Smaller Rare-earth Ions Research Articles

Origin of metal–insulator transition in the weak-ferromagnetic superconductor system RuSr2RCu2O8 (R = rare earths)

For the oxygen-annealed weak-ferromagnetic superconductor system RuSr2RCu2O8 (R=rare earths), superconducting transition temperature Tsc decreases steadily from maximum 56K for smaller rare earth Gd3+ (ionic radius r=0.105nm), to 54K for (Eu0.5Gd0.5)3+, 36K for Eu3+, 8K for (Sm0.5Eu0.5)3+, and metallic but not superconducting for larger Sm3+ (r=0.108nm), with a metal–insulator transition for even larger rare earth ions Nd3+ (r=0.112nm) and Pr3+ (r=0.113nm). Powder X-ray diffraction Rietveld refinement study indicates that the insulating phase is stabilized in the undistorted tetragonal phase (space group P4/mmm) with the larger tetragonal lattice parameter a∼0.390–392nm, which gives a reasonable Ru5+–O bond length of d∼0.197nm. On the other hand, the metallic phase with smaller rare earth ions can be stabilized only in the distorted tetragonal phase (space group P4/mbm), with the smaller a/√2∼0.383–0.385nm but still provide a reasonable Ru–O bond length through RuO6 octahedron rotation. The metal–insulator transition as well as the variation of superconducting Tsc is closely related to oxygen deficiency content δ which control the variation of mobile hole concentration and structural variation in this hole-doped superconductor system.

Energetics of rare-earth-doped hafnia

The enthalpies of formation of rare-earth (RE)-doped Hf1−xRExO2−x/2 solid solutions (RE = Sm, Gd, Dy, Yb; x = 0.25 to 0.62) with respect to the oxide end members, monoclinic HfO2 and C-type REO1.5, were determined using oxide melt solution calorimetry. The enthalpies of formation fit a function quadratic in composition. The strongly negative interaction parameters in all solid solutions confirm a strong tendency for short-range order. Though strongly negative for all systems, the interaction parameters become less negative with increasing ionic potential (decreasing RE radius). Crystallization energetics were investigated for amorphous coprecipitation products with x = 0.4. The energy difference between the crystalline (fluorite and pyrochlore) and amorphous phases decreases with decreasing dopant radius. This suggests that systems doped with small RE ions have more similar local structures in the fluorite and amorphous phases. These observations may result in a smaller kinetic barrier to recrystallization and account for the greater radiation resistance of materials with smaller RE cations.

Organolanthanide-Catalyzed Hydroamination/Cyclization Reactions of Aminoalkynes. Computational Investigation of Mechanism, Lanthanide Identity, and Substituent Effects for a Very Exothermic C−N Bond-Forming Process

This contribution focuses on organolanthanide-mediated catalytic hydroamination processes and analyzes the exothermic hydroamination/cyclization of a prototypical aminoalkyne, H2N(CH2)3C⋮CR, mediated by Cp2Sm− complexes, using density functional theory. The reaction is found to proceed in two discrete steps, namely, cyclization with concerted Ln−C and C−N bond formation and subsequent Ln−C protonolysis. Dissociation of the cyclized amine then follows to regenerate the active catalyst. Analysis is carried out for (i) insertion of the triple bond moiety into the Sm−N bond via a four-center transition state, (ii) subsequent Sm−C protonolysis by a second substrate molecule, (iii) the effects of other Ln+3 ions and aminoalkyne R substituents on the reaction energetics, and (iv) comparison to the analogous, essentially thermoneutral process for aminoalkenes. DFT energetic profiles are computed for the turnover-limiting aminoalkyne C⋮C triple bond insertion into the Ln−NH− linkage, and the geometries and stabilities of reactants, intermediates, and products are analyzed. The picture that emerges involves concerted, rate-limiting, exothermic insertion of the alkyne fragment into the Ln−N(amido) bond via a highly organized, seven-membered chairlike cyclic transition state (ΔH⧧calcd = 4.6 kcal/mol, ΔS⧧calcd = −11.9 eu). The resulting cyclized complex then undergoes exergonic protonolysis to yield an amine−amido complex, the likely resting state of the catalyst. Large rate accelerations effected by smaller lanthanide ions and certain alkyne substituents can be understood in terms of approach distances and charge buildup in the cyclization transition state.

A molecular dynamics model of the atomic structure of Tb metaphosphate glass(Tb2O3)0.25(P2O5)0.75

There have been many recent experimental studies of rare earth (RE) phosphate glasses,(RE2O3)x(P2O5)1−x, but only two previous reverse Monte Carlo (RMC) modelling studies. The current study reportsthe first molecular dynamics model of an RE phosphate glass, for Tb metaphosphate glass, withx = 0.25. The model is in good agreement with experimental results for nearest-neighbourdistances and coordination numbers, and in reasonable agreement for x-rayand neutron diffraction structure factors. There is a tetrahedral phosphatenetwork, with marked splitting of distances from P to bridging oxygen(Ob) and from P tonon-bridging oxygen (Onb). The phosphate network has tetrahedra denotedQn (wheren is the number ofOb) with an averageof n = 2.1 and mostlyQ2 groups, butwith some Q1 andQ3 groups. Most Tb arecoordinated to six Onb, andthe average coordination is NTbOnb = 5.7, which compares favourably with experimental results that indicateNRE−O∼6 in metaphosphate glasses with small RE ions. The great majority ofOnb are bonded to only one Tb, but there are a few sharedOnb occurringin Tb–Onb–Tb configurations. These cause a small peak in Tb–Tb correlations around 4 Å, prior to themain peak around 6 Å. The corresponding Tb–Tb partial structure factor shows promisingagreement with a recent experimental measurement using magnetic neutron scattering.

Work function of the mixed-valent manganese perovskites

We have performed resistivity and work function measurements on a series of samples with Ln0.7D0.3MnO3 composition, where Ln is a lanthanide (La, Pr, or Nd) and D is a nominally divalent ion (Sr, Ca, or Pb). The resistivity measurements are, within some small variation, in agreement with those previously reported for polycrystalline samples of similar composition. The measurement of a work function in air is often difficult to reproduce, mainly owing to the fact that it is a surface probe and can be modified significantly by attachment of contaminants from the atmosphere. Here we have obtained reproducible results that are slightly more stable than the reference metals used in the measurements. This required the introduction of a higher order correction to a common Kelvin probe experimental procedure. Another factor contributing to the high quality of the results is the low carrier density of the conducting oxides. This allows greater penetration of quasistatic electric fields. The work function results here are in overall agreement with previous photoemission work that determined an approximate electron binding energy. We find the best correlation of increased work function is to strain introduced by larger divalent ions and smaller Lanthanide ions. We conclude that the work function is dominated by electrons near the divalent site, and this is interpreted in terms of strain modifying the coulomb energy of the electronic states.

Crystal Structures and Thermal Behaviour of Lanthanide(III) Hexanoate 1, 10‐Phenanthroline Complexes, [M(C5H11CO2)3(phen)] and [Tm(C5H11CO2)2(NO3)(phen)

AbstractLanthanide(III) hexanoate 1, 10‐phenanthroline complexes crystallise in the space group P21/n. The compounds consist of dimers, whereby two lanthanide ions are held together by two bidentate bridging and two tridentate bridging carboxylate groups. The first coordination sphere of the lanthanide ions is completed by one bidentate chelating carboxylate group and by one bidentate 1, 10‐phenanthroline molecule, resulting in the coordination number 9. The dimers have a spherical form, which has important consequences for the thermal properties of complexes. The basic idea behind the preparation of this type of compounds is the stabilisation of the ionic lanthanide layer, so that the smaller lanthanide ions (from which the normal alkanoates do not show mesomorphism because they are too small) show liquid crystallinity. The stabilisation of the ionic layer was successful, expressed by the high melting temperatures, but mesomorphism is not observed. The absence of mesomorphism is related to the isotropic structure of the compounds. A lower symmetry is obtained when a hexanoate group is replaced by a nitrate group. Thulium(III) dihexanoate nitrate 1, 10‐phenanthroline crystallises in the space group P1¯. However, this compound also shows a spherical dimeric structure, but no mesomorphism.

Computer-Simulation Study of the Orthorhombic−Hexagonal Phase Change in Lanthanide Manganates (LnMnO3)

Static lattice simulation techniques are applied to modeling the structures of the lanthanide manganates (LnMnO3). New interatomic potentials are developed based on the Born model of the ionic solid supplemented by the angular overlap model to describe the nonspherical components of the Mn3+−O2- interactions. The new model correctly predicts the greater stability of hexagonal compared with orthorhombic structures for the system containing the smaller lanthanide ions. A detailed comparison of the simulated structures with experiment is presented.

Acid-amide calixarene ligands for uranyl and lanthanide ions: synthesis, structure, coordination and extraction studies

The synthesis of a series of calix[4]arene ligands bearing lower rim 1,3-acid-amide functionality for the two-phase solvent extraction of lanthanide and uranyl ions from aqueous wastes is described. X-Ray crystal structure studies demonstrate binding of lanthanide ions at the calixarene lower rim through phenolate, carboxylate and carboxamide oxygen atoms. The crystal structures of a centrosymmetric, dimeric lanthanum(III) and non-centrosymmetric dimeric sodium(I) complexes of a 1,3-acid-dihexylamidecalix[4]arene ligand are detailed. Electrospray mass spectrometry confirms retention of this 2 ∶ 2 dimeric structure for large lanthanide ions and that the smaller lanthanide ions e.g. Lu3+ form monomeric 1 ∶ 1 calixarene ∶ lanthanide complexes in solution. Extraction studies demonstrate a pH dependent uptake of lanthanide and uranyl cations by 1,3-acid-amide ligands with the nature of the carboxamide substituent largely irrelevant to extraction efficiency. Upper rim modified 1,3-dinitro and de-tert-butylcalix[4]arene ligands were prepared and NMR and ES-MS studies confirm uranyl coordination by these novel acid-amide ligands. The 1,3-dinitro and de-tert-butyl ligands exhibit superior and more efficient extraction of uranyl cations at lower pH values.

Compositional Effects on the Properties of Si‐Al‐RE‐Based Oxynitride Glasses (RE = La, Nd, Gd, Y, or Lu)

A series of silicon‐aluminum oxynitride‐glass compositions containing selected rare‐earth (RE) additions were prepared to examine the effects of specific RE, as well as Si:Al:RE and N:O ratio, on properties. The properties that were characterized included density, thermal expansion coefficient (α), glass‐transition temperature (Tg), hardness (H), and Young's modulus (E). The compositions (in equivalent percent) selected included: 55 Si‐20 RE‐25 Al oxide and 80 O‐20 N oxynitride, and 45 Si‐30 RE‐25 Al oxide and 70 O‐30 N glasses. The results show that the density increased significantly with an increase in the RE atomic mass and slightly with an increase in N:O ratio. For each of the fixed Si‐Al‐RE‐O‐N compositions, the E, H, and Tg values were each increased by substituting smaller RE ions, whereas the α value was decreased. For each specific cation composition and RE, increasing the N:O ratio systematically led to a decrease in the α values but an increase in the E, H, and Tg values. The observed response in the glass properties to changes in composition appears to reflect modifications in the bonding within the glass network.

Role of Rare Earth Ion in Spin Glass Behavior for R0.7Sr1.3MnO4

Magnetic and structural properties for melt-grown single crystals of monolayered manganites R0.7Sr1.3MnO4 (R = La, Pr, and Nd), possessing the K2NiF4 type structure, have been investigated. On the bases of magnetization, neutron powder diffraction, and relaxation measurements, it is revealed that all the three compounds display spin glass behaviors in the low-temperature region. In La0.7Sr1.3MnO4, spin glass behaviors are observed isotropically both in the ab plane and along the c axis. In contrast, Pr0.7Sr1.3MnO4 and Nd0.7Sr1.3MnO4 show anisotropic spin glass transition. The salient feature of spin glass behavior found in the Pr- and Nd-doped compounds can be understood in terms of the enhanced lattice distortion induced by small rare earth ion as well as the magnetic anisotropy of the rare earth ions.

Structures and catalytic properties of triphenylphosphine oxide complexes of scandium and lanthanide triflates

The complexes M(OTf) 3(Ph 3PO) 4 where M=lanthanide metals and scandium have been prepared and characterised in the solid state by single crystal X-ray diffraction, infrared spectroscopy and elemental analysis. All the complexes have the ionic structure [M(OTf) 2(Ph 3PO) 4] +[OTf] −. Single crystal X-ray structures for M=Sc, Nd and Lu are reported. In the complexes of the early lanthanides (LaNd) the metals are seven-coordinate with one monodentate, one bidentate triflate. The smaller lanthanide ions, and scandium, have six-coordinate structures with an approximate octahedral coordination about the metal where both triflates are bound as monodentate ligands. The solution structures have been analysed by electrospray mass spectrometry, which indicates that extensive ligand redistribution occurs. The complexes catalyse the Friedel Crafts acylation, alkenylation and alkylation of activated aromatics with modest increases in selectivity compared to the same reactions using scandium triflate.

Metal-Nonmetal Changeover in Pyrochlore Iridates

We report the low temperature properties of a new series of pyrochlore iridates R 2 Ir 2 O 7 ( R =rare-earth elements). We found that the compounds with R =Pr, Nd, Sm, and Eu exhibit metallic conductivity, whereas those with smaller rare-earth ions are nonmetallic. Such metal-nonmetal changeover is attributable to the importance of electron correlation among the Ir 5d electrons. Reflecting the geometrical frustration in the pyrochlore lattice, the rare-earth moments do not exhibit magnetic ordering to temperatures well below the antiferromagnetic Weiss temperature. We did not find any sign of superconductivity down to 0.3 K in these compounds.

Effect of crystal symmetry on O′-phase weak ferromagnetism

Correlation between copper weak-ferromagnetic/canted-antiferromagnetic (WF/CAF) order and structural symmetry of the Gd 2−x R x CuO 4 ( R= Eu and Tb ) systems is reported. With smaller rare earth ions, lattice mismatch between (Gd,R) 2O 2 and CuO 2 layers results in a small oxygen distortion in square CuO 2 plane. The distortion lowers the crystal symmetry from tetragonal T′- to an orthorhombic O′-phase. The weak ferromagnetism is a direct result of such a non- 180° α (Cu–O–Cu) coupling with CAF alignment and a WF saturation moment m s ( Cu 2+)=μ( Cu 2+) sin θ , with canting angle θ=(π− α)/2. Magnetic and crystallographic data indicate that the magnetic moment m( T) and the canting angle θ decrease (increase) with larger (smaller) ion doping.

Variation of oxygen stoichiometry in BaRE(Cu0.5Fe0.5)2O5+δ double perovskites upon heat treatments under ambient and high pressure

The tunability of the oxygen stoichiometry in the BaRE(Cu0.5Fe0.5)2O5+δ double-perovskite structure, stable with rare earth elements RE=Lu–Pr, was studied by both normal-pressure thermogravimetric oxidative (O2) and deoxidative (Ar) post-annealings and high-pressure heat treatments carried out in a cubic-anvil-type high-pressure apparatus. Under reducing conditions the BaRE(Cu0.5Fe0.5)2O5+δ double perovskite is obtained at δ≈0 irrespective of the size of the RE constituent. In the structures with RE=Pr and Nd the oxygen stoichiometry can be continuously varied by normal-pressure post-annealings up to δ≈0.24 and 0.35, respectively, whereas with a smaller rare earth ion, i.e., RE=Lu–Sm, the amount of excess oxygen raises from δ≈0 only upon high-pressure heat treatment.

Role of calcium in the evolution of superconductivity in a (La2-xRx)Ba2(CayCu4+y)Oz (R = Y, Er, Gd) system

It has been reported that, by adding equal amounts of CaO and CuO to non superconducting La3Ba3Cu6O z (La-336), a series of superconductors with nominal compositions La3Ca y Ba3Cu6+yO z were prepared with maximum Tonc ∼ 80K. Similar studies on addition of CaO and CuO in nonsuperconducting LaBaCu2O z (La-112) resulted into superconducting LaCaBaCu3O z (La-1113). To date no attempt has been made to synthesize La2CaBa2Cu5O z (La-2125) superconducting phase by addition of CaO and CuO to non superconducting La2Ba2Cu4O z (La-224) system. Also no reports are published to study the effect of replacing larger La3+-ions (1.01A) by smaller rare earth ions viz Y3+(0.89A), Er3+(0.91A), Gd3+(0.91A) on the structural and superconducting properties of (La2−xR x )Ba2(Ca y Cu4+y)O z (LRBCaC); 0.0 ≤ x ≤ 0.5; y=2x system. In this paper, we report the method of synthesis, structural and superconducting property characterization using X-ray diffraction, oxygen content measurements using iodometry, resistivity measurements using d.c. four probe technique and a.c. susceptibility measurements in the temperature range RT to 15K. Also a comparative study, on the evolution of superconducting phase with Ca-concentration for different rare earth substitutions for LRBCaC system in the context of hole doping mechanism, is carried out.

High-Pressure Synthesis and Magnetic Properties of Layered Double Perovskites Ln2CuMO6 (Ln = La, Pr, Nd, and Sm; M = Sn and Zr)

Ln{sub 2}CuSnO{sub 6} (Ln = Pr, Nd, and Sm) and La{sub 2}CuZrO{sub 6} have been stabilized in the same layered double perovskite structure as La{sub 2}CuSnO{sub 6} at high pressures of 6--8 GPa. Their crystal structures and magnetic properties were investigated. Replacement of La{sup 3+} by smaller lanthanide ions and Sn{sup 4+} by larger Zr{sup 4} increased the buckling of the Cu-O-Cu bonds in the CuO{sub 2} layers. In Nd{sub 2}CuSnO{sub 6}, both the Cu and Nd sublattice were found to exhibit weak ferromagnetism owing to spin canting. The magnetization data showed an anomalous transition at 110 K.

Reactions of rare earth acetates with aluminum isopropoxide in ethylene glycol: Synthesis of the garnet and monoclinic phases of rare earth aluminates

The reactions of mixtures of rare earth (RE) acetates and aluminum isopropoxide (Al/RE = 3/5) in ethylene glycol at 300°C afforded amorphous products. The IR spectra of these products were essentially identical with each other and indicated the presence of an acetate group and ethylene glycol moiety. When the content of RE (La–Gd) acetate in the starting mixture was increased, a crystalline glycol complex of RE oxide acetate was also formed together with the amorphous phase, but the reactions of acetates of the smaller RE ions (Tb–Lu) yielded amorphous products with wider compositions (Al/RE = 5/3–1/6). Phase-pure RE3Al5O12 (garnet) and RE4Al2O9 (monoclinic) were obtained by calcination of the amorphous products with the corresponding stoichiometric compositions at low temperatures.

Formation and Stability Regions of the High‐Temperature Fluorite‐Related Phase in the R2O3‐Ta2O5 System (R = La, Nd, Sm, Ho, Er, and Yb)

Solid‐state reaction and annealing of melts of samples in the R2O3‐Ta2O5system (R is a rare‐earth element, i.e., La, Nd, Sm, Ho, Er, and Yb) revealed a defect‐fluorite phase (F‐phase) at high temperatures. The formation region of this F‐phase was in the region of ˜70‐80 mol% R2O3for small rare‐earth ions, such as erbium and ytterbium, but only in the region of ˜80 mol% R2O3at temperatures of >1800°C for large rare‐earth ions, such as the lanthanum‐through‐samarium series. This F‐phase exhibited disordered cation and anion sublattices, such as (R0.8Ta0.2)(O1.7box0.3). The F‐phase decomposed to R2O3 and an ordered phase–R3TaO7 (orthorhombic weberite)–through a eutectic reaction at temperatures in the range of 1500°‐1700°C for gadolinium or larger rare‐earth ions, whereas the F‐phase was stable at 1500°C for the small rare‐earth ions (the erbium‐through‐ytterbium series).

Thermally stimulated depolarization currents and ionic conductivity of cubic lead fluoride containing small rare earth ions

Abstract This work is concerned with thermally stimulated depolarization current (TSDC) and ionic conductivity studies in lead fluoride containing the small rare earths Dy, Ho, Er and Yb. The TSDC scans from 80 to 300 K show two peaks. For Pb1−xErxF2+x one is located at 106 K, and another, which is much stronger, occurs at about 160 K. The former is associated with a dipolar defect containing at least two rare earths and the latter is attributed to the development of F− space charge during polarization of the sample. The activation energies obtained from both the high temperature TSDC peak and the ionic conductivity are the same, which corroborates the latter assignment. In addition, the ionic conductivity is shown to be independent of concentration. Those results can be understood of rare earth clustering, which is either absent or is unobservable dielectrically for large rare earths such as lanthanum, occurs extensively even at very low concentrations of the small rare earths. The explanation is that the majority of fluorine charge compensators are trapped by clusters.

Der Halbleiter-Metall-Übergang in Pyrochloren des Systems Ln2-xCuxRu2O7-z (Ln = Pr, Sm, Eu, Gd, Dy) / The Semiconductor-Metal Transition in Pyrochlors of the System Ln2-xCuxRu2O7-z (Ln = Pr, Sm, Eu, Gd, Dy)

In the pyrochlor system Ln2-xCuxRu2O7-z a partial substitution of Pr bv Cu is observed for x ≾ 0.375. With increasing x a gradual semiconductor-metal transition occurs. The change of sign of TCR from negative to positive is observed for ρ ∾ 6 × 10-3 Ω cm (x = 0.2). The presence of variable range hopping in the insulating regime strongly suggests that the metal-semiconductor transition is of Anderson type. For the pyrochlores of type Ln1.625Cu0.375Ru7-z with the smaller rare earth ions Sm, Eu, Gd, Dy the metallic conductivity is absent and localization of Anderson type occurs.