Cerium Valence Research Articles

Fabrication and Scintillation Performance of Nonstoichiometric LuAG:Ce Ceramics

Nonstoichiometric LuAG:Ce Ceramics ([Lu(1–x)Cex]3Al5O12, x = 0.005) with different excess of Lu3+ were designed on the basis of Lu2O3‐Al2O3 phase diagram and fabricated by a solid‐state reaction method. Without using any traditional sintering aids, pure phase and good optical performance were obtained in such a Lu‐rich LuAG:Ce ceramics. In addition, scintillation efficiency and light yield of 1% excess of Lu3+ ceramic sample were found 16 times and 1.82 times higher than that of commercial Bi4Ge3O12 (BGO) single crystals, respectively. Such values are comparable or even better than those in most of LuAG:Ce single crystals. However, antisite defects were also induced by excess of Lu doping, whose luminescence was found at 350–410 nm in Lu‐rich LuAG:Ce ceramics. The relationship of excess content of Lu and the microstructure, optical quality, and scintillation performance were clarified and discussed. Furthermore, by utilizing X‐ray absorption near edge spectroscopy technique, the charge state stability of cerium in Lu‐rich LuAG:Ce ceramics was examined. It appears that the excess of isovalence Lu3+ doping has a negligible effect on the cerium valence instability and creation of stable Ce4+ center.

The effect of cerium valence states at cerium oxide nanoparticle surfaces on cell proliferation

Understanding and controlling cell proliferation on biomaterial surfaces is critical for scaffold/artificial-niche design in tissue engineering. The mechanism by which underlying integrin ligates with functionalized biomaterials to induce cell proliferation is still not completely understood. In this study, poly-l-lactide (PL) scaffold surfaces were functionalized using layers of cerium oxide nanoparticles (CNPs), which have recently attracted attention for use in therapeutic application due to their catalytic ability of Ce4+ and Ce3+ sites. To isolate the influence of Ce valance states of CNPs on cell proliferation, human mesenchymal stem cells (hMSCs) and osteoblast-like cells (MG63) were cultured on the PL/CNP surfaces with dominant Ce4+ and Ce3+ regions. Despite cell type (hMSCs and MG63 cells), different surface features of Ce4+ and Ce3+ regions clearly promoted and inhibited cell spreading, migration and adhesion behavior, resulting in rapid and slow cell proliferation, respectively. Cell proliferation results of various modified CNPs with different surface charge and hydrophobicity/hydrophilicity, indicate that Ce valence states closely correlated with the specific cell morphologies and cell–material interactions that trigger cell proliferation. This finding suggests that the cell–material interactions, which influence cell proliferation, may be controlled by introduction of metal elements with different valence states onto the biomaterial surface.

Origin of improved scintillation efficiency in (Lu,Gd)3(Ga,Al)5O12:Ce multicomponent garnets: An X-ray absorption near edge spectroscopy study

In the recent successful improvement of scintillation efficiency in Lu3Al5O12:Ce driven by Ga3+ and Gd3+ admixture, the “band-gap engineering” and energy level positioning have been considered the valid strategies so far. This study revealed that this improvement was also associated with the cerium valence instability along with the changes of chemical composition. By utilizing X-ray absorption near edge spectroscopy technique, tuning the Ce3+/Ce4+ ratio by Ga3+ admixture was evidenced, while it was kept nearly stable with the Gd3+ admixture. Ce valence instability and Ce3+/Ce4+ ratio in multicomponent garnets can be driven by the energy separation between 4f ground state of Ce3+ and Fermi level.

The influence of Gd/Ce substitution on the magnetic properties, electronic structure and the valence of cerium in the solid solution CexGd1−xNi3

The polycrystalline compounds CexGd1−xNi3 with a rhombohedral PuNi3 (x ≤ 0.2) and hexagonal CeNi3 (x ≥ 0.8) – type of crystal structures have been obtained. The change of the lattice parameters may suggest an unstable valence of cerium in the studied system. The effect of partial substitution of Gd by Ce is reflected in a change of the cell volume V(x), the Curie temperature TC(x) and in the temperature dependence of the magnetic susceptibility and electrical resistivity. Thus, TC(x) decreases from 115 K (x = 0) to ∼6.8 K (x = 0.8). The XPS spectra have been measured at room temperature. The valence band spectra (VB) as well as the core level lines have been analysed as influence of the Gd/Ce substitution on the electronic structure. The VB near the Fermi level (EF) is dominated by Ni3d states. In the VB region some slight effects of hybridization and changes of intensity of states on EF have been noticed. The analysis of Ce core level lines reveals the occurrence of possible intermediate valence. The values of the Ce4f - state occupation parameter (nf) and the hybridization energy (Δ) have been estimated. The gradual filling of the Ni3d band is revealed by a reduction of the 6 eV satellite intensities in the Ni2p core level spectrum. All these effects can modify the magnetic properties of the investigated compounds.

The Solid Solutions (Ce1-xLax)RuSn

X-Ray-pure samples of the solid solutions (Ce1-xLax)RuSn were obtained up to x=0.5. Powder diffraction data show the CeRuSn-type superstructure up to x ≈ 0:3 and the CeCoAl-type subcell for higher lanthanum contents. The structure of a single crystal with x=0.5 was refined on the basis of single-crystal X-ray diffractometer data: CeCoAl type, C2=m, a=1160.8(2), b=477.6(1), c=511.6(1) pm, β = 102:97(2)°, wR=0.0510, 444 F2 values, 20 variables. Magnetic investigations were performed for all samples up to a lanthanum content of x=0.4. No cooperative phenomena could be observed, and all samples show Curie-Weiss behavior above a certain temperature. The cerium valence is about 3.32(2) for all samples of the solid solution. Hence, La3+ has to replace Ce3+ as well as Ce4+ in a particular quantity. The electrical resistivity measurements confirm the suppression of the magnetic ordering and the structural transition upon replacement of cerium by lanthanum. 119Sn Mössbauer spectra of samples with x=0.2 and 0.5 are indicative of single tin sites with isomer shifts of δ =1.86(1)mm s-1 for x=0.2 and δ =1.88(1)mm s-1 for x=0.5. Both signals are subject to significant quadrupole splitting, a consequence of the low site symmetry. Results of XANES measurements are perfectly in line with the cerium valences determined by susceptibility measurements and yield a constant value of 3.16(1) for all investigated compounds

Synergistic effect between MnO and CeO2 in the physical mixture: Electronic interaction and NO oxidation activity

MnO and CeO2 powders were mechanically mixed by a spatula and by milling to obtain loose-contact and tight-contact mixed oxides, respectively. The monoxides and their physical mixtures were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS), Raman, O2 temperature-programmed desorption (O2-TPD), H2 temperature-programmed reduction (H2-TPR) and NO temperature-programmed oxidation (NO-TPO). The MnOx-CeO2 solid solutions did not form without any calcination process. The oxidation state of manganese tended to increase while the ionic valence of cerium decreased in the mixed oxides, accompanied with the formation of oxygen vacancies. This long-ranged electronic interaction occured more significantly in the tight-contact mixture of MnO and CeO2. The formation of more Mn4+ and oxygen vacancies promoted the catalytic oxidation of NO in an oxygen-rich atmosphere.

Microstructure and Optical Characteristics of Rod-Like Nanoscale CeO<SUB>2</SUB> Synthesized by Hydrothermal Method

The rod-like nanoscale CeO2 were successfully synthesized by an ethanediamine-assisted hydrothermal synthesis process using CeCl3 x 7H2O as cerium source and N2H4 x H2O as mineralizer. The morphology, microstructure, and optical properties of nanoscale CeO2 were characterized by the scanning electron microscope, transmission electron microscope, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectrometer, UV-Vis spectrophotometer and photoluminescence spectrometer. The lattice parameter decreases with the increase of crystallite size due to the effect of lattice relaxation and Ce ions. Suitable amount of ethylenediamine is beneficial for the growth of CeO2 nanorods. The as-growth rod-like nanoscale CeO2 is polycrystalline materials, and the main valence of cerium was +4. The spectra exhibit a strong absorption band at the near UV region due to the charge-transfer transitions from O 2p to Ce 4f. The band gap energy increases with the increase of ethylenediamine from 0 up to 3 mL and then decreases. Photoluminescence properties were measured using the 325 nm line of He-Cd laser and were also discussed.

Characterization and optical properties of pole-like nano-CeO2 synthesized by a facile hydrothermal method

CeO2 nanopoles have been successfully synthesized by a hydrothermal method using CeCl3·7H2O as cerium source, NaOH as mineralizer, and ethylenediamine as complexant. The pole-like nano-CeO2 was examined by XRD, TEM, XPS, UV–vis, PL, and Raman scattering. The results show that the crystallinity of the nanopoles is not so good, and the main valence of cerium is +4. The estimated direct band gaps are 3.014, 3.099, 2.931, and 2.927eV for the samples synthesized within 2, 18, 50, and 100h, respectively. These band gaps are smaller than bulk CeO2, oxygen vacancies and Ce3+ ions result in an effective red shifting of the band gap. The visible luminescence exhibits similar emission peaks of room temperature photoluminescence and the emission intensity increases with the increase of concentration of oxygen vacancies and Ce3+ ions.

Cerium Valence Change in the Solid Solutions Ce(Rh1-xRux)Sn

The solid solutions Ce(Rh1-xRux)Sn were investigated by means of susceptibility measurements, specific heat, electrical resistivity, X-ray absorption spectroscopy (XAS), and 119Sn Mössbauer spectroscopy. Magnetic measurements as well as XAS data show a cerium valence change in dependence on the ruthenium content. Higher ruthenium content causes an increase from 3.22 to 3.45 at 300 K. Furthermore χ and χ-1 data indicate valence fluctuation for cerium as a function of temperature. For example, Ce(Rh0:8Ru0:2)Sn exhibits valence fluctuations between 3.42 and 3.32 in the temperature range of 10 to 300 K. This could be proven by using the interconfiguration fluctuation (ICF) model introduced by Sales and Wohlleben. Cerium valence change does not influence the tin atoms as proven by 119Sn Mössbauer spectroscopy, but it influences the electrical properties. Ce(Rh0:9Ru0:1)Sn behaves like a typical valence fluctuating compound, and higher ruthenium content causes an increase of the metallic behavior

Spatial Distribution of Cerium Valence in Model Planar Pd/Ce0.7Zr0.3O2 Catalysts

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

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.

Change of the cerium valence with temperature – Structure and chemical bonding of HT-CeRhGe

Polycrystalline CeRhGe was prepared via arc-melting of the elements. Its TiNiSi-related structures (space groups Pnma) were studied by powder diffraction using synchrotron radiation over the temperature range of 315–770 K. CeRhGe shows a first-order structural phase transition at 520 K upon heating. Ab initio elelectronic structure calculations give evidence for the depletion of the cerium 4f band in HT-CeRhGe and in consequence a redistribution of the electron density from the cerium to the rhodium atoms. Purely trivalent cerium atoms in the low-temperature modification (LT) change to intermediate-valent cerium in the high-temperature modification (HT). The integrated crystal orbital Hamilton populations show an enhancement of the Ce–Rh bonding in HT-CeRhGe. The three-dimensional [RhGe] polyanionic network shows drastic puckering of the [Rh3Ge3] hexagons in LT-CeRhGe and a flattening in HT-CeRhGe. The cerium valence change is accompanied by a drastic jump in the lattice parameters: a = 7.42249(8), b = 4.46699(5) and c = 7.1276(1) Å at 315 K vs. a = 7.24579(6), b = 4.47506(4) and c = 7.43579(6) Å at 570 K. Large shifts occur for the x parameter of the rhodium and the z parameter of the cerium atomic positions (Wyckoff sites 4c (x 1/4 z)).

Spatial distribution of cerium valence in model planar Pd/Ce0.7Zr0.3O2 catalysts

The spatial distribution of cerium valence in H2-reduced model planar Pd/Ce0.7Zr0.3O2 catalysts was determined by electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). Spherical reduction zones were found beneath each Pd particle, consistent with a Pd-catalyzed, oxygen anion diffusion-limited bulk oxide reduction process. Evidence was also found for a more extended surface reduction process, possibly associated with hydrogen spillover from the Pd particles.

Intermediate cerium valence intermetallics Ce4RuMg, Ce23Ru7Mg4, CeRu2Mg5, and Ce2Ru4Mg17: Electronic structures and chemical bonding from DFT

The electronic structures of the intermediate-valent cerium compounds Ce4RuMg, Ce23Ru7Mg4, CeRu2Mg5, and Ce2Ru4Mg17 were studied ab-initio by DFT calculations. Common structural characteristics are short Ce∼4+–Ru bonds accompanied by intermediate-valent or almost tetravalent cerium. The degree of depletion of the cerium 4f shell is well monitored in the DOS plots, paralleled by strong covalent Ce–Ru bonding. We also derive magnitudes for the electronic contribution to the specific heat of these compounds in order to appreciate their intermediate valence behaviours.

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).

Intermediate-valent Cerium in Ce2Ru4Mg17 and a Group-Subgroup Scheme for La9Ru4In5 and Ce9Ru4Ga5

Ce2Ru4Mg17 was synthesized by high-frequency melting of the elements in a sealed tantalum ampoule. This magnesium-rich compound crystallizes with a new tetragonal structure type: I4̄2m, a = 986.75(8), c = 1008.7(1) pm, wR2 = 0.0513, 909 F2 values and 34 variables. The striking structural motifs in the Ce2Ru4Mg17 structure are slightly bent CeRu2 units with short Ce-Ru distances of 231 pm and additionally a short Ce-Ce distance of 307 pm. These features are a direct consequence of the cerium valence. The CeRu2 units are embedded in a magnesium-rich matrix with a broad range of Mg-Mg distances (291 - 361 pm). Temperature-dependent magnetic susceptibility data show intermediate-valent behavior of the cerium atoms (0.23(5) μB per Ce atom) and no magnetic ordering down to 3 K, indicative of almost tetravalent cerium in Ce2Ru4Mg17. The ceriumrich gallide Ce9Ru4Ga5 shows an unusually short Ce-Ru distance of 237 pm for the Ce2 position as a result of an intermediate cerium valence. The structural distortions are discussed on the basis of a group-subgroup scheme for La9Ru4In5 (space group I4/mmm) and the superstructure variant Ce9Ru4Ga5 (space group I4mm).

CaBe2Ge2 type phosphides REIr2P2 (RE = La–Nd, Sm) and arsenides REIr2As2 (RE = La–Nd): synthesis, structure, and solid state NMR spectroscopy

The phosphides REIr(2)P(2) (RE = La-Nd, Sm) and arsenides REIr(2)As(2) (RE = La-Nd) were synthesized by a ceramic route via precursor compounds REIr(2) with phosphorus and arsenic, respectively. Well-shaped single crystals were obtained from lead and bismuth fluxes, respectively. The nine pnictides crystallize with the tetragonal CaBe(2)Ge(2) type structure, space group P4/nmm. The structures of CeIr(2)P(2), SmIr(2)P(2), LaIr(2)As(2) and CeIr(2)As(2) were refined from single crystal X-ray diffractometer data. The structures are composed of three-dimensional [Ir(2)P(2)] and [Ir(2)As(2)] networks in which the rare earth atoms fill cavities of coordination number 16 (8 P + 8 Ir). The phosphorus and arsenic atoms have tetrahedral and square pyramidal iridium coordination. Temperature dependent magnetic susceptibility measurements show intermediate cerium valence for CeIr(2)P(2). The rare-earth and phosphorus local environments in LaIr(2)P(2) are characterized further by (139)La and (31)P single and double resonance solid state nuclear magnetic resonance (NMR) spectroscopy. Strong (31)P Knight shifts and extremely short spin-lattice relaxation times indicate that the bonding character of the phosphide species is strongly metallic. The two crystallographically distinct phosphorus sites are well-resolved in the (31)P magic-angle spinning (MAS) spectrum and also differ significantly with respect to their effective magnetic shielding anisotropies. Unambiguous site assignments are accomplished on the basis of homonuclear (31)P-(31)P magnetic dipole-dipole interactions, which can be measured in a site-resolved fashion in this compound using static (31)P spin echo decay spectroscopy. The highly symmetric La environment in LaIr(2)P(2) is characterized by a sharp (139)La MAS-NMR spectrum, revealing rather weak nuclear electric quadrupole coupling. Furthermore, a second local environment is detected, which is characterized by stronger quadrupolar coupling and similar dipolar coupling strength with (31)P as the regular site, according to (139)La{(31)P} rotational echo double resonance (REDOR) NMR results. On the basis of these data we attribute this site to a La species next to a phosphorus vacancy. From the signal area of this resonance we deduce a composition LaIr(2)P(1.90).

Tuning Ceria Nanocrystals Morphology and Structure by Copper Doping

Ceria (CeO2) nanocrystals have unique and highly attractive properties that depend on the morphology and structure. In this paper, we demonstrate that by controlling the addition of copper to growing ceria nanocrystals it is possible to modulate the resulting structure properties. We show also by means of X-ray diffraction, high-resolution transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray fluorescence spectrometry, and X-ray absorption spectroscopy that copper configuration affects both the morphology and the electronic structure of cerium oxide nanocrystals. Indeed, external and internal copper doping on ceria nanocrystals leads to structural transitions, including a morphological transition from cubic to truncated octahedron, along with cerium valence changes. The results of this study, optimizing the structural properties of such nanocrystals, may certainly trigger new opportunities and, ultimately, new applications.

Electronic structure and chemical bonding of α- and β-CeIr2Si2 intermediate valence compounds

The dimorphism of the intermediate valence ternary cerium silicide CeIr2Si2 in the ThCr2Si2 (α) and CaBe2Ge2 (β) modifications is addressed in the framework of the density functional theory. The geometry optimization is in good agreement with the experiment and the subsequent establishment of the energy–volume equation of state (EOS) indicates a stabilization of the β-type relative to the α-type concomitant with the trend of the cerium valence, changing to tetravalent in β-CeIr2Si2. This is equally shown from the site projected DOS and from the large increase of the electronic contribution to the specific heat. The chemical bonding indicates the strongest bonding interactions within the Ir–Si substructure in both varieties. Stabilization of β-CeIr2Si2 with almost tetravalent cerium is in good agreement with ThIVIr2Si2 which exclusively crystallizes in the CaBe2Ge2 type. The EOS behavior of different RIr2Si2 (R=Th, Ce, La) is comparatively discussed.

Effect of cerium valence on As(V) adsorption by cerium-doped titanium dioxide adsorbents

In this study, three titanium dioxide (TiO 2) adsorbents doped with Ce(III) (Ce 3), Ce(IV) (Ce 4) and H 2O 2 oxidized Ce(III) (Ce 3O), respectively, were prepared and the effect of cerium valence on arsenate (As(V)) adsorption was investigated. The Ce 3, Ce 4 and Ce 3O adsorbents all existed in amorphous form and Ce 3 exhibited a higher micropore surface area and micropore volume than both Ce 4 and Ce 3O. The adsorption capacity of As(V) on Ce 4 and Ce 3O decreased significantly when the solution pH was increased, while the adsorption capacity of As(V) by Ce 3 was higher and more stable at pH values ranging between 3.7 and 7.0. The adsorption kinetics of As(V) on Ce 3 was better fit to pseudo-second order model while both the pseudo-second order and the pseudo-first order model described the adsorption of As(V) on Ce 4 and Ce 3O well. The equilibrium adsorption data for As(V) on Ce 3 was well fit by the Langmuir model, and the adsorption of As(V) on Ce 4 and Ce 3O was described well by both the Langmuir and Freundlich models. FTIR analysis indicated that the hydroxyl groups on the three adsorbents’ surfaces were involved in As(V) adsorption. The results suggested that the dominant chemical state of cerium valence plays an important role in affecting the adsorption behavior of As(V) by cerium-doped TiO 2 adsorbents.