Intermediate Valence Compound Research Articles

Valence and Structural Transitions in the Pseudo-Ternary Germanide Ce(Rh0.69Ir0.31)Ge

Ce(Rh0.69Ir0.31)Ge has been investigated by magnetization measurements, differential scanning calorimetry, and X-ray diffraction on a single crystal at 300, 150, and 100 K. For the first time, a decrease of the valence of cerium with decreasing temperature has been observed. This first-order valence transition evidenced at 236−258 K by magnetization measurements is correlated to the occurrence of a structural transition appearing between 300 and 150 K. The structural properties versus temperature of Ce(Rh0.69Ir0.31)Ge are discussed in relation to those determined on single crystals on the antiferromagnet CeRhGe and on the intermediate valence compound CeIrGe. The structure refinements revealed significantly different distortions for the [RhGe] and [IrGe] polyanionic networks, which are a direct consequence of the different electron densities at the rhodium and iridium sites, respectively. The crystal chemical peculiarities of the CeRhGe, Ce(Rh0.69Ir0.31)Ge, and CeIrGe structures are discussed.

Crystal-field effects in the mixed-valence compoundsYb2M3Ga9(M=Rh,Ir)

Magnetic susceptibility, heat capacity, and electrical resistivity measurements have been carried out on single crystals of the intermediate valence compounds ${\mathrm{Yb}}_{2}{\mathrm{Rh}}_{3}{\mathrm{Ga}}_{9}$ and ${\mathrm{Yb}}_{2}{\mathrm{Ir}}_{3}{\mathrm{Ga}}_{9}$. These measurements reveal a large anisotropy due apparently to an interplay between crystalline electric field (CEF) and Kondo effects. The temperature dependence of magnetic susceptibility can be modelled using the Anderson impurity model including CEF within an approach based on the non-crossing approximation.

XAFS-spectroscopy of EuCu 2Si 2

We report the results of the intermediate valence compound EuCu 2Si 2 local electron and crystalline structure studies by means of L 3-Eu XANES and K-Cu EXAFS spectroscopy. Temperature dependence of the europium valence has been defined. In EXAFS studies, we have analyzed Cu atoms local environment and have revealed bond length and rigidity parameters of Cu–Si, Cu–Cu and Cu–Eu coordination spheres.

Rare Beryllium Icosahedra in the Intermediate Valence Compound CeBe13.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Low-temperature properties ofYbAl2

Measurements on the intermediate-valent intermetallic compound $\mathrm{Yb}{\mathrm{Al}}_{2}$ are presented. We discuss magnetization, resistivity, magnetoresistance and specific heat in the temperature range between 1.5 and $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in magnetic field up to $14\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. Our samples, with a high residual resistivity ratio of nearly 50, display a ${T}^{3.5}$ temperature dependence of the resistivity and a strong magnetoresistance, reaching $\ensuremath{\Delta}\ensuremath{\rho}∕{\ensuremath{\rho}}_{0}\ensuremath{\approx}1.5$ at $12\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ and $3\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The low-temperature specific heat and magnetization of $\mathrm{Yb}{\mathrm{Al}}_{2}$ remain unchanged in high fields, as expected for this intermediate-valent compound with a high fluctuation temperature.

Influence of single‐site and cooperative magnetic effects on phonons in CeNi‐based compounds

A study of the temperature evolution of acoustic phonons and Ce valence value in the intermediate-valence compound Ce0.85La0.15Ni have been performed. The substitution of Ce by La does not suppress the intermediate valence state but decreases the valence value. A significant softening for most of the acoustic modes has been found in Ce0.85La0.15Ni with respect to reference compound LaNi at room temperature. The analysis of obtained data using analogous ones for CeNi allow to distinguish between effects of influence on CeNi phonon frequencies caused by intermediate-valence single-ion phenomenon and magnetic correlation between Ce unstable-valence ions. It appears that intermediate valence effect results in reduction of phonon frequencies in general, while inter-ion magnetic interaction leads to an extra contribution to the value of frequencies for selected phonon modes. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Rare Beryllium Icosahedra in the Intermediate Valence Compound CeBe13

Single-crystal X-ray diffraction experiments show that the Be atoms in CeBe13 form a Be12 icosahedra, which is a very unusual structural feature due, in part, to the remarkably low valence electron count of Be. Magnetization studies show that CeBe13 displays intermediate valence behavior, in which valence fluctuations between the Ce 4f0 and 4f1 states give rise to enhanced electronic specific heat and magnetic susceptibility. Calculations using ab initio theory were used to determine the electronic structure and bonding and to give insight into the relationship between the crystal structure, the bonding, and the intermediate valence behavior of CeBe13. The hybridization between the localized f electrons and the conduction electrons is responsible for the large values of the electronic specific heat coefficient (gamma approximately 100 mJ/mol K2) and magnetic susceptibility (chi approximately 1 x 10-3 emu/mol), which is in marked contrast to those of ordinary metals that have gamma approximately 1 mJ/mol K2 and chi approximately 1 x 10-5 emu/mol values. The magnetic susceptibility, chi = M/H versus T, of a single crystal of CeBe13 exhibits a broad maximum at Tmax approximately 130 K and is typical of intermediate valence systems with an unusually large energy scale (Kondo), TK approximately 500 K.

Hydrogenation of the ternary compounds CeNiX (X=Al, Ga, In, Si, Ge and Sn): influence on the valence state of cerium

The hydrogenation process of the intermediate valence compounds CeNiAl, CeNiGa, CeNiIn, CeNiSi and CeNiGe and of the Kondo semiconductor stannide CeNiSn has been investigated. Magnetization and electrical resistivity measurements reveal interesting physical transition induced by the insertion of hydrogen: (i) the Kondo temperature decreases from 600(20) to 220(10)K in the sequence CeNiGe → CeNiGeH1.6(1); (ii) the hydrides CeNiAlH1.93(5) and CeNiGaH1.1(1) contain cerium in trivalent state and the former shows the presence of magnetic correlations around 3.6(2)K; (iii) in contrast, CeNiInH1.8(2) presents a ferromagnetic transition at TC=6.8(2)K and a magnetic transition Kondo semiconductor → antiferromagnet (TN=4.5(2)K) → ferromagnet (TC=7.0(2)K) is evidenced in the CeNiSnHY system. These results are explained by a modification of the competition between RKKY magnetic interaction and Kondo interaction induced by the hydrogenation of these intermetallics.

Pseudogap Formation and Heavy-Carrier Dynamics in Intermediate-Valence YbAl3

Infrared optical conductivity [σ(ω)] of the intermediate-valence compound YbAl 3 has been measured at temperatures 8–690 K to study its microscopic electronic structures. Despite the highly metallic characteristics of YbAl 3 , σ(ω) exhibits a clear pseudogap (strong depletion of spectral weight) of about 60 meV below 40 K. It also shows a strong mid-infrared peak centered at ∼0.25 eV. The energy-dependent effective mass and scattering rate of the carriers obtained from the data indicate the formation of a heavy-mass Fermi liquid. These characteristic results are discussed in terms of the hybridization states between the Yb 4 f and the conduction electrons. It is argued, in particular, that the pseudogap and the mid-infrared peak result from the indirect and direct gaps, respectively, within the hybridization state.

Polarized neutron diffraction study of CePd 3

We have performed polarized neutron diffraction measurements on the intermediate valence compound CePd 3 using the D3 spectrometer at the ILL. The results show that at low temperatures, 1.7 K , as well as at 100 K , the field-induced magnetic amplitudes measured in a field of 4.6 T could be interpreted in terms of two contributions: a 4 f-type contribution with a Ce 3+ form factor, and an extra delocalized contribution (relatively narrow in q-extent). In particular, we find that the values of the 4 f moment deduced from the extrapolation of the neutron data to Q=0 are systematically lower than the values deduced from bulk magnetization measurements performed on the same sample. This difference can be ascribed to a possible positive polarization of the conduction band which is similar at both temperatures.

Valence changes in YbAl 2 under pressure: a resonant inelastic X-ray emission study

We used X-ray absorption in the partial fluorescence yield mode (PFY-XAS), and resonant X-ray emission (RXES) to study the pressure dependence of Yb valence in the intermediate valence compound YbAl 2. We find that the Yb valence changes from 2.25 at ambient pressure to 2.95 at 385 kbar, and that the valence transition is completed around 200 kbar. The combined use of complementary spectroscopic techniques considerably reduces the uncertainties in the extracted valence values.

Magnetic, Thermal and Transport Properties of Ce(Ni1-xCux)2Al3: The Dominant Role of Electronic Change

The Ni sites of an intermediate-valence compound CeNi 2 Al 3 can be partially substituted with Cu atoms up to 40%. Both the unit-cell volume and the number of conduction electrons increase with the substitution. The magnetic susceptibility χ( T ), specific heat C ( T ), electrical resistivity ρ( T ) and thermoelectric power S ( T ) were measured for the Ce(Ni 1- x Cu x ) 2 Al 3 system. With increasing x , the χ( T ) curve changes from that of a Pauli paramagnet to a Curie–Weiss type showing Ce 3+ local moments above T = 50 K. Huge thermoelectric power is observed in all the samples of our study. The great peak over 50 µV/K in S ( T ) curve and the broad hump in ρ( T ) curve shift down to lower temperatures, meanwhile, a ten-fold enhancement of both the electronic specific-heat coefficient γ and the linear coefficient α of S ( T ) takes place simultaneously, with increasing x . These results reveal that the hybridization between 4f and conduction electrons is weakened by introducing the Cu atoms into the ...

Nature of the magnetic excitation spectrum in (Sm,Y)S: CEF effects or an exciton?

The dynamic magnetic susceptibility spectrum in a single-crystal sample of the intermediate-valence compound Sm0.67Y0.33S is studied by inelastic neutron scattering with neutron momentum transfer and sample temperatures varying over wide ranges. Two coupled collective modes have been found in the spectrum. Unlike the higher energy mode, whose intensity approximately follows the form factor of Sm2+, the lower energy mode exhibits a stronger angular dependence than could be expected from the form factor for the localized f electrons. The total intensity of the inelastic component of the magnetic response decreases with increasing temperature; this is accompanied by the appearance of a broad quasi-elastic signal of a magnetic nature at significantly lower temperatures than follows from the calculated intensities of the transitions within the excited multiplet of the Sm2+ ion. An analysis of the observed features allows the suggestion to be made that the fine structure of the magnetic excitation spectrum in (Sm,Y)S is associated with the formation of an exciton-like intermediate-valence state on Sm ions rather than with the crystal-electric-field effects.

Determination of pressure-induced valence changes inYbAl2by resonant inelastic x-ray emission

Novel x-ray spectroscopies such as high-resolution x-ray absorption (XAS) performed in the partial fluorescence yield mode (PFY), and resonant emission (RXES) offer new opportunities to probe the bulk electronic configuration in strongly correlated systems. We performed an Yb ${\mathrm{L}}_{3}$ PFY-XAS and RXES investigation of the intermediate valence compound ${\mathrm{YbAl}}_{2}$ under external pressure. We found that the Yb valence increases from 2.25 at ambient pressure to 2.9 at 385 kbar. This experiment has extended the range of pressures explored until now, in order to cover the whole valence transition, which we find to be completed around 200 kbar. Moreover, the new data considerably reduce the experimental error bars on previous estimates and provide the most accurate description to date of this transition. We discuss the relative merits of various approaches to extract the $4f$ occupancy from the spectroscopic data.

The new hydrides CeNiGeH 1.6 and CeCuGeH 1.0 crystallizing in the derivative hexagonal ZrBeSi-type structure

The ternary germanides CeNiGe and CeCuGe absorb hydrogen in the temperature range 393–473 K. X-ray powder diffraction and transmission electron microscopy show that the hydride CeNiGeH 1.6(1) adopts the hexagonal ZrBeSi-type whereas CeCuGeH 1.0(1) crystallizes in a superstructure of this type. Magnetization, electrical resistivity and thermopower measurements reveal that CeNiGeH 1.6(1) is an intermediate valence compound having a Kondo temperature ≅220(10) K smaller than that observed ≅600(20) K for CeNiGe. On the contrary, a transition from ferromagnetism to non-magnetic ordering above 1.8 K is evidenced during the hydrogenation of CeCuGe.

Dependence of the effective masses in YbAl3 on magnetic field and disorder.

The susceptibility and specific heat--and hence the effective mass--of the intermediate valence compound YbAl3 show anomalous enhancement below the Fermi liquid temperature T(coh) approximately 40 K. We show that these anomalies are suppressed by alloying in Yb1-xLuxAl3 indicating high sensitivity to lattice coherence. The de Haas-van Alphen effective masses for key branches of the Fermi surface are reduced by magnetic fields B>40 T. We argue that this reduction does not arise from 4f polarization but reflects renormalization of the quasiparticle states by the field.

Transport and magnetic properties of CeNiGe2$minus;xSix single crystals

The electrical resistivity and the magnetic susceptibility have been measured for the series of intermetallic compounds CeNiGe 2− x Si x . CeNiGe 2 is an antiferromagnetic compound with T N =3.3 K . CeNiSi 2 is an intermediate-valence compound with cerium valence varying from 3.4 at room temperature to 3.75 at 1.8 K . With increasing x , the coupling constant J is enhanced and thus the Kondo temperature T K increases.

Structure and magnetic properties of the ternary gallides CeMGa (M=Mn, Co and Cu) and their hydrides

The ternary gallides CeMGa present various structural properties since CeMnGa, CeCoGa and CeCuGa crystallize, respectively, in the cubic MgCu2-, monoclinic CeCoAl-, and orthorhombic CeCu2-type structure. These compounds absorb hydrogen at room temperature but hydrogenation induces a change of the crystalline structure: (i) CeCoGaH3.0 and CeCuGaH0.8 adopt the hexagonal AlB2-type; (ii) on the contrary, the cubic MgCu2 structure is preserved for CeMnGaH1.6. Moreover, in some cases hydrogenation leads to a valence change for Ce. For instance, CeCoGa is considered as intermediate valence compound, whereas CeCoGaH3.0 contains Ce3+. This paper demonstrates that hydrogenation strongly influences the Kondo effect in these intermetallics.

Transport and magnetic properties of CeCoxNi1$minus;xGe2 system

Electrical resistivity and the magnetic susceptibility have been measured for the series of intermetallic compounds CeCo x Ni 1− x Ge 2 ( x =0.1, 0.2, 0.5, 0.7, 0.8, 0.9 and 1). CeNiGe 2 is an antiferromagnetic compound with T N =3.3 K . CeCoGe 2 is an intermediate-valence compound with cerium valence varying from 3.2 at room temperature to 3.8 at 1.8 K . The valence fluctuations are quenched at x =0.5, while the heavy Fermion behavior appears in compositions with x <0.5.

A new energy scale at low temperatures in YbAl 3

We observed high magnetic field de Haas–van Alphen signals up to 57 T at low temperatures down to 0.5 K in the intermediate valence compound YbAl 3. High-field magnetization and magnetic susceptibility measurements imply that new electronic states in YbAl 3 are created with an energy scale of 40 K . In the 〈1 1 1〉 direction, the effective masses, which are 13.5 m 0 for the α-branch and 23.9 m 0 for the β-branch at low magnetic fields (<40 T) , are strongly reduced to 5.25 m 0 for both branches at high magnetic fields (>40 T) .