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Achieving low thermal conductivity (κ) is an ideal goal for porous ceramics in high-temperature thermal insulation applications. In this study, a novel high-entropy (La 0.2 Ce 0.2 Sm 0.2 Eu 0.2 Gd 0.2)PO 4 (HE (5RE 1/5)PO 4) ceramic possessing the lowest intrinsic thermal conductivity has been screened from 21 kinds of potential high-entropy phosphate with five components. The porous HE (5RE 1/5)PO 4 ceramics with 30-50 wt% solid loading are fabricated by the particle-stabilized foaming method and the corresponding thermophysical properties are systematically investigated. The porous HE (5RE 1/5)PO 4 ceramics with 35 wt% solid loading display a better balance between compressive strength (1.51 MPa) and thermal conductivity (0.086 W m−1 K−1). Furthermore, porous HE (5RE 1/5)PO 4 ceramics possess a thermal insulating capability over 878 °C, indicating excellent thermal insulation performance. These excellent characteristics enable porous HE (5RE 1/5)PO 4 ceramics to be an excellent candidate material for high-temperature thermal insulation applications in the future. [ABSTRACT FROM AUTHOR]

High-entropy rare earth phosphates (HE REPO 4 , RE = Ho, Tm, Yb, Lu, Dy, Er and Y) with xenotime structure have been successfully synthesized and comprehensively characterized to evaluate their feasibility as thermal/environmental barrier coatings (T/EBCs) applied in SiC f /SiC ceramic matrix composites (SiC-CMCs). HE REPO 4 show good chemical compatibility with SiO 2 at 1400 °C and excellent high-temperature stability up to 1750 °C. The thermal expansion coefficient (TEC) of HE (6RE 1/6)PO 4 is 5.9 × 10−6/°C ranging from room temperature to 900 °C, which is close to that of SiC-CMCs. Compared with single-component REPO 4 , the thermal conductivity of HE (6RE 1/6)PO 4 (from 5.1 W·m−1·K−1 at 100 °C to 2.8 W·m−1·K−1 at 1300 °C) is significantly reduced benefiting from larger lattice distortion. These allow HE REPO 4 to be a potential candidate as T/EBCs applied in SiC-CMCs. [ABSTRACT FROM AUTHOR]

Single phase (Lu 0.2 Yb 0.2 Er 0.2 Y 0.2 Gd 0.2)PO 4 was synthesized, and its thermal properties and CMAS resistance were investigated to explore its potential as an environmental barrier coating (EBC) candidate. The high entropy phosphate (Lu 0.2 Yb 0.2 Er 0.2 Y 0.2 Gd 0.2)PO 4 displays a lower thermal conductivity (2.86 W m−1 K−1 at 1250 K) than all the single component xenotime phase rare-earth phosphates. Interaction of (Lu 0.2 Yb 0.2 Er 0.2 Y 0.2 Gd 0.2)PO 4 pellets with CMAS at 1300 °C led to the formation of a dense and uniformed Ca 8 MgRE(PO 4) 7 reaction layer, which halted the CMAS penetration into the bulk pellet. At 1400 and 1500 °C the (Lu 0.2 Yb 0.2 Er 0.2 Y 0.2 Gd 0.2)PO 4 -CMAS corrosion showed CMAS penetrating beyond the reaction layer into the bulk pellet via the grain boundaries, and SiO 2 precipitates remaining at the pellet surface. The effects of duration, temperature, and compositions on the resistance against CMAS corrosion are discussed within the context of optimizing materials design and performance of high entropy rare-earth phosphates as candidates for advanced EBC applications. [ABSTRACT FROM AUTHOR]

This paper reports on the successful preparation of LaPO4-x wt.% LiF (x = 0–5) ceramics using the traditional solid-state reaction method. The crystal structures, sintering behaviors, and dielectric response at microwave and terahertz frequencies were investigated. XRD results indicate that all the diffraction peaks were attributed to LaPO4, and no secondary phase was observed. Rietveld refinement was conducted to analyze the variation of the crystal structure of LaPO4-x wt.% LiF. SEM indicates that the addition of LiF significantly decreased the grain size while increasing the apparent density of the ceramics. When x = 3, the optimum microwave dielectric properties εr = 10.03, Q × f = 81,467 GHz, and τf = −43.79 ppm/°C were achieved in LaPO4-3 wt.% LiF ceramic at 750 °C. The infrared reflectance spectrum and terahertz time-domain spectroscopy were analyzed and compared with the dielectric properties measured at microwave frequency to investigate the inherent dielectric response. The findings indicate that the dielectric constant attributed to ionic displacement polarization and oxygen vacancy is an essential factor affecting dielectric loss. Moreover, it is worth noting that the LaPO4-3 wt.% LiF ceramic demonstrates excellent compatibility with silver powders, suggesting its immense potential as a dielectric material in LTCC applications. [ABSTRACT FROM AUTHOR]

The Na3RE(PO4)2 phosphates (where RE: La, Ce, Nd, Gd or Er) were obtained by precipitation from sols under mild hydrothermal conditions. Characterization of the powders was carried out by XRD, SEM, ICP/OES and DSC/TG methods. The resulted molar ratio of Na:RE:P in the precipitates was equal 3:1:2 and proved the successful synthesis of Na3RE(PO4)2 compounds. The powders consisted of non-agglomerated spindle or flower-like shaped grains with the size up to 5 microns. The most of the obtained powders have a trigonal structure, which is a characteristic feature of high-temperature polymorphs of Na3RE(PO4)2. An exception is the Na3La(PO4)2 compound, which crystallized in an unidentified type of structure. Under heating, the Na3RE(PO4)2 phosphates change their structure. Their polymorphic transformation path, i.e. temperatures of transitions and their structural changes, is dependent on RE3+ radius size. For La, Ce and Nd compounds heated at high temperatures, only orthorhombic polymorphs have been observed, whereas monoclinic or orthorhombic polymorphs of Na3Er(PO4)2 and Na3Gd(PO4)2 phosphates can form in the course of sintering. [ABSTRACT FROM AUTHOR]

This paper reports on the successful preparation of LaPO4-x wt.% LiF (x = 0–5) ceramics using the traditional solid-state reaction method. The crystal structures, sintering behaviors, and dielectric response at microwave and terahertz frequencies were investigated. XRD results indicate that all the diffraction peaks were attributed to LaPO4, and no secondary phase was observed. Rietveld refinement was conducted to analyze the variation of the crystal structure of LaPO4-x wt.% LiF. SEM indicates that the addition of LiF significantly decreased the grain size while increasing the apparent density of the ceramics. When x = 3, the optimum microwave dielectric properties εr = 10.03, Q × f = 81,467 GHz, and τf = −43.79 ppm/°C were achieved in LaPO4-3 wt.% LiF ceramic at 750 °C. The infrared reflectance spectrum and terahertz time-domain spectroscopy were analyzed and compared with the dielectric properties measured at microwave frequency to investigate the inherent dielectric response. The findings indicate that the dielectric constant attributed to ionic displacement polarization and oxygen vacancy is an essential factor affecting dielectric loss. Moreover, it is worth noting that the LaPO4-3 wt.% LiF ceramic demonstrates excellent compatibility with silver powders, suggesting its immense potential as a dielectric material in LTCC applications.

La(PO 3) 3 activated with different concentrations of Pr3+ was synthesized by a solid-state process. The obtained powder was composed of chunks of several microns in size, typical for this type of synthesis. X-ray diffraction confirmed an orthorhombic crystal structure with the C 222 1 space group. The electronic band structures and density of states of La(PO 3) 3 and La(PO 3) 3 :Pr3+ are calculated and presented. The visible and near-infrared emissions from Pr3+ [Xe]4f2 → [Xe]4f2 electronic transitions were detected under 442 nm excitation. The red emission from the 1D 2 state dominates visible spectra while emissions from the 3P 0,1,2 states have smaller intensities. The 1D 2 emission centered around 1020 nm is detected in the near-infrared spectral range. Intensities of 3P 0 emissions increased with an increase in Pr concentration, while 1D 2 emissions decreased in intensity. We found that the main process responsible for the 1D 2 state decay is the Pr(1D 2) + Pr(3H 4) → Pr(1G 4) + Pr(3F 3,4) cross-relaxation between two Pr ions of electric dipole-dipole character. This finding is supported by 1D 2 emission decay measurements, which revealed a reduction of average decay constants from 242 μs for a 0.5 mol% doped sample to 11 μs for a 10 mol% doped sample and an increasingly stronger non-exponential behavior of emission decay patterns with an increase in Pr3+ concentration. [Display omitted] • La(PO 3) 3 :Pr3+ powders were synthesized by a solid-state process. • Visible and near-infrared emissions from Pr3+ were analyzed. • Electronic band structures and density of states of La(PO 3) 3 :Pr3+ were calculated. • Pr3+ (1D 2) + (3H 4) → (1G 4) + (3F 3,4) cross-relaxation process was identified. [ABSTRACT FROM AUTHOR]

Spectroscopic properties of YPO4 nanoparticles doped with Yb3+, Tm3+, and Er3+ ions have been studied in detail. These multiemitting materials are promising not only for photonic or electronic use but especially for anticounterfeiting applications. The nanopowders are synthesized by the facile coprecipitation method with annealing in the air atmosphere at 1000 °C. The structural and morphological studies reveal pure tetragonal nanocrystallites with an average size of 20–30 nm. Most interestingly, under NIR excitation, the samples exhibit intense upconversion (UC) luminescence where the color can be tuned by changing the laser source, switching the excitation wavelengths between 800, 975, 1208, and 1532 nm, double‐wavelength excitation, and by changing laser power density. As a result, a very high color shift, being the result of intensity changes in the emission bands of Er3+ (green and red) and Tm3+ (blue and red) is obtained. The luminescence lifetimes, temporal evolution, and the pump power dependences are also measured to propose the mechanisms responsible for the observed UC emission. [ABSTRACT FROM AUTHOR]

Hexagonal KCaGd(PO4)2:RE3+ (RE=Ce,Tb,Eu,Tm,Sm) were synthesized by coprecipitation method and their vacuum ultraviolet–ultraviolet (VUV-UV) spectroscopic properties were investigated. The bands at about 165 nm in the VUV excitation spectra are attributed to the host lattice absorptions. For Ce3+-doped samples, the bands at 207, 256, 275, and 320 nm are assigned to the 4f-5d transitions of Ce3+ in KCaGd(PO4)2. For Tb3+-doped sample, the bands at 203 and 222 nm are related to the 4f-5d spin-allowed transitions. For Eu3+-doped sample, the O2-–Eu3+ charge-transfer band (CTB) at 229 nm is observed, and the fine emission spectrum of Eu3+ indicates that Eu3+ ions prefer to occupy Gd3+ or Ca2+ sites in the host lattice. For Tm3+- and Sm3+-doped samples, the O2-–Tm3+ and O2-–Sm3+ CTBs are observed to be at 176 and 186 nm, respectively. From the standpoints of the absorption band, color purity, and luminescent intensity, Tb3+-doped KCaGd(PO4)2 is a potential candidate for 172 nm excited green plasma display phosphors. [ABSTRACT FROM AUTHOR]

Silver terbium diphosphate AgTbP2O7 was prepared by solid state reaction at T=500° and characterized by combining SEM-EDS morphological investigation and semi-quantitative chemical analysis via Scanning Electron Microscopy (SEM) equipped with energy dispersive X-Ray spectroscopy (EDS), structure solution via Rietveld refinement of powder X-ray diffraction (PXRD) data, and Fourier transform infrared (FTIR) spectroscopy. The synthesized compound is monoclinic, P21/n space group, with cell parameters: a=11.9108(3) Å, b=6.90265(16) Å, c=6.62030(15) Å, β=90.3702(5)° and V=544.28(2) Å3. The crystal structure consists of a network of zig-zag chains of TbO8 polyhedra extending along the b axis, bridged by P2O7 diphosphate groups and by chains of AgO7 polyhedra running along the c crystallographic direction. MIR (medium infrared) spectrum shows bands typical of P–O modes of diphosphate compounds. Observed band multiplicity is in agreement with the symmetry reduction with respect to orthorhombic rare earth diphosphates with similar stoichiometries reported in the literature. [ABSTRACT FROM AUTHOR]

Silver terbium diphosphate AgTbP2O7 was prepared by solid state reaction at T=500° and characterized by combining SEM-EDS morphological investigation and semi-quantitative chemical analysis via Scanning Electron Microscopy (SEM) equipped with energy dispersive X-Ray spectroscopy (EDS), structure solution via Rietveld refinement of powder X-ray diffraction (PXRD) data, and Fourier transform infrared (FTIR) spectroscopy. The synthesized compound is monoclinic, P21/n space group, with cell parameters: a=11.9108(3) Å, b=6.90265(16) Å, c=6.62030(15) Å, β=90.3702(5)° and V=544.28(2) Å3. The crystal structure consists of a network of zig-zag chains of TbO8 polyhedra extending along the b axis, bridged by P2O7 diphosphate groups and by chains of AgO7 polyhedra running along the c crystallographic direction. MIR (medium infrared) spectrum shows bands typical of P–O modes of diphosphate compounds. Observed band multiplicity is in agreement with the symmetry reduction with respect to orthorhombic rare earth diphosphates with similar stoichiometries reported in the literature. [ABSTRACT FROM AUTHOR]

Geological disposal of nuclear waste relies upon a multiple barrier approach in which canisters containing nuclear wasteforms are proposed to be stored in geological repositories. In the event of a failed container situation, the groundwater present near the repository may penetrate the waste containment system and eventually come in contact with the nuclear wasteform. Assessing the chemical durability of nuclear wasteforms is of utmost importance and in this study, leaching experiments were conducted on rare-earth phosphate materials adopting the monazite- (LaPO 4 ), xenotime- (YbPO 4 ), and rhabdophane- (GdPO 4 .H 2 O) type structures. Monazite and xenotime are abundant rare-earth minerals containing varying amounts of actinides incorporated within their crystal structure and are proposed as a potential host matrix for the immobilization of actinides. Rhabdophane is a hydrous rare-earth mineral that forms on the surface of chemically altered monazite mineral and is proposed to act as a protective barrier by preventing the release of actinides to the biosphere. The as-synthesized materials were exposed to deionized water for a total period of seven months and the concentration of the leached elements in the water solution were determined using inductively coupled plasma – mass spectrometry (ICP-MS). In this study, the normalized leach rates of LaPO 4 , YbPO 4 , and GdPO 4 .H 2 O materials were found to be low and indicate the chemical durability of these materials. Structural characterization of materials before and after leaching was performed using powder X-ray diffraction (XRD) and X-ray absorption near-edge spectroscopy (XANES). Analysis of the powder XRD diffractograms and XANES spectra has shown that the long-range and local structures of monazite-, xenotime-, and rhabdophane-type materials remain unaffected after exposure to water for seven months. [ABSTRACT FROM AUTHOR]

Modifier-mixing provides a useful tool for tailoring the structural dynamics of oxide glasses. Its physical basis, however, remains subject to intense discussion. Here, we extend our previous consideration of Ca-Mg mixing in alkaline earth metaphosphate glasses [J. Non-Cryst. Solids 468, 74–81 (2017)] across the broader range of alkaline earth species. We report on the effects of divalent modifier mixing for pairs selected from the group of Mg, Ca, Sr and Ba. For this, short-range structural features as obtained from nuclear magnetic resonance, Raman and far-infrared spectroscopy are related to various physical properties, including ionic mobility. Depending primarily on the difference in ion size and bond localization among the modifier cation species, we find different extents of the mixed-modifier effect. This agrees with the dynamic structure model in which the mismatch of cation sites governs non-linear mixing effects. [ABSTRACT FROM AUTHOR]

Rare-earth phosphate with the general formula NaLn(PO) (Ln = lanthanoids) have been known as good luminescent materials for a long time, but their crystal structures have not been recognized completely. In this work, compound NaDy(PO) was prepared using molten salt (flux) method and was structurally determined on X-ray single-crystal diffraction for the first time. Interestingly, its structure solution was complicated by non-merohedral twinning, whose twin law can be characterized by a 180° rotation about the (1 0 0) direction of reciprocal lattice. Furthermore, the excitation and emission spectra, decay time and CIE chromaticity index of NaDy(PO) were studied. Under near-UV light excitation (350 nm), the powdered NaDy(PO) sintered at 1000 °C shows the strongest emission at around 484 and 574 nm, which originates from the F→H and F→H transformations of Dy ion, respectively. The decay curve at 350 nm was measured and the decay curve can be well fitted by bi-exponential function, which can be explained by two crystallographic sites of Dy atom in the structure. [ABSTRACT FROM AUTHOR]

Ultrafine particles of pure and gadolinium (Gd)-doped neodymium phosphate nanoparticles with general formulae GdNdPO where x = 0.00, 0.01, 0.03, 0.05 were prepared by wet chemical co-precipitation technique. X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), ultraviolet spectroscopy (UV) and thermogravemetric analysis (TG) were performed to characterize the samples. XRD analysis suggests that the material belongs to the monoclinic crystal system. Scanning electron microscopic (SEM) and transmission electron microscopic (TEM) studies depict that all the as-synthesized samples show spherical morphology with the formation of slight clusters. The energy-dispersive X-ray analysis (EDAX) gives elemental composition present in the grown material. FTIR analysis determines the presence of various atomic bonds within a molecule, and the present study depicts the presence of water molecule along with the phosphate group and a slight shift in wave number with doping. Thermogravemetric analysis (TG) suggests that the structural phase transition takes place above 800 °C. Raman spectra give insight into the order-disorder in the system and show relevant peaks for symmetric and antisymmetric of PO, O-P-O bending mode, rare earth-oxygen vibrations. UV spectroscopy depicts that various absorption bands fall in the ultraviolet range and the absorption edge shifts toward the lower wavelength side (blue shift) with increase in dopant concentration. [ABSTRACT FROM AUTHOR]

Dyrnaesite-(La), Na7.89(Ce0.94Ca0.06)Σ1.00(La1.14Ce0.40Pr0.10Nd0.24Ca0.12)Σ2.00(PO4)6 is orthorhombic, Pnma, a = 18.4662(7), b = 16.0106(5), c = 7.0274(2) Å, V = 2077.7(1) Å3, Z = 4. The crystal structure is related to the group of Na3REE(XO4)2 compounds (with X = P, V, As), based on the aphthitalite/glaserite structural type. Dyrnaesite is distinct in having ordered Na vacancies, and a rare-earth element (REE) site occupied preferentially by Ce4+. This also distinguishes it from closely related vitusite-(Ce) [Na3REE (PO4)2]. The relation of their unit cells is: ad = bv, bd = 3av, cd = 1/2cv. The distinct Ce4+ site in dyrnaesite-(La) has smaller coordination with shorter bond lengths than the other REE site in the same structure or the REE sites in vitusite-(Ce). It is adjacent to the predominately vacant Na site, which in its turn has the largest coordination of all Na sites in the structure. REE sites, or Na sites in a [010] row (similar to [100] in vitusite-(Ce)) assume two types of coordinations with and without mirror symmetry and two different configurations of surrounding PO4 tetrahedra. This summarizes the topological difference to vitusite-(Ce) where the corresponding coordinations are similar in the same row and intermediate in character to the two types in dyrnaesite-(La). [ABSTRACT FROM AUTHOR]

Herein, we present the rare-earth (RE) activated Ca0.5Y1−x(WO4)2:xEu3+ phosphors prepared using a surfactant-assisted hydrothermal route. From the synthesized powder samples of Ca0.5Y1−x(WO4)2:xEu3+, nanosized, thin phosphor films were fabricated based on the pulsed laser deposition (PLD). Structural studies suggest that Ca0.5Y1−x(WO4)2 doped with Eu3+ belongs to the scheelite tetragonal crystal structure. The powder phosphors show signs of bi-pyramid-like micro-architectures. The down-conversion luminescence studies suggest the presence of a predominant transition attributed to the red color in the visible region. The Judd–Ofelt theory has been incorporated to estimate the photophysical parameters and luminescence quantum efficiencies of both Ca0.5Y1−x(WO4)2:xEu3+ powder and thin film phosphors. The color chromaticity coordinates and luminescence decay times were estimated. The effect of alkali compounds on the prepared powder phosphors was investigated. The results indicate that both the powder and thin film phosphors are the best candidates for display and electro/cathodo-luminescence applications. [ABSTRACT FROM AUTHOR]

Calcium-magnesium-alumina-silicate (CMAS) attack has been considered as a significant failure mechanism for thermal barrier coatings (TBCs). As a promising series of TBC candidates, rare-earth phosphates have attracted increasing attention. This work evaluated the resistance characteristics of LnPO 4 (Ln = Nd, Sm, Gd) compounds to CMAS attack at 1250 °C. Due to the chemical reaction between molten CMAS and LnPO 4 , a dense, crack-free reaction layer, mainly composed of Ca 3 Ln 7 (PO 4 )(SiO 4 ) 5 O 2 apatite, CaAl 2 Si 2 O 8 and MgAl 2 O 4 , was formed on the surface of compounds, which had positive effect on suppressing CMAS infiltration. The depth of CMAS penetration in LnPO 4 (Ln = Nd, Sm, Gd) decreased in the sequence of NdPO 4 , SmPO 4 and GdPO 4 . GdPO 4 had the best resistance characteristics to CMAS attack among the three compounds. The related mechanism was discussed based on the formation ability of apatite phase caused by the reaction between molten CMAS and LnPO 4 . [ABSTRACT FROM AUTHOR]

Rare earth phosphate nanomaterials doped with lanthanide ions (Yb 3+ , Ho 3+ , Er 3+ , Tm 3+ , Tb 3+ and Eu 3+ ) showing upconversion (UC) luminescence may play an important role in photonics, optics, electronics and in biomedical diagnostics. Year by year, the significance of rare earth compounds increases. Here, we present structural, morphological and spectroscopic investigations on REPO 4 nanoparticles, which were successfully synthesized by the co-precipitation method followed by annealing at 1000°C. Obtained tetragonal (YPO 4 , LuPO 4 ) and monoclinic (LaPO 4 , GdPO 4 ) nanocrystals were single phase and homogeneous, with dimensions not exceeding 75 nm. These samples exhibited strong luminescence when excited in NIR range (975–978 nm) and the UC mechanisms were proposed. After initial photon absorption by Yb 3+ ions, the energy transfer (ET) to emitting ions (Ho 3+ , Er 3+ , Tm 3+ , Tb 3+ , Eu 3+ ) occurred. Luminescence rise and decay times were short with the exception for Tb 3+ and Eu 3+ ions, which showed up to 5 ms long luminescence. Dependencies of integral luminescence intensity on laser energy indicated that, most of the transitions were two-photon what is typical value for this kind of ions. However, some of transitions were three- and even four-photon (especially in Tm 3+ ions) and some of slope values were lower than two, what was caused by the saturation and local thermal effects. [ABSTRACT FROM AUTHOR]

We investigated the crystal structure, defect structure, and thermal stability of the rare-earth phosphate proton conductors (La,M)PO4 (where M = Sr, Ca) and obtained the thermal expansion coefficients, surface topography, size distribution, and proton conductivity. The study employed neutron powder diffraction (NPD) at elevated temperatures up to 800 °C, combined with powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS). Although the proton-oxygen site is located on the corners of the PO4 tetrahedra, the NPD shows an average bond length distortion in the hydrated 4.2% Sr/Ca-doped LaPO4. We investigated the proton dynamics by EIS and previously by Quasi-Elastic Neutron Scattering (QENS), and determined the bulk diffusion and the self-diffusion coefficients. Our results showed that QENS and EIS probe fundamentally different proton diffusion processes, where the EIS data reflect long-range intertetrahedral diffusion, whereas QENS probes more local diffusive motions. [ABSTRACT FROM AUTHOR]

Since their discovery in the late 19th century, cerium (IV) phosphates have been a very poorly studied class of rare earth compounds. In this paper, a facile method of amorphous cerous phosphate gels preparation is proposed, starting from nanocrystalline ceria and allowing for large-scale synthesis of these compounds. The use of this method provides deeper insight into the chemical composition and structure of cerous phosphate gels, and enables the peculiarities of their crystallization under hydrothermal conditions in H 2 O and H 3 PO 4 aqueous solutions to be traced. Only Ce + 3 compounds crystallize in a neutral medium if they are present in the composition of the cerous phosphate gels. For Ce + 4 compounds to crystallize, a significant amount of orthophosphoric acid should exist in a reaction mixture. These features are likely to be related to the fact that crystalline Ce + 4 orthophosphates can only exist in the form of acid salts. [ABSTRACT FROM AUTHOR]

Phosphate rocks such as fluorapatite often contain significant amounts of rare earth minerals and considered as rare earth ores. They can be processed to produce phosphoric acid as well as rare earth metals. The mineralization, however, is commonly associated with other rare earth minerals such as monazite ((Ce,La,Th,Nd,Y)PO 4 ), florencite ((La,Ce)Al 3 (PO 4 ) 2 (OH) 6 ), xenotime (YPO 4 ) and cheralite ((Ca,Ce)(Th,Ce)(PO 4 ) 2 ). The treatment of fluorapatite for rare earth extraction commonly requires a pre-leach stage with a mineral acid. Calcium, sodium, magnesium, aluminium, potassium, iron, manganese and a range of other metals including uranium and thorium may enter the solution depending upon the oxide/phosphate/silicate mineralogy. Further processing may involve partial neutralisation to precipitate any rare earth metals which may have solubilised during pre-leach, acid bake of the residue or precipitate with sulphuric acid, water leach followed by purification and precipitation. This paper describes results from a comparative study conducted on pre-leaching a phosphate rare earth concentrate using perchloric, hydrochloric, nitric and phosphoric acids under various leach conditions including different acid concentrations, temperatures and solid/liquid ratios. Through equilibrium constants and kinetic data including measured leachability of relevant metal ions, the study suggests an alternative process route which involves a selective phosphoric acid pre-leach causing low deportment of rare earth elements, uranium and thorium leading to a potentially more efficient downstream process. [ABSTRACT FROM AUTHOR]

In this article we report a comparison of structural and up-conversion (UC) luminescence properties of three compounds used as hosts for Yb 3+ and Er 3+ ions: YPO 4 , LaPO 4 and GdPO 4 phosphates. A co-precipitation method connected with annealing at 900 °C was used in order to synthesize nanocrystalline REPO 4 :Yb 3+ ,Er 3+ materials (RE=Y, La, Gd). The X-ray diffraction patterns collected for the prepared materials indicated their high crystallinity and phase purity for the wide range of used dopants concentrations (0.25–7.5% of Er 3+ and 5–30% of Yb 3+ ions). Transmission electron microscopy images showed small nanocrystal sizes: around 20 nm for YPO 4 -based products, 30–50 nm for LaPO 4 or 40–150 nm for GdPO 4 -based materials. The nanomaterials prepared showed yellow UC under 980 nm laser irradiation, which was assigned to Er 3+ electronic transitions. The optimal dopants concentration were determined, giving the highest UC intensity: 25% of Yb 3+ ions (or 20% in the case of LaPO 4 ) and 1% of Er 3+ ions. Also the mechanism of observed UC was analyzed. Two- and three-photon excitation and energy transfer between Yb 3+ and Er 3+ were confirmed. The effect of crystal cell volume on the green to red ratio of Er 3+ UC and the presence of Er 3+ –Er 3+ energy transfer were found. [ABSTRACT FROM AUTHOR]

The pyrochemical process for the recycling of used nuclear fuels generates LiCl and LiCl–KCl waste salt during a series of processes. In particular, for the reduction of LiCl–KCl waste volume, radioactive rare earth nuclides in the LiCl–KCl waste salt are separated by a phosphorylation precipitation reaction, generating rare earth phosphate waste in a powder form. In this study, for the immobilization of rare earth phosphate wastes, rare earth calcium phosphate (RCP) glass waste forms have been fabricated using a CaO–P 2 O 5 glass system under a moderate vitrification temperature of below 1200 °C. Vitrification properties of NdPO 4 , which is a representative rare earth phosphate material have been investigated through XRD, SEM, EDX, DSC, IR, and density measurements. The RCP glass waste form fabricated from the 4CaO–6P 2 O 5 system showed an immobilization capacity of about 32 wt.% NdPO 4 waste loading. Also, chemical durabilities of RCP glass waste forms are investigated by PCT-A test. [ABSTRACT FROM AUTHOR]

The selective leaching of thorium and uranium from monazite with ammonium oxalate was studied at near ambient temperatures, as a function of pH, particle size, phosphate and oxalate concentration, and temperature. The thermodynamic speciation showed that the presence of phosphate seriously restricted the solubility of both metals, while the solubility of thorium was also lowered by the formation of solid thorium oxalate in the moderately to strongly acidic pH region (pH < 3). Experimental evidence indicated that once the optimum pH was determined, the extent and velocity of both thorium and uranium extractions were modestly affected by the temperature, but greatly by the particle size. Regardless of conditions, complete recovery of thorium and uranium could not be achieved, apparently because of passivation of unreacted monazite cores by recrystallized rare earth phosphates or precipitated oxalates. [ABSTRACT FROM AUTHOR]

An aqueous solution of sodium dihydrogen phosphate was mixed with a aqueous solution of lanthanum nitrate and stirred for 24 h, and the pH was adjusted to 11 using ammonia. The obtained phosphates were analyzed using X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetry-differential thermal analysis, and scanning electron microscopy. The lanthanum phosphate gel was obtained with a large amount of water. The fluorescence of the gels was investigated by substituting a part of the lanthanum cations with cerium, terbium, and europium cations. UV-vis reflectance and fluorescence spectra of these substituted materials were obtained and analyzed. Rrare-earth phosphate gels with large amounts of water exhibited bluish purple, green, and red fluorescence when cation ratios of La/Ce = 70/30, La/Ce/Tb = 55/30/15, and La/Eu = 95/5 were used, respectively. [ABSTRACT FROM AUTHOR]

An equation for the calculation of the detection limit c is presented. Relations for the limit of detection c, limit of determination c, and relative error V of analyte determination are derived. The performance characteristics of lead determination in rare earth phosphates on a JXA-8100 microanalyzer are found. Under the typical conditions of the determination of monazite age (15 kV, probe current of 250 nA, count time in the positions of the line and background 400 s each) c using the 2σ-criterion is 0.006 wt % for an ordinary spectrometer and 0.004 wt % for a high-aperture one. In the simultaneous registration of lead by the scheme 'two ordinary plus one high-aperture channel,' c is reduced to 0.003%. For c = 0.006%, the acceptable error of age estimates V of 1-3 relative percent is attained at c = 0.3-0.08%, respectively. As was found, the lowest capacity of microanalyzers for geochronology needs is 0.02-0.03% of PbO. Equations of constraints for the performance characteristics of methods ensure the revelation of the necessary conditions of analysis and compliance with them. [ABSTRACT FROM AUTHOR]

A novel method is employed for the formation of rare earth phosphate solid solution compounds with unique mesoscopic structures. Europium- and lanthanum-doped sodium borate glass microspheres and particles, ranging in sizes from 50 to 300 μm, were reacted in 0.25 M K2 HPO4 solution to form hollow spheres of nanocrystalline rare earth phosphate compounds by dissolution-precipitation reactions. The initially X-ray amorphous precipitated rare earth phosphate materials were heat-treated at 700°C for 2 h to form nanocrystalline compounds. Differential thermal analysis (DTA) experiments yield an average activation energy for crystallization of 394 ± 26 kJ/mol. X-ray diffraction (XRD) data indicate that samples crystallized to the monazite structure (monoclinic P21/n) with unit cell volumes ranging from 306.5 Å3 for LaPO4 to 282.5 Å3 for EuPO4 and with crystallite grain sizes of 56 ± 14 nm. Compositions containing both rare earth elements formed solid solutions with the composition La(1− x) Eu x PO4. Raman spectroscopy indicates that the P-O symmetric stretching vibrations (ν1) change systematically from 963 cm−1 for LaPO4 to 986 cm−1 for EuPO4, consistent with a systematic decrease in average P-O bond length. Photoluminescence measurements show maximum emission intensity for the La0.65 Eu0.35 PO4 composition. [ABSTRACT FROM AUTHOR]

Nanoparticles of YPO:Eu, YPO:Eu:Ce, YPO:Tb, and YPO:Ce:Tb with different amount of Eu or Tb dopant ions were synthesized by a hydrothermal route. Particle structures and morphologies were investigated and it was found that the increase of Eu or Tb ions, and co-doping with Ce ions resulted in a transition of the crystal structure from tetragonal to hexagonal phase. The phase transition and the morphology transformation should be ascribed to the change of the average radius of cation by doping Ce, or to the changing the amount of Eu or Tb. The energy transfer from Ce ions to Eu and Tb ions was tested by studying the photoluminescence lifetime of YPO:Eu, YPO:Eu:Ce, YPO:Tb, and YPO:Ce:Tb nanoparticles, and their energy transfer results in the improvement of the luminescence intensity. A schematic diagram for the electronic transitions of the studied systems is presented. [ABSTRACT FROM AUTHOR]

Abstract: Structural, optical, and electronic properties of a new alkali metal-rare earth polyphosphate, RbHo(PO3)4, have been investigated by means of single-crystal X-ray diffraction, power X-ray diffraction, elemental analysis, and spectral measurement. RbHo(PO3)4 crystallizes in the monoclinic with space group P21/n and Z =4. It is described as a three-dimensional (3D) architecture built up of PO4 double spiral chains and HoO8 polyhedra by corner-sharing. The 11-coordinated rubidium atoms are located in infinite tunnels. Additionally, in order to gain further insight into the relationship between property and structure of RbHo(PO3)4, theoretical calculation based on the density functional theory (DFT) was performed using the total-energy code CASTEP. [Copyright &y& Elsevier]

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A series of isotypic rare earth phosphites (RE = Ce(III), Pr(III), Nd(III), Pu(III), or Am(III)) with the general formulas RE2(HPO3)3(H2O) along with a Pu(IV) phosphite, Pu[(HPO3)2(H2O)2], have been prepared hydrothermally via reactions of RECl3 with phosphorous acid. The structure of RE2(HPO3)3(H2O) features a face-sharing interaction of eight- and nine-coordinate rare earth polyhedra. By use of the crystallographic data from the isotypic series along with data from previously reported isotypic series, the ionic radii for higher coordinate Pu(III) and Am(III) were calculated. The VIIIPu(III) radius was calculated as 1.112 ± 0.004 Å, and the IXPu(III) radius was calculated to be 1.165 ± 0.002 Å. The VIIIAm(III) radius was calculated as 1.108 ± 0.004 Å, and the IXAm(III) radius was calculated as 1.162 ± 0.002 Å. [ABSTRACT FROM AUTHOR]

A detailed survey of REE distribution in the xenotime and florencite paragenetic association from quartz veins of Au-REE mineral occurrences of the Nether-Polar Urals revealed maximum contents of Ce, Nd, Sm, Gd, Dy, and Y. Four of these elements form their own minerals: florencite-( Ce), -( Nd), and -( Sm) and xenotime-( Y) containing up to 25% GdO. The compositions of the minerals are isomorphous mixtures of two different groups: Yb (Y, Ho, Er, Yb, Lu, Nd, and As) and Gd (Gd, Tb, Dy, Eu, and Sm) in xenotime and La (La, Ce, Pr, Eu, and As) and Sm (Nd, Sm, Gd, Sr, Ca, and S) in florencite. Both minerals display strong heterogeneities in the distribution of isomorphous components and two types of zoning, oscillatory and trend. The crystal cores are enriched in HREE. Variations in the composition of isomorphous mixtures are accompanied by a change in the dominant crystal forms of florencite. Redeposited varieties are distinguished by homogeneous composition, the absence of impurities, and weak correlations between elements. The REE fractionation is interpreted in terms of quantum mechanics. [ABSTRACT FROM AUTHOR]

Nano-sized GdPO4:Eu3+ hierarchical hollow spheres were successfully synthesised through chemical conversion using Gd(OH)CO3:Eu3+ nanospheres as templates. The obtained hollow spheres were composed of primary nanowires. Based on the time-dependent experiments, the hollow shells were proposed to be formed through chemical conversion from the precursor spheres. The as-prepared GdPO4:Eu3+ hollow spheres exhibit good photoluminescence performance. Furthermore, the current method can serve as a general way for the synthesis of rare-earth phosphates hollow spheres. [ABSTRACT FROM AUTHOR]

Abstract: Hexagonal and monoclinic phase yttrium/cerium phosphates with terbium dopants have been prepared successfully through co-precipitation and calcination approaches. Their crystal structure properties were studied by X-ray diffraction (XRD), Scanning Electronic Microscope (SEM) and Dynamic Light Scattering (DLS). Fluorescence excitation and emission spectra exhibited improved luminescence properties by increasing the terbium content in the micron-meter crystals. In addition, Y0.3Ce0.7PO4: Tb was embedded in colloidal sols and strong green emission was obtained in the presence of water atoms or hydroxyl groups. [Copyright &y& Elsevier]

Abstract: A simple spectrophotometric method is developed here for the determination of phosphate present in the samples of soil, detergents, water, bone and food based on the formation of phosphomolybdate complex with the added molybdate followed by the reduction of the complex with thiourea in aqueous sulfuric acid medium. The system obeys Beer''s law at 840nm in the phosphate concentration range, 0.5–10.0μg/ml. Molar absorptivity, correlation coefficient and Sandell''s sensitivity values are found to be 1.712mol−1 cm−1, 0.9769 and 0.0555μgcm−2 respectively. For a comparison of the results determined from the developed method, phosphate present in the same set of samples is determined separately following an official method. The results of the developed method are agreeing well with those of the official phosphomolybdate method. [Copyright &y& Elsevier]

Flower-like LaPO4:Ln3(Ln = Ce, Tb) hierarchical architectures composed of polygon prisms originated from the same center have been prepared by direct precipitation in solution-phase under moderate conditions. CTAB and citric acid were chosen as surfactant and chelating reagent, respectively. It is found that the addition of CTAB plays an important role in controlling the flower-like morphology of the products. The factors that influence the morphology and the luminescent properties of the products have been investigated in detail. The evolution of the morphology of the products has been studied and a possible formation mechanism of the flower-like structure was proposed according to the time dependent experimental results and analysis. [ABSTRACT FROM AUTHOR]

Abstract: Two new isostructural rare earth phosphates Na7Mg13 Ln(PO4)12 (Ln=La, Eu) have been synthesized and investigated by X-ray diffraction and optical measurements. They crystallize in the orthorhombic system with the Cmc21 space group (Z=4). The crystal structure exhibits a new type of framework built up from LnO8 (Ln=La, Eu), MO6 (M=0.5Mg+0.5Na) and MgO x (x=5, 6) polyhedra and PO4 tetrahedra linked by common corner, edge or face. It can be described in terms of [Mg4MP4O22]∞ layers stacked along the a direction. These layers are interconnected by [Mg4 LnP4O36]∞ undulating chains spreading along the b direction. This framework delimits 6 distinct cavities occupied by Na+ cations. The results of the optical study of Na7Mg13La1−x Eu x (PO4)12 (x=0, 0.02, 0.1, 1) reveal the presence of two different Eu3+ ion environments whereas the X-ray study predicts the existence of only one Eu site. This difference can be explained by the possible presence of the europium element in the sodium sites with small occupancies which cannot be detected by the X-ray structural determination. [ABSTRACT FROM AUTHOR]

Abstract: We report a TEM microstructural and compositional study of proton-conducting (La,Ca)P3O9 (Al-free) and (La,Ca,Al)P3O9 (Al-containing) glass-ceramics, prepared by the crystallization of quenched glass melts. Bright-field Transmission Electron Microscopy imaging, annular dark-field Scanning Transmission Electron Microscopy, and energy dispersive x-ray compositional analysis were used to characterize the structure and chemistry of the glass and crystalline phases of the samples that developed following the treatments at various temperatures, heating-rate and dwell-time. TEM chemical characterization revealed that the crystallized domains were mainly Ca-doped LaP3O9, while the remaining phase was a Ca-rich phosphate glass. Crystallization improved conductivity first (up to a conductivity of nearly 10−4.5 S/cm at 550°C in air). However, the conductivity dropped when the samples were treated at higher temperature or for longer dwell-times, due to a continued Ca-departure from the La-rich crystalline into glass phase. Possible proton transfer mechanisms and strategy to improve conductivity in this doped rare earth metaphosphate glass-ceramic are discussed. [Copyright &y& Elsevier]

Abstract: Monazite growth and composition has been modeled in the system K2O–Na2O–CaO–MnO–MgO–FeO–Al2O3–SiO2–H2O–Y2O3–Ce2O3–P2O5–F and a pseudosection is presented that is contoured for monazite modal amount and YPO4 composition, garnet amount and YAG composition, and xenotime amount. Monazite amount and composition is strongly controlled by the presence of xenotime and when xenotime is absent, by the growth or consumption of garnet. Monazite grows continuously in the subsolidus region over most reasonable metamorphic P–T paths with YPO4 content generally increasing with increasing temperature, especially where xenotime is present. When melt is present, monazite is consumed with increasing melting and, in the absence of xenotime, decreases in YPO4 content. Two prograde forward models are presented to illustrate the effect of garnet fractional crystallization on the results. When xenotime is present and the modal amount of garnet is low (e.g., at low pressure), fractional crystallization has little effect on phase amount or composition. Where xenotime is absent and garnet is more abundant (e.g., at intermediate to high pressure), garnet fractionation dominates both monazite growth history and composition. In particular, YPO4 content of monazite drops to nearly zero at intermediate grades in a rock in which garnet has sequestered the Y as YAG component. This Y may be released by diffusion at higher temperature and promote monazite growth with a higher YPO4. Therefore, the growth history of monazite as inferred from YPO4 zoning may be quite complex and reflect different reactions and processes, even in a simple bulk composition. [Copyright &y& Elsevier]

Uniform rare earth phosphate (REPO4, RE = La−Tb) nanocrystals were successfully synthesized in a properly designed TBP/[Omim]Cl/H2O (tributylphosphate/1-octyl-3-methyl-imidazolium chloride/water) microemulsion system. The phosphoryl groups anchored the TBP molecules on the surfaces of the nanocrystals, and this made the nanocrystals easily dispersed in some imidazolium-based ILs. LaPO4:Eu3+and CePO4:Tb3+nanocrystals capped with TBP showed bright red and green emission under UV excitation, with enhanced emission intensity and lifetimes compared with the uncapped ones. [Omim]Cl and TBP were found to play the key roles in the nucleation and growth of the nanocrystals. The designed strategy provided a facile way for the fabrication of TBP capped REPO4nanocrystals, which may also be applicable for the synthesis of other inorganic nanocrystals. [ABSTRACT FROM AUTHOR]

Abstract: Al2O3 was added to a 2CaO–La2O3–5P2O5 metaphosphate, to replace 10% of the Ca2+ ions by Al3+, forming a phosphate with the nominal composition 1.8CaO–0.1Al2O3–La2O3–5P2O5. The effect of Al2O3 addition and heat treatment on the microstructure and conductivity of the resulting glass–ceramics was investigated by XRD, SEM, TEM, and AC impedance spectroscopy. Upon transformation from glass to glass–ceramic, conductivities increased significantly. The glasses were isochronally transformed at 700 and at 800 °C for 1 h or 5 h, in air, following heating at 3 or 10 °C/min. With Al2O3 addition, after a heat treatment at 700 °C, 100–300 nm nano-domains of LaP3O9 crystallized from the glass matrix. Annealing at 800 °C produced a further order of magnitude conductivity increase for the Al-free glass, but less so for the Al-containing glass. [Copyright &y& Elsevier]

Abstract: SF6 was selectively hydrolyzed to SO3 over rare earth (RE) phosphates above 800 K. CePO4 was the most active catalyst, followed by GdPO4, YbPO4, DyPO4, ErPO4, SmPO4, PrPO4, TbPO4, NdPO4, and LaPO4. The middle RE phosphates were found to be more active than the light RE phosphates, but the reason for the high activity of CePO4, which belongs to the light RE group, is not clear. The catalytic activity was independent of the specific surface area (SSA), acid amount, and acid concentration of the catalysts. RE phosphates were single-phase, and a broad inversely proportional relation was observed between the crystallite size and SSA, except in the case of CePO4. The combination of highly active AlPO4 and CePO4 creates synergetic effects in the catalytic activity and SO3 selectivity over all ranges of composition. Binary catalysts were a mixture of small crystalline AlPO4 and CePO4. The addition of Ce promoted the crystallization of AlPO4, which was controlled to about 10 nm at 10–50% Ce content. The turnover frequency for SF6 decomposition was proportional to the surface concentration of hydroxyls of binary catalysts. Therefore, synergy effects may come from the number of hydroxyl (OH) pair sites on which a bidentate intermediate of hydrolysis of SF6 may be formed by the moderate crystallization of AlPO4. The addition of Ce, Pr, or Y to AlPO4 brings about a small promotion effect for SF6 decomposition, but the addition of La, Nd > Gd > Yb diminishes the activity. The addition of Gd, Pr, or Nd greatly improved the SO3 selectivity. A linear relationship between catalytic activity and the concentration of surface hydroxyls of the catalysts supports a reaction mechanism in which two F atoms of an SF6 molecule interact with two surface hydroxyls to form a bidentate intermediate. [Copyright &y& Elsevier]

Abstract: The luminescent characteristics of RE (RE3+ =Eu, Tb, Dy, Sm and Tm)-doped K2GdZr(PO4)3 have been investigated. The band in the range of 130–157nm in the VUV excitation spectra of these compounds is attributed to the host lattice or PO4 3− group absorption and the band from 157nm to 215nm with the maximum at 188nm is due to the O–Zr charge transfer transition. For Eu3+-doped sample, the relatively weak band of O2−–Eu3+ charge transfer (CTB) at 222nm is observed and for Tb3+-doped sample, the band at 223nm is related to the 4f–5d spin-allowed transition of Tb3+. For Dy3+- and Sm3+-doped samples, the O2−–Dy3+ and O2−–Sm3+ CTBs have not been observed, probably due to the 2p electrons of oxygen tightly bound to the zirconium ion in the host lattice. In Tm3+-doped sample, the weak O2−–Tm3+ CTB is located at 170nm. It is observed that there is energy transfer between the host and the luminescent activators (e.g. Eu3+, Tb3+ and Sm3+) except for Tm3+. [Copyright &y& Elsevier]

Abstract: Various rare earth orthophosphates, such as monazite and xenotime RePO4 (Re=La, Ce, Nd, Sm, Tb, Dy, Y, Yb) were prepared by a conventional solid-state reaction method. The crystal structure and microstructure of the sintered ceramics were investigated by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM), respectively. The dielectric properties were measured in the microwave region using a network analyzer. It was found that the monazite RePO4 could be sintered near 1400°C, although the xenotime RePO4, which had a smaller Re3+ ionic radius, could be sintered above 1600°C. The permittivity (ɛ r) of the monazite structures was higher than that of the xenotime structures. This difference was explained by the differences in ionic polarizability and bond strength. Both monazite and xenotime rare earth orthophosphates, however, exhibited a high quality factor (Q × f), where Q =1/tan δ and f is the measuring frequency, of greater than 60,000GHz. The temperature coefficient of resonant frequency (τ f ) was a negative value, ranging between −17 and −56ppm/°C. [Copyright &y& Elsevier]