Gd Pairs Research Articles

Intrinsic spin shielding effect in platinum-rare-earth alloy boosts oxygen reduction activity.

Oxygen reduction reactions (ORRs) involve a multistep proton-coupled electron process accompanied by the conversion of the apodictic spin configuration. Understanding the role of spin configurations of metals in the adsorption and desorption of oxygen intermediates during ORRs is critical for the design of efficient ORR catalysts. Herein, a platinum-rare-earth-metal-based alloy catalyst, Pt2Gd, is introduced to reveal the role of spin configurations in the catalytic activity of materials. The catalyst exhibits a unique intrinsic spin reconfiguration because of interactions between the Gd-4f and Pt-5d orbitals. The adsorption and desorption of the oxygen species are optimized by modifying the spin symmetry and electronic structures of the material for increased ORR efficiency. The Pt2Gd alloy exhibits a half-wave potential of 0.95V and a superior mass activity of 1.5 A·mgPt-1 in a 0.1M HClO4 electrolyte, as well as higher durability than conventional Pt/C catalysts. Theoretical calculations have proven that the spin shielding effect of Gd pairs increases the spin symmetry of Pt-5d orbitals and adsorption preferences toward spin-polarized intermediates to facilitate ORR. This work clarifies the impact of modulating the intrinsic spin state of Pt through the interaction with the local high spin 4f orbital electrons in rare-earth metals, with the aim of boosting the spin-related oxygen reduction reaction, thus fundamentally contributing to the understanding of new descriptors that control ORR activity.

Role of spin-glass behavior in the formation of exotic magnetic states in GdB6

Randomness and frustration are believed to be two crucial criteria for the formation of spin glass state. However, the spin freezing occurs in some well-ordered crystals below the related temperature Tf due to the instability of each spin state, which induces the variation of either magnetic moment value or exchange energy. Here we explore the new mechanism of the in-site originated disorder in antiferromagnets Gd0.73La0.27B6 and GdB6, which is caused by the random mutual shifts of Gd3+ spins from the centrally symmetrical positions in the regular cubic lattice. The universal scaling of ESR linewidth temperature dependencies to the power law ΔH(T) ~ ((T − TD)/TD)α with α = − 1.1 ± 0.05 in the paramagnetic phase of both compounds demonstrates the identity of the origin of magnetic randomness. In Gd0.73La0.27B6 the resulting random spin configurations freeze at Tf ≈ 10.5 K where the maximum of magnetization is observed. Below Tf the splitting of ZFC and FC magnetization curves takes place as well as the magnetic state depends on the antecedent sample history. In the case of GdB6 the coherent displacement of Gd ions compete with these random shifts forming an antiferromagnetic (AFM) phase at TN = 15.5 K, which prevails over the spin freezing at Tf ≈ 13 K, expected from the ESR data. The observation of the hysteresis of the ESR spectrum in the AFM phase suggests that its properties may be determined by the competition of two types of AFM orders, which results in formation of stable magnetic domains with nonequivalent positions of AFM Gd pairs at T < 10 K.

Structural origin of high-density Gd2O3–MoO3–B2O3 glass and low-density β′-Gd2(MoO4)3 crystal: a study conducted using high-energy x-ray diffraction and EXAFS at high temperatures

In this study, the short- and medium-range ordering of 21.25Gd2O3–63.75MoO3–15B2O3 glass (GM15B glass) was studied at temperatures ranging from 297 K to above the crystallization temperature. This was achieved using extended x-ray absorption fine structure and high-energy x-ray diffraction analyses to elucidate the structural origin of the higher glass density (d) as compared to that of precipitated crystals, i.e. GM15B glass with d = 4.762 g cm−3 and β′-Gd2(MoO4)3 crystal with d = 4.555 g cm−3, and the self-powdering phenomenon in GM15B glass. The bond lengths in the short-range order, i.e. Gd–O, Mo–O, and Gd–Mo in GM15B, were extremely similar to those in the β-phase; Gd–Gd and Mo–Mo bonds in Gd–O–Gd and Mo–O–Mo, respectively, were not found. The distance between the Gd–Gd pairs in Gd–O–Mo–O–Gd in GM15B glass, r(Gd–Gd) = 6.19 Å, was considerably smaller than that in β-Gd2(MoO4)3 crystals (6.71 Å) in the crystallized glass at 808 K. This indicates that the glass had a more packed structure than did the precipitated crystals. The probable origin of the self-powdering phenomenon in GM15B glass was the large expansion of the structure during crystallization, which is due to the formation of an open structure with long Gd–Gd pairs and varying bond angles.

Noticeable red emission and Raman active modes in nanoscale gadolinium oxyfluoride (Gd4O3F6) systems with Eu3+ inclusion

Eu3+ doped gadolinium oxyfluoride (Gd4O3F6, GOF) nanoscale systems have been synthesized following a modified Pechini method. While exhibiting a tetragonal crystal structure, the GOF nanosystem gave an average crystallite size (d) of ~21–26 nm. The Lotgering factor (LF), which is a measure of orientation of crystallites along the preferred direction was found to vary between 0.22 and 0.48. In the photoluminescence spectra, ~595 and ~613 nm peaks were identified as magnetically driven (5D0 → 7F1) and electrically driven (5D0 → 7F2) transitions with latter (red emission) being strongly manifested with Eu3+ doping concentration and intrinsic defects. Moreover, several Raman active modes have been probed in the Raman spectra with low frequency peaks (<300 cm−1) and moderate frequency peaks (~481 and 567 cm−1) assigned to observable vibration of heavy atom Gd–Gd pairs and Gd–O groups, respectively. Apart from manifestation of phononic features, inclusion of Eu3+ in the host lattice would bring new insight on improving the red emission response prior to concentration quenching.

Studies on selection of solvent extractant system for the separation of trivalent Sm, Gd, Dy and Y from chloride solutions

In this paper, solvent extraction of mixture of rare-earths (Sm(III), Gd(III), Dy(III) and Y(III)) from chloride solutions using acidic, neutral and basic type extractants and their mixtures has been investigated. The main objective of the work was to select a suitable solvent extractant system for the separation of mixture of rare-earths using organophosphoric acid (TOPS 99), phosphonic acid (PC 88A), phosphinic acid (CYANEX® 272), monothiophosphinic acid (CYANEX® 302), phosphine oxide (CYANEX® 923, CYANEX® 921) and amine based (ALAMINE® 336, ALIQUAT™ 336) extractants. The composition of the synthetic mixture of rare-earths was Sm — 1.835g/L, Gd — 0.862g/L, Dy — 0.303g/L, Y — 0.589g/L which was prepared according to the leach liquor of the Korean domestic ore. From the effect of aqueous phase acidity and extractant concentration on the extraction efficiency of rare-earths, PC 88A (0.1mol/L) considered as better extractant with high separation factor for Dy–Y pair with Sm–Gd pair. Mixture of extractants has shown no significant effect on the separation factors of rare-earths than individual extractants.

Pairs of Domains Where All Intermediate Domains are Treed

For a ring extension \({R \subseteq S, (R, S)}\) is called a treed pair if every ring \({T, R \subseteq T \subseteq S,}\) is treed. When R is a field it is shown that the only treed pairs are those satisfying tr.deg[S : R] ≤ 1. For dim(R) ≥ 1 several satisfactory results are given. We establish a relationship between treed pairs, GD pairs and residually algebraic pairs.

First-principles calculation on the Curie temperature of GdFeSi

Electronic structure and magnetic properties for GdFeSi have been studied theoretically from a first-principles density functional calculation. The energy band structure is calculated in a local spin density approximation (LSDA)+Hubbard U approach (LSDA+U). For Gd atoms, seven spin up 4f bands are fully occupied, while the spin down 4f hole levels are completely unoccupied and well above the Fermi level. The calculated magnetic moment of Gd is 7.20 μB, leading to a total magnetization of 7.1 μB per formula unit due to the small induced negative moments at Fe and Si atoms. The intersites exchange coupling parameters Jij of the Heisenberg model have been derived using a linear-response method based on the calculated band structure. The exchange coupling parameters for the nearest Gd–Gd pair (JGd–Gd) is 0.19 mRy. JGd–Gd decreases rapidly with increasing the intersites distance and shows a Ruderman-Kittel-Kasuya-Yoshida-like oscillation. The estimated Curie temperature is about 145 K from the calculated exchange coupling parameters based on the mean-field approximation, in good agreement with the experimental value (TCexp.=135 K).

First-principles calculation on the Curie temperature ofGd3NiSi2

The electronic structure and magnetic properties forGd3NiSi2 have been studied theoretically from a first-principles density functional calculation. Theenergy band structure is calculated in a local spin density approximation (LSDA), and in aLSDA+Hubbard U approach (LSDA+U), respectively. ForGd atoms, in the LSDA+U approximation, seven spin-up 4f bands are fully occupied and situated at the bottom of Si sstates, while the spin-down 4f hole levels are completely unoccupied and well above theFermi level. The calculated magnetic moments for the three Gd sites vary from 7.13 to7.16 µB, leading to a totalmagnetization of 21.5 µB per formula unit including the small induced moments at Ni and Siatoms. The exchange coupling parameters for the nearest Gd–Gd pairs (JGd−Gd) are 0.16 mRyd, 0.14 mRyd and 0.19 mRyd in the three Gdsub-lattices, respectively. The inter-site distance dependence ofJGd−Gd shows a RKKY-like oscillation. The estimated Curie temperature is about251 K from the calculated exchange coupling parameters based on themean-field approximation, in good agreement with the experimental value (TCexp. = 215 K).

A single molecule magnet (SMM) with a helicate structure

The X-ray structure determination of [L2Cu2Gd2(NO3)6(H2O)2](0.5H2O) (L = bis-[(2-hydroxy-3-ethoxyphenyl)methylene]-4,4′-methylenedianiline) confirms the existence of a tetranuclear double-stranded homotopic and unsaturated helicate. In each Cu–Gd pair, the reduced Cu⋯Gd distance allows ferromagnetic Cu–Gd superexchange interactions through two phenoxo oxygen atoms. Replacement of gadolinium ions by terbium ions yields the first example of a helicate behaving as a single molecule magnet (SMM).

Temperature dependence of excited-state relaxation processes of Pr3+ in Pr-doped and Pr, Gd-doped CsCdBr3

Abstract The temperature dependence and mechanisms for relaxation of the 1 D 2 and 3 P 1 states of Pr 3+ in symmetrical Pr–Pr and Pr–Gd pairs in 0.12% Pr:CsCdBr 3 and 0.025% Pr:1.5% Gd:CsCdBr 3 , respectively, are presented. Both single-ion and pair relaxation mechanisms are considered. The temperature dependences of the energy-transfer processes in Pr–Pr pairs are all well-explained in terms of resonance considerations for the donor and acceptor zero-phonon crystal-field transitions.

Assembling ferromagnetically coupled (Cu, Gd) pairs: toward higher spin entities

Abstract We have previously shown that in strictly dinuclear copper–gadolinium complexes, ferromagnetism is an intrinsic property of the Cu–Gd pair, yielding a S = 4 ground state. We want now to show that similar dinuclear complexes obtained with an original ligand having a supplementary oxygen donor atom are able to assemble in order to yield higher spin entities with a S = 8 ground state. These dimeric entities have four metallic centres, two Cu(II) and two Gd(III) ions, arranged at the vertices of a rectangle in such a way that two different ferromagnetic Cu-Gd interactions are operative. The stronger one (7 cm–1) is supported by a double phenolato bridge (CuO2Gd) and the weaker one (0.6 cm–1) by a single amido bridge (Cu–N–C–O–Gd).

Molecular dynamics simulation of gadolinia-doped ceria

Abstract Oxygen diffusion in the oxygen ion conductor of gadolinia-doped ceria (CeO 2 ) 1− x (Gd 2 O 3 ) x /2 was investigated by means of the molecular dynamics simulation. The atomic arrangement with Gd–Gd pairs in gadolinia-doped ceria was found to be energetically more stable and more realistic than that with isolated Gd atoms from the calculation of the enthalpy of formation and the lattice parameter. The compositional dependence of the diffusion constant was in good agreement with the experimental one obtained from the data of electrical conductivity. The Gd–O nearest neighbor distance was longer than that of Ce–O and they were decreased with the increase of the gadolinia content in an excellent agreement with an EXAFS study. The oxygen coordination number of Gd was about seven except for a very small gadolinia content, suggesting that Gd ions are apt to form a Gd 3+ –vacancy–Gd 3+ cluster. The reason why the diffusion constant at high gadolinia contents decreases was discussed in terms of short range and long range interactions. Comparison of diffusion behavior with yttria stabilized zirconia was also discussed.

Isotope effects of the mechanical relaxation of H and D in dilute PdY and PdGd alloys

The internal friction spectra of dilute PdY and PdGd alloys charged with H or D show three relaxation maxima. The maxima attributed to the stress induced ordering of H or D pairs (Zener maximum) and to the reorientation of H–Y or H–Gd pairs (D–Y or D–Gd pairs, respectively) (reorientation maximum) show clear isotope effects concerning their temperature position as well as their activation parameters. In case of the reorientation maximum (activation enthalpy Ha,HM2=0.18–0.26 eV, Ha,DM2=0.20–0.26 eV, depending on H or D as well as Y or Gd concentration) which occurs at temperatures slightly above 100 K the pre-exponential factor of the jump frequency is of the same order of magnitude as the Debye frequency of the host lattice (νDeb=5.7·1012 s−1) for D, whereas it is one order of magnitude smaller for H. In case of the Zener maximum (Ha,HM1=0.11 eV, Ha,DM1=0.15 eV) which occurs at temperatures of about 80 K the pre-exponential factors for D and H are lower by two and four orders of magnitude, respectively, as compared to the Debye frequency. These results are interpreted in terms of thermally activated tunnelling jump processes which occur either adiabatically for higher temperatures or non-adiabatically for lower temperatures.

Very High Mobility in Semimagnetic Semiconductors with Rare Earth

AbstractA semimagnetic semiconductor with rare earth like magnetic ion is studied: Pb1-xGdxTe. Very high Hall mobilities were found up to 107 cm2/V.s. The magnetization is well fitted at 4.2K by a Curie-Weisslaw, with a Gaj's parameter of 1K. The value of JNN' the exchange in a Gd pair deduced from the susceptibility measurements is small, as expected, due to the specific character of Gadolinium (S state and 4f shell): JNN/k = - O.3°K for x = 0.025.

Model Calculations for the Anisotropy Formation by Resputtering Processes during the Growth of Amorphous GdCo Films

AbstractIn order to understand the formation of a magnetic anisotropy during the preparation of amorphous GdCo films by bias sputtering, model calculations are carried out for simple cubic or hexagonal close packed lattices. A structural anisotropy is always obtained, if differences in the binding energies of CoCo, GdCo, and GdGd pairs are assumed. The anisotropy can be described as a difference of the pair numbers parallel and perpendicular to the film plane. Sign and magnitude of the anisotropy depend strongly on the surface configurations involved in the resputtering process (adatoms or inner atoms) and on the binding energies assumed for the different pairs.

High frequency specific heat measurements and exchange constants of GdCl 3

Improved high frequency susceptibility experiments have been performed to determine the magnetic specific heat C M of GdCl 3 at 77° and 20.4°K. The results are compared with an extrapolation of calorimetric measurements below 4.2°K and a high temperature calculation using the exchange constants J nn and J nnn derived from EPR measurements on GdGd pairs. Correlating all results leads to an unambiguous choice for the two exchange constants.

Augmenting the separation of adjacent pairs of rare earths in elutions with hydroxyethylethylenediaminetriacetate (HEDTA)

Rare-earth mixtures have been eluted with ammonium HEDTA at 92° with substantial gains in the separation of ErHo, HoDy, DyTb and TbGd pairs. Individual stability constants at 92° have been derived from the observed separation factors, and a simple mechanism has been proposed to account for a drastic drop in chelate stability which occurs when only the lighter rare-earth chelate species are heated from 15° to 92°. A scheme for economical recovery of pure yttrium from yttrium-lanthanide mixtures is discussed.