentration of 10 g L^-1 revealed that the extracted complexes, Ce₂O·6NO₃·3TBP_(org) or Ce₄O₄·8NO₃·6TBP_(org), are reduced during the combustion. Compositions of the resulting ashes and soot were analyzed and highlighted the presence of pyrophosphates and polycyclic aromatic hydrocarbons, with some traces of cerium. Ce(IV) is not suitable to represent Pu(IV) from a chemical point of view in HNO₃/TBP-HTP solutions.

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

Rare-earth phosphates with the general formula REEPO₄·nH₂O belong to four distinct structural types: monazite, rhabdophane, churchite, and xenotime. We report herein the first direct comparison between vibrational spectra of these compounds for the same metal cation i.e. gadolinium. The four GdPO₄·nH₂O samples were prepared through wet chemistry methods and first characterized by X-ray diffraction. Three distinct spectral domains, associated to the deformation and stretching modes of phosphate tetrahedra (PO₄) and to water molecules vibrations were then analyzed from FTIR and Raman data, and discussed regarding the structural characteristics of each sample. The most obvious differences between the spectra were associated to δ(H₂O) and δ_s(PO₄) modes and led to propose a simple method to rapidly and unambiguously discriminate the four polymorphs.

Crystal Chemistry and Stability of Hydrated Rare-Earth Phosphates Formed at Room Temperature

In order to understand the crystal chemical properties of hydrous rare-earth (RE) phosphates, REPO4,hyd, that form at ambient temperature, we have synthesized REPO4,hyd through the interaction of aqueous RE elements (REEs) with aqueous P at room temperature at pH < 6, where the precipitation of RE hydroxides does not occur, and performed rigorous solid characterization. The second experiment was designed identically except for using hydroxyapatite (HAP) crystals as the P source at pH constrained by the dissolved P. Hydrated RE phosphate that precipitated at pH 3 after 3 days was classified into three groups: LREPO4,hyd (La → Gd) containing each REE from La-Gd, MREPO4,hyd (Tb → Ho), and HREPO4,hyd (Er → Lu). The latter two groups included increasing fractions of an amorphous component with increasing ionic radius, which was associated with non-coordinated water. REallPO4,hyd that contains all lanthanides except Pm transformed to rhabdophane structure over 30 days of aging. In the experiments using HAP, light REEs were preferentially distributed into nano-crystals, which can potentially constrain initial RE distributions in aqueous phase. Consequently, the mineralogical properties of hydrous RE phosphates forming at ambient temperature depend on the aging, the pH of the solution, and the average ionic radii of REE, similarly to the well-crystalline RE phosphates.