New reagents for the chemical synthesis of fine rare-earth orthoferrite powders

Ultrafine (≤150 nm) rare-earth orthoferrites (RFeO₃ where R = Sm, Nd, and Gd) with good purity and chemical homogeneity were prepared through two solution-based chemical routes using some new and versatile chemical reagents. The first route involved coprecipitation of mixed metal nitrates from their aqueous solutions, in the presence of water soluble polyvinyl alcohol (PVA), using triethylammonium carbonate solution. The other process involved complete evaporation of a mixture of optimum amounts of PVA and the desired aqueous metal nitrate solutions, with the addition of the optimum amount of urea when the mixture was evaporated to a pasty mass. In addition, detailed study of the reported potassium ferricyanide route was also carried out for the production of the same powders. The precursor as well as the heat-treated RFeO₃ powders, prepared through these three routes, were compared by physical characterization studies involving thermal gravimetry and differential scanning calorimetry (TG/ DSC), Infrared spectroscopy (IR), X-ray powder diffraction (XRD), energy dispersive X-ray spectroscopy (EDXS), transmission electron microscopy (TEM), and room temperature magnetic measurements.     

Chemical synthesis of nanosized oxides

Fine powder of single and binary mixed oxides can be produced by decomposition of the respective metal nitrates and polyvinyl alcohol (PVA) or, a mixture of PVA and polyacrylic acids. These mixtures, after spray drying, yield a brown fluffy mass, which is spontaneously combustible and the heat liberated is sufficient for the crystallization of the desired oxide phase. The rate of combustion controls the growth of the particles. This can be manifested by combustion of the mixture in controlled atmosphere. The nanoparticles of the oxide system studied are: spinels [MFe₂O₄ where M = Ni(II), Co(II), Zn(II), Mg(II)]; orthoferrites [MFeO₃ where M = Gd(III), Sm(III)]; LaAlO₃, NdGaO₃, CaO/MgO/Y₂O₃ stabilized zirconia (ZrO₂); lead zirconate titanate (PZT), lanthanum modified lead zirconate titanate (PLZT) and BaTiO₃.     

Influence of Temperature on the Mutual Solubility of Components in the Binary Systems Tetradecane—Rare-Earth Metal (Nd,Sm, Gd,Y, Lu) Nitrate Solvate with TBP

The phase diagrams of binary liquid systems tetradecane—rare-earth metal (Nd, Sm, Gd, Y, Lu) nitrate solvate with TBP were studied in the temperature range 273.15–373.15 K. These diagrams contain the fields of homogeneous solutions and the field of separation into two liquid phases (I, II). Phases I and II are enriched, respectively, in tetradecane and Ln(NO₃)₃(TBP)₃ (Ln = Nd, Sm, Gd, Y, and Lu). With increasing temperature, the fields of separation into two liquid phases contract. The upper critical solution temperatures are dependent on the kind of rare-earth metal(III).     

Extraction of Am(III) and Rare-Earth Elements from Nitric Acid Solutions with (Diphenylphosphinylmethyl)phenylphosphinic Acid

Extraction of microamounts of Am, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu nitrates from aqueous HNO₃ with solutions of (diphenylphosphinylmethyl)phenylphosphinic acid in dichloroethane was studied. The stoichiometry of the extractable complexes was determined. Synergistic enhancement of the extraction of rare-earth elements in the presence of tetraphenylmethylenediphosphine dioxide was revealed.     

Extraction of rare-earth elements from nitric acid solutions with bis(dioctylphosphinylmethyl)phosphinic acid

The distribution of microamounts of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y nitrates between aqueous HNO₃ solutions and solutions of bis(dioctylphosphinylmethyl)phosphinic acid in organic diluents was studied. The stoichiometry of the extractable complexes was determined, and the diluent effect on the efficiency of extraction of rare-earth elements from nitric acid solutions was examined. Addition of a neutral solvating component, tetraphenylmethylenediphosphine dioxide, to the organic phase enhances the extraction of rare-earth elements, exerting a synergistic effect.     

Structural, Transport, and Magnetic Properties of (Ru0.9Nb0.1)Sr2(Gd0.67R0.67Ce0.66)Cu2O z (R = Nd, Gd, and Tb)

Samples with nominal compositions of (Ru_0.9Nb_0.1)Sr₂(Gd_0.67R_0.67Ce_0.66)Cu₂O _z (R = Nd, Gd, Tb) were prepared, and the influence of doped rare-earth element on the structural, magnetic and superconducting properties of these samples has been investigated. Resistivity and room-temperature thermoelectric power measurements showed that the superconductivity is mainly affected by the change in the hole concentration induced by doping of the rare-earth element. All of these samples exhibited weak ferromagnetic behavior at temperatures below approximately 90 K with the branching of zero-field-cooled (ZFC) and field-cooled (FC) magnetization. In the ZFC magnetization measurements, diamagnetic transition was observed at temperatures below approximately 15 K for the R=Gd and Nd samples. Magnetization measurements also revealed that the partial substitution of Tb for Gd results in a significant enhancement of weak-ferromagnetic component of the FC magnetization, as well as an increase in the magnetic ordering temperature up to 20 K. A quite opposite behavior was observed in the FC magnetization for the case of the R=Nd sample. The experimental results are discussed in conjunction with the local structural changes in the Ru sublattice, which are induced by doping of the rare-earth element, based on the results of Rietveld refinement of the X-ray diffraction data.     

A chemical synthetic route for the preparation of fine-grained metal molybdate powders

Nanocrystalline metal molybdates [i.e., AMoO₄, where A=Ca(II), Co(II), Cu(II), Ni(II), and Zn(II)] powders have been prepared from the complete evaporation of a polymer-based metal-complex precursor solution. The metal ions are in aqueous solution through complexion with triethanolamine (TEA) and are dispersed in a polymeric reagent composed of an aqueous solution mixture of sucrose and polyvinyl alcohol (PVA). The mesoporous carbon-rich precursor powders has been obtained on complete dehydration of the precursor solution and generates the respective metal molybdate phase after complete removal of carbonaceous residue at a heat treatment temperature less than 500 °C. X-ray diffractometry (XRD), Differential Thermal Analysis and Thermogravimetric Analysis (TG/DTA), and Transmission Electron Microscopy (TEM) have been used to characterize the preprecursors and the heat-treated final powders. The average particle sizes as measured from X-ray line broadening and transmission electron microscopy studies are around ∼10–30 nm. The particles having sizes of the same order with that of crystalline indicates the poor agglomeration of crystallites.     

Electrical conductivity of Ln2CuO4 compounds

Electrical conductivity of Ln₂CuO₄ compounds (where Ln = La, Pr, Nd, Sm, Eu and Gd) have been studied in air from 100° to 900°C. Results show that La₂CuO₄ is metallic showing an anomaly in its conductivity at about 310°C, while the conductivity of Pr₂CuO₄ enhances rapidly with temperature showing a saturation behaviour at high temperature. All other compounds are semiconducting. At high temperatures the Nd, Sm, Eu and Gd compounds have the same activation energy for conduction of 0.82 ± 0.02 eV.     

Studies on the solid solutions of Ln1−xSrxVO3−0.1x

Solid solutions (Ln_1?xSr_xVO_3?0.1x; Ln:La, Nd, Eu, Gd, Er or Yb) composed with rare-earth vanadites (LnVO₃) and strontium vanadite (SrVO_2.9) and resulted from the reaction of Ln₂O₃, V₂O₃ and SrO were studied. If the value of x in Ln_1?xSr_xVO_3?0.1x was lower than a certain value, crystals isostructural with LnVO₃ (Ln:La, tetragonal cell; Ln:Nd, Eu or Gd, orthorhombic cell) were obtained; if the value of x was higher than that, the products belonging to a cubic system isomorphous with SrVO_2.9, were formed in the case of La, Nd, Eu or Gd. As the ionic radius of the incorporated rare-earth ion becomes larger, the region of the cubic-phase formation increases in size.     

Synthesis of the new layered oxides NaRbLnMO5 (Ln = La, Nd, Sm, Eu, Gd; M = Nb, Ta)

The new layered transition metal oxides NaRbLnMO₅ (Ln = Nd, Sm, Eu, Gd; M = Nb, Ta) were synthesized by direct solid-state reaction. NaRbLaNbO₅ crystallizes with a tetragonal unit cell [a=5.839(6) Å, c=8.313(1) Å] analogous to that of the related compound NaKLaNbO₅, while NaRbLaTaO₅ indexes to a larger monoclinic unit cell [a=9.577(2) Å, b=5.834(1) Å, c=8.323(2) Å, β=93.00(2)]. NaRbLnNbO₅ can be prepared for Ln = Nd, Sm, Eu, Gd, and NaRbLnTaO₅ can be prepared for Ln = Nd, Sm. Both series of compounds show the expected decrease in unit cell volume as the size of the lanthanide decreases. NaRbLaNbO₅ is also amenable to ion exchange, forming Li_2−xRb_xLaNbO₅ upon reaction with molten lithium nitrate.     

Catalytic activity of rare-earth orthoferrites and orthochromites

The catalytic activity of LnFeO₃ and LnCrO₃ (Ln=LaGd) for methanol oxidation has been studied by the measurement of conductivity change on methanol adsorption in the mixture gas (O₂(0.5%) + N₂(99.5%)); The activity of these compounds appeared above the Neel points. The sequence of the activity for LnFeO₃ was Gd > Eu > Sm > Nd > Pr > La, which was correlated with the Néel temperature; GdFeO₃, which has the lowest Néel temperature, showed the highest activity. However, a relationship between activity and Néel temperature was not clearly observed in LnCrO₃.     

Improved flux growth of rare-earth arsenates,RAsO4

The tetragonal rare-earth arsenates (R = Lu to Sm) have previously been grown as small crystals from Pb₂As₂O₇ as flux.Attempts to prepare larger crystals by modifying the flux and controlling evaporation by means of a crystalline seal are described. The crystal size was increased by 1 to 2 orders of magnitude.The preparation of the monoclinic arsenates (R = Nd to La) is also described.     

Improvement of the magnetic and electric properties of Cu-Zn ferrites

We study Cu-Zn polycrystalline ferrites substituted with rare-earth ions. For substituted samples (R = Nd, Sm, and Gd), the initial magnetic permeability and homogeneity increase as compared with nonsubstituted samples. At the same time, the magnetization and energy losses decrease for all substituted samples. The specific resistance increases only for samples with R = La and Nd. The grain sizes increase for all substituted samples (as compared with nonsubstituted) except the samples with R = La and Nd. It is shown that the Curie temperature is almost constant and does not depend on the type of substituted rare-earth ions.Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 4, pp. 84–88, July–August, 2004.     

Rare-earth (Nd, Sm, Eu, Gd and Y) enhanced CeO2 solid solution nanorods prepared by co-precipitation without surfactants

Rare-earth doped CeO₂ solid solution nanorods were successfully prepared via a simple co-precipitation method without surfactants at room temperature and pressure. The products were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The effects of the doping contents, pH values, aging times and the precipitation agents on the structure and morphology were investigated. We found that the yield and uniformity of the nanorods were significantly improved by doping with optimum contents of Nd, Sm, Eu, Gd or Y. The intrinsically anisotropic structure of the rare-earth hydroxides is the driving force for the growth of the nanorods. Raman spectra show a great increase in oxygen vacancy concentrations on the doped CeO₂ solid solutions compared with that of pure CeO₂.     

On atoms charge states in RBa2Cu3Ox high-temperature superconductors

The distribution of charge states of all atoms in the unit cell of RBa₂Cu₃O_x; x=6,7 (R = Yb, Er, Ho, Y, Gd, Eu, Sm, Nd, Pr) is determined on the basis of comparison of the theoretical calculated electric field gradient tensor and existing experimental data. The variation of the charge states of the atoms as a function of the ionic radius is analyzed.     

Structural properties of inorganic materials: tungsten bronze SBN ceramics

Rare-earth doped strontium barium niobates were synthesized using usual ceramic technique. The dopants are La, Ce, Gd, Sm and Nd. The materials were characterized by XRD and density measurements. The grain sizes were determined from SEM analysis. Lattice parameters changed uniformly with rare-earth dopants in unfilled structures. Density measurements and SEM analysis confirmed only minute changes in the densities of the ceramics.     

Enhancing the oxygen permeability of Ba0.5Sr0.5Co0.8Fe0.2O5 + δ membranes by coating RBaCo2O5 + δ(R= Pr, Nd, Sm, Gd) layers

The effects of a thin RBaCo₂O_5 + δ (R= Pr, Nd, Sm, Gd) layer coating on the oxygen permeation flux through Ba_0.5Sr_0.5Co_0.8Fe_0.2O_5 + δ(BSCF) membrane were investigated. Due to the high oxygen adsorption and desorption rate constants k_a and k_d of the RBaCo₂O_5 + δ (R112) materials and the porous structure of the coating layers, the oxygen permeation flux through the BSCF membranes can be enhanced remarkably. It was found that the reaction between Pr112 and BSCF also has significant influence on the oxygen permeation flux. The reaction between Pr112 and BSCF forms impurities which may block oxygen permeation flux. However, Nd112, Sm112, and Gd112 do not react seriously with BSCF, therefore, coating layers of these materials can significantly enhance the oxygen permeation flux of BSCF membranes.     

Etude de la transformation ordre-desordre de la structure fluorite a la structure pyrochlore pour des phases (1−x) ZrO2  x Ln2O3

Pyrochlore type phases of formula Ln₂Zr₂O₇ (Ln = La, Nd, Sm, Gd) have been studied by X ray diffractometry, Raman spectroscopy and electron microscopy. A disorder appears at high temperature for some of these phases and yields an oxygen deficient fluorite type structure. Important modifications of the Raman spectra associated with this order - disorder transformation, are related to the observation of anti-phase domains.     

Surface characterization and catalytic activity of Ln2Ti2O7 (Ln=Y,Sm, Gd and Tb)

The surface of the catalyst, Ln₂Ti₂O₇ where Ln=Y, Sm, Gd and Tb was analysed by X-ray photoelectron spectroscopy. Terbium is found to be present in 3⁺ and 4⁺ states while Y, Sm and Gd are present in the 3⁺ state. The catalytic activity of these oxides was tested using decomposition of N₂O as the test reaction and the catalytic activity was correlated to the surface properties.     

Crystal structure and optical properties of oxynitride rare-earth tantalates RTa-(O, N) (R = Nd, Gd, Y)

X-ray absorption and photoluminescence were used to investigate the structure of rare-earth tantalum oxynitrides RTa-(O, N) (R = Nd, Gd, Y). Owing to the size of the rare-earth element, the crystal structure tends towards the pyrochlore or defect fluorite-type structure. EXAFS suggested neodymium and yttrium atoms are coordinated either by 6 + 2 or 6 oxygen/nitrogen atoms in the Nd or Y respective oxynitrides although the coordination number of tantalum is six in both compounds. Photoluminescence for 5 at.% doped europium showed the spectra compatible with the point symmetry C_3v lower than O_h in fluorite and D_3d in pyrochlore structure type for both the Gd and Y tantalum oxynitrides. These measurements supported that their structure types were basically pyrochlore for Nd and Gd tantalum oxynitrides and defect fluorite for Y tantalum oxynitride but they are highly defective.