Valence Compounds Research Articles

Electron-spin-resonance in intermediate valent and heavy-electron compounds

We report on electron-spin-resonance of Gd-impurities in intermediate valent CePd 3 and CeOs 2 as well as in the heavy-electron compounds CeCu 2Si 2 and CeAl 3. In all investigated compounds at high temperature, the thermal broadening d(ΔH) dT of the Gd-resonance is comparable to isostructural reference compounds. At low temperature, the intermediate valent compounds show a reduced thermal broadening due to hybridization between Ce-4f and conduction electrons. In the heavy fermion compounds, the low temperature broadening is enhanced due to slow fluctuating Ce moments and the enhanced under hydrostatic pressure ( p ≤ 17 kbar) confirmes the relation between slowly fluctuating Ce moments and the enhanced slope d(ΔH) dT .

Acoustic Mode Vibrational Anharmonicity in the Intermediate Valent Compound Samarium Sulfide

AbstractAnharmonic elastic properties of SmS in its semiconducting form are analysed in terms of third order elastic constants and pressure derivatives of second order elastic constants calculated using an interionic force model. This potential model consists of long‐range Coulomb and three‐body interactions and short‐range overlap repulsion, effective up to second neighbour ions. Repulsive parameters (b, ϱij) are determined using the equation of equilibrium condition and overlap integrals. The results reported are compared with experimental values and fairly good agreement is obtained.

Heat capacity and magnetic susceptibility of mixed valent YbPt 2Si 2

From heat capacity and magnetic susceptibility results, we conclude that YbPt 2Si 2 is a homogeneously mixed valent compound.

Thermal decomposition of ammonium fluorooxovanadates(IV)

The thermal decomposition of (NH4)2[VOF4], (NH4)2[VOF4·H2O] and (NH4)3[VOF5] has been investigated in air and argon by simultaneous TG, DTG and DTA. Chemical analysis. X-ray data, and IR spectroscopy have been employed to characterize the intermediate phase for all compounds. The complexes start decomposing by giving off NH4F. NH4VOF3 has been found as an intermediate phase for all compounds. In argon atmosphere NH4VOF3 is reduced to a mixture of lower valent compounds, with the main component VF3. By carrying out the decomposition in air, the oxidation of vanadium to V2O5 has been observed.

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.

Theory of Photoemission, Photoabsorption and Inverse Photoemission in CeO2 and Metallic Ce Compounds

Ce 3d core X-ray photoemission (3d-XPS), 3d core photoabsorption (3d-XAS), 3d core resonant photoemission (3d-RXPS) and bremsstrahlung isochromat spectra (BIS) in a mixed valent insulator CeO2 and typical metallic mixed valent Ce compounds such as CeRh3 are studied theoretically on the basis of the Anderson model. By assuming that the weight of 4f0 configuration in the ground state, the 4f-valence band mixing and the 4f-4f Coulomb interaction for CeO2 are considerably larger than those for metallic Ce compounds, a remarkable difference in 3d-XPS and an apparent similarity in 3d-XAS between the two systems are consistently explained. Furthermore differences in 3d-RXPS and in BIS between the two systems expected from the present analysis are predicted. A preliminary discussion about the effect of 4f-core hole and 4f-4f exchange interactions on core absorption spectra in mixed valent systems is also presented.

On the nature of the valence transition in Eu(Pd 1− xAu x) 2Si 2

The intermediate valent compound series Eu(Pd 1− x Au x ) 2Si 2 has been studied by Mössbauer effect measurements on 151Eu( T = 4.2−300K) and 197Au (4.2K) and by X-ray diffraction (10K, 300K). The temperature induced valence transition Eu 2+ → Eu 3+ for x < 0.175 is not of first order type, as suggested in a previous phase diagram [1]. The valence change of the Eu-ion is reflected also in the isomer shift of the 197Au Mössbauer-resonance.

Gd3+-ESR in the intermediate valent cerium compounds Ce x La1-x Os2

In the intermediate valent compounds Ce x La1−x Os2 the Gd3+-ESR shows a non-linear increase of the linewidth ΔH(T) in the temperature range 4.2K<T<300K for 0.3≦x≦1.0. This non-linearity of the thermal broadening is strongly altered by changing the Ce-concentration. The experimental results ΔH(T,x) can quantitatively be described via hybridization of conduction electrons with Ce4f electrons: Due to hybridization, the Ce4f states are broadened but remain localized. The conduction electron density of states is reduced in the vicinity of the 4f states. In addition we observe a maximum in the electrical resistivity ϱ(T) of CeOs2 at 270 K.

ATOMIC RELAXATION IN MIXED-VALENT SYSTEMS DEDUCED FROM EXAFS SPECTROSCOPY

We will present how it is possible, from EXAFS spectroscopy, to estimate atomic relaxation effects even in homo.geneous mixed valent compounds, for which these relaxations are known to be very small. EXAFS experiments, analysed by our procedure, can then be used to decide if the ground state is homogeneous or inhomogeneous. At least, the procedure will be applied to the Tm Se system. We conclude that the homogeneous character of the mixed valent stzte is preserved in the non-stoichiometric compounds.

The static susceptibility of the periodic Anderson hamiltonian

A calculation of the temperature and f-level dependence of the static f-electron susceptibility of the periodic Anderson Hamiltonian (PAH) for the cases of one (Nt=1) and two (Nt=2) electrons per atom and in the strongly mixed valent regime is presented. Correlations are included beyond meanfield. Evidence for a tendency towards ferromagnetic order is found for some parameter values in the case Nt=1. For Nt=2, the susceptibility exhibits a peak at finite temperature, a feature typical of many mixed valent compounds, and this peak correlates with the appearance of a pseudo-gap in the f-electron density of states. The theory is correct at high temperature, where the known perturbative results are correctly reproduced.

Theory of the electronic properties of mixed valent compounds

A self-consistent Green function theory of the Anderson lattice Hamiltonian is presented. The approximations include spin and charge fluctuations beyond mean field and are similar to previous approximations made for the impurity problem by Theumann (1969). The self-energy integral equations are solved exactly and the static physical properties are obtained for almost the entire strongly mixed valent regime. The temperature dependence of the electronic specific heat, C(T), gamma (T)=C(T)/T, densities of states, Fermi level and valence are discussed for both one and two electrons per atom. The behaviour of gamma (T) is similar to that found experimentally.

Xanes study of trivalent and homogeneous mixed valent rare earth systems

We report for the first time a systematic study of X-ray absorption structures 20–30eV above the L III edge (XANES) of homogeneous mixed valent (SmB 6, YbAl 2, CePd 3...) and trivalent reference Rare-Earth compounds (e.g. GdB 6, LaAl 2, NdPd 3). The XANES of integral valent compounds in simple structures (NaCl, AuCu 3...), which is due to multiple scattering of the photoelectron on the neighbouring atomic shells, is characterized by a single peak. On the contrary the XANES of mixed-valent materials presents a splitted structure which replicates the edge structures. We interpret this splitting as a direct evidence for the existence of two threshold energies and corresponding presumably to two different Rare-Earth metal distances in agreement with 4 f fluctuations. Finally, we shall propose a consistent picture for core hole spectroscopies (L III edge, XANES, EXAFS and XPS) in mixed-valent materials.

Non-adiabatic breathing shell model for the lattice dynamics of intermediate valent compounds

We present a model for the lattice dynamics of an intermediate-valent impurity where the degree of freedom of its electronic shell has monopolar fluctuations with an effective mass and damping. Explicit consideration of this additional degree of freedom accounts for the experimentally observed gap-mode in Sm 1− x R x ( R = Y, La, Pr, Gd, Tb, Dy, Tm) in contrast to previous models which assign this mode to the isotopic defect of ion R.

Light scattering from crystalline field and intra-ionic excitations in metallic rare earth compounds

Abstract Recent developments in applying inelastic light scattering to investigate localized electronic excitations in rare earth Kondo-type and intermediate valence (IV) compounds are briefly summarized. The Г 8 −Г 7 crystalline-electric-field (CEF) excitation of CeB 6 is found near 372 cm -1 (530 K) with a Г 8 ground state dynamically split by 20 cm -1 (30 K). Resonantly coupled phonon-CEF excitations have been investigated in CeAl 2 and CeD x (1.95≦ x ≦2.91); a phonon bound state is found in CeAl 2 . Intra-configurational spin-orbit split ( J ) multiplet excitations have been observed in the IV compounds CePd 3 , EuPd 2 Si 2 and EuCu 2 Si 2 . In the latter two compounds the temperature dependence of the inter-configurational excitation energy E x and of the fluctuation temperature T f have been obtained from the Roman peak positions and widths, respectively. The spectroscopic data provide the first direct proof of the ionic interconfigurational fluctuation model.

Nmr of 31P in the mixed valent compound, EuNi 2P 2

Nmr of 31P in the mixed valent compound, EuNi 2P 2

Valence inhomogeneities in intermediate-valent Eu compounds

Abstract In each of the intermediate valent compounds EuNi 2 P 2 , EuPd 2 Si 2 , and EuPd 6 B 4 , the line width of the 151 Eu-Mossbauer resonance attains a maximum when its isomer shift varies most strongly with temperature. We associate this anomalous line broadening with a distribution of the local Eu valence, which may be described by an inhomogeneous width, Δ, of the interconfigurational excitation energy, E exc , in addition to the homogeneous fluctuation width, k B T f . Applying this model to the analysis of isomer shift, line width, and magnetic susceptibility, the model parameters Δ, E exc , and k B T f are obtained. Δ is found to attain maximal values at temperatures where E exc varies most steeply with temperature.

Temperature and pressure dependence of lattice parameters of the XCu2Si2 compounds (X=Ce, Eu, Yb, La, Gd, Lu, Ca, Th)

The RECU2Si2(RE=Rare Earth) series shows pronounced anomalies of the lattice parameters. In those stable valent compounds with 4f orbital angular momentumL≠0 thec/a ratio shows an anomalous temperature dependence due to crystal field effects. The compounds with unstable valence (RE=Ce, Eu, Yb) show less or no such effects but on the other hand show anomalies of the volume which are temperature dependent. Using these volume anomalies of the unstable valent systems and the measured bulk moduli of the reference compounds we have extracted the valence as function of temperature.

Electronic configuration of cerium in amorphous alloys (invited; abstract)

We have investigated the effect of structural disorder on the valence of cerium in different families of amorphous alloys prepared by coevaporation on to cryogenic substrates. The Ce electronic configuration was characterized by LIII x-ray absorption spectroscopy and bulk magnetization measurements. Special attention was paid to the amorphous analogs of prototypic Kondo and strongly mixed valent compounds in the crystalline state, such as Ce Si2 (TK≂200 K), CemNin, CemCon. The cerium was found to be purely trivalent in amorphous CexSi1−x (0≤x≤0.75) alloys over both the metallic and nonmetallic concentration ranges, so that the weakly mixed valent state in crystalline CeSi2 is believed to rely on details of the crystalline symmetry and band structure. In contrast, the cerium was found to remain mixed valent in amorphous CexCo1−x (0.15≤x≤0.75) and CexNi1−x (0.30≤x≤0.60) alloys, without any significant effect of the structural disorder. The concentration dependence of the Ce valence anomaly was found to be different in the Ce Ni and Ce Co alloys. This different behavior is discussed in terms of electronic band structure.

Point contact spectroscopy of YbB 12 and CeAl 3

It will be shown that the first derivative of the current-voltage characteristic in point contact spectroscopy (PCS) is a function of the electronic density of states at the Fermi level Ef. PCS has been used to investigate an intermediate valent compound, YbB 12, and a heavy Fermion system, CeAl 3. It has been found that YbB 12 has a hybridization gap and CeAl 3 a density of states peak at Ef.

Photoemission spectra of Ce compounds and their relation to ground-state and dynamical properties

Abstract 4f-derived features in photoemission spectra of Ce compounds are described by using a Ce-ligand cluster model which takes into account strong 4f-electron correlation, 4f-ligand hybridization and band-structure effects. In terms of this model it is shown that ground-state properties such as the disappearance of the local magnetic moment and the small lattice volume results from the 4f-ligand orbital hybridization in mixed valent and tetravalent Ce compounds. Applicability of the present and other models for spectroscopic, ground-state and low energy dynamical properties of Ce compounds is discussed.