Mixed Valence Phenomena Research Articles

Water-Soluble and Ultrastable Ti4L6 Tetrahedron with Coordination Assembly Function.

We have successfully constructed a tetrahedral Ti4L6 cage with calixarene-like coordination-active vertices. It further features high solubility and stability in H2O and DMF/H2O solution, affording an interesting stepwise assembly function with other metal ions. Through trapping of different amounts of Co or Ln ions, the Ti4L6 tetrahedra can be organized into various dimensional architectures, including a Ti4L6-Co3 cage, a Ti4L6-Ln2 cage, a Ti4L6-Ln2 chain, and a three-dimensional Ti4L6-Ln framework. An unusual mixed-valence phenomenon was observed in the Ti4L6 cage, whose Ti3+/4+ compositions were adequately identified by electron spin resonance and X-ray photoelectron spectroscopy analyses. More remarkably, the calixarene-like oxygen vertices of the Ti4L6 cage can also be used for the recognition of C3-symmetric dye molecules through N-H···O hydrogen bonding. Accordingly, driven by visible light, selective and efficient homogeneous photodecomposition of acid blue 93 and alkali blue 4B were successfully achieved. Therefore, this work not only represents a milestone in constructing symmetric Ti-based cages with interesting coordination assembly function but also provides a new method for preparing technologically important soluble photoactive cages.

Role of spin dependent f–d interaction (Hund coupling) in mixed-valence phenomena

The effect of spin dependent f–d interaction (Hund coupling) in mixed-valence phenomena has been investigated within Falicov–Kimball model extended by the spin interaction. Calculations have been performed both at zero and at finite temperatures using exact diagonalization method. It is observed that spin dependent f–d interaction (1) shifts the d-level energy and increases the width of valence transition, (2) increases disorder in the system, (3) causes both single peak and double peak structures to appear in specific heat curves, (4) increases the growth of antiferromagnetic correlations.

ChemInform Abstract: Photoswitchable Mixed Valence

AbstractReview: 60 refs.

Falicov-Kimball model extended by next–nearest–neighbor (NNN) hopping of d-electrons

The role of next–nearest–neighbor (NNN) hopping of the d-electrons in the mixed-valence phenomena has been investigated. Falicov-Kimball model extended by NNN hopping has been considered. Calculations have been made both at zero and at finite temperatures. It has been observed that NNN hopping (i) shifts the d-level energy, (ii) makes the system more disordered, (iii) decreases degeneracy of the ground state and (iv) has important effects on the spin susceptibility. Antiferromagnetic type exchange interaction has also played similar role as NNN hopping in the spin susceptibility χ.

Photoswitchable mixed valence

For a molecular electronics technology to be fully serviceable, switching functions will be indispensable. Specifically, it will be desirable to control the conductivity of a given molecule using an external stimulus. This tutorial review discusses photoswitchable mixed valence systems that are comprised of a reversibly photoisomerizable bridging unit connecting two redox-active moieties, and as such represent some of the most simple chemical systems in which switching of charge delocalization can be explored. As photoisomerizable units, dithienylethenes have received much attention in the context of photoswitchable mixed valence, but there are also more exotic examples such as norbornadiene- and dimethyldihydropyrene-based switchable systems. As redox-active units responsible for the mixed valence phenomenon, both metal-containing as well as purely organic moieties have been employed. Typical investigations in this area involve the comparison of cyclic voltammograms and (near-infrared) optical absorption spectra of the two isomeric forms of a given system. The magnitude of the comproportionation constant and evaluation of intervalence absorption bands using appropriate theoretical models yield information regarding the extent of charge delocalization in the two isomeric forms. In several of the compounds investigated so far, the light stimulus induces a substantial increase of charge delocalization, or in the terminology commonly used in mixed valence chemistry, a changeover from class I to class II or even class III behavior.

Role of correlated hopping in mixed valence phenomena

Role of correlated hopping is studied using extended Falicov–Kimball model in a small cluster. A discontinuous insulator-to-metal transition is observed at a critical f-level energy. Transition is sharper for larger correlated hopping. In the specific heat curves a two-peak structure consisting of a sharp peak followed by a Schottky-type broad peak is exhibited. In a limited parameter region, some heavy-fermion like characteristics have been observed.

Theory of XAS and XMCD for the field-controlled valence mixed state in RE compounds

A valence fluctuating phenomenon, which is one of the most interesting subjects in strongly correlated electron systems, has been studied theoretically in EuNi2(Si0.18Ge0.82)2, in which the field-induced valence transition from a nonmagnetic Eu3+ state (4f6) to a magnetic Eu2+ state (4f7c1) was observed at around 40 Tesla by means of X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) at Eu L-edges. Here, c denotes a hole in a conduction band. Both XAS and XMCD in fields exhibit two peaks corresponding to the two valence states, although one of them is nonmagnetic. In this paper, we propose a model to explain the field-induced valence transition, in which the hybridization between the 4f6 state and the 4f7c1 state plays an essential role. Comparing XAS and XMCD calculated with experimental data, we find that XAS and XMCD provide us with detailed microscopic information on the mixed valence state through the polarized 5d electrons. Finally, we demonstrate how effective XAS and XMCD are for studying such mixed valence phenomena.

Mixed valency in rare-earth fullerides

Mixed-valence phenomena associated with the highly correlated narrow-band behaviour of the 4f electrons in rare earths are well documented for a variety of rare-earth chalcogenides, borides and intermetallics (Kondo insulators and heavy fermions). The family of rare-earth fullerides with stoichiometry RE2.75C60 (RE=Sm, Yb, Eu) also displays an analogous phenomenology and a remarkable sensitivity of the rare-earth valency to external stimuli (temperature and pressure) making them the first known molecular-based members of this fascinating class of materials. Using powerful crystallographic and spectroscopic techniques which provide direct indications of what is happening in these materials at the microscopic level, we find a rich variety of temperature- and pressure-driven abrupt or continuous valence transitions-the electronically active fulleride sublattice acts as an electron reservoir that can accept electrons from or donate electrons to the rare-earth 4f/5d bands, thereby sensitively modulating the valence of the rare-earth sublattice.

Magnetic spectral response and lattice properties in mixed-valenceSm1−xYxSsolid solutions studied with x-ray diffraction, x-ray absorption spectroscopy, and inelastic neutron scattering

Mixed-valence phenomena occurring in the "black" (B) and "gold" (G) phases of Sm1-x Yx S have been studied by x-ray diffraction, x-ray absorption spectroscopy, and inelastic neutron scattering. Lattice-constant and phonon-dispersion results confirm that the valence instability occurs already inside the B phase. On the other hand, pronounced temperature anomalies in the thermal expansion \alpha(T), as well as in the Sm mean-square displacements denote the onset of the B-G transition for the compositions x = 0.33 and 0.45. It is argued that these anomalies primarily denote an effect of electron-phonon coupling. The magnetic spectral response, measured on both powder and single crystals, is dominated by the Sm2+ spin-orbit component close to 36 meV. A strongly overdamped Sm3+ contribution appears only for x >= 0.33 near room-temperature. The quasielastic signal is strongly suppressed below 70 K, reflecting the formation of the singlet mixed-valence ground state. Quite remarkably, the signal around 36 meV is found, from the single-crystal spectra, to arise from two distinct, dispersive, interacting branches. The lower peak, confirmed to exist from x = 0.17 to x = 0.33 at least, is tentatively ascribed to an excitation specific to the mixed-valence regime, reminiscent of the "exciton" peak reported previously for SmB6 .

Possible mixed valence behavior of CeIr 2Ga and YbIr 2Ga

We report possible mixed valence behavior in the new ternary compounds CeIr 2Ga and YbIr 2Ga. These materials crystallize in the hexagonal Na 3As structure of space group P6 3 cm. Magnetization, specific heat and transport measurements show reduced magnetic moments and the absence of magnetic order above 0.04 K. The quasi-2D structure of these compounds offers the possibility for investigating the role of spatial dimensionality on mixed valence phenomena.

Phase transitions in mixed valence systems

The experimental properties of URu 2Si 2 are briefly discussed to demonstrate that the lattice degrees of freedom may be tightly involved into the mixed valence phenomenon and the heavy fermion behavior. Instead of describing the f-electrons in terms of rigid single ion orbitals, we emphasize the importance of the lattice effects in localization of an f-electron near the given site. In view that so many pertinent interactions and parameters are involved, a phenomenological approach to the problem is discussed in terms of the generalized Anderson model for single centers. Implications of the picture to the symmetry considerations are discussed in some more details for URu 2Si 2. Some features of the Kondo insulator state are also discussed to indicate that they may also be compatible with the developed views.

Thermoelectric power behaviour of CeRh2− xNixSi2 alloys

The thermoelectric power (TEP) behaviour in the temperature range 2–300 K is investigated in the pseudoternary system CeRh 2− x Ni x Si 2, in which the interplay among magnetic ordering, heavy fermion and mixed valence phenomena is recently established. The present study provides a way to the understanding of the TEP behaviour in Ce systems, where several phenomena compete.

The determinant method and quantum simulations of many-body effects in a single impurity Anderson model

We present a short description of a quantum Monte Carlo technique that has proved useful for simulating many-body effects in systems of interacting fermions at finite temperatures. We then report our preliminary results using this technique on a single impurity Anderson model. Examples of such many-body effects as local moment formation, Kondo behavior, and mixed valence phenomena found in the simulations are shown.

Mixed-valence phenomena in neutron scattering

Abstract We briefly review the results obtained by three classes of neutron scattering experiments on mixed-valence compounds: induced moment form factors, paramagnetic spectra, and phonon dispersion curves. Whenever appropriate, the results are compared to those obtained on dense Kondo and heavy-fermion compounds, two classes of materials closely related to the mixed-valence compounds. Future directions for research in these areas are indicated.

Coherent and incoherent Ce heavy fermion systems: The very nearly trivalent end of the mixed valence phenomenon

Evidence is presented for the notion that Ce heavy fermion behavior corresponds to the n f ≈ 1 limit of mixed valence and hence the T K → 0 limit of the Kondo regime in both Ce lattice systems and in systems in which the Ce ions are diluted and hence don't form a regular sublattice.

Systematic trends in the LIII-edge absorption spectra of the rare-earth based systems

We have investigated the LIII-edge absorption spectra of the metallic systems, RA12 (R = lanthanide except Pm, Ho, and Tm), and the insulating systems, RF3 (R = lanthanide except Pm). From analysis of the data, we have found that the “white lines” (WLs) are wider for the metallic systems as compared to that for the insulating systems. We interpret this result in terms of stronger hybridization in the RA12 systems, as expected from the smaller R-R distances in RA12, relative to that in the insulating RF3 systems. We discuss the EuA12 and YbA12 systems in the context of mixed-valence phenomena. The EuA12 spectrum indicates the system to be weakly mixed valent, whereas the broad WLs encountered in the RA12 series mask the bimodal character of the YbA12 spectrum.

The order-parameter in the mixed-valence phenomenon and the boomerang valence

We report a new phenomenon, namely, that upon cooling the valency of the rare-earth ion goes from one value to another at a characteristic temperature which upon further cooling returns to the original value at another characteristic temperature. We call this happening as the boomerang valence phenomenon which occurs because of our invention of a new order parameter.

Phenomenological thermodynamics of mixed valence phenomenon

The fluctuation of valence in some rare-earth (RE) compounds is described in terms of the effective potential seen by the RE ion. The nearly degenerate 4ƒ n+1(5d6s) 2 and 4ƒ n(5d6s) 3 levels of the RE ion split in energy in the presence of a repulsive potential. The energy separation (Δ E) between these levels is a function of external variables such as temperature, pressure or composition, which change the effective potential seen by the ion. The variation of Δ E with temperature is obtained for four Europium compounds from 151Eu Mössbauer isomer shift data. The temperature dependence of susceptibility is then obtained from the same (Δ E) variation and compared with experimental results. A characteristic temperature ( T υ) is found below and above which (Δ E) behave as ΔE = αT and ΔE = βT+ γ respectively.

A study of decomposition of Ce(In 1- xSn x) 3 in air accompanied by whisker growth and measurement of electrical resistivity

A study of decomposition of Ce(In 1- xSn x) 3 in air accompanied by whisker growth and measurement of electrical resistivity

Exact results in the theory of magnetic alloys

Abstract Recently it has been shown that many traditional models used for a description of dilute magnetic alloys are completely integrable and may be solved exactly without any approximation. In this article we summarize the results which have been obtained in this way. The main part of the article is devoted to the consistent and detailed account of the Bethe-Ansatz technique for the s-d exchange (Kondo) model with arbitrary impurity spin, s-d model with anisotropic exchange, degenerate exchange model and for the canonical Anderson model. The thermodynamic properties of a magnetic impurity in a non-magnetic metal host obtained by the Bethe method are considered in detail. Mainly attention is paid to the analysis of singularities associated with the formation of a localized moment, Kondo effect and mixedvalence phenomenon, which can be treated analytically. In the introductory part of the article we discuss the applicability of the models which we study in the paper to the real alloys and consider some m...