Influence of rare earth addition in cobalt ferrite thin films obtained by pulsed laser deposition

The influence of rare earth (RE = Dy, Gd, Yb) ion doping on the structural, magnetic and optical properties of cobalt ferrite thin films was studied. CoFe_2-xRE_xO₄ (x = 0.01; 0.03; 0.05; 0.1; 0.2; 0.3) films were obtained by pulsed laser deposition on silicon substrates. The X-ray diffraction and Raman spectroscopy results on the bulk materials, which were used as targets during deposition, revealed the formation of residual phases as the concentration of RE dopant was increased. However, the annealed thin films presented diffraction lines and vibrational modes corresponding only to cobalt ferrite. A lower crystallinity of the deposited samples was observed when higher RE concentrations were used. Due to the substitution of Fe by RE elements which possess large ionic radii, the lattice parameter of the thin films presented a monotonous increase. The magnetic response of the nanostructures was correlated to the magnetic moment of the RE dopant and to the structural properties of the films. The optical bandgaps derived from the reflectance spectra presented an increasing trend as the Yb and Dy concentrations were augmented.     

Theoretical exploration of the abnormal trend in lattice thermal conductivity for monosilicates RE2SiO5 (RE = Dy,Ho, Er,Tm, Yb and Lu)

Rare earth monosilicates RE₂SiO₅ have been considered as promising environmental barrier coating materials for silicon-based ceramics due to their low thermal conductivity and good high-temperature stability. We herein performed a systematic study of the lattice dynamics for RE₂SiO₅ (RE?=?Dy, Ho, Er, Tm, Yb and Lu) using first-principles calculations. The loosely bound rare earth atoms provide large Grüneisen parameters and low phonon group velocities, both of which determine the low thermal conductivity. Theoretical exploration predicts an anomalous increase of lattice thermal conductivity with increment of RE atomic number and the mechanism is explained by the stronger atomic bonding and weaker phonon anharmonicity. Although incorporating heavier atoms has long been considered as an effective way to reduce lattice thermal conductivity, this work addresses the importance of bonding heterogeneity and anharmonicity rather than atomic mass variation. This theoretical study suggests an alternative approach towards the design of new thermal insulating materials.     

Raman scattering study of RETiTaO6 dielectric ceramics

In this work we have investigated microwave-dielectric ceramics RETiTaO₆ (RE=Al, Y, In, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er or Yb) by Raman scattering measurements at room temperature. The observed Raman-active phonons were analysed in conformity with two orthorhombic structures proposed for these ceramics, namely aeschynite (for lanthanide ions with atomic number between 58 and 66) or euxenite (for Y, Ho, Er and Yb). The results indicate that the phonon spectra remained relatively unchanged with respect to the RE ion substitution within each structure. For the three remaining materials with RE=La, Al and In, which are known to present multiphases, the Raman phonon spectra showed significant differences in comparison with those of ceramics with aeschynite and euxenite structure.     

Electrical properties of (Bi2O3)0.75(RE2O3)0.25 ceramics (RE=Dy, Y, Ho, Er and Yb)

Five kinds of rare earth stabilized bismuth oxide ceramics, (Bi₂O₃)_0.75(RE₂O₃)_0.25 (RE=Dy, Y, Ho, Er and Yb), were synthesized by sintering a mixture of Bi₂O₃ and RE₂O₃ at 900–1100 °C and their electrical properties were investigated. The bulk density and the lattice constant linearly increased with an increase in the atomic weight of RE and the ionic radius of RE³⁺, respectively. The electrical conductivity at 300 °C slightly increased with the increasing ionic radius of RE³⁺, while at 500 and 700 °C, it was constant regardless of the ionic radius of RE³⁺. The migration activation energy and the association activation energy showed a maximum value and a minimum value at RE=Er, respectively.     

Theoretical investigation of phonon contributions to thermal expansion coefficients for rare earth monosilicates RE2SiO5 (RE = Dy,Ho, Er,Tm, Yb and Lu)

Owing to the superior chemical and thermal compatibility with silicon-based ceramics, rare earth (RE) monosilicates RE₂SiO₅ are promising as environmental barrier coating materials for the applications in high-temperatures aero-engines. Herein, thermal expansion and phonon characteristics of RE₂SiO₅ (RE = Dy, Ho, Er, Tm, Yb and Lu) were investigated by first-principles calculations. The thermal expansion coefficients were predicted as a function of temperature and ranged from 8.87 to 7.72 × 10⁻⁶ /K at 1600 K for the studied RE₂SiO₅. The isolated RE and O5 atoms determine the thermal expansion behaviours of RE₂SiO₅ especially at low phonon frequencies; while the [SiO₄] tetrahedrons contribute positively, but less significantly to the thermal expansion. The balance between negative contribution from distortion of RE polyhedrons and positive contribution from stretching of REO bonds is identified as the key to tune the thermal expansion coefficient of RE silicates.     

Crystal growth and phase formation of high-entropy rare-earth monoclinic aluminates

For the first time, high-entropy rare-earth monoclinic aluminate crystals were grown via directional solidification using the micro-pulling-down method. Five high-entropy compositions were formulated with a general formula RE₄Al₂O₉, where RE is an equiatomic mixture of five rare-earth elements. The rare-earth elements included were Lu, Yb, Er, Y, Ho, Dy, Tb, Gd, Eu, Sm, Nd, and La. High-temperature powder X-ray diffraction and Rietveld structure refinement indicated that all crystals were a single monoclinic phase and that rare-earth average ionic radius did not affect phase purity. At room temperature, the refined lattice parameters increased consistently with increasing average ionic radii of the five compositions. One of the crystals had a typical high-temperature phase transition of single-RE RE₄Al₂O₉ in the range of 1100–1150°C, which consisted of a lattice contraction upon heating. Differential scanning calorimetry indicated a thermal event corresponding to that phase transition. Electron probe microanalysis revealed Al-rich inclusions on the surface of the crystals. Crystals containing Tb had dark surface features that became lighter after annealing in a reducing atmosphere, which indicated that Tb⁴⁺ may be responsible for the dark features.     

Rare‐Earth Dopant Effects on the Structural,Energetic, and Magnetic Properties of Alumina from First Principles

Density functional theory was used to study the effect of rare‐earth dopants on the structure, phase stability, and magnetic properties of α‐ and θ‐Al₂O₃. Lanthanide series rare‐earth dopants (Pr, Nd, Gd, Er, and Yb) were considered at a doping concentration of 0.83 at.%. Incorporation of rare‐earth dopants was found to increase the lattice parameters and exaggerate the local structural distortion around the dopant. The extent of local lattice distortion was correlated with the dopant ionic radii. The phase stability of rare‐earth‐doped Al₂O₃ was assessed by comparing cohesive and defect formation energies for doped and undoped α‐ and θ‐Al₂O₃. Rare‐earth dopants increased the relative stability of the metastable θ‐Al₂O₃, although doped α‐Al₂O₃ remained more stable. The total magnetic moment of the doped Al₂O₃ was shown to correlate with the number of unpaired electrons. The magnetic moment was also found to be strongly localized on the rare‐earth dopant for Er, Gd, Nd, and Pr‐doped Al₂O₃. In contrast, the Yb dopant induced a delocalized magnetic moment on ~80% of the oxygen atoms. These results further the understanding of dopant incorporation mechanisms, as well as the doping effect on phase stability and magnetic properties that may be applied to advanced field‐assisted material synthesis and processing for enhanced properties.     

Thermochemistry of stoichiometric rare earth oxyfluorides REOF

Rare earth oxyfluorides (REOF) have potential applications in luminescent devices and energy storage and are important for synthesis and properties of RE-doped oxyfluoride glasses. This study evaluates the thermodynamic properties of stoichiometric REOF with varying RE (RE = La, Nd, Gd, Ho, Y, and Yb) elements using high temperature oxide melt solution calorimetry and investigates the phase transitions of YbOF by differential scanning calorimetry. The formation enthalpies from binary oxides and fluorides are exothermic and become less exothermic with the decrease of ionic radius of RE³⁺, showing that REOF compounds are energetically stable relative to equal molar mixtures of RE₂O₃ and REF₃ and the RE–O interactions contribute to most of their thermodynamic stabilities. Different from REOF with large RE ions (La–Er and Y) having the rhombohedral phase, REOF with small cations (e.g., Yb) possesses the monoclinic structure as the more energetically stable phase at room temperature. However, the slow transition between the monoclinic and cubic phases easily generates the rhombohedral phase, which is favored by kinetics.     

Rare earth metals' influence on the heat generating capability of cobalt ferrite nanoparticles

The aim of this study is to synthesize and assess the potential applications of rare earth doped cobalt ferrite nanoparticles in cancer treatment through hyperthermia.The synthesis of CoFe_2−xRE_xO₄ (where RE=Yb, Dy, Gd and x=0.01–0.3) through the co-precipitation method is presented. The composition and properties of the nanoparticles where investigated and evaluated in correlation with their heat generating capability. The XRD and EDX analysis indicated phase separation for high rare earth content with the appearance of Gd₂O₃ and Dy₂O₃ secondary phases, which leads to unwanted changes in the nanoparticles' magnetic properties and consequently of the specific absorption rate. All the nanoparticles present functional group belonging to the surfactant as determined by FT-IR and Raman. Magnetic and specific adsorption rate measurement suggest increases in saturation magnetization and SAR value in doped ferrites, compared to CoFe₂O₄ with as much as 26% and 15% for Dy doped and Gd doped samples respectively.     

Rare-earth nitrate additives for the sintering of silicon carbide

Fourteen rare earth elements in their nitrate form were evaluated as sintering additives for β-SiC. All rare earth nitrates transformed to oxides by a reaction with the surface-adsorbed thin SiO₂ during heat treatment, which enhanced the density of the SiC monolith without decomposing SiC. In particular, Sc, Yb, Tm, Er and Ho were quite effective sintering additives; a > 99% relative density was observed by the addition of 5 wt.% rare earth oxide, whereas the other rare earth additives (Lu, Dy, Tb, Gd, Eu, Sm, Nd, Ce and La) revealed 77–92% density. Moreover, a fine 156 nm-sized SiC grain could be acquired by Sc addition, whereas the other additives showed a SiC grain size of approximately 1 μm. The mean hardness and K_Ic of the dense SiC containing rare earth elements were 24–27 GPa and 3.3–5.0 MPa m^1/2, respectively.     

Theoretical and experimental analysis for site preference of rare earth elements in BaTiO3

Rare earth (RE) elements such as Y, Ho, and Dy are considered to play an important role in the long-term reliability of multilayer ceramic capacitors (MLCCs). Theoretical calculations of the site preference of RE elements in BaTiO₃ are useful tools for understanding this mechanism, especially when the RE elements are co-doped with other additive elements. In this study, the site preference of RE elements doped with and without Mg in BaTiO₃ is investigated by performing first-principles calculations and experiments. The calculation results indicate that the site preference of RE elements is strongly related to their ionic radii. In the simulation of Dy, Dy atoms replace both Ba and Ti atoms without Mg co-doping. On the other hand, when Dy is co-doped with Mg, Dy atoms prefer to occupy the Ba site instead of the Ti site in BaTiO₃. Experiments were also performed to analyze element concentrations in Dy-doped BaTiO₃ with and without Mg co-doping by scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy. The theoretical calculation and experimental measurement agree quite well on the site preference of Dy with and without Mg co-doping. Our theoretical and experimental results reveal the influence of Mg co-doping on the site preference of the RE elements in BaTiO₃.     

Hot corrosion behavior of Ba2REAlO5 (RE = Dy,Er, Yb) ceramics by vanadium pentoxide at 900–1000 °C

Hot corrosion behavior of Ba₂REAlO₅ (RE = Dy, Er, Yb) ceramics exposed to V₂O₅ molten salt at 900 °C and 1000 °C was investigated, providing a better understanding of their corrosion resistance as promising thermal barrier coatings. Obvious surface reactions occurred forming continuous, dense reaction layers on the top surfaces of the samples, the types of corrosion products being temperature and time independent. After heat treatment for 4 h and 20 h in V₂O₅ salt at the two temperatures, the corrosion products consisted of REVO₄, Ba₂REV₃O₁₁ and BaAl₂O₄ (RE = Dy, Er, Yb). Prolonged heat treatment and elevated temperature promoted the growth of Ba₂REV₃O₁₁ and REVO₄ grains. The reaction layer had a positive function on suppressing further penetration of the molten salt. The mechanism by which the corrosion reaction occurs is proposed based on Lewis acid-base rule, phase diagrams and thermodynamics.     

Effect of Rare Earth Doping on the Catalytic Activity of Copper‐Containing Hydrotalcites in Phenol Hydroxylation

The copper‐containing hydrotalcites (Cu‐HTLcs) are synthesized under microwave irradiation, the effect of rare earth elements (RE) on the synthesis of Cu‐HTLcs, wherein RE stands for rare earth elements, e.g., La, Y, Sm and Ce, is also investigated in the present work. The hydrotalcite structure of the synthesized samples is verified by XRD and FT‐IR. The results of their catalytic performances in phenol hydroxylation show that the doped rare earth elements can promote the catalytic activity of Cu‐HTLcs, exhibiting a good trend as follows: La>Y>Sm>Ce. XPS results show that the Cu⁺ species are produced after the interaction of La‐Cu‐HTLcs with H₂O₂. Combining with the catalytic test results, we propose a new mechanism about the generation of HO ^. radicals in phenol hydroxylation, it is assumed that HO‐Cu⁺‐OH species are first formed by the reduction of HO‐Cu²⁺‐OH located in hydrotalcites in the presence of H₂O₂, and then react with H₂O₂ to give rise to HO^. radicals.     

Effects of various rare-earth additives on the sintering and transmittance of γ-AlON

Several rare earth elements (Sc, La, Pr, Sm, Gd, Dy, Er, and Yb) were evaluated to develop a sintering additive for transparent γ-AlON. After adding 0.2 wt. % of each of the rare earth additives to the α-Al₂O₃ and AlN starting material, hot pressing was performed at 1850 °C for 1 h under 20 MPa as a screening test, whereby the γ-AlON with Pr-nitrate showed the highest transmittance of 60.4 % at 632 nm. Two-step pressureless sintering was conducted as a separate test for the same starting material after adjusting the amount of Pr-nitrate to 0, 0.1, or 0.2 wt. % to enhance transmittance. The γ-AlON with 0.1 wt. % Pr-nitrate showed the highest transmittance of 80.36 % after 1st and 2nd sintering steps at 1610 and 1940 °C, respectively, indicating that Pr can be an alternative sintering additive to conventional Y₂O₃, MgO, and La₂O₃ for the fabrication of transparent γ-AlON.     

The effect of rare-earth (La,Sm, Dy,Ho and Er) and Mg on the microstructure in BaTiO3

The effect of rare-earth elements, La, Sm, Dy, Ho and Er, on the microstructure in BaTiO₃(BT)–MgO–rare-earth oxide based system was studied. Larger amount of MgO was required to suppress the grain growth for the larger ionic radius rare-earth ion (La, Sm)-doped samples than for the smaller ion (Dy, Ho, Er)-doped samples. Also, substitution modes of rare-earth elements and Mg in BT lattice were investigated. The solubility of rare-earth ions in BT lattice and the substitution ratio of rare-earth ions into Ba-site decreased as ionic radius decreases. This suggests that the formation of core-shell structures in BT–MgO–rare-earth oxide based system is dependent on the substitution ratio of rare-earth ions into Ba-site.     

Crystal structure of rare-earth silicon-oxynitride J-phases,Ln4Si2O7N2

Rare-earth silicon-oxynitride J-phases, Ln₄Si₂O₇N₂ (Ln=Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), were prepared by the N₂ gas-pressured sintering method at 1 MPa of N₂ and 1500–1700 °C. The Rietveld analysis was carried out for X-ray powder diffraction data measured at room temperature. The crystal structures of Ln₄Si₂O₇N₂ were refined with the structure model of La₄Si₂O₇N₂ for Ln=La, Pr, Nd, and Sm, and with that of Lu₄Si₂O₇N₂ for Ln=Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The refined monoclinic unit-cell parameters (lengths a, b, c, angles β, and volume V) increased linearly in their two series of Ln with increasing ionic radii of rare-earth atoms. Discontinuities of the unit-cell parameters were found between the two Ln series.     

Solvent Extraction of Heavy Lanthanides(III) with 5,7‐Dibromo‐8‐hydroxyquinoline from Aqueous Perchlorate Solution

Abstract

The extraction of Tb(III), Dy(III), Ho(III), Er(III), Tm(III), and Yb(III) with 5,7‐dibromo‐8‐hydroxyquinoline (Hdbq or HA) in chloroform from aqueous perchlorate solutions was investigated. The formation of the LnA₃ species (where Ln = Tb, Dy, Ho, Er, Tm, and Yb) in the organic phase was supported by the data. The parameters of the extraction processes were determined, and the separation factors between two adjacent lanthanides(III) were calculated.     

Atmospheric Pressure Synthesis of Heavy Rare Earth Sesquioxides Nanoparticles of the Uncommon Monoclinic Phase

We report, for the first time, the atmospheric pressure synthesis of nonagglomerated nanoparticles (20–60 nm in diameter) of the uncommon monoclinic phase of some heavy rare earth sesquioxides RE₂O₃ (RE=Dy, Ho, Er, Tm, and Yb). The RE₂O₃ nanoparticles, prepared by a flame synthesis process, were characterized by X-ray diffraction, transmission electron microscopy, and electron energy loss spectroscopy. Monoclinic nanoparticles were formed when the flame temperature was sufficiently high; lower temperatures led to the formation of the normal cubic (C-type) phase. We explain the formation of the uncommon monoclinic phase on the basis of pressure–temperature phase equilibria, and the extra internal pressure induced by surface curvature of the nanoparticles.     

EXTRACTION AND SEPARATION OF HEAVY RARE EARTH(III) WITH EXTRACTION RESIN CONTAINING DI(2,4,4-TRIMETHYL PENTYL) PHOSPHINIC ACID (CYANEX 272)

ABSTRACT

Extraction resins, of the type of Levextrel, (which is a collective term for styrene/divinylbenzene based copolymers of predominantly macroporous structure that contain a selective extractant) are important for the recovery and separation of metal ions, as they combine features of solvent extraction and ion exchange resins. This paper presents the results of the adsorption of heavy rare earth ions (Ho(III), Er(III), Tm(III), Yb(III), Lu(III) and Y(II1)) from hydrochloric acid solutions at 0.2 mol/L ionic strength and 50°C by the extraction resin containing di (2,4,4-trimethyl pentyl) phosphinic acid (Cyanex 272) and the chromatographic separation of (Er(III), Tm(III) and Yb(III)). Technological separation products, with purity and yield of Tm₂O3 >99.97%, >80%; Er₂O₃ >99.9%, >94% and Yb₂O₃ >99.8%, >80% respectively, have been obtained from a feed having the composition Tm₂O₃ 60%, Er₂O₃ 10%, and Yb₂O₃ 3%, the others 27%.

The distribution coefficients, extraction equilibrium constants and separation factors have been determined as a function of acidity, loading of the resin and rare earths, flow rates and column ratios. The resolutions and efficiencies of separation of Er/Tm/Yb each other have been calculated. The stoichiometry of the extraction of rare earth ions has been suggested as well.     

Dopant Site Occupancy and Chemical Expansion in Rare Earth‐Doped Barium Zirconate

Rare earth‐doped BaZrO₃ is a very attractive material in electrochemical applications due to its proton conductive property. In this work, powder X‐ray diffraction patterns of BaZr_0.8M_0.2O_3?δ (M = Sc, Eu, Sm, Dy) were collected using synchrotron radiation, and also using characteristic X‐ray of CuKα in dry and wet atmospheres at high temperature. Then, a combined interpretation of the diffraction patterns was established by using Rietveld refinement. The results revealed that an obvious lattice expansion was observed for BaZr_0.8M_0.2O_3?δ (M = Sc, Eu, Sm, Dy) in wet O₂ compared with the case in dry condition, indicating a chemical expansion effect on lattice volume by incorporating water into lattice. Eu, Sm, and Dy cations occupied both A‐ and B‐sites of BaZrO₃ crystalline lattice, whereas Sc cations were determined to occupy B‐site only. These results indicate clearly an increasing tendency toward A‐site occupation for the rare earth cations in BaZrO₃ with an increasing radius.