Valence State Of Cerium Research Articles

Thermal expansion behaviour of a versatile monazite phase with simulated HLW: A high temperature x-ray diffraction study

Abstract Synthetic analogues of monazite are considered as an alternate ceramic host for the immobilization of high level waste (HLW) generated in the nuclear fuel cycle. The Ca doped CePO 4 (Ce 0.8 Ca 0.2 PO 4 ) is expected to be a versatile monazite phase for this purpose due to the mixed valence state of cerium in this compound which assists in accommodating cations of any valence by initiating the internal redox reaction, viz., Ce 3+ ⇌ Ce 4+ + e − , as required by the incoming cation for charge neutrality of the lattice. The thermal expansion of this versatile monazite phase with 20 wt.% simulated HLW was measured using high temperature x-ray diffractometer (HTXRD) in the temperature range of 298–973 K and are reported here. The thermal expansion behaviour is found to be similar for CePO 4 , Ce 0.8 Ca 0.2 PO 4 and Ce 0.8 Ca 0.2 PO 4 with 20 wt.% simulated HLW with the average volume expansion coefficient of 27.4, 26.5 and 24.1 × 10 −6 K −1 respectively in the temperature range of 298–973 K.

Novel ternary compound Ce2RuAl: Synthesis, crystal structure, magnetic and electrical properties

The intermetallic phase Ce2RuAl was synthesized by arc melting the elemental components and subsequent annealing of the product at 823 K. Its crystal structure was determined from the single-crystal X-ray diffraction data and confirmed by powder X-ray diffraction at room temperature. The compound is of a new structural type related to the LaF3-type of structure: space group P63cm, Z = 6, a = 7.8362(7) Å, c = 9.7510(9) Å, Pearson symbol hP24. Some Ce–Ru distances in the unit cell are unusually short being equal to 2.4821(6) Å. This structural feature brings about instability in the cerium valence state, directly evidenced in the XANES data and corroborated in the study of bulk physical behavior in Ce2RuAl.

Chemical state identification of Ce3 +/Ce4 + in the Sm0.2Ce0.8O2 − δ electrolyte for an anode-supported solid oxide fuel cell after long-term operation

Direct evidences of cerium valence state transformation from Ce4+ to Ce3+ in an anode-supported solid oxide fuel cell after long-term operation have been identified. Single cell with samarium-doped ceria (Sm0.2Ce0.8O2−δ, SDC) thin film electrolyte is prepared with maximum power density of 608mWcm−2 at 650°C when the fuel/oxidant flow rates are 335/1005sccm, respectively. The long-term durability tests are executed by fixed-current operation for 950h. The power density and voltage degradation are observed and critically attributed to the chemical state transformation of cerium in the electrolyte. From X-ray photoelectron spectroscopy (XPS), the Ce3+ ratio increases from 30.3 to 52.9% in the electrolyte before and after operation. The focused ion beam (FIB)/transmission electron microscopy (TEM) techniques are utilized to investigate the structure variation and the selected area diffraction patterns of the chosen grains towards electrodes identify the phase transformation from CeO2 to Ce2O3, suggesting that the Ce3+ species increase at the near-anode side. The direct evidence of Ce3+ presence shows one of the key factors for the degradation of a low temperature solid oxide fuel cell (LT-SOFC).

Physical properties and valence state of cerium in the filled skutterudite CePt4Ge12

Electronic, magnetic, and transport properties of the filled platinum–germanium skutterudite CePt4Ge12 are investigated. High resolution x-ray absorption spectroscopy measurements at the cerium LIII edge demonstrate that Ce in this compound has a temperature-independent valence close to three. However, magnetic susceptibility, thermopower, Hall effect, and electronic specific heat reveal a broad maximum at Tmax = 65–80 K, suggesting the presence of valence fluctuations. The Sommerfeld coefficient γ = 105 mJ mol−1 K−2, deduced from specific heat, indicates moderately enhanced band masses for CePt4Ge12. We discuss these findings and conclude that CePt4Ge12 represents a system at the border between intermediate valence (IV) and Kondo lattice behavior. In addition, the lattice specific heat and the thermal conductivity are discussed with respect to the vibrational dynamics of Ce in the [Pt4Ge12] framework.

Correlation of magnetic properties with the local features of the electronic and crystal structure in the Ce2Fe17 − x Mn x intermetallide: XAFS data analysis

The rearrangement of the local environment of cerium has been examined using extended X-ray absorption fine structure spectroscopy above the K-Ce absorption edge for various manganese contents and temperatures in Ce2Fe17 − x Mn x intermetallics (x = 0, 1, 2). The valence state of cerium has been studied simultaneously by X-ray absorption near-edge structure spectroscopy above the L 3-Ce absorption under the same conditions. A correlation has been revealed between the changes observed in the local electronic and crystal structure of Ce2Fe17 − x Mn x compounds and the types of magnetic states in them.

Surface ligand assisted valence change in ceria nanocrystals

In this report, we used x-ray absorption, electron energy-loss, and magnetic analyses to demonstrate the effect of modification of ceria (CeO 2-δ) nanocrystals with a particle size D= 3.5-14 nm by oleic acid surface ligands. We observe a shift of the electronic state of the cerium ions from tetra to trivalent state as the surface density of the ligand molecules is increased. The valence state of cerium in the nanocrystal is strongly correlated with the number of ligand molecules bound to the surface of the ceria nanocrystals. One oxygen vacancy is stabilized by 5-6 ligand molecules bound on a CeO2-δ nanocrystal. © 2011 American Physical Society.

27Al and 63Cu NMR studies on intermetallic Kondo compound CeCu 3Al 2

Detailed 27Al and 63Cu nuclear magnetic resonance (NMR) experiments have been performed in between 4.5 and 295 K on a polycrystalline sample of CeCu 3Al 2 which has a Kondo temperature ∼ 11 K . The NMR spectral features and Knight shift measurements show that in this compound, Cu occupy both c and g lattice sites, while Al occupy only the g sites. From the temperature dependence of Knight shift ( K), the hyperfine fields for 63Cu and 27Al at the g sites have been estimated as 2.02(5) and 1.67 ( 5 ) kOe / μ B , respectively, and that for 63Cu at c sites as − 3.83 ( 5 ) kOe / μ B . Nuclear spin-lattice relaxation time ( T 1) measurements indicate that though the relaxation process is governed by fluctuations of magnetic interaction, nuclear quadrupolar interaction also has a significant contribution, especially towards relaxation of 63Cu in both c and g sites. For 27Al, K and T 1 have yielded the effective 4f-spin correlation rate 1 / τ 4 f . The behavior of its temperature dependence indicates that at low temperatures in CeCu 3Al 2, the valence state of cerium might not be stable.

Detailed study on the role of oxygen vacancies in structural,magnetic and transport behavior of magnetic insulator: Co–CeO2

Room temperature ferromagnetism in polycrystallineCoxCe1−xO2−δ (0.001≤x≤0.10) bulksamples has been investigated. Annealing in the forming gas transformed the as-prepared paramagneticinto a ferromagnetic insulating material with over two orders of magnitude enhancement (from3.7 × 10−2 to1.24 μB/Co) in the magnetization. Structural characterization of both the as-prepared andH2-treated samples showed a single phase material. The incorporation of Co with the formation ofoxygen vacancies in the oxide lattice was revealed by x-ray photoelectron spectroscopy (XPS).The presence of oxygen vacancies is indicated by the existence of mixed valence states of cerium(Ce4+ and Ce3+) in the high resolution XPS 3d spectrum. The role of the donor defects (oxygen vacancies)has been verified through the removal of oxygen vacancies. The ferromagneticinsulating ground state has been explained in terms of the interaction of theF+ center and 3d magnetic cations. The connection between magnetic properties, electronicstructure of the magnetic impurity and donor defect has been established. First principlecalculations have been performed using the full potential linearized augmented plane wavemethod within the density functional theory (DFT) framework; these support ourexperimental findings. Both the experiment and calculations reinforced the crucial role ofoxygen vacancies.

Interplay of negative pressure and hydrogen chemical effects in CeRhSn from first principles

Investigations within the local spin density functional theory (LSDF) of the intermetallic hydride system CeRhSnHx were carried out for discrete model compositions in the range 0.33 ≤xH ≤ 1.33 with the purpose of assessing the change of the cerium valence state in the neighborhood of the experimental hydride composition, CeRhSnH0.8. In agreement with experiment, the analyses of the electronic and magnetic structures and of the chemical bonding properties point to trivalent cerium for 1 ≤xH ≤ 1.33. In contrast, for lower hydrogen amounts the hydride system stays in an intermediate-valent state for cerium, like in CeRhSn. The influence of the insertion of hydrogen is addressed from both the volume expansion and chemical bonding effects. The latter are found to have the main influence on the change of Ce valence character. Spin polarized calculations point to a finite magnetic moment carried by the Ce 4f states; its magnitude increases with xH in the range 1 ≤xH ≤ 1.33.

The Valance Determination of Cerium Ions in α-SiAlON by Electron Energy Loss Spectroscopy Analysis

SiAlON ceramics have found applications in many different areas due to their excellent engineering properties such as high hardness, fracture toughness, good thermal shock and oxidation resistance. SiAlON exist mainly in two different polymorphs: a (MxSi12-(m+n)Al(m+n)OnN16-n; M: metal and rare earth cations, x≈0,35 and n≤1,35) and β (β-Si6-zAlzOzN8-z; 0≤z≤4). In general, stable alpha and beta phases separately as well as in combination of α and β are obtained by incorporation of metal and rare earth cations as sintering additives. The metal cations such as Li, Mg, Ca, Y, and most lanthanide cations with the exception of La, Ce, Pr and Eu are able to stabilise α-SiAlON structure. Ekstrom et al. 1991 found that cerium can not occupy interstitial sites in α-SiAlON structure due to the fact that ionic radius of Ce3+ (0.103 nm) is too large, whereas ionic radius of Ce4+ (0.080 nm) is too small to stabilise α-SiAlON structure. After this work, several studies carried out to incorporate cerium cations into α-SiAlON structure. It was shown that cerium cation alone can be incorporated into α-SiAlON if the samples are either fast cooled after sintering, or when the samples are spark plasma sintered. On the other hand, cerium can also be incorporated into the α-SiAlON structure when it is used as a sintering additive together with a smaller α-SiAlON stabiliser cation such as Yb or Ca. Similar results were observed in other multi-cation doped SiAlONS that non α-SiAlON stabiliser cations like Sr2+ (0.112 nm) and La3+ (0.106 nm) are able to stabilise α-SiAlON when used together with α-SiAlON stabiliser cations such as Ca or Yb. Although it was shown that cerium existed in mixed valance state at domain boundaries in Ce-doped and spark plasma sintered α-SiAlON, there is no work on the valance determination of cerium in sintered α-SiAlON which has no domain boundaries. Therefore, in this study; it was aimed to incorporate cerium into α-SiAlON structure by combining with Yb3+ and the determination of possible cerium valence states (Ce3+/Ce4+) in both α-SiAlON grains and secondary phases.

Formation of coloring complexes in glass colored with cerium and titanium oxides

The valence state of cerium and titanium ions in barium-strontium aluminosilicate glass with a different ratio of cerium and titanium oxides was investigated and the possible composition of the cerium-titanium complexes formed in silicate glass was analyzed. It was found that an active separate reaction between cerium and titanium oxides with the appearance of complex color centers takes place in synthesis of the glass. It is hypothesized that color centers primarily form between tetravalent cerium and tetravalent titanium in glass-forming melts.

Cytochemical behavior of rare earth ions in Euglena gracilis studied by XAFS

The uptake of rare earth elements (REE) by Euglena gracilis cells has been investigated in Fudan University. The remarkable ability to transport REEs to these cell’s compartments had been observed. X-ray absorption fine structure experiments (XAFS) of cerium in Euglena gracilis were performed at Beijing Synchrotron Radiation Facility (BSRF) to directly determine the cerium valence state and coordination structure in situ. Extended X-ray absorption fine structure (EXAFS) derived calculation showed that cerium was surrounded by 8 N atoms with bond length of 0.258 nm. Combining with other measurements, it may indicate that most likely REEs are mainly located in chlorophyll molecules.

Novel valence state of cerium in Ce 2Zr 2O 7.5 elucidated by electron energy-loss spectroscopy under electron channeling conditions

Rare earth compounds are known to often exhibit peculiar valence states. In the present study, we show, by electron energy loss spectroscopy (EELS) measurements under electron channeling conditions, the existence of a novel valence state for the cerium atoms in Ce 2Zr 2O 7.5, where the valence state is split into two states, with the ions exhibiting a trivalent character (Ce 3+) when facing oxygen planes with oxygen vacancies, and a tetravalent character (Ce 4+) when facing oxygen planes with no oxygen vacancies. This result should prompt reconsideration of the conventional concept of the valence fluctuation in rare earth compounds.

XANES and XMCD study of CePtSn

We present the results of X-ray magnetic circular dichroism (XMCD) studies of the intermetallic Kondo antiferromagnet CePtSn just above the Néel temperature under a magnetic field. The X-ray absorption near-edge structure (XANES) and XMCD spectra at the Ce L 2,3-edges confirm the trivalent valence state of cerium in this compound. The observation of the XMCD signal at the Pt L 2,3-edges indicates that the 5d states of Pt are involved in the magnetic interactions in CePtSn.

Preparation of Nano-Sized Doped Ceria for SOFC Anodes

ABSTRACTNano-particles of Pr and Mn-doped ceria have been prepared by mixing nitrate solutions and hexamethylenetetramine as an oxidizing agent, and their thermal stability has been investigated. The particle size was evaluated by means of XRD, BET and dynamic light scattering techniques. The particle size of Pr and Mn-doped ceria was found to be approximately 3 nm as prepared. Even though the particle size of doped ceria increases with increasing annealing temperature, the size of less than 15 nm was kept after annealing at 873 K for 10 h. On the other hand, the lattice constant of nano-sized ceria, for example 0.544 nm for 3 nm-size particles, decreases and reaches to the value of bulk material (0.541 nm) as annealing temperature increases. This suggests that the valence state of cerium and the amount of oxygen vacancies strongly depend on the particle size.

Cerium LIII-Edge XAS Investigation of the Structure of Crystalline and Amorphous Cerium Oxides

Cerium oxide solid samples were prepared via precipitation from aqueous solution of hydrous cerium(III) nitrate in the presence of different percentages of hydrogen peroxide (H2O2) as model corrosion inhibiting coatings materials for aluminum alloys. X-ray absorption spectroscopy at the Ce LIII-edge was applied for the characterization of crystalline anhydrous CeO2, nanocrystalline hydrous CeO2, nanocrystalline CeO2 sample I precipitated in the presence of H2O2, and an amorphous CeO2 sample II precipitated at a higher H2O2 concentration. An analysis by X-ray absorption near-edge structure (XANES) for cerium oxides did not indicate a broad variation in cerium valence state in the precipitated samples as compared to anhydrous CeO2. Furthermore, XANES analysis revealed a decrease in the intensity of the white line peaks for the precipitated samples relative to those in anhydrous CeO2. The EXAFS spectra of the oxides showed that H2O2 reduced the precipitate's particle diameter and bulk crystallinity. Growth i...

A magnetic, neutron diffraction and X-ray absorption spectroscopy study of Ce(Fe, Si) 11C x compounds ( x = 0.5, 1.0, 1.5)

Three samples of nominal compositions CeFe 9Si 2C x ( x = 0.5, 1.0, 1.5) were prepared by arc-melting. The crystallographic structure and the magnetic properties of the obtained compounds were investigated by means of X-ray and neutron powder diffraction, and magnetic measurements. They crystallise within a filled variant of the tetragonal BaCd 11-type structure, with Ce in 4(a) position and Fe in the 4(b), 32(i) and 8(d) sites, the Si atoms being located on the remaining part of the 8(d) sites and the carbon atoms occupying 8(c) octahedra formed by four Fe and two Ce atoms. From the neutron diffraction study, the composition of the x = 1.0 compound is thus found to be close to CeFe 9.8Si 1.2C x. At 2 K, a significant difference is observed in the Fe moment amplitudes between the 32(i) site ( μ Fe ∼ 1.6 μ B ) on the one hand, and the 4(b) ( μ Fe ∼ 3.0 μ B ) and 8(d) sites ( μ Fe ∼ 2.5 μ B ) on the other hand. No magnetic moment was detected on the Ce sublattice. So X-ray absortion experiments were performed on the three compounds to analyse the cerium valence state. They led to characterize an intermediate valence state of Ce, with a partial relocalisation of the 4f electronic orbitals when the carbon concentration increases.

Hydrogenation of the ternary compounds CeNiX (X=Al, Ga, In, Si, Ge and Sn): influence on the valence state of cerium

The hydrogenation process of the intermediate valence compounds CeNiAl, CeNiGa, CeNiIn, CeNiSi and CeNiGe and of the Kondo semiconductor stannide CeNiSn has been investigated. Magnetization and electrical resistivity measurements reveal interesting physical transition induced by the insertion of hydrogen: (i) the Kondo temperature decreases from 600(20) to 220(10)K in the sequence CeNiGe → CeNiGeH1.6(1); (ii) the hydrides CeNiAlH1.93(5) and CeNiGaH1.1(1) contain cerium in trivalent state and the former shows the presence of magnetic correlations around 3.6(2)K; (iii) in contrast, CeNiInH1.8(2) presents a ferromagnetic transition at TC=6.8(2)K and a magnetic transition Kondo semiconductor → antiferromagnet (TN=4.5(2)K) → ferromagnet (TC=7.0(2)K) is evidenced in the CeNiSnHY system. These results are explained by a modification of the competition between RKKY magnetic interaction and Kondo interaction induced by the hydrogenation of these intermetallics.

Crystallographic and hydriding properties of the system La 1− xCe xY 2Ni 9 ( xCe=0, 0.5 and 1)

The structural properties of the system La 1− x Ce x Y 2Ni 9 with x Ce=0, 0.5 and 1 have been investigated by electron probe microanalysis, powder X-ray diffraction and absorption spectroscopy. The compound LaY 2Ni 9 adopts a rhombohedral structure of PuNi 3-type ( R-3 m space group, Z=3). It can be described as an intergrowth between RM 5 (Haücke phase) and RM 2 (Laves phase) type structures. Among the two available crystallographic sites for R atoms, lanthanum occupies preferentially the site 3 a leading to a partially ordered ternary compound. Substitution by cerium involves anisotropic variations of the cell parameter with a decrease of a and an increase of c leading to an overall cell volume reduction. Increasing cerium content does not induce any symmetry change but leads to a statistical distribution of the rare earths over the two sites 3 a and 6 c involving an evolution toward a pseudo-binary compound. This behavior is related to the intermediate valence state of cerium observed by X-ray absorption spectroscopy. The hydriding properties of the two compounds LaY 2Ni 9 and CeY 2Ni 9 are described in relation with their crystallographic structure.

Ce 3d XPS study of composite Ce xMn 1−xO 2−y wet oxidation catalysts

X-ray photoelectron spectroscopy (XPS) was used to investigate the valence state of cerium in unsupported composite Ce x Mn 1− x O 2− y catalysts. Manganese–cerium composite oxides are being widely used in sub- and supercritical catalytic wet oxidation for the treatment of wastewater containing toxic organic pollutants. The performance of such catalysts depends, among others, on the redox reactions involving CeO 2− x suboxides and manganese oxides. In order to investigate the surface chemistry of such environmental catalysts, 10 samples with Mn/Ce ratios ranging from 0/10 to 9/1—molar basis—were synthesized by co-precipitation and characterized as-prepared and after gold coating. Principal component analysis and curve fitting of Ce 3d and O 1s core-level spectra were used to determine the types of valence states and their proportions on the catalysts’ surface. The study revealed valence state modifications in cerium and changes in the amount of lattice oxygen depending on the Mn/Ce ratio and on whether or not the as-prepared catalysts were subjected to gold deposition. The proportion of surface Ce 3+ species in the non-stoichiometric CeO 2− x oxides was successfully quantified through linear correlation between v 2′ peak area and integral of the Ce 4+ 3d spectrum.