Mixed Valence Ce Research Articles

Electronic structure of Ce xSn y compounds

We present the structural properties, the electronic structure, and the 3d and 4d X-ray photoemission (XPS) spectra for eight cerium stannides of the Ce x Sn y -type. Previous magnetic investigations showed that Ce in CeSn 3, Ce 2Sn 5 and Ce 3Sn 7 has a mixed valence whereas Ce 5Sn 3 and Ce 3Sn are known as heavy fermions. The 3d and 4d XPS spectra obtained for Ce x Sn y series indicate a mixed valence of Ce. The 3d XPS spectra and LMTO calculations indicate a strong hybridization energy Δ of the f-orbitals with conduction band. Both the quantity: hybridization energy Δ , as well as the valence of Ce, depend on the number of Ce atoms and give evidence, that the type of crystallographic structure and the number of nearest Ce-neighbors decide about the ground state properties of these compounds.

Synthesis of (Ca,Ce 3+,Ce 4+) 2Ti 2O 7: a pyrochlore with mixed-valence cerium

Pyrochlore with mixed-valence Ce was synthesized by firing and annealing Ce(NO 3) 4, TiO 2, and Ca(OH) 2 with a stoichiometry of CaCeTi 2O 7 at 1300 °C. The product contains Ce-pyrochlore, Ce-rich perovskite, CeO 2 (cerianite), and minor CaO. Electron energy-loss spectroscopy (EELS) revealed both Ce 4+ and Ce 3+ in the Ce-pyrochlore with a Ce 4+ to total Ce (Ce 4+/ΣCe) of 0.80 giving ( Ca 0.87 Ce 0.21 3 + Ce 0.86 4 + ) Ti 2.05 O 7 . Cerium in the perovskite and cerianite is dominated by Ce 3+ and Ce 4+, respectively. High-resolution transmission electron microscope (HRTEM) images show that the boundary between Ce-pyrochlore and Ce-rich perovskite is semi-coherently bonded. The orientational relationship between the neighboring Ce-pyrochlore and Ce-rich perovskite is not random. Ce-pyrochlore (CaCeTi 2O 7) is a chemical analogue for CaPuTi 2O 7, which is a proposed ceramic waste form for deposition of excess weapons-usable Pu in geological repositories. It is postulated, based on the presence of Ce 3+ in the Ce-pyrochlore, that neutron poisons such as Gd can be incorporated into the CaPuTi 2O 7 phase.

Mixed valence state in Ce and Yb compounds studied by magnetic susceptibility

To investigate the mixed valence state, the magnetic susceptibility χ( T) of CeRu 3Si 2 has been measured. The temperature dependence of χ( T) T/ C in several mixed valence Ce and Yb compounds is plotted against temperature, where C is the Curie constant of Ce 3+ or Yb 3+ ion. The spin fluctuation temperature T sf has been estimated. The scaling of χ( T) T/ C vs. T/ T sf is found good in both Ce and Yb compounds. CeRu 3Si 2 is found to be a typical mixed valence system. The difference in the low-temperature susceptibility χ(0), electronic specific heat coefficient γ and superconducting transition temperature T s among Ce and Yb mixed valence compounds are discussed in terms of T sf and Wilson ratio R w.

Experimental study of the physical properties in the complex magnetic phase diagram ofCe1−xLaxRhSn

We have investigated the electronic structure and magnetic properties of the new ternary Ce 1 - x La x RhSn system. The 3d and 4d x-ray photoelectron spectroscopy (XPS) spectra indicate a mixed valence of Ce. Analysis of the 3d 9 4f 2 weight in the 3d XPS spectra using Gunnarsson-Schonhammer theory suggests a hybridization interaction A of about 120 meV for different compositions x of the system. The magnetic measurements indicate the existence of a highly anomalous weak ferromagnetic state below ∼220 K in Ce 1 - x La x RhSn for x 0.5 is dominated by spin fluctuations.

Electronic structure studies of CeRhSb with Rh substituted by Pd and Ni

We have investigated the influence of the valence of Ce ions on the gap formation in CeRhSb-type Kondo insulators. We present the $3d$ x-ray photoemission spectroscopy (XPS) spectra for CeRhSb, ${\mathrm{CeRh}}_{1\ensuremath{-}x}{\mathrm{Pd}}_{x}\mathrm{Sb},$ and ${\mathrm{CeRh}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}\mathrm{Sb}$ compounds. From the experimental Ce $3d$ XPS spectra we extracted the spectral weight of the different ${3d}^{9}{f}^{0},$ ${3d}^{9}{f}^{1},$ and ${3d}^{9}{f}^{2}$ final states. We used the Doniach-\ifmmode \check{S}\else \v{S}\fi{}unji\ifmmode \acute{c}\else \'{c}\fi{} method to separate the overlapping peaks in the $3d$ XPS spectra. Analysis of the ${3d}^{9}{f}^{2}$ weight using Gunnarsson-Sch\"onhammer theory suggests a hybridization interaction $\ensuremath{\Delta}$ of about 150 meV for CeRhSb. The hybridization energy $\ensuremath{\Delta}$ obtained for CeNiSb and CePdSb is $\ensuremath{\sim}3$ times smaller than that of the Kondo insulating compound CeRhSb. Analysis of the ${3d}^{9}{f}^{0}$ component indicates a mixed valence of Ce in CeRhSb, and a stable configuration of Ce in the Ni or Pd alloys. The experimental results indicate the influence of atomic disorder on the magnetic behavior of CePdSb and CeNiSb.

First mixed valence cerium–organic trinuclear cluster [Ce3(OBut)10NO3] as a possible molecular switch: synthesis, structure and density functional calculations

The crystalline trinuclear cerium cluster complex [{(Ce(OBut)2}2(μ-OBut)3(μ3-OBut)2{Ce(OBut)(NO3)}] 1 has been obtained in good yield from 2 [Ce(OBut)4(thf)2], [Ce(OBut)3(NO3)] and Sn(C5H3But2-1,3)Me3 by refluxing in hexane, and was characterised by microanalysis, 1H NMR, IR and UV–Vis spectra and MS data. Its structure and bonding have been analysed by X-ray diffraction. Density functional calculations were carried out on the model compound 2, in which each OBut substituent was replaced by OH, and 3, related to 2 but with OH instead of NO3. The geometric parameters showed good agreement with experiment. It is concluded that 1 is a mixed valence Ce(III)[Ce(IV)]2 cluster, that the single f electron is localised on the NO3-bearing Ce atom, and that excitation of this electron to a low energy alternative state is facile. Hence the cluster may offer potential as an f-electron molecular switch.

Theory of resonant spectroscopies in nickel, cerium compounds and other systems

Abstract In this paper, we discuss theoretically information about electronic structures in strongly correlated systems obtained from resonant X-ray spectroscopies. From a many-body point of view on the basis of an impurity Anderson model or a cluster model including the intra-atomic multipole interaction between electrons, we calculate spectra by exactly diagonalizing our Hamiltonian. First, for ferromagnetic Ni, through an analysis of magnetic circular dichroism in 3 d resonant photoemission spectroscopy (RPES) and resonant inverse photoemission spectroscopy (RIPES), it is shown that, in addition to 3 d 9 and 3 d 10 , the 3 d 8 configuration with weight of about 20% is contained as electron configuration of a Ni atom in Ni metal. The 3 d 8 configuration is furthermore shown to be mainly composed of 3 F and its relation to the origin of ferromagnetism is discussed. Second, for mixed valent Ce compounds, including CeNi 2 , it is shown, through an analysis of RIPES, tht the 4 f -conduction band hybridization must be reduced by the presence of a core hole. Third, for some 3 d transition metal (TM) oxides, 3 d RPES at TM ion 2 p threshold is shown to be a powerful method for estimating the magnitude of 3 d –3 d interaction strength and the charge transfer energy.

On the change of electronic states at the Fermi level by Ce doping in the intermetallic LaRu 2

Variation of electronic structure at the Fermi level in superconducting LaRu 2 intermetallic system has been studied by X-ray absorption near edge structure (XANES) spectroscopy. The high resolution Ru K-edge XANES have been measured by fluorescence yield method using multi-element solid state detector assembly allowing to reach a signal to noise ratio of the order of Δ α/ α∼10 −4. The results show that a doping by mixed valence Ce in place of La in the LaRu 2 directly modifies the electronic states at the Fermi level leading to a decrease of the superconducting transition temperature.

High energy resonant photoemission and resonant Auger spectroscopy in mixed valent Ce compounds

Abstract In CeRh3 and Ce7Rh3 compounds, strong resonant effects are observed in the Ce 3d and 4d photoemission spectra at the Ce L3 and M4,5 absorption edges respectively. These resonances show unambiguously that interference effects exist in core level spectroscopies. In the CeRh3 mixed valent compound, the resonances of photoemission peaks corresponding to 4f0, 4f1 and 4f2 final state character occur at different photon energies, making it possible to unravel the X-ray absorption profiles. Moreover, resonant L3M4,5N4,5 Raman Auger lines are observed up to 10 eV above the L3 absorption edge, showing the strong localized character of the core excited state.

Surface effects in the spectroscopy of mixed-valent Ce compounds

The electronic properties of correlated mixed-valent Ce compounds have been studied in detail over the last two decades by means of a variety of electron spectroscopies. This work has been carried out in connection with their peculiar ground state properties, deriving from the hybridization of the Ce 4f wavefunctions with the conduction band states. However, the relevance of surface effects, in the interpretation of the spectral line-shapes has been recognized only very recently. These are related to the reduction of 4f hybridization at the surface. In this review I will give a summary of the main achievements in this area, showing the procedures to separate the surface from the bulk spectral contributions for both the Ce 4f spectral functions and the Ce 3d core levels. In addition, a systematic analysis of the surface and bulk electronic properties of a number of different compounds is reported. This new information can in some cases reconcile the results obtained by different spectroscopies in terms of theoretical models based on the Anderson single impurity Hamiltonian. However, when comparing spectroscopies which rise to considerably different final states this may no longer be valid and some possible solutions are outlined. Furthermore, preliminary results on single crystals are presented. They seem to show somewhat unexpected features which need to be studied in deeper detail in the future.

Superconductivity at an elevated temperature in a new class of quaternary rare earth-transition metal boro-carbides

Our discovery of superconductivity at an elevated temperature (∼ 12 K) in the system YNiBC has opened up an entirely new area of research in superconductivity of intermetallics. We briefly present the results of our recent studies on magnetic history effects in YNi 2B 2C, the coexistence of superconductivity and magnetism in ErNi 2B 2C, the effect of substituents on superconductivity in YNi 2B 2C, the mixed valence of Ce in CeNi 2B 2C and superconductivity in Th-based borocarbides.

Valence-fluctuation behavior of Yb ions and YbCuGa.

The compound YbCuGa has been synthesized and is found to crystallize in the orthorhombic CeCu 2 -type structure. Its magnetic susceptibility has been measured between 4.2 and 300 K and shows a broad maximum at about 190 K. This maximum is a characteristic feature of the mixed-valent Ce- and Yb-based compounds. The temperature-dependent resistivity exhibits an S-type behavior. The thermoelectric power is negative between 20 and 300 K and also shows a broad minimum at about 190 K. L 3 x-ray-absorption near-edge-structure (XANES) measurements show the two-peak pattern that corresponds to two different valence states of Yb. Thus, magnetic susceptibility, electrical resistivity, thermoelectric power, and L β XANES measurements all point to the mixed-valence nature of Yb ions in YbCuGa.

Circular Dichroism in the 3dand 4dCore Photoabsorption for Ferromagnetic Ce Compounds -Interplay of Hybridization, Spin Orbit Interaction and Crystal Field-

The Ce 3 d - and 4 d -core photoabsorption spectra (3 d XAS and 4 d XAS) of ferromagnetic mixed valent Ce compounds are calculated on the basis of the impurity Anderson model. The model takes into account the Coulomb interaction producing multiplet structures, the spin-orbit interaction, the crystal field effect and the molecular field acting on the 4 f spin. The calculation shows a strong circular dichroism even for the Ce magnetic moment of ∼0.5 µ B . The dichroism is furthermore shown to be a powerful method to measure the spin and orbital contributions to the 4 f moment. On the basis of the calculation, the 4 f magnetic state of the ferromagnet CeRh 3 B 2 is discussed.

Circular dichroism in the 3d- and 4d-core photoabsorption for ferromagnetic mixed valent Ce compounds

Effects of symmetry breaking in the 4f orbital occupancy of Ce atom in ferromagnetic Ce compounds on the multiplet structures in the Ce 3d- and 4d-core photoabsorption spectra are discussed based on the impurity Anderson model. The calculation predicts a strong circular dichroism in the spectra even for a molecular field producing the 4f magnetic moment of ~0.5µB. The interplay of the 4f-conduction band hybridization and the 4f spin-orbit interaction in the ground state is discussed in detail and the present dichroism is shown to be a good method to measure the spin and orbital contributions to the 4f magnetic moment.

Electronic Spectroscopies of the Mixed Valence Ce Compounds: A Slave Boson Approach to the Impurity Model11Project supported in part by National Natural Science Foundation of China under Grant No. 1891101 and Grant No. 1870702.

A slave boson approach to the electronic spectroscopies for the mixed valence Ce compounds in the impurity model is presented. Three valence configurations of Ce ion ( and ) are considered so the framework of the theory can be used for culculating both XPS and BIS. The spectrum under mean field approximation is presented.

Multiplet Structures in 3dand 4dCore Photoemission Spectra for Ce Compounds

The Ce 3 d and 4 d core X-ray photoemission spectra (3 d and 4 d XPS) of Ce compounds are studied based on the impurity Anderson model considering atomic multiplets arising from electron-electron interaction. We make a consistent analysis of 3 d and 4 d XPS of trivalent Ce compounds CeF 3 , CeCl 3 and Ce 2 O 3 , which can only be made by considering the atomic multiplets. We also apply the approach to the interpretation of the spectra of mixed valent Ce systems Ce–Ni compounds and CeO 2 . We show that the analysis of the multiplet structure gives information about many-body electronic states of the system.

Pressure-induced changes inLIIIx-ray-absorption near-edge structure of CeO2and CeF4: Relevance to4f-electronic structure

X-ray-absorption near-edge structure (XANES) studies were performed at the ${L}_{\mathrm{III}}$ thresholds of Ce${\mathrm{O}}_{2}$ and Ce${\mathrm{F}}_{4}$ as a function of external pressure up to \ensuremath{\cong}267 kbar at room temperature. In both cases, the ${4f}^{0}$-related higher-energy component of the essentially double-peaked XANES structure decreases in relative intensity with pressure, opposite to what is generally observed for metallic mixed-valent Ce compounds. This supports the proposed peak assignment on the basis of core-hole induced many-body effects due to $4f$-ligand covalency compatible with traditional tetravalency of these compounds.

Coexistence of strong 3d antiferromagnetism and mixed-valent Kondo regime transition in Ce(MnxCr

Structural, thermal, magnetic, transport, and Ce ${L}_{3}$-edge spectroscopic measurements on the Ce(${\mathrm{Mn}}_{\mathrm{x}}$${\mathrm{Cr}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${)}_{2}$${\mathrm{Si}}_{2}$ series are presented. These results all indicate that Cr substitution drives this system from a mixed-valent Ce state at x=1.0 to a Kondo or heavy-fermion state with increasing x. The reflection of the Ce valence-state variation (from ${L}_{3}$ measurements) in the electronic specific heat coefficient and lattice parameters is discussed. The magnetic and transport measurements indicate that the Ce spin-flip scattering processes (i.e., the Kondo effect) apparently thrive within the Mn-moment antiferromagnetically ordered phase of these compounds.

Multiplet structure of core photoabsorption spectra in mixed valent Ce compounds

The effect of the 4f-valence band hybridization on the multiplet structure in core X-ray absorption spectra (XAS) of mixed valence systems is discussed on the basis of the impurity Anderson model incorporated with the 4f-core hole exchange interaction. A qualitative interpretation for 4d-XAS in the prethreshold region og CeCo2 and metallic Ce compounds is presented.

Evolution of magnetism from mixed-valent CeIr3B2 to trivalent CeIr3Si2.

The ternary compound Ce${\mathrm{Ir}}_{3}$${\mathrm{B}}_{2}$ is a mixed-valent material crystallizing in a monoclinic structure, closely related to the hexagonal Ce${\mathrm{Co}}_{3}$${\mathrm{B}}_{2}$-type structure. The structural and magnetic phase diagrams of the systems $\mathrm{Ce}{\mathrm{Rh}}_{3\ensuremath{-}y}{\mathrm{Ir}}_{y}{\mathrm{B}}_{2}$ ($y=0\ensuremath{-}2$) and $\mathrm{Ce}{\mathrm{Ir}}_{3}{\mathrm{B}}_{2\ensuremath{-}x}{\mathrm{Si}}_{x}$ ($x=0\ensuremath{-}2$) have been investigated. Properties of the new compound Ce${\mathrm{Ir}}_{3}$${\mathrm{Si}}_{2}$ have been reported for the first time. It is shown that the structure of Ce${\mathrm{Ir}}_{3}$${\mathrm{B}}_{2}$ transforms to the hexagonal type on substitution of Rh and Si at Ir and B sites, respectively. Accompanying the structural transformation is an electronic transition in which the mixed-valent Ce atoms become more magnetic. The observed ferromagnetic ground state is similar to the one seen in Ce${\mathrm{Rh}}_{3}$${\mathrm{B}}_{2}$ (${T}_{c}=115$ K and ${\ensuremath{\mu}}_{\mathrm{sat}}=0.4{\ensuremath{\mu}}_{B}$). The role of Ce-atom ligands and their hybridization is thought to be the primary cause for the unusual nature of magnetism in Ce${\mathrm{T}}_{3}$${\mathrm{B}}_{2}$-type compounds.