Valence Force Research Articles

Absence of magnetic field effect on the cerium valence in CeCu2Si2 at its optimum superconducting critical temperature

The archetypical heavy-fermion superconductor $\mathrm{Ce}{\mathrm{Cu}}_{2}{\mathrm{Si}}_{2}$ is known to present two distinct superconducting phases under pressure. In the low-pressure region, the superconductivity is mediated by spin fluctuations while the superconducting phase observed in the high-pressure region could be associated with a first-order valence transition (FOVT). However, the critical end point (CEP) of the FOVT was shown to be located at negative temperature and only a continuous valence change (crossover regime) was so far observed at 14 K, i.e., far above the optimal superconducting temperature $({T}_{c}=2.3\phantom{\rule{0.16em}{0ex}}\mathrm{K})$. Here we present x-ray absorption measurements under pressure and applied magnetic field at the Ce ${L}_{3}$ edge at 2.7 K, i.e., close to the optimal ${T}_{\mathrm{c}}$. It was expected that the applied magnetic field could shift the CEP to positive temperature with the possibility to observe a FOVT. Our data indicate the valence of Ce increases continuously (crossover regime) from 3.11 at ambient pressure up to 3.20 at 8.5 GPa likewise for any applied magnetic field up to 6 T.

Coercivity enhancement by adjusting the ratio of La to Ce in REFeB films

High abundant La and Ce elements have been supposed be a potential developing direction as the partial alternative to crucially relied Nd/Pr in the 2:14:1-type permanent magnets. In this work, we reported that adjusting the substitution ratio of La to Ce for inducing Ce valence is an effective strategy to prepare perpendicular magnetic anisotropic (LaxCe1-x)9Nd9Fe72B10 films (0 ≤ x ≤ 1) with good magnetic properties. Both X-ray diffraction and transmission electron microscopy results illustrate the lattice expansion of RE2Fe14B tetragonal phase after lanthanum partially replaces cerium element. The increase of La content in the composition range of x ≤ 0.7 benefits Ce valence shifting towards the favorable Ce3+ state as verified by X-ray photoelectron spectroscopy results, leading to the enhancement of coercivity. Combining with characterization of domain morphology and micromagnetic theory analysis, the coercivity mechanism of domain reversal has been confirmed that with the increasing La content, it gradually tends to a dominant domain-wall-pinning mechanism from a mixed mechanism of domain-wall-pinning and nucleation behaving together. Our results provide a feasible method to enhance magnetic properties of high abundant rare-earth permanent magnetic films by manipulating doping element contents and are helpful to understand magnetization reversal behaviors in the films.

The polarization contribution and effect mechanism of Ce-doped 0.65BaTiO3-0.35Sr0.7Bi0.2TiO3 Pb-free ferroelectric ceramics for dielectric energy storage

In the current study, a series of CeO2-doped (x = 0.01, 0.015, 0.02, 0.025 mol) ceramic samples were prepared, and the valence of Ce in the perovskite was analyzed in detail, revealing the polarization contribution and effect mechanism of the doped Ce element in the 0.65BaTiO3–0.35Sr0.7Bi0.2TiO3 ceramic. The phase structure and electrical stored energy performance of the resulting ceramics were also characterized. It was found that adding an appropriate amount of CeO2 to BaTiO3 was helpful for improving the relaxor behavior, promoting electric storage, and releasing ceramic materials. Particularly, the electric breakdown strength of the 0.02Ce-doped specimen reached 330 kV/cm, and the corresponding density and efficiency were 2.57 J/cm3 and 81.30%, respectively. In addition, doping with Ce caused the ceramics to exhibit good stability at frequencies of 1–300 Hz and over a temperature range of 30–150 °C. A high current density (CD) and power density (PD) were obtained in charge–discharge tests.

Visible to near-infrared broadband fluorescence from Ce-doped silica fiber

We investigate the fluorescence characteristics of a purely Ce-doped silica fiber and demonstrate broad-bandwidth fluorescence across the visible and near-infrared. The Ce-doped fiber is fabricated using standard modified chemical vapor deposition technology. Trace metal analysis by inductively coupled plasma mass spectrometry confirmed the purity of Ce-doping. The Ce valence state of 3+ was revealed by X-ray photoelectron spectroscopy. The optimum pump wavelength for the broadest luminescence from a fiber is scanned between 405 nm to 440 nm wavelength of diode lasers operating under continuous-wave regime. The strongest pump absorption is observed at the wavelength of 405 nm. Variation of pump power and fiber length results in the demonstration of broad-bandwidth fluorescence with spectral widths up to 301 nm (at -10 dB). The measured fluorescence spectra cover the wavelength range from ∼458 nm to ∼819 nm with spectral power densities of up to 2.4 nW/nm.

Phases formation in cerium-doped hydroxyapatite

Cerium-substituted hydroxyapatites (Ce-HAs) with an estimated content of Ce3+ of 0; 0.1; 0.25; 0.5 mol. % were prepared by two different synthesis methods: precipitation from aqueous solutions of salts and mechanochemical synthesis. The powders after synthesis were heat-treated at 1250 °C to achieve well-crystallized phases. The X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), energy dispersive analysis (EDA), and dilatometry methods revealed that cerium ions didn’t not fully incorporate into the HA structure, but rather were transform to the CeO2 phase. It was attributed to the transition of Ce valency (III)→(IV) through the oxidation reaction under air conditions during materials heating. Two different synthesis methods provided similar phase composition of the obtained compounds, though different microstructure of the resulted ceramics. It was shown that cerium introduction prevents HA sintering during thermal activation, possibly due to contribution of large atomic mass of cerium ions and formation of cerium (IV) oxide at the crystal boundaries.

Study of the electronic structure of Ce0.95Fe0.05O2-δ thin film using X-ray photoelectron spectroscopy

In the present work, the effect of the oxygen partial pressure (OPP) has been studied on the electronic structure of the Ce0.95Fe0.05O2-δ thin film (TF) through Raman spectroscopy, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The varying OPP, from 200 mTorr to 1 mTorr, was observed to vary the oxidation conditions during TF deposition. The oxygen valency was estimated to be reduced along with the generation of oxygen vacancies as determined via Raman analysis. The AFM analysis revealed the lowered values of the roughness parameters which indicated that reduction in OPP is associated with the smoothness of the film surface. The film surface was found to be reduced, as characterized by XPS investigation, as a result of the change in valency of Ce from +4 to +3 due to reduced OPP. The concentration of Ce3+ state was estimated to increase from 19.5% to 33% with reducing OPP. This evidently supports the formation of oxygen vacancies since the reduction of the valence state of Ce is known to be accompanied by the generation of oxygen vacancies. The O1 s spectra confirmed the oxygen non-stoichiometry as a function of OPP, whereas, Fe 2p suggested the mixed-valence state of Fe (Fe2+/Fe3+) in Ce0.95Fe0.05O2-δ (TF). To put in brief, the variation in OPP affects the chemical composition of the material by modifying its electronic structure via controlling the exchange interactions in the core structure of the compound.

Correlation between Fabrication and Operation Conditions for Ce1-XZrxO2-σ (x~0.2) Stability

CeO2 shows high oxygen storage capacity (OSC) by changing the valency of Ce depending on oxygen partial pressures. The addition of Zr into CeO2 (CZO) improves OSC further and has been used in three-way catalysts (TWCs). Recently, researchers applied CZO as an anode in solid oxide fuel cells (SOFCs). Because the operating temperature of SOFCs (~1000K) is higher than that of TWCs (~600K), the thermal stability of CZO becomes an issue. When CZO is fabricated in oxidizing condition, the distribution of metal atoms in CZO would be less stable under reducing operation condition. In this study, we studied the structural stability of CZO as a function of oxygen partial pressure for fabrication and operation conditions; the oxygen partial pressure was varied by changing the number of oxygen vacancies in the model structures. To find energetically favorable structures efficiently among the huge number of possible structures, we used genetic algorithm and lattice statics.

Atomistic origin of high-concentration Ce3+ in {100}-faceted Cr-substituted CeO2 nanocrystals

Improving the potential of promising CeO2-based nanocatalysts in practical applications requires an atomic-scale analysis of the effects of active dopants on the distribution of Ce valence states and the formation of oxygen vacancies (VOs). In this study, a Cr dopant is introduced into the cubic {100}-faceted CeO2 nanocrystals (NCs) with an average size of 7.8 nm via supercritical water. The Cr dopants substitute Ce sites in the amount of approximately 3 mol%. Based on the aberration-corrected STEM-EELS, the effects of Cr dopant on the distribution of cerium valence states are investigated layer by layer across the ultrafine Cr-substituted CeO2 NC perpendicular to the {100} exposed facet. It is found that an increased amount of Ce3+ cations is present in Cr-substituted CeO2 NCs, particularly in the internal atomic layers, compared to the pristine CeO2 NCs. The atomic-scale analysis of the local structure combined with theoretical calculations demonstrates that Cr dopant reduces the formation energy of VOs and increases the mobility of oxygen atoms for the nano-sized CeO2. These effects, in principle, result in an improved oxygen storage capacity and provide a fundamental understanding of role of the dopant in the formation and distribution of VOs in the doped CeO2 NCs.

The low temperature reducibility of Ce4+ ions in Ce0.7Yb0.2Pd0.1O2-δ: in situ XANES study

Cerium mixed oxides are interesting materials for the oxygen storage capacity due to the Ce ability to valence state changes and oxygen vacancy formation. The mixed-valence state Ce4+/Ce3+ in Ce0.7Yb0.2Pd0.1O2-δ has been studied by in situ Ce L3 X-ray Absorption Near Edge Structure (XANES) spectroscopy during the reduction from room temperature to a 500 °C. The cerium ions start to reduce in the mixed oxide lattice at a temperature of 70 °C. 13% of Ce3+ has been detected in the studied sample during the reduction at a temperature of 70 °C. 26% of Ce4+ were reduced to Ce3+ at a temperature of 200 °C. No other changes were found in the Ce valence state up to the temperature of 500 °C. The most interesting result is that the reduction of Ce4+ to Ce3+ starts clearly before the reduction of the noble metal. The redox activity temperature range of Ce in mixed oxides is significantly lower than reported by other authors.The total density of states (DOS) of Ce0.7Yb0.2Pd0.1O2-δ is very similar to that of Ce0.875Pd0.125O2. The theoretical calculations using the Density Functional Theory method supported the experimental Ce L3 XANES spectra. It was shown that the presence of Yb and Pd ions could enhance the formation of oxygen vacancies in cerium mixed oxides.

Magnetic properties and electronic structure of CeFe2−xMnx and CeFe2Mnx compounds

The effect of manganese alloying on the structure and magnetic properties of CeFe2 has been studied on two isostructural series: quasibinary CeFe2−xMnx and nonstoichiometric CeFe2Mnx alloys. The single-phase bcc MgCu2-type structure is formed at x≤0.5 in both systems. At x=0.5, the lattice parameter is increased by ∼0.3%. The Mn alloying leads to a nonmonotonic variation of magnetic moment and gradual decrease in the Curie temperature from 230 K to 150 K and 167 K for CeFe1.5Mn0.5 and CeFe2Mn0.5, respectively. For x≥0.3, the magnetization data indicate the formation of noncollinear magnetic structure. The binary CeFe2 and nonstoichiometric CeFe2Mn0.15 have been studied using Mössbauer effect and X-ray photoelectron spectroscopy. The Ce valence state remains unchanged upon the Mn alloying. The parameters of hyperfine interactions have been determined in paramagnetic state at room temperature and in magnetically ordered state at 78 K. The Mössbauer data revealed a difference in the quadrupole coupling constants of CeFe2 and CeFe2Mn0.15, which is associated with the difference in the local distortions of the lattice.

Intrinsic properties, phase constitution and microstructure of novel RE-Fe-B strip-casting alloys based on misch metal

In order to reduce the RE-Fe-B material cost and balance the utilization of rare earth sources, misch metal substitution for Nd in the sintered magnet becomes of many interests. Under the process of fabricating high magnetic properties magnet, the strip-casting (SC) technique plays a key role, a series of rapidly solidified [(PrNd)1-xMMx]13.8FebalCo0.5Cu0.1Al0.6Ga0.07B5.9 (x = 0 ~ 1.00) (MM, Misch Metal) strips were manufactured. Subsequently, the phase constitution, microstructure and the intrinsic magnetic properties were measured. There are always stable RE2Fe14B phase in the strips with MM, which is similar to Nd-Fe-B strip. Whereas, when x ≥ 0.50, there exists REFe2 phase inevitably, and the amount of REFe2 phase gradually increases with increasing MM content. Magnetic measurements indicated that intrinsic magnetic properties, such as the saturation magnetization Ms and magneto-anisotropy field HA, deteriorate gradually by substituting PrNd with MM except for x = 0.35. For x = 0.35, the abnormal phenomenon is ascribed to Ce valence shifting toward +3 that is caused by lattice expanding. In addition, when x ≥ 0.50, La and Ce is prior to aggregate in the RE-rich phase clusters, and the content of Fe rises with the increase of MM, moreover, the interfaces between of RE2Fe14B phase and RE-rich phase become indistinct.

Local structure displacements in La[formula omitted]Ce[formula omitted]OBiSSe as a function of Ce substitution

We have investigated the local structure of layered La1−xCexOBiSSe system by Bi L3-edge extended X-ray absorption fine structure (EXAFS) measurements for different Ce substitutions. Ce L3-edge X-ray absorption spectroscopy (XAS) has been used to evaluate the Ce valence responsible for the self-doping in this system. We have found that the local distortion, determined by the separation between two Bi-Ch distances within the BiCh2-layer (Ch=S,Se), is quickly suppressed by Ce substitution while the axial Bi-S2 bond elongates. Ce L3-edge XAS reveals a coexistence of Ce3+ and Ce4+ in which the Ce4+ weight decreases, an indication of a partial breaking of RE-S-Bi (RE=La/Ce) charge transfer channel with Ce substitution. The results suggest that interaction between REO spacer layer and BiCh2 layer, dictated by the out-of-plane Bi-S2 distance, has a significant role in triggering superconductivity in the title system, with the in-plane distortion controlling the charge mobility within the BiCh2-layer.

Mixed Valence of Ce and Its Consequences on the Magnetic State of Ce9Ru4Ga5: Electronic Structure Studies.

We report on X-ray photoelectron spectroscopy (XPS) and ab initio electronic structure investigations of a novel intermetallic material CeRuGa. The compound crystallizes with a tetragonal unit cell (space group I4) that contains three inequivalent Ce atoms sites. The Ce core level XPS spectra indicated an intermediate valence (IV) of selected Ce ions, in line with the previously reported thermodynamic and spectroscopic data. The ab initio calculations revealed that Ce1 ions located at Wyckoff positions possess stable trivalent configuration, whereas Ce2 ions that occupy site are intermediate valent. Moreover, for Ce3 ions, located at different position, a fractional valence was found. The results are discussed in terms of on-site and intersite hybridization effects.

Phase Stability of Ce-Substituted Magnetoplumbite-Type Sr Ferrite

We studied the phase stability of Ce-substituted magnetoplumbite-type Sr ferrite under various oxygen pressures. We have successfully obtained Ce-substituted samples as revealed from X-ray diffraction and wavelength dispersion X-ray analyses. The solubility limit of Ce in Sr1-xCexFe12O19 was estimated to be $x = 0.24$ at ambient pressure, which is reduced to 0.05-0.10 under higher oxygen pressures. The variation of the $c$ -axis against $x$ and the reduction of the solubility by increasing oxygen pressure suggests that the valence of Ce is not 4+ but 3+. The solubility limit of Ce is dominated not only by the ionic radius but also by the valence instability of Ce.

Effect of NH4F treatment on the chromaticity of the Ba2+-doped γ-Ce2S3 red pigment

The Ba2+-doped cerium precursor powder, (Ce, Ba)CO3, was successfully prepared via a co-precipitation reaction method using Ce(NO3)3·6H2O, Ba(NO3)3, and (NH4)2CO3 as raw materials. The above powder was placed in a muffle furnace and calcined at 1000°C for 120 min to obtain the corresponding oxide, (Ce, Ba)O2, subsequently, and the oxide was vulcanized at 850°C for 150 min with CS2 as a sulfur source to obtain the Ba2+-doped γ-Ce2S3 red pigment. The effects of different NH4F concentrations (1.0, 2.5, and 4.0 M) and treatment times (30, 60, 90, and 120 min) on the phase composition and chromaticity of the pigment were studied by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffractometry (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and CIELAB colorimetry. The SEM and EDS analyses revealed the presence of irregular flaky particles on the pigment surface after the NH4F treatment; the amounts of particles gradually increased with the NH4F concentration. The XRD and Raman spectra confirmed that these particles were CeF3 and CeFS heterophases. Furthermore, the Raman spectra denoted the presence of amorphous carbon. The XPS analysis demonstrated that the NH4F treatment did not change the valence of Ce (Ce3+ for γ-Ce2S3). The chromaticity of a sample reached its maximum (L* = 34.24, a* = 38.19, and b* = 39.95) after it was treated using 2.5 M NH4F for 60 min, indicating that the elimination of amorphous carbon can effectively improve the pigment chromaticity.

AxCe9(IO3)36 (A = K, La): the effects of a change in the intermediate valence on the second-harmonic generation response.

Two new isostructural non-centrosymmetric cerium iodates KxCe9(IO3)36 (x≈ 3) and LaxCe9(IO3)36 (x≈ 0.3) were synthesized via a hydrothermal method and systematically characterized. They are intermediate valence compounds with the same band gap, showing similar microscopic net dipole moments per unit volume, but different macroscopic second-harmonic generation (SHG) responses under 1064 nm laser irradiation. This research gives a new view for understanding the effects of electronic structure on the SHG response.

Atomistic Origin of High-Concentration Ce <sup>3</sup>+ in {100}-Faceted Cr-Doped CeO <sub>2</sub> Nanocrystals

Improving the potential of promising CeO2-based nanocatalysts in practical applications requires an atomic-scale analysis of the effects of active dopants on the distribution of Ce valence states and the formation of oxygen vacancies (Vos). In this study, a Cr dopant is introduced into the cubic {100}-faceted CeO2 nanocrystals (NCs) with an average size of 7.8 nm via supercritical water. The Cr dopants substitute Ce sites in the amount of approximately 3 mol%. Based on the aberration-corrected STEM-EELS, the effects of Cr dopant on the distribution of cation valence states in the Cr-doped CeO2 NCs are investigated layer by layer across the ultrafine Cr-doped CeO2 NC perpendicular to the {100} exposed facet. It is found that an increased amount of Ce3+ cations is present in Cr-doped CeO2 NCs, particularly in the internal atomic layers, compared to the non-doped CeO2 NCs. The atomic-scale analysis of the local structure combined with theoretical calculations demonstrates that Cr dopant reduces the formation energy of Vos and increases mobility of oxygen atoms for the nano-sized CeO2. These effects, in principle, result in an improved oxygen storage capacity and provide a fundamental understanding of role of the dopant in the formation and distribution of VOs in the doped CeO2 NCs.

Spectroscopic insight into the interplay between structural disorder and oxidation degree in melt‐grown Ce0.5Zr0.5O2‐y compounds

AbstractThe oxygen storage capacity of CexZr1‐xO2‐y compounds arises from the mixed‐valent character of cerium, which makes the system sensitive to external factors such as temperature and oxygen partial pressure through their influence in the Ce valence, cation ordering and oxygen content. In this work, we analyze the phase relations in Ce0.5Zr0.5O2‐y compounds fabricated by the laser floating zone technique in either oxidising or reducing atmosphere. Chemical, structural, and microstructural properties are studied by X‐ray diffraction, thermogravimetry, scanning electron microscopy (energy dispersive X‐ray spectroscopy and electron backscatter diffraction), and Raman spectroscopy. The latter proves to be determinant to discriminate between phases arising from subtle variations of atomic distribution that are hardly distinguished by X‐ray diffraction or energy dispersive X‐ray analyses. Cation disordered, fluorite‐like phases are obtained when processing is performed in air, whereas cation‐ordered, pyrochlore‐related phases are formed when processing in Ar+5%H2 atmosphere. Processing in a low but non‐negligible pO2 yields, besides the pyrochlore and fluorite‐like phases, an intermediate state with partial disorder that may be relevant for catalysis applications. The vibrational Raman spectrum of the Ce2Zr2O7 pyrochlore has been fully interpreted through polarisation measurements at room and low temperature. Bands arising from crystal field transitions within the ground and first excited multiplets of Ce3+ ions are also identified. The electronic Raman spectrum is found to be a sensitive probe of the degree of cation and oxygen order in these compounds.

Pressure-driven valence increase and metallization in the Kondo insulator Ce3Bi4Pt3

We report the results of high pressure x-ray diffraction, x-ray absorption, and electrical transport measurements of Kondo insulator Ce$_3$Bi$_4$Pt$_3$ up to 42 GPa, the highest pressure reached in the study of any Ce-based KI. We observe a smooth decrease in volume and movement toward intermediate Ce valence with pressure, both of which point to increased electron correlations. Despite this, temperature-dependent resistance data show the suppression of the interaction-driven ambient pressure insulating ground state. We also discuss potential ramifications of these results for the predicted topological KI state.

A study on the structure of tungsten by the addition of ceria: Effect of monomeric structure over W/Ce/TiO2 catalyst on the SCR reaction

In this study, we investigated the structural characteristics of catalysts and selective catalytic reduction (SCR) reaction activities depending on the Ce valence state and Ce content of the Ce/TiO2 support in order to examine the influence of the W structural characteristics of W/Ce/TiO2 catalyst on SCR reaction. Raman, X-ray photoelectron spectroscopy (XPS) and H2-temperature programmed reduction (H2-TPR) were performed in order to investigate the physicochemical characteristics of the catalyst·NH3-temperature programmed desorption (NH3-TPD) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) experiments were performed to investigate the reaction characteristics of the catalyst. As a result, SCR reaction activity was excellent when the W structure was present as monomeric W, and that this structure was formed by Ce2(WO4)3 compound, which was a binding species of WO42− monomeric W and Ce3+. In addition, the high Ce4+ ratio of the Ce/TiO2 increased the formation of monomeric W, which was found to be optimal when the loading of Ce was 10 wt%. Finally, the catalyst having the structure of monomeric W showed higher SCR reaction activity at low temperature because of increased NO adsorption and formation of unstable NOx adsorption species.