Rare-earth Oxyfluoride Research Articles

Sodium Assists Controlled Synthesis of Cubic Rare-Earth Oxyfluorides Nanocrystals for Information Encryption and Near-Infrared-IIb Bioimaging.

Rare-earth oxyfluoride (REOF) colloidal nanocrystals (NCs) suffer from a low photoluminescence efficiency due to their small size with poor crystallinity and a detrimental surface quenching effect. Herein, we introduce an innovative approach that involves doping sodium ions into REOF NCs to produce monodisperse, size-controllable, well-crystallized, and highly luminescent colloidal REOF core/shell NCs. The Na+ doping allows for successfully synthesizing the cubic REOF NCs with a tunable size from 6 to 30 nm. Further fabrication of the core/shell NCs doped with Na+ results in enhancements up to 1062 (Ho3+), 1140 (Er3+), and 2212 (Tm3+) folds in upconversion luminescence and 17.7 folds (Er3+) in downconversion luminescence compared to that of core/shell NCs without doping Na+ ions. These NCs were subsequently developed into multicolor luminescent inks, demonstrating significant potential application for information security, and used for near-infrared-IIb (NIR-IIb) (1500-1700 nm) in vivo imaging, which exhibits a high-resolution in vivo dynamic imaging capability with a signal-to-noise ratio of 5.28. These results present the way to the controlled synthesis of efficient luminescent cubic LuOF: RE3+/LuOF core/shell NCs, expanding the toolkit of rare-earth doped NCs in diverse applications such as advanced encoding encryption, varied fluorescence imaging, and biomedicine.

Rapid green preparation of Lu7O6F9:RE (RE = Eu3+/Gd3+, Yb3+/Er3+) phosphors using ionic liquids

Rare earth oxyfluoride materials are a fascinating class of host matrixes for various applications. However, rapid green preparation of multifunctional rare earth oxyfluoride phosphors remains a huge challenge. In this work, we report the fast synthesis of Lu7O6F9 using ionic liquids (ILs), which enables the precursors to be obtained at 10 min. The effects of calcination temperature, reaction temperature, reaction time and fluorine source on the phase and morphology of precursors and products are elaborated and the growth mechanism is put forward. Eu3+ alone and Yb3+, Er3+ co-doped samples were prepared to investigate the down-conversion (DC) and up-conversion (UC) properties, respectively. Besides, Gd3+ ions were introduced to the Eu3+ doped products, endowing the phosphors more intense luminescence and paramagnetic properties. The proposed facile synthesis route, excellent luminescence, thermal stability and paramagnetic properties open up the way to obtain rare earth fluoride and oxyfluoride for multifunctional applications.

Phase change, micro-structure and reaction mechanism during high temperature roasting of high grade rare earth concentrate

The deposit of Bayan Obo in Inner Mongolia is the world's largest rare earth element (abbreviated as REE) resource. The exploration of the theory of mineral formation of Bayan Obo is an important foundation for mineralogical research, and is the scientific basis for mining, industrial beneficiation, smelting and extraction, and processing and utilization. With the rapid development of science and technology, the demand for the utilization of rare earth elements is increasing, and the separation process between rare earth elements needs to be developed. The purpose of this paper is to provide high temperature experimental information for the formation and application of rare earth minerals. To this end, the mineral evolution of high-grade rare earth concentrates with increasing temperature and the migration of rare earths at different stages and their reaction mechanisms were studied. According to thermogravimetric analysis and differential scanning calorimetry (TG-DSC), calcination was carried out at different temperature ranges, and the calcined products were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope and energy dispersive spectrometer (SEM-EDS) and other analytical techniques. The results are shown in this process, the rare earth phase is first converted into rare earth oxide and rare earth oxyfluoride. As the temperature increases, Ca5(PO4)3F and a large number of self-shaped spherical Ca-RE-OF and Ca-RE-PO4 particles are formed, and the separation of La and Ce elements is discovered. According to the phase diagram analysis, the production of Ca5(PO4)3F is due to the reaction of monazite and fluorite, and the phases CeF2 and CeF3 are formed during the reaction. When it reaches 1500 °C, barium ferrite is produced and a new substance containing Ba2+ is formed.

A green method of respectively recovering rare earths (Ce, La, Pr, Nd) from rare-earth tailings under super-gravity.

A green method was proposed for respectively recovering rare earths (REEs) under super-gravity from rare-earth tailings which are massively stockpiled in the Bayan Obo tailings dams. Firstly, the REEs (Ce, La, Pr, Nd) were discovered to be precipitated as the rare earth oxyfluoride, rare earth ferrate and britholite phases respectively at various temperature ranges of 1773-1673 K, 1673-1473 K and 1473-1373 K. However, the Re-rich phases were intimately intertwined with each other in the normal-gravity. Consequently, respective recovery of REEs (Ce, La, Pr, Nd) at their corresponding precipitation temperatures was conducted under the super-gravity. 98.38% of (Ce) were firstly enriched into the rare earth oxyfluoride and separated from the tailings as driven by the super-gravity, 97.70% of (La) were enriched into the rare earth ferrate and separated subsequently, and the residual REEs were precipitated further into britholite. Accordingly, high-purity of rare earth oxyfluoride, rare earth ferrate and britholite phases were attained respectively, achieving the green and efficient recovery of REEs (Ce, La, Pr, Nd) from the tailings with no additives, no hazardous wastes and no secondary pollution.

Thermal decomposition and oxidation of bastnaesite concentrate in inert and oxidative atmosphere

To clearly elucidate the oxidative roasting behaviors of the bastnaesite, the thermal decomposition and oxidation of the bastnaesite concentrate in inert and oxidative atmosphere have been investigated in detail. Experimental data indicated that the initial decomposition temperature of the concentrate under N2 atmosphere is 150 °C higher than that under O2 atmosphere, most likely because the oxidation of the cerium induces the decomposition of the concentrate. For the roasted samples under N2 atmosphere at 500 °C and above, the oxidation efficiency of the cerium is 19.8%–26.8% because of the fact that rare-earth fluorocarbonate is first decomposed to form rare-earth oxyfluoride and CO2, and the cerium oxyfluoride is then partially oxidized by the CO2 gas. The rest cerium in these samples can be further oxidized in air at room temperature, with the oxidation efficiency of the cerium gradually increasing to above 80% in 7 d. This can be attributed to the obvious changes in the inner morphology of the roasted samples under N2 atmosphere at high temperatures, which largely induce the diffusion of the air and improves the oxidation activity of CeOF, and further induces the oxidation of CeOF by the air. XRD and XPS techniques were used to further verify the significant differences in the thermal decomposition behaviors of the bastnaesite concentrate under N2 and O2 atmosphere. Moreover, no oxidation of Pr3+ to Pr4+ in the roasted samples under both N2 and O2 atmosphere is observed. This gives an overall understanding of the oxidative roasting of the bastnaesite concentrate without additives.

Phase transition of bastnaesite concentrate in calcification process

The phase transformation in calcification process was investigated by X-ray diffraction (XRD) and differential scanning calorimeter (DSC), and the effect of calcification on the leaching rate of rare earth was analyzed. The results show that bastnaesite transforms into rare earth hydrate at the calcification temperature range of 225–300 °C. However, this transition is verified to be an efficient reaction for the activating bastnaesite when the temperature is higher than 200 °C. The leaching rate of rare earth increases to 89.17 % for activating bastnaesite from 36.27 % for the bastnaesite, and it is the highest with calcification temperature of 250 °C, which is consistent with the result of DSC analysis. The transition of rare earth oxyfluoride into RE(OH)3 is accelerated by the addition of NaOH according to the experiments of different calcification systems. In the calcification process, the diffraction peak corresponding to RE(OH)3 appears and the diffraction peak corresponding to REOF disappears for activated ores. However, for bastnaesite, there is no apparent phase transition at the temperature of 270 °C, except that the weak diffraction peak corresponding to RE(OH)3 appears

Structure and distortion of lead fluoride nanocrystals in rare earth doped oxyfluoride glass ceramics

A series of rare earth (RE) doped oxyfluoride glasses with the composition of (45-x) SiO2-5Al2O3-40PbF2-10CdF2-xRe2O3 (x = 1, 5, 10, 15) (mol%) were prepared by a traditional melt-quenching method. Glass ceramics (GCs) were obtained after thermal treatment and characterized by X-ray diffraction (XRD) to investigate the nanocrystal structure and distortion. Both the dopant type and the doping level play an important role in the distortion of the PbF2-RE lattice. It is found that a cubic Pb3REF9 phase forms in low doping GCs, a tetragonal PbREF5 phase forms in middle doping GCs and cubic PbRE3F11 forms in high doping GCs. Accordingly, the site symmetry of RE(3+) dopants in β-PbF2 nanocrystal undergoes a transition of Oh···D4h···Oh with the increase of doping level. The change in the ligands coordinating the RE(3+) ions was further illustrated by the optical changes in Yb-doped GCs. This paper provides insights on the nanocrystal structure of RE at the atomic level and tries to make a complete description of the nanocrystal structure and distortion in these glass-ceramic materials, which will benefit the optimization of optical properties.

Effect of DyF 3 additions on the coercivity and grain boundary structure in sintered Nd–Fe–B magnets

Coercivity of Nd–Fe–B sintered magnets was obviously enhanced by DyF3 additions with little sacrifice of remanence and squareness factor. An ordered rare-earth oxyfluoride grain boundary phase containing ∼30 at.% F was formed. In addition, the presence of F decreased the thickness of the region of reduced anisotropy of the Nd2Fe14B grain. Dy segregated on the surface of the Nd2Fe14B grains. The individual effects of F and Dy elements on the increased coercivity are discussed based on these results.

Blue and white upconversion emissions of rare-earth ions-doped oxyfluoride phosphors

Abstract A blue emitting upconversion phosphor based on Yb 3+ and Tm 3+ codoped oxyfluoride is reported. Under the excitation of a single 980 nm diode laser with the power density of 5.56 W/cm 2 , the upconversion luminescence brightness can reaches to 13053 mcd/m 2 , which is much stronger than that of commercial Y 2 O 2 S:Yb, Tm phosphors (8194 mcd/m 2 ). The research of upconversion mechanism indicates that cooperating sensitization dominates the emissions at 479 and 645 nm, while the emission at 454 nm results from the combination of cooperating sensitization and phonon-assisted energy transfer from Yb 3+ to Tm 3+ . Also, bright upconversion white light is obtained by tri-doping of Er 3+ , Tm 3+ and Yb 3+ . Because the blue (main emission at 479 nm), green and red emissions are two-photon process, the white light is not sensitive to the pumping power.

Increased coercivity in sintered Nd–Fe–B magnets with NdF 3 additions and the related grain boundary phase

Enhanced coercivity was achieved by adding NdF 3 to both Dy-free and Dy-containing Nd–Fe–B sintered magnets with little decrease in remanence. NdF 3 reacts with the Nd-rich grain boundary phase and a new rare-earth oxyfluoride (ROF) phase with ordered face-centered cubic structure is formed. Dy was detected in the ROF phase in the Dy-containing magnet. This suggests that the increase of coercivity can be related to the ordered grain boundary phase as well as to Dy segregation.

Luminescent Properties of Rare-Earth Oxyfluoride Nanofibers Prepared via Electrospinning

Luminescent rare-earth oxyfluoride nanofibers were first prepared used electrospinning technique in this paper. The formation of rare-earth oxyfluoride nanofibers was carried out by calcining rare-earth trifluoroacetic acid/Poly(vinyl pyrrolidone) [R (CF3CO2)3/PVP (R = Eu, Ho)] nanofibers, which were electrospun from the mixture solution of R (CF3CO2)3 and PVP. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) analyses demonstrated the morphology of the Rare-Earth Oxyfluoride (ROF) nanofibers, thestructure and the cubic crystal in ROF fibers. The characteristic photoluminescence from ROF nanofibers were observed with laser excitation at 290 nm.

Crystal fields in ROF:Tb3+ (R = La, Gd)

Laser excited luminescence and site selective excitation spectra of trivalent terbium (Tb3+) in two rare earth oxyfluoride (ROF) matrices, LaOF and GdOF, were detected at temperatures down to liquid helium. From the analyses of spectra, the crystal field (c.f.) fine structure of the 7FJ (J=0–6) ground multiplet was obtained for the 4f8 electron configuration. The energy level schemes comprising 26 c.f. sublevels out of 33 allowed for the C3v point symmetry of the R3+ site were simulated according to a phenomenological model taking into account only the c.f. effect.The simulation including six c.f. parameters (B02, B04, B34, B06, B36, and B66) reproduced the experimental energy level schemes in a satisfactory manner with rms deviations of 5 and 6cm−1 for the LaOF and GdOF hosts, respectively. The c.f. parameter sets are characterized by B02 values close to zero while the fourth and sixth rank parameters assumed rather high values. The Oh to C3v descending symmetry method revealed a slight trigonal distortion from the cubic fluorite CaF2-type structure. The c.f. parameters vary only slightly as a function of the host matrix. The 7F6 level was not simulated correctly probably because of the electron–phonon interaction.

Fabrication of Nanoparticulate Porous LaOF Films through Film Growth and Thermal Decomposition of Ion-Modified Lanthanum Diacetate Hydroxide

This paper first reports fabrication of macro/nanotextured rare-earth oxyfluoride films. Usage of ion-modified lanthanum diacetate hydroxide (LDAH) as self-templates was successful in producing nanoparticulate lanthanum oxyfluoride (LaOF) films. LDAH template films were deposited on glass substrates through a chemical bath deposition in solutions composed of lanthanum acetate sesquihydrate, methanol, trifluoroacetic acid, and aqueous ammonia. The LDAH films had a unique, nestlike morphology owing to a two-dimensional hexagonal crystal growth. Modification of LDAH with trifluoroacetate ions led to formation of LaOF after pyrolyzing the template films at temperatures of 400-600 degrees C in air. The resultant LaOF films had a nanoparticulate porous microstructure, maintaining the morphology of the original LDAH template films. It was also successful to incorporate Eu3+ ions into LaOF through deposition of the LDAH film in a solution containing europium acetate tetrahydrate. The characteristic photoluminescence from Eu(3+) was observed with an ultraviolet-light excitation at 273 nm, indicating that Eu3+ was homogeneously distributed in LaOF host crystals. Thus the ion-modification of LDAH was also demonstrated to be a useful method for preparing nanostructured rare-earth oxyfluoride materials havingvarious cationic compositions.

Sol-Gel Processing and Luminescent Properties of Rare-Earth Oxyfluoride Materials

Luminescent rare-earth oxyfluoride materials were prepared by a sol-gel method using trifluoroacetic acid as a fluorine source. Crystalline (La, Eu)OF powders and thin films were obtained by heating gels at typically 600–800°C. Transparent SiO2-LaOF glass-ceramic thin films were also prepared by mixing the trifluoroacetate sols and silica sols, spin-coating on silica glass substrates and heating at temperatures 600–900°C. Eu3+ ions doped in the oxyfluoride materials exhibited a strong red 5D0 → 7F2 emission (611 nm) by a charge-transfer (O2−-Eu3+) excitation with ultra-violet radiation (265 nm). It was strongly suggested that the Eu3+ activators were preferentially incorporated into the crystalline LaOF phase in the SiO2-LaOF glass-ceramics.

PERFORMANCE CHARACTERIZATION OF CERIUM OXIDE ABRASIVES FOR CHEMICAL-MECHANICAL POLISHING OF GLASS

Investigation of the nature of chemomechanical polishing (CMP) for glass requires an in-depth knowledge of the influences of abrasive physical properties and slurry chemistry. In the present work, eight cerium oxide abrasives were used to polish fused silica on a polyurethane pad. Results from pre-polished surfaces re-polished in aqueous and non-aqueous suspensions and from corrosion testing of solutions recovered from abrasive suspensions were used to discern chemical, chemomechanical, and mechanical equivalents for each abrasive. Performance indices (i.eabrasive quality values) were generated from the equivalents and related to conventional polishing of lapped surfaces. Optimally performing abrasives were found to require a balance of equivalents (i.eset ratio) as dictated by inherent chemical activities, heat treatments, and milling. From solution chemistry testing, particle/workpiece attraction via surface charge was found to increase the chemomechanical effect. The CMP mechanism is concluded to be a chemical reaction at the particle/workpiece interface promoted primarily by mechanical interactions such as abrasion, with both cerium oxide and rare-earth oxyfluoride constituents promoting the reaction.

Study of phase transition and volume thermal expansion in a rare-earth (RE) oxyfluoride system by high-temperature XRD (RE=La, Nd, Sm, Eu and Gd)

Volume thermal expansion behaviour of a number of rare-earth oxyfluorides (REOF) have been studied using high-temperature X-Ray diffractometry (HT-XRD) in the 298–1075 K range in air. The studies revealed an anomalous expansion for each compound associated with phase transition. The phase transition temperature and the coefficient of volume thermal expansion of five compounds in REOF series determined by this method are reported and compared with our earlier results obtained by dilatometry.

Oxidative dehydrogenation of ethane over BaF 2 promoted SmO 3[sbnd]LaF 3 catalysts

Oxidative dehydrogenation of ethane (ODE) was investigated on Sm 2O 3 LaF 3 and BaF 2 promoted Sm 2O 3 LaF 3 catalysts. Under the reaction conditions: 973 K, reactant mixture (C 2H 6 : O 2: N 2=2 : 1 : 7) flow rate 40 ml/min, 89.7% ethene selectivity with 22.1% ethane (C 2H 6) conversion was obtained on Sm 2O 3 LaF 3 catalyst (molar ratio 1 : 1). The addition of BaF 2 significantly improved the catalytic performance. On 16.7 mol%Ba/F 2Sm 2O 3 LaF 3 (molar ratio 1 : 1), 83.9% ethene selectivity with 42.0% ethane conversion was obtained. XRD measurement revealed that partial cation or anion substitution occurred between Sm 2O 3 and LaF 3 phases (for Sm 2O 3 LaF 3 catalysts) as well as Sm 2O 3 LaF 3 and BaF 2 phases (forBa/F 2Sm 2O 3 LaF 3 catalysts). Structure defects such as anionic vacancies, O − and F-center might be formed by the substitution. Rhombohedral Sm 2/3La 1/3OF was the main component in catalysts of Sm 2O 3 LaF 3 (molar ratio 1 : 1) and 16.7 mol%Ba/F 2Sm 2O 3 LaF 3 (molar ratio 1 : 1). The formation of this kind of binary rare earth oxyfluoride might be one of the factors that are responsible for the high C 2H 4 selectivity. XPS results that the binding energies of La3d 5/2 and Sm3d 5/2 in catalysts of Sm 2O 3 LaF 3 are 0.5-1.5 eV negative shifts compared with the standard binding energies of La3d 5/2 and Sm3d 5/2 in La 2O 3, LaF 3 and Sm 2O 3 show that the electron donating ability of Sm 2O 3 LaF 3 catalysts is higher than pure La 2O 3, LaF 3 and Sm 2O 3. Adding 16.7 mol% BaF 2 to Sm 2O 3 LaF 3 catalysts, although the binding energy of La3d 5/2 varied little compared with that in Sm 2O 3 LaF 3, the difference between the binding energies of La3d 5/2 and La3d 3/2 decreased slightly. The contraction of cubic BaF 2 and the decrease of the difference of the binding energy of La3d 5/2 and La3d 5/2 suggested that the isomorphous substitution between BaF 2 and LnOF (Ln La and/or Sm) lattice took place and more structure defects were formed. These might be the cause of the improvement of ethane conversion.

Oxidative dehydrogenation of ethane over LaF 3[sbnd]CeO 2 catalysts

A new series of very selective rare earth oxyfluoride LaF 3 CeO 2 catalysts was prepared and used for the oxidative dehydrogenation of ethane (ODE). At 953 K and C 2H 6 : O 2 : N 2=2 : 1 : 7, C 2H 4 selectivity of 94.5% with C 2H 6 conversion of 23.4% were achieved over LaF 3 CeO 2 (1 : 1 in mole) catalyst. XRD and BET measurements showed that LaF 3 dissolved in CeO 2 and acted as an isolation material as well as partial anions (F −, O 2−) and/or cations (La 3+, Ce 4+) exchange between LaF 3 and CeO 2 phases might be responsible for the high selectivity.

Study of ODE Reaction over Binary Rare Earth Oxyfluoride Sm2O3-LaF3 Catalysts

Abstract The oxidative dehydrogenation of ethane (ODE) reaction has been investigated over Sm2O3-LaF3 and BaF2 promoted Sm2O3-LaF3 catalysts. Over Sm2O3-LaF3(molar ratio=1:1) and 16.7 mol% BaF2/Sm2O3-LaF3(molar ratio=1:1) high C2H4 selectivities of 89.7% and 83.9% were achieved respectively. XRD and XPS characterizations show the electron donating ability of the catalysts are stronger that corresponding rare earth oxide and fluoride. The addition of BaF2 can increase the number of the structure defects that may enhance the electron donating ability and increase the surface adsorbed oxygen concentration, thus should be favorable to improving the catalytic performance. However, the higher concentration of surface adsorbed oxygen is unfavorable for the ethene formation.

Systematic analysis of the optical spectra of selected RE3+ ions in rare-earth oxyfluoride

The c.f. structure of the2S+1LJ levels of the Pr3+, Nd3+, Sm3+, Eu3+, Tb3+, and Dy3+ ions in hexagonal rare-earth oxyfluoride (REOF) matrices was derived from an analysis of the optical absorption, luminescence, excitation, and inelastic neutron-scattering spectra. The energy-level schemes were simulated with a phenomenological model which included the parametrized electrostatic (the Racah parameters E0–3) and the two- and three-body configuration interactions (Tree's parameters α, β, and γ as well as Judd's parameters Tk) as well as spin-orbit coupling (the coupling constant ζ4f). The effect of the neighboring ions was taken into account by a crystal field of C3v symmetry. All parameter values were determined unambiguously by least-squares calculations by considering simultaneously both the free-ion and c.f. interactions. The standard deviations achieved in some simulations were low, considering the restricted degrees of freedom due to the limited basis set of experimentally observed levels. The B q k parameters were found to evolve smoothly across the RE3+ series. For the lighter RE3+ ions the shifts in the c.f. parameter values correlated well with the model emphasizing the effect of the increasing nuclear charge, whereas beyond Tb3+ additional effects may be needed to describe the evolution of the B q k values. Further work is hence needed for the heavier RE3+ ions in RE-oxyfluoride hosts in order to assess the importance of, e.g., the two-electron c.f. interactions and the correlation between the c.f. effect and the crystal structure.