Rare-earth Radius Research Articles

Магнитные фазовые диаграммы R-=SUB=-1-x-=/SUB=-Sr-=SUB=-x-=/SUB=-(Mn-=SUB=-1-x/2-=/SUB=-Sb-=SUB=-x/2-=/SUB=-)O-=SUB=-3-=/SUB=- (R=La, Pr, Nd, Sm, Eu) с ионами марганца в трехвалентном состоянии

AbstractThe results of neutron-diffraction, magnetic, and magnetotransport studies of La_1 ‒ x Sr_ x (Mn _1- x /2 ^3+ Sb _ x /2 ^5+ )O_3 ( x ≤ 0.7) pervoskites, which contain only Mn^3+ ions, are reported. It is demonstrated that the system undergoes a transition from the antiferromagnetic state to the ferromagnetic one at x ∼ 0.2. Its structure then changes from O ′-orthorombic to O -orthorombic, which is attributable to orbital disordering. At x ≥ 0.6, the competition between antiferromagnetic and ferromagnetic interactions and strong diamagnetic dilution trigger the emergence of the cluster spin-glass phase. As the ionic radius of rare-earth ions in R_0.7Sr_0.3Mn_0.85Sb_0.15O_3 decreases, the ferromagnetic state transforms gradually into spin glass (R = Sm). All ferromagnetic compositions are dielectric in the magnetically ordered phase with strong magnetoresistive effects. It is assumed that ferromagnetism is induced by the strong covalency and orbital disordering.

Seven-coordinate lanthanide complexes with a tripodal redox active ligand: structural, electrochemical and spectroscopic investigations.

The tripodal ligand TREN-(3,5-di-tert-butylsalicylidene)3 (H3L) was synthesized and its tris(phenolato) lanthanide complexes L-Ln (Ln = NdIII, EuIII, TbIII, GdIII, ErIII, YbIII and LuIII) were prepared. The X-Ray crystal structures confirm that each metal ion resides in a similar monocapped octahedral geometry, excluding water molecules from the coordination sphere. The coordination bond distances are in agreement with the lanthanide contraction, with Ln-O bond lengths in the range 2.139-2.216 Å. The complexes show three reversible monoelectronic oxidation waves, which are assigned to the successive oxidation of the phenolate moieties to phenoxyl radicals. The L-Nd complex is the easiest to oxidize, with E = 0.11, E = 0.21 and E = 0.34 V vs. Fc+/Fc, due to the larger size of the lanthanide ion. The ΔE1/2 value (ΔE1/2 = E-E) is correlated to the lanthanide radius, with values of 0.10 V for L-Nd and 0.22 V for L-Lu. The monoradical species were persistent in solution, allowing for their characterisation. All exhibit a distinct absorption band at around 445 nm due to the phenoxyl π-π* transitions. The EPR spectrum of L-Lu+ consists of a single resonance at giso = 1.999, confirming the radical nature of the oxidized product. Most of the other complexes (L-Gd, L-Er, L-Yb) show a quenching of the LnIII-based resonances upon oxidation, indicative of magnetic interactions between the metal and the radical spins. The L-Ln (L = Nd, Er, Yb) complexes exhibit a metal-based luminescence upon excitation of the ligand. A significant quenching of the luminescence was observed upon radical formation: 92%, 83% and 79% respectively for L-Nd+, L-Er+ and L-Yb+.

Energetic characteristics of barium cerates doped by yttrium, gadolinium and luthetium oxides

Abstract For the first time solution enthalpies of BaCe0.8Y0.1Gd0.1O2.9, BaCe0.8Y0.1Lu0.1O2.9 and mixtures of BaCl2 + 0.8CeCl3 + 0.1YCl3 + 0.1GdCl3, BaCl2 + 0.8CeCl3 + 0.1YCl3 + 0.1LuCl3 in 1 mol dm−3 HCl with 0.1 mol dm−3 KI have been measured. On the basis of experimental data obtained and literature data the standard molar enthalpies of formation and stabilization energies were calculated. Using Born-Haber cycle the lattice energies were calculated. It was established that lattice energy was increased from BaCe0.8Y0.1Gd0.1O2.9 to BaCe0.8Y0.1Lu0.1O2.9. It correlates with decreasing of ion radius of rare earth element from gadolinium to lutetium. Increasing of lattice energy with decreasing of ion radius of rare earth element was explained with modified formula of Kapustinskii.

Photoluminescent properties of three lanthanide compounds of formulae LnCl3(diphenyl((5-phenyl-1H-pyrazol-3-yl)methyl)phosphine oxide)2, Ln = Sm, Eu and Tb: X-ray structural, emission and vibrational spectroscopies, DFT and thermogravimetric studies

Abstract The synthesis and properties of a new sensitizing bidentate ligand, diphenyl((5-phenyl-1H-pyrazol-3-yl)methyl)phosphine oxide, 2, capable of demonstrating the emissive properties of lanthanide elements, is described. Two ligands are attached to LnCl3 moieties, Ln = Sm, Eu and Tb, resulting in complexes of formulae LnCl3(2)2. These complexes were structurally characterized by single-crystal X-ray crystallography and have geometries that are pentagonal bipyramidal with the two bidentate ligands in a cisoid conformation on the equatorial plane and the three chloride ligands in a mer-configuration. The coordination bond distances decrease from Sm to Tb paralleling the decrease in lanthanide ionic radii. The photoluminescent properties of these complexes were examined and, one of them, TbCl3(2)2, was assessed a ΦF of ca. 1.00. DFT calculations were conducted to understand the mechanism of proton transfer in ligand 2 for a hydrogen atom hopping between nitrogen atoms on the pyrazolyl moiety and, to assess aspects of the coordination sphere of the lanthanide complexes.

Role of Nd-Ni on structural, spectral and dielectric properties of strontium-barium based nano-sized X-type ferrites

The influence of neodymium and nickel substitution on structural and dielectric parameters was investigated in strontium-barium X-type hexagonal ferrites having composition SrBaCu2−xNixNdyFe28−yO46 (x = 0, 0.2, 0.4, 0.6, 0.8, 1 and y = 0, 0.02, 0.04, 0.06, 0.08, 0.1). Sol-gel method was employed for synthesizing these hexagonal ferrites. The XRD plots of all studied materials which were annealed at 1250°C show single phase characteristics. Lattice parameter ‘c’ increased as a consequence of larger radius of rare earth ion (Nd3+) as compared to (Fe3+), while lattice parameter ‘a’ showed very small variation. The cell volume was obtained in the range 2508.32–2523.75 (Å3). The inclusion of Nd-Ni also affected X-ray density, bulk density and porosity. The FTIR spectroscopy indicated the particular absorption peaks of hexagonal ferrites and it was performed in the range of 500–700cm−1. On account of Nd-Ni doping, the dielectric constant, dielectric loss and AC-conductivity showed decreasing trend. The occupancy of Nd3+ ions at octahedral site impedes the valence alternation of Fe3+; therefore there was decrease in dielectric permittivity. Ac conductivity has been decreased from 9.14 to 6.49 (Ωcm)−1 at frequency of 2.7GHz. The Cole-Cole plots of synthesized materials noticeably revealed grain boundary contribution. The appearance of single semi-circle in impedance Cole-Cole graphs confirms the exceptional role of grain boundaries in the conduction process. The considerably lower dielectric parameters of investigated nano X-type ferrites propose their feasibility for high-frequency applications (phase shifters, dielectric resonators, stealth technology etc).

Polarized Raman scattering on single crystals of rare earth orthochromite RCrO3 (R = La, Pr, Nd, and Sm)

AbstractWe report on polarized Raman scattering on orthorhombic RCrO3 (R = La, Pr, Nd, and Sm) single crystals at room temperature. The analysis of polarized Raman spectra as a function of polarizer and analyzer angles with respect to crystallographic axes allowed to observe in these flux grown crystals phonon modes that were not detected yet. On the basis of both Raman selection rules and lattice dynamics calculations within density functional perturbations theory framework, the observed Ag, B1g, B2g, and B3g phonon modes were unambiguously assigned. Phonon mode wave number evolutions as a function of orthorhombic distortion, related to rare earth ionic radius, were investigated and highlighted a complex behavior for several low wave number modes allowed crossing. For the modes of same symmetry, a mode mixing behavior is observed, and numerical calculations based on two or three coupling systems were performed to fit experimental data and to extract the coupling constants.

Crystal chemistry and the role of ionic radius in rare earth tetrasilicates: Ba2RE2Si4O12F2 (RE = Er3+-Lu3+) and Ba2RE2Si4O13 (RE = La3+-Ho3+).

Structural variations across a series of barium rare earth (RE) tetrasilicates are studied. Two different formulas are observed, namely those of a new cyclo-silicate fluoride, BaRE2Si4O12F2 (RE = Er3+-Lu3+) and new compounds in the Ba2RE2Si4O13 (RE = La3+-Ho3+) family, covering the whole range of ionic radii for the rare earth ions. The Ba2RE2Si4O13 series is further subdivided into two polymorphs, also showing a dependence on rare earth ionic radius (space group P{\overline 1} for La3+-Nd3+, and space group C2/c for Sm3+-Ho3+). Two of the structure types identified are based on dinuclear rare earth units that differ in their crystal chemistries, particularly with respect to the role of fluorine as a structural director. The broad study of rare earth ions provides greater insight into understanding structural variations within silicate frameworks and the nature of f-block incorporation in oxyanion frameworks. The single crystals are grown from high-temperature (ca 953 K) hydrothermal fluids, demonstrating the versatility of the technique to access new phases containing recalcitrant rare earth oxides, enabling the study of structural trends.

Study of variation in thermophysical properties of RE6UO12(s) along the lanthanide series

The variation in thermophysical properties along the lanthanide series for RE6UO12(s) was investigated. Structural characterization of Er6UO12(s) was carried out by Rietveld refinement method. The trend in heat capacity along the rare earth series for RE6UO12(s) and RE2O3(s) as function of ionic radius of RE3+ was found to be similar. Hence, the sole cause of excess heat capacity in rare earth uranates was concluded to be the rare earth ion. Linear relation was found between the thermal expansion coefficient of RE6UO12(s) (RE = La, Gd, Lu) and the ionic radii of rare earth ion.

Mott transition and anomalous resistive state in the pyrochlore molybdates

The rare-earth based pyrochlore molybdates involve orbitally degenerate electrons Hund's coupled to local moments. The large Hund's coupling promotes ferromagnetism, the superexchange between the local moments prefers antiferromagnetism, and Hubbard repulsion tries to open a Mott gap. The phase competition is tuned by the rare-earth ionic radius, decreasing which leads to change from a ferromagnetic metal to a spin disordered highly resistive ground state, and ultimately an “Anderson-Mott” insulator. We attempt a quantitative theory of the molybdates by studying their minimal model on a pyrochlore geometry, using a static auxiliary field based Monte Carlo. We establish a thermal phase diagram that closely corresponds to the experiments, predict the hitherto unexplored orbital correlations, quantify and explain the origin of the anomalous resistivity, and present dynamical properties across the metal-insulator transition.

IR absorption spectroscopic characteristics of peripherally substituted thiophenyl phthalocyanine in sandwich bis(phthalocyaninato) complexes

The IR spectra data for a series of thirteen rare earth double-deckers M[Pc(SPh)8]2 (M=Y, Ce⋯Lu, except La and Tm) have been collected and detailed characterized. The electronic absorption spectra showed that the Soret and Q bands are blue-shifted, and especially the Q bands at 705–726nm clearly become stronger along with the decrease of rare earth ion radius. For MIII[Pc(SPh)8]2, the Infrared characteristic absorption peaks for the phthalocyanine anion radical [Pc(SPh)8]− were observed at 1311–1323cm−1 as the strongest absorption bands, which can be ascribed to the pyrrole stretching. As for the Ce[Pc(SPh)8], the marker absorption bands at 1335cm−1 was observed. In addition, the typical IR absorption bands of phthalocyanine radical anion [Pc(SPh)8]− move to the high energy as the decrease of rare earth metal ionic radius. These facts suggest that the π-π electron interaction in these double-deckers becomes stronger along with the lanthanide contraction.

Non-isothermal crystallization kinetics of the heavy-group lanthanide dititanates

The systematic investigation of crystallization kinetics of heavy-group lanthanide dititanate pyrochlores, HL2Ti2O7 (HL=Lu, Tm, Er, Ho, Y, Dy, and Tb) is presented. Materials are prepared by the Pechini-type polymerized complex route, and their thermal properties were studied by differential thermal analysis under non-isothermal conditions. For this group, the crystallization temperature became larger with increasing the ionic radius of lanthanides, from 1066 K for Lu2Ti2O7 to 1125 K for Tb2Ti2O7. The linear dependence of crystallization temperature on radius of heavy lanthanide ions is found to follows Tc[K]=168.86+920.89×rion[Å] semi-empirical equation. Apparent activation energies of crystallization were obtained from calculations based on the Arrhenius equation, isoconversional method and Pérez-Maqueda method, and are ∼805 kJ mol−1 calculated by three different methods. The crystallization kinetics followed the so-called Avrami–Erofeev (A3) model, as confirmed using several criteria, namely, the Málek, master plot, and Pérez-Maqueda methods. The crystallization mechanism of all lanthanide dititanate pyrochlores is found to be the same, and the crystallization process involves a constant rate of nucleation and two-dimensional growth of nuclei.

Phase transition-induced magnetocaloric effects in R2Mo2O7 (R = Er, Dy, Gd and Y) molybdates

We have investigated the magnetic properties and magnetocaloric effects (MCEs) of R2Mo2O7 (R=Er, Dy, Gd and Y). Magnetic ground states of R2Mo2O7 largely depend on the radius of rare earth ion. Er2Mo2O7, Dy2Mo2O7 and Y2Mo2O7 with small rare earth radius exhibit spin-glass state around 20K, while Gd2Mo2O7 with larger rare earth radius undergoes a ferromagnetic state below 78K and a reentrant spin-glass behavior below 34K. Magnetic phase transition leads to magnetocaloric effect. In particular, a natural table-like -ΔSM and a very large refrigerant capacity (RC) value of 94.4J/kg was observed in a field change of 0–10.5kOe for Gd2Mo2O7 due to two successive magnetic transitions at ∼78 and ∼10K. These notable magnetocaloric properties make the Gd2Mo2O7 a potential candidate for low-field Ericsson-cycle magnetic refrigeration in the low temperature range.

Phase structural transition and microwave dielectric properties in isovalently substituted La1−xLnxTiNbO6 (Ln=Ce, Sm) ceramics

Aeschynite-type La1−xLnxTiNbO6 (Ln=Ce, Sm, x=0–1) ceramics were prepared via a conventional solid state method. Analysis of X-ray diffraction, Raman, infrared reflectivity spectra and the microstructures revealed a series of composition-induced phase evolution in sequence: monoclinic→coexistence of monoclinic and orthorhombic→orthorhombic structure, i.e. M→M+O→O. The critical compositions of distinguishing the dominant M or O phase were x=0.15 in La1−xCexTiNbO6 and x=0.10 in La1−xSmxTiNbO6 ceramics, exactly corresponding to the average ionic radius of rare earth ions (IR) ~1.027Å. The crystal structure and microwave dielectric properties of RETiNbO6 (RE=rare earth) ceramics strongly depended on IR. Near-zero τf was achieved in the Ce-sample with x=0.153 (εr=28.9, Q×f=17,275GHz@6.54GHz) as well as in the Sm-sample with x=0.098 (εr=28.2, Q×f=19,186GHz@6.78GHz). Eventually, RETiNbO6 would form O euxenite (-τf), O aeschynite (+τf), and M aeschynite (-τf), as IR<0.945Å, 0.945Å <IR<1.027Å, and 1.027Å<IR<1.032Å, respectively. The infrared reflectivity study also confirmed that the structural phonon oscillations in the infrared region dominated the dielectric performance in the microwave region for this system.

Reproducibility and off-stoichiometry issues in nickelate thin films grown by pulsed laser deposition

Rare-earth nickelates are strongly correlated oxides displaying a metal-to-insulator transition at a temperature tunable by the rare-earth ionic radius. In PrNiO3 and NdNiO3, the transition is very sharp and shows an hysteretic behavior akin to a first-order transition. Both the temperature at which the transition occurs and the associated resistivity change are extremely sensitive to doping and therefore to off-stoichiometry issues that may arise during thin film growth. Here we report that strong deviations in the transport properties of NdNiO3 films can arise in films grown consecutively under nominally identical conditions by pulsed laser deposition; some samples show a well-developed transition with a resistivity change of up to five orders of magnitude while others are metallic down to low temperatures. Through a detailed analysis of in-situ X-ray photoelectron spectroscopy data, we relate this behavior to large levels of cationic off-stoichoimetry that also translate in changes in the Ni valence and bandwidth. Finally, we demonstrate that this lack of reproducibility can be remarkably alleviated by using single-phase NdNiO3 targets.

Phase transformation and interface segregation behavior in Si3N4 ceramics sintered with La2O3–Lu2O3 mixed additive

AbstractMicrostructure and mechanical property of silicon nitride (Si3N4) ceramic are strongly dependent on the selection of sintering additives. When rare‐earth (RE) oxide is used as the sintering additive, segregation of RE ions at interface between Si3N4 grain and intergranular glassy film (IGF) is believed to play a critical role. Although the ionic radius of RE ion is known to be an empirical parameter to modify the mechanical property, the correlation between the segregated ions and their ionic radii is still under controversy. In order to address this issue, (i) rate of α‐β phase transformation and (ii) segregation behavior at the interface were studied for Si3N4 ceramics sintered using mixture of La2O3 and Lu2O3 as additives in this study. Specimens of Lu content 30% and higher exhibited lower activation energies for the α‐β phase transformation as compared with those of Lu content 20% and lower. In terms of the segregation behavior, La was preferably segregated at one site and Lu at the other site along β‐Si3N4/IGF interface in the specimens of Lu content 30% and higher. It is understood from these results that Lu segregation site should be more closely related with grain growth.

Magnetoelectric effect in multiferroic NdMn2O5 **Project supported by Higher Education Commission (HEC) of Pakistan through its IRSIP scholarship program (IRSIP 27 PS 28).

We have measured the dielectric constant for NdMn2O5 in an external magnetic field to map out the magnetoelectric phase diagram. The phase diagram corresponds well with the previously reported data of neutron diffraction and magnetic susceptibility. Our main finding is the observation of a dielectric anomaly in the low temperature phase with a strong magnetoelectric effect, which is attributed to the independent Nd ordering. Moreover, the absence of the dielectric anomaly in the paramagnetic phase is discussed, keeping in view the exchange interaction and its dependence on the rare-earth ionic radius.

Conventional synthesis and characterization of rare earth doped barium borophosphates, BaBPO5:RE (Y, Gd and La)

Purpose: of this research, our target is synthesis and characterization of rare earth metalssuch as Y, Gd and La doped barium borophosphate compounds which are applicable innon-linear optics industry.Design/methodology/approach: The starting materials rare earth oxides, bariumcarbonate, boric acid and ammonium dihydrogen phosphate as analytically grade weighed0.01:1:1:1 molar ratio and homogenized in an agate mortar. The mixture placed into aporcelain crucible to heat in high temperature oven step by step. First, mixtures were waitedat 400°C for 2 hours for calcination process, subsequently heated 900°C with step rate10°C/m for 8 hours, and finally cooled down to room temperature with step rate 10°C/m.After many grindings final product get ready for characterization. X-ray powder diffraction(XRD) analysis was performed using PANanalytical X’Pert PRO Diffractometer (XRD) withCu Kα (1.5406 Å, 45 kV and 30 mA) radiation. Fourier transform infrared spectroscopy(FTIR) was taken on a Perkin Elmer Spectrum 100 FTIR Spectrometer from 4000 to 650cm-1. Scanning electron microscopy was achieved in SEM JEOL 6390-LV. Luminescenceproperties were performed by Andor Solis Sr 500i spectrophotometer. Conventional solidstate syntheses were done in Protherm furnace.Findings: The powder XRD patterns of the samples show that there is no impurity related todoping materials mean all diffractions corresponding to host material barium borophosphatecrystallized in hexagonal system with unit cell parameters a=7.1003 and c=6.9705 Å. Theunit cell parameters of rare earth doped barium borophosphates were calculated and displayboth increase and decrease depends on ionic Radius of rare earths. The other supportingmethods confirm the crystal structure and luminescence properties.Research limitations/implications: The synthesis method has some disadvantagessuch as low homogeneity, non-uniform product etc. We tried to minimize these negativeaspects in our research and succeeded.Practical implications: Phosphor materials Y:BaBPO5, Gd:BaBPO5 and La:BaBPO5(ICSD 51171) were synthesized by conventional solid state method and characterizationswas mainly based on powder X-ray diffraction pattern. Also, morphological and luminescenceproperties were completed to get the highest knowledge.Originality/value: Of the paper is first time conventional synthesis of Y, Gd and La dopedBaBPO5 compounds, calculation of unit cell parameters, and investigation of morphologicaland luminescent properties.

Effect of A cation size on structural, magnetic and electrical properties of K2NiF4-type oxide LaSrFeO4

K2NiF4-type oxides La0.8R0.2SrFeO4 (R=La, Nd, Gd, Dy) have been synthesized by Pechini method and their structures were established by Rietveld refinements of the XRD data in the space group I4/mmm. Analysis of cell parameters shows that the decrease in the c from La to Dy compound is much larger than that in a. All the samples show antiferromagnetic behavior with increase in value of Θ with decreasing average <La/R/Sr> site radius. For all compounds, semiconducting behavior persists in the whole temperature range and can be expressed by two conduction mechanisms – Arrhenius model and polaron hopping model. The results suggests that the transport properties are dominated by the adiabatic small polaron hopping mechanism. Both activation energy (Ea) and polaron hopping energy (Ep) decrease with decreasing average ionic radius of rare earth ion at A-site.

Catalytic combustion of soot particulates over rare-earth substituted Ln2Sn2O7 pyrochlores (Ln = La, Nd and Sm)

Catalytic combustion is one of the most promising methods for diesel soot removal. Ln2Sn2O7 pyrochlores substituted with different rare-earth (RE) elements (Ln=La, Nd and Sm) were prepared through co-precipitation method for catalytic combustion of soot particulates. The structural, textural and redox properties, together with the oxygen vacancy of the catalysts were investigated systematically. Their catalytic activities were evaluated by both temperature-programmed oxidation and isothermal reaction techniques. With the increasing in RE ionic radius (r), the SnO bond strength in Ln2Sn2O7 pyrochlores evaluated from the stretching IR band was decreased, resulting in the improved reducibility and enhanced oxygen vacancies of catalysts. The increase of oxygen vacancy concentration was further confirmed by photoluminescence (PL) investigations wherein upon excitation with UV radiation, the pyrochlores nanoparticles exhibited strong and sharp transition at 408nm attributed to oxygen vacancies. Catalytic combustion and isothermal reactions revealed that the ignition activity (ignition temperature, T5) and the intrinsic activity (turnover frequency, TOF) were shown to depend correlatedly on redox properties and oxygen vacancy concentrations, both of which were influenced by the substitution of different RE elements. Among the pyrochlore oxides, the as-synthesized La2Sn2O7 sample displayed relatively the highest ignition activity and the largest intrinsic activity with TOF of 2.33×10−3s−1.

Hybridization between microstructure and magnetization improvement in lead and RE co-doped BiFeO3

The crystal structure, magnetic and electrical properties of Bi0.96Pb0.04FeO3 and Bi0.92Pb0.04 RE0.04FeO3 (RE=La, Sm, Dy and Yb) polycrystalline samples were prepared by the flash autocombustion technique. X-ray diffraction (XRD) measurements show that the rare-earth doped compositions crystallized in rhombohedral symmetry of space group R3c. The undoped sample consisted needle shape particles while rare earth substitution preferred platelet like particles as clarified from high resolution transmission electron microscopy (HRTEM). Morphological features were examined using field emission scanning electron microscopy (FESEM). Magnetization measurements showed that Yb3+ samples possessed the highest room temperature saturation magnetization while when Bi3+ ions were substituted by La3+ ions, a smaller MS (0.28 emu/g) was obtained. The coexistence of ferroelectric and magnetic transitions was detected using DSC and χM, indicating the multiferroic characteristics of Bi0.92Pb0.04RE0.04FeO3 crystallites. The Néel temperature shifted upwards with decreasing the ionic radius of rare earth ion. Nice correlation was established between microstructure, morphology and magnetic properties in view of the contribution of magnetocrystalline and shape anisotropy in the magnetic parameters values.