Rare Earth Niobium Research Articles

Ultrafast synthesis and luminescence properties of rare earth orthoniobates RENbO4 (RE = La, Eu, Gd, Yb, Lu)

Rare earth orthoniobates (RENbO4) are one kind of important functional materials due to its applications in solid-state phosphors, thermal barrier coatings, and microwave dielectric ceramics. The synthesis of rare earth niobates often needs high reaction temperatures (1300 °C–1700 °C) and long processing times (from hours to tens of hours) in solid-state reactions, which can increase the study time of the relationship between structure and properties. In this work, we used ultrafast high-temperature sintering method to synthesize RENbO4 (RE = La, Eu, Gd, Yb, Lu), and found specific structure and properties in these materials obtained with specific synthetic techniques. Based on the electronegativity scale, the charge transfer energy of lanthanide ions in the YNbO4 crystal was calculated. The rapid synthesis of RENbO4 in a vacuum atmosphere generated more oxygen vacancies, and the structures of [REO8] and [NbO6] were distorted. The shortening of the fluorescence lifetime of LaNbO4 and EuNbO4 was related to the formation of self-trapped excitons facilitated by lattice distortion. The emission peak of LuNbO4 at about 530 nm is attributed to the oxygen vacancy in the niobate group. The reported synthetic methods can provide a fast materials screening route for high melting point inorganic materials.

Rapid and Precise Approaches for XRF Analysis of Rare Earth Niobates

Rapid and Precise Approaches for XRF Analysis of Rare Earth Niobates

New insights into the bonding, electronic and optical properties of fergusonite and scheelite ReNbO4 (Re = Y, La and Lu) compounds

A systematic study of the structural, bonding, electronic, and optical properties of the ReNbO4 (Re = Y, La, and Lu) compounds with the fergusonite (space group I/2a) and scheelite (space group I41/a) structure was performed using density functional theory calculations. The aim of these calculations was to resolve some doubt about these properties for the fergusonite phase and to fill the existing knowledge gap for the scheelite phase. To carry out geometry optimization, the generalized gradient approximation for solids was used. Based on these optimized crystal structures, the bonding, electronic and optical properties were determined using the modified Becke and Johnson exchange potential. An analysis of the bonds between Nb and O atoms showed that Nb atoms in the fergusonite structure of the rare-earth niobates were octahedrally coordinated to oxygen atoms, unlike in the scheelite phase, which had tetrahedral coordination. The calculated values of the direct energy band gap for the fergusonite YNbO4, LaNbO4 and LuNbO4 compounds were 4.435, 4.427 and 4.431 eV, respectively. The optical spectrum for these systems is characterized by a small exciton binding energy, which affects the experimental prediction of the energy band gap when the Tauc plot method is used. The values for the energy band gap in the scheelite phase were 4.625 eV for YNbO4, 4.844 eV for LaNbO4 and 4.611 eV for LuNbO4 and all were indirect. The real and imaginary parts of the dielectric tensor were computed up to 25 eV for both phases, and were interpreted based on their electronic structure. The lower energy regions of the optical spectra of these systems are mainly due to electronic transitions within the NbO6 polyhedron (for the fergusonite phase) and the NbO4 polyhedron (for the scheelite phase), i.e., electronic transitions from the occupied O-2p states at the top of the valence band to the unoccupied Nb-4d states at the bottom of the conduction band.

Suppressing the oxygen-ionic conductivity and promoting the phase stability of the high-entropy rare earth niobates via Ta substitution

Improving and optimizing the target properties of ceramics via the high entropy strategy has attracted significant attention. Rare earth niobate is a potential thermal barrier coating (TBCs) material, but its poor high-temperature phase stability limits its further application. In this work, four sets of TBCs high-entropy ceramics, (Sm1/5Dy1/5Ho1/5Er1/5Yb1/5)(Nb1/2Ta1/2)O4 (5NbTa), (Sm1/6Dy1/6Ho1/6Er1/6Yb1/6Lu1/6)(Nb1/2Ta1/2)O4 (6NbTa), (Sm1/7Gd1/7Dy1/7Ho1/7Er1/7Yb1/7Lu1/7)(Nb1/2Ta1/2)O4 (7NbTa), (Sm1/8Gd1/8Dy1/8Ho1/8Er1/8Tm1/8Yb1/8Lu1/8) (Nb1/2Ta1/2)O4 (8NbTa) are synthesized using a solid-state reaction method at 1650 °C for 6 h. Firstly, the X-ray diffractometer (XRD) patterns display that the samples are all single-phase solid solution structures (space group C2/c). Differential scanning calorimetry (DSC) and the high-temperature XRD of 8NbTa cross-check that the addition of Ta element in 8HERN increases the phase transition temperature above 1400 °C, which can be attributed to that the Ta/Nb co-doping at B site introduces the fluctuation of the bond strength of Ta-O and Nb-O. Secondly, compared to high-entropy rare-earth niobates, the introduction of Ta atoms at B site substantially reduce thermal conductivity (reduced by 44 %, 800 °C) with the seven components high entropy ceramic as an example. The low thermal conductivity means strong phonon scattering, which may originate from the softening acoustic mode and flattened phonon dispersion in 5–8 principal element high entropy rare earth niobium tantalates (5–8NbTa) revealed by the first-principles calculations. Thirdly, the Ta/Nb co-doping in 5–8NbTa systems can further optimize the insulation performance of oxygen ions. The oxygen-ion conductivity of 8NbTa (3.31 × 10−6 S cm−1, 900 °C) is about 5 times lower than that of 8HERN (15.8 × 10−6 S cm−1, 900 °C) because of the sluggish diffusion effect, providing better oxygen barrier capacity in 5–8NbTa systems to inhibit the overgrowth of the thermal growth oxide (TGO) of TBCs. In addition, influenced by lattice distortion and solid solution strengthening, the samples possess higher hardness (7.51–8.15 GPa) and TECs (9.78 × 10−6 K−1–10.78 × 10−6 K−1, 1500 °C) than the single rare-earth niobates and tantalates. Based on their excellent overall properties, it is considered that 5–8NbTa can be used as auspicious TBCs.

Phase transformation inhibition and improved thermo-mechanical properties of rare earth niobates for thermal barrier coatings

Rare earth niobate Re3NbO7 (Re=Rare earth element) possesses ultra-low thermal conductivity for application as thermal barrier coatings (TBCs). However, Re3NbO7 with large rare earth ion radius (ReLa, Nd, Sm) exhibits both low temperature phase transition and poor facture toughness which is detrimental to the durability of the coatings. In this work, an effective method was proposed to solve those problems by doping. Considering the low-temperature phase transition at about 340K, La3NbO7 is selected as representative material. In order to suppress phase transition, Y3+ with relatively smaller ionic radius is used as a doped ion, and (La1-xYx)3NbO7 (x = 0.0,0.1,0.3,0.5) were prepared and investigated. It is shown that, low temperature phase transition of La3NbO7 is effectively suppressed due to Y3+ doped. Facture toughness is increased significantly from 0.5 to 2.3 MPa m1/2 with the increase of doping concentration. Furthermore, the Vickers hardness is also improved by doping. Considering the phase stability and ultra-low thermal conductivity, (La1-xYx)3NbO7 solid solution (x = 0.5) maybe promising candidates for high temperature thermal insulation materials such as TBCs, thermoelectric materials, etc.

Enhancing the thermodynamic properties of rare-earth niobates through high-entropy and composite engineering

High-entropy rare-earth niobates have garnered significant attention in the field of thermal barrier coatings (TBCs) due to their ultra-low thermal conductivity. However, the exceptional thermal properties of fluorite-phase high-entropy rare-earth niobates and the outstanding mechanical properties of monoclinic-phase high-entropy rare-earth niobates cannot be simultaneously harnessed. To address this limitation, this paper investigates and synthesizes five single-phase high-entropy rare-earth niobates: (RE1/5Ho1/5Er1/5Y1/5Yb1/5)3NbO7 (RE = Ce, Sm, Eu, Dy, Lu), and five composite-phase high-entropy rare-earth niobates with a molar ratio of (RE1/5Ho1/5Er1/5Y1/5Yb1/5)NbO7: (RE1/5Ho1/5Er1/5Y1/5Yb1/5)NbO4 = 1:2 (RE = Ce, Sm, Eu, Dy, Lu). The objective is to fabricate niobate coatings that exhibit both outstanding thermal and mechanical properties.The test results reveal that the intrinsic thermal conductivity of the composite-phase samples stands at a mere 1.46 W m−1·K−1 at 1000 °C. Comparatively, the experimental thermal conductivity of the single-phase samples slightly surpasses that of the composite-phase at 1000 °C, registering a minimum of 1.57 W m−1·K−1. Importantly, both values adhere to the TBC standard. The mechanisms behind the low thermal conductivity of the composite-phase and single-phase samples are elucidated by the phonon scattering mechanism and the Cahill model, respectively. Furthermore, the thermal expansion coefficient of the composite-phase samples exceeds that of the single-phase samples overall, and they demonstrate exceptional mechanical properties, offering nearly twice the fracture toughness of the single-phase samples while maintaining high hardness. Experimental evidence shows that composite-phase niobates exhibit enhanced thermodynamic properties compared to single-phase niobates. This work serves as a valuable reference for the utilization of high-entropy rare-earth niobates in the realm of thermal barrier coatings.

Green-emitting lanthanum niobate mesoporous nanospheroids for near-infrared light responsive cancer theranostics

Biomedical applications require nonlinear optical materials with high performance and specific morphology. Among these materials, rare-earth niobates are very promising candidates. However, synthesizing pure-phase rare-earth niobates with controlled shape is challenging. To overcome this obstacle, we present a feasible technique to prepare biocompatible LaNbO4:Er3+/Yb3+ mesoporous nanospheroids with strong upconversion luminescence. We use a pre-Ouzo effect assisted hydrothermal process to obtain the monoclinic phase nanospheroids, which exhibit two-photon excitation induced green emission under 980 nm laser irradiation. We also investigate the energy transfer mechanism between Yb3+ and Er3+ in the LaNbO4 matrix and also explored their pathways in energy level diagram. The cytocompatibility of the LaNbO4:Er3+/Yb3+ mesoporous nanospheroids is verified by Cell Counting Kit-8 assay. Moreover, the cytoplasm and nucleoplasm of LaNbO4:Er3+/Yb3+ mesoporous nanospheroids stained MDA-MB-231 breast cancer cells exhibit upconversion luminescence, indicating that the mesoporous nanospheroids can enter them effectively. Additionally, these nanospheroids have a good ability to deliver DOX to the cancer cells. These results suggest that the LaNbO4:Er3+/Yb3+ mesoporous nanospheroids are promising candidates for luminescence imaging-based cancer diagnosis and therapy.

Crystal structure and ionic conductivity of novel rare-earth niobates LnNbO4 (Ln = Nd, Sm, Eu, Gd) by substituting Nb with W

Crystal structure and ionic conductivity of novel rare-earth niobates LnNbO4 (Ln = Nd, Sm, Eu, Gd) by substituting Nb with W

Accurate Determination of the Bandgap Energy of the Rare-Earth Niobate Series.

We report diffuse reflectivity measurements in InNbO4, ScNbO4, YNbO4, and eight rare-earth niobates. A comparison with established values of the bandgap of InNbO4 and ScNbO4 shows that Tauc plot analysis gives erroneous estimates of the bandgap energy. Conversely, accurate results are obtained considering excitonic contributions using the Elliot-Toyozawa model. The bandgaps are 3.25 eV for CeNbO4, 4.35 eV for LaNbO4, 4.5 eV for YNbO4, and 4.73-4.93 eV for SmNbO4, EuNbO4, GdNbO4, DyNbO4, HoNbO4, and YbNbO4. The fact that the bandgap energy is affected little by the rare-earth substitution from SmNbO4 to YbNbO4 and the fact that they have the largest bandgap are a consequence of the fact that the band structure near the Fermi level originates mainly from Nb 4d and O 2p orbitals. YNbO4, CeVO4, and LaNbO4 have smaller bandgaps because of the contribution from rare-earth atom 4d, 5d, or 4f orbitals to the states near the Fermi level.

Tailoring thermal and mechanical properties of rare earth niobates by coupling entropy and composite engineering

In this paper, a series of high-entropy rare earth niobates, including fluorite RE3NbO7 (HE317), monoclinic RENbO4 (HE114) and RENbO4/RE3NbO7 composite (HE-composite), are prepared via solid state reaction, following by a study about their thermal and mechanical properties. The high-entropy rare earth niobates exhibit excellent phase stability after thermally exposed to 1300 °C for 100 h, indicating entropy can stabilized high-entropy rare earth niobates. Compared with the single element rare earth niobates, high-entropy rare earth niobates have higher fracture toughness and hardness. The high-entropy RENbO4/RE3NbO7 composite has the best mechanical properties, with a fracture toughness of 2.71 ± 0.17 MPa·m1/2 and hardness of 9.46 ± 0.24 GPa, respectively. The high-entropy niobates exhibit high coefficients of thermal expansion which is close to 7 wt% Y2O3 stabilized ZrO2. It is also proved that the configurational entropy has little effect on the critical temperature from monoclinic to tetragonal phase transition. The thermal conductivity of HE-composite is lower than HE114, indicating the combination of HE114 and H317 is a more efficient strategy to decrease the thermal conductivity of HE114 than entropy engineering.

Short-range order and origin of the low thermal conductivity in compositionally complex rare-earth niobates and tantalates

Rare-earth niobates and tantalates possess low thermal conductivities, which can be further reduced in high-entropy compositions. Here, a large number of 40 compositions are synthesized to investigate the origin of low thermal conductivity. Of these, 29 possess single (nominally cubic) fluorite phases and most of them are new compositionally complex (medium- or high-entropy) compositions. Furthermore, doping 2% of light element cations can further reduce thermal conductivity. This large data set enables the discovery of a negative correlation between the thermal conductivity and averaged radius ratio of the 3+/5+ cations. While this ratio is still below the threshold for forming long-range ordered weberite-type phases, this correlation suggests the reduced thermal conductivity is related to short-range weberite-type order, which is indeed revealed by diffuse scattering in X-ray diffraction and neutron total scattering. Specifically, neutron total scattering is used to characterize five selected specimens. A better fit to a weberite-type structure is found at the nanoscale. The characteristic length (domain size) is appears to be larger in more insulative materials. As it approaches the Ioffe-Regel limit, the phonon limit breaks down and “diffusons” give rise to the observed amorphous-like thermal conductivity.

Highly efficient thermal insulation in crystalline weberites RE3NbO7 (RE=La, Nd, Sm, Eu, Gd) with glass-like thermal conductivity

Thermal energy has been considered to propagate by wave-like phonons with well-defined wave vectors in crystalline materials. In this work, we demonstrate a completely different thermal conduction scenario in crystalline rare earth niobates, RE 3 NbO 7 (RE=La, Nd, Sm, Eu, Gd), having ultralow thermal conductivities of 1.0–1.5 Wm -1 K -1 . Especially, the rare earth niobates exhibit glass-like temperature dependence of thermal conductivity from room temperature to 1273 K, indicating significant suppression of both phonon and radiation heat transfer. According to Cahill model based on amorphous theory, minimum thermal conductivity of rare earth niobate was calculated. It is shown that the glass-like thermal conductivity result from a wide range of amorphous solids due to phonon scattering by multiple lattice imperfections. Furthermore, phonon mean free path of RE 3 NbO 7 is comparable to the atomic spacing of crystal indicating that energy carried by phonons can only transmission between a few atomic spacings leading to the ultralow thermal conductivity. The mechanical properties of rare earth niobates are also discussed.

Hydrochloric Acid Decomposition of the Niobium–Rare-Earth Slag Produced by Reducing Roasting of the Rare-Earth Ore from the Chuktukon Deposit

Hydrochloric Acid Decomposition of the Niobium–Rare-Earth Slag Produced by Reducing Roasting of the Rare-Earth Ore from the Chuktukon Deposit

Ultra-low thermal conductivity and enhanced mechanical properties of high-entropy rare earth niobates (RE3NbO7, RE = Dy, Y, Ho, Er, Yb)

A new entropy-stabilized rare earth niobates (RE3NbO7, RE = Dy, Y, Ho, Er, Yb) system with disordered defective fluorite structure was prepared by solid-state method. XRD, Raman and EDS results confirmed the homogeneous distribution and equimolar doping of rare earth cations. High-entropy RE3NbO7 ceramics showed smaller grain size compared with single rare earth niobates owing to the sluggish grain diffusion. Due to the increased configuration entropy, (Dy0.2Ho0.2Er0.2Y0.2Yb0.2)3NbO7 (5RE3NbO7) has enhanced mechanical properties (Hv = 9.51 GPa, KⅠC = 2.13 MPa m0.5), a high thermal expansion coefficient (10.2 × 10−6 K-1, 1200℃) and ultra-low thermal conductivity (0.724 W m-1 K-1, 25℃). These results suggest that the material can be used as a new type of thermal barrier coatings as alternative to yttria-stabilized zirconia.

High entropy defective fluorite structured rare-earth niobates and tantalates for thermal barrier applications

Rare-earth tantalates and niobates (RE3TaO7 and RE3NbO7) have been considered as promising candidate thermal barrier coating (TBC) materials in next generation gas-turbine engines due to their ultra-low thermal conductivity and better thermal stability than yttria-stabilized zirconia (YSZ). However, the low Vickers hardness and toughness are the main shortcomings of RE3TaO7 and RE3NbO7 that limit their applications as TBC materials. To increase the hardness, high entropy (Y1/3Yb1/3Er1/3)3TaO7, (Y1/3Yb1/3Er1/3)3NbO7, and (Sm1/6Eu1/6Y1/6Yb1/6Lu1/6Er1/6)3(Nb1/2Ta1/2)O7 are designed and synthesized in this study. These high entropy ceramics exhibit high Vickers hardness (10.9–12.0 GPa), close thermal expansion coefficients to that of single-principal-component RE3TaO7 and RE3NbO7 (7.9×10−6-10.8×10−6C−1 at room temperature), good phase stability, and good chemical compatibility with thermally grown Al2O3, which make them promising for applications as candidate TBC materials.

Reducing Roasting of the Niobium–REM Ores of the Chuktukon Deposit with the Production of Phosphorus Cast Iron and Niobium–REM Slag

The results of studying the reducing roasting of the high-iron (65% Fe2O3) niobium–rare earth ore of the Chuktukon deposit with the production of high-phosphorous cast iron and niobium–rare earth slag are discussed. The optimum reducing roasting conditions are found, and fairly complete separation of the roasting product into metal and slag phases is shown to be achieved at 1400°C. Under these conditions, 88% of phosphorus passes into cast iron, and 95% of niobium and 85% of manganese are concentrated in a rare-earth slag. The yield of phosphorous cast iron (up to 3.5% P) is about 45% and that of slag is 25% of the ore mass. The main slag phases are glass and, crystals with perovskite, loparite and spinel structures. Niobium and rare earth metals are concentrated in the first three phases, and the spinel phase is represented by an MnAl2O4–MgAl2O4 solid solution. After the preliminary removal of iron and phosphorus from the ore in the form of a metallic product, the material flows in the hydrometallurgical stage of processing are found to be reduced fourfold. Moreover, the entire process can become environmentally friendly with high technical and economic indicators.

Liquid phase synthesis and sintering of Y3NbO7

Rare earth niobates (RE3NbO7) are materials that present interesting magnetic [[1], [2], [3]], dielectric [4,5], photocatalytic [6,7], electric or ionic conductivity [[8], [9], [10], [11], [12]] and optical properties [13,14]. Such properties could be related to the crystal structures of RE3NbO7 that exists in three different crystalline forms depending on the ionic radii of the rare earth atoms [3,[15], [16], [17], [18], [19]]. Orthorombic (Pnma) and cubic fluorite pyrochlore (Fdm) structures are found for the largest cations, while the smallest radii RE (Dy, Ho, Er, Tm, Eu, Yb …) crystallize into a defect cubic fluorite structure (Fmm) [15,17]. Rare earth and niobium atoms are statistically distributed on cationic sites, as well as oxygen vacancies on anionic sites. This relative disordered structure leads to peculiar optical properties with a significant broadening of the 4f-4f transitions observed in an Eu-doped Y3NbO7 compound...

Mineralogy and geochemistry of the Morro dos Seis Lagos siderite carbonatite, Amazonas, Brazil

The Morro dos Seis Lagos niobium rare earth element, Ti-bearing lateritic deposit (Amazonas, Brazil) is derived from a primary siderite carbonatite. The complex is the only example of a Nb deposit in which Nb-rich rutile is the main Nb ore mineral. Apart from the laterites, at the current level of exposure the complex consists only of siderite carbonatite; silicate rocks are absent. Three types of siderite carbonatite are recognized: (1) a brecciated and oxidized core siderite carbonatite consisting of up to 95 vol% siderite together with: hematite; pyrochlore; Nb-brookite; Ti-maghemite; and thorobastnäsite; (2) a REE- and P-rich variety of the core siderite carbonatite consisting of siderite (up to 95 vol%), hematite, minor pyrochlore, monazite and bastnäsite; (3) a border hydrothermal siderite carbonatite with ~70 vol% siderite, barite (~15 vol%), gorceixite (~7 vol%) and minor rhabdophane and pyrochlore. The country rock gneiss in which the carbonatite was emplaced was affected by potassic fenitization, with the formation of phlogopite and orthoclase together with monazite, fluorapatite and bastnäsite. The siderite carbonatites exhibit a wide variation of δ13C (−5.39‰ to −1.40‰), accompanied by a significant variation in δ18O (17.13‰ to 31.33‰), especially in the REE-rich core siderite carbonatite, and are explained as due to the presence of both H2O and CO2 in the magma. The core siderite carbonatite is the richest in Fe (48.64–70.85 wt% Fe2O3) and the poorest in Ca (up 0.82 wt% CaO) example of a siderite carbonatite yet recognized The ferrocarbonatite has significant contents of Mn, Ba, Th, Pb and LREE, and a very high Nb (up to 7667 ppm) content due to the presence of Nb-brookite. The substitution 3Ti4+ = Fe2+ + 2Nb5+ recognized in Nb-rich brookite explains enrichment of Nb in the core siderite carbonatite and indicates formation in a reducing environment. The high Nb/Ta ratio (1408–11,459) of the carbonatite is compatible with residual liquids derived by fractional crystallization. The 87Sr/86Sr (0.70411–0.70573) and 144Nd/143Nd (0.512663–0.512715) isotopic data suggest the carbonatite is mantle-derived with essentially no crustal contamination and is younger than the maximum age of 1328 ± 58 Ma (UPb in zircon). We suggest that the Morro dos Seis Lagos carbonatite complex represents the upper-most parts of a differentiated carbonatite magmatic system, and that the siderite carbonatite is related to late-magmatic-to-carbo-hydrothermal processes.

Thermal and mechanical properties of ferroelastic RENbO4 (RE = Nd, Sm, Gd, Dy, Er, Yb) for thermal barrier coatings

A new series of rare-earth niobates (RENbO4) are synthesized and investigated for potential thermal barrier coating (TBC) application. The thermal conductivities of RENbO4 are much lower than yttria stabilized zirconia (YSZ) mainly due to the chemical inhomogeneity. The lattice distortion contributes to the higher coefficients of thermal expansion (CTEs, 12.8 × 10−6 K−1) of RENbO4 below the critical temperature. Especially, RENbO4 exhibits high fracture toughness (1.93–2.77 MPa m0.5) comparable to YSZ, owning to the absorption of crack energy by the ferroelastic domain switching. The excellent thermal and mechanical properties give RENbO4 great potential as new durable TBC materials.

Mechanical properties, oxygen barrier property, and chemical stability of RE3NbO7 for thermal barrier coating

AbstractRare earth niobate (RE3NbO7, RE = Dy, Y, Er, Yb) ceramics have shown extremely low thermal conductivity but remain questionable in high temperature thermal barrier coating (TBC) applications with high thermal, mechanical, and chemical loads. Herein, we comprehensively characterize the properties of rare earth niobates, including mechanical properties, oxygen barrier properties, chemical stability, etc. It is found that the oxygen conductivities of the rare earth niobates are three orders of magnitude lower than 7wt.% yttria‐stabilized zirconia (YSZ), indicating a remarkable oxygen barrier property to avoid oxidation of underlying metallic components. The corrosion resistance of rare earth niobate against calcium‐magnesium‐aluminum silicate (CMAS) is also significantly better than that of YSZ. Together with the extremely low thermal conductivity, the rare earth niobates exhibit a combination of excellent high temperature properties, which may become a promising candidate material of high temperature TBC of next generation gas turbines.