Intermediate Valence Systems Research Articles

Crystal structure and low-temperature magnetic properties of R2Au2Pb (R = Y, La–Nd, Sm, Gd–Lu) plumbides

Polycrystalline samples of the ternary compounds R2Au2Pb (R = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) were studied by means of powder X-ray diffraction and magnetic measurements. All these materials were found to crystallize with a tetragonal crystal structure of the Er2Au2Sn type (space group P42/mnm, Pearson code tP20). The Y-, La-, and Lu-based phases were established to be diamagnetic, while Yb2Au2Pb was characterized as an intermediate valence system due to an unstable 4f shell. All the other R2Au2Pb ternaries were found to order antiferromagnetically at low temperatures (TN in the range 2–30 K) due to the localized magnetic moments carried on trivalent R3+ ions.

Anisotropy-driven quantum criticality in an intermediate valence system

Intermetallic compounds containing f-electron elements have been prototypical materials for investigating strong electron correlations and quantum criticality (QC). Their heavy fermion ground state evoked by the magnetic f-electrons is susceptible to the onset of quantum phases, such as magnetism or superconductivity, due to the enhanced effective mass (m*) and a corresponding decrease of the Fermi temperature. However, the presence of f-electron valence fluctuations to a non-magnetic state is regarded an anathema to QC, as it usually generates a paramagnetic Fermi-liquid state with quasiparticles of moderate m*. Such systems are typically isotropic, with a characteristic energy scale T0 of the order of hundreds of kelvins that require large magnetic fields or pressures to promote a valence or magnetic instability. Here we show the discovery of a quantum critical behaviour and a Lifshitz transition under low magnetic field in an intermediate valence compound α-YbAlB4. The QC origin is attributed to the anisotropic hybridization between the conduction and localized f-electrons. These findings suggest a new route to bypass the large valence energy scale in developing the QC.

Antiferromagnetic Correlations in Strongly Valence Fluctuating CeIrSn.

CeIrSn with a quasikagome Ce lattice in the hexagonal basal plane is a strongly valence fluctuating compound, as we confirm by hard x-ray photoelectron spectroscopy and inelastic neutron scattering, with a high Kondo temperature of T_{K}∼480 K. We report a negative in-plane thermal expansion α/T below 2K, which passes through a broad minimum near 0.75K. Volume and a-axis magnetostriction for B∥a are markedly negative at low fields and change sign before a sharp metamagnetic anomaly at 6T. These behaviors are unexpected for Ce-based intermediate valence systems, which should feature positive expansivity. Rather they point towards antiferromagnetic correlations at very low temperatures. This is supported by muon spin relaxation measurements down to 0.1K, which provide microscopic evidence for a broad distribution of internal magnetic fields. Comparison with isostructural CeRhSn suggests that these antiferromagnetic correlations emerging at T≪T_{K} result from geometrical frustration.

Near zero thermal expansion in metal matrix composites based on intermediate valence systems: Al/SmB6

This work is focused on the fabrication and characterization of a new type of composite invar materials combining near-zero thermal expansion and functional properties that are important for applications. This is accomplished through the use of particles of SmB6, an intermediate valence system with negative thermal expansion, embedded in Al matrix. The composite based on SmB6-21 vol% was fabricated by hot pressing and was characterized by XRD, optical/electron microscopy, X-ray computed tomography and capacitive dilatometry. The study of thermal expansion revealed that the sample exhibits invar behaviour up to ~60 K with a zero value of the coefficient of thermal expansion near 45 K. In comparison to pure aluminum, the temperature range has increased by about 20 K. A quantitative analysis of dilatometric experimental data performed on the basis of widely used theoretical models showed that the thermal expansion of the Al/SmB6 composite was well reproduced within the Turner model.

Anomalous thermal expansion in intermediate valence compound Ce2Ni3Si5

The linear thermal expansion coefficients in the intermediate valence system Ce2Ni3Si5 were determined along the main crystallographic directions by means of neutron diffraction in the temperature range 10–300 K. The anomalous contribution to thermal expansion due to valence instabilities of cerium was estimated by employing the virtual mix approach. The magnitude of the anomalous magnetic contribution to the volume coefficient of thermal expansion in this intermetallic system is about 1.2× 10−5 K−1. The observed effect is caused by a strong temperature dependence of the degree of the partial delocalization of the cerium 4f electrons in the intermediate valence regime. The effective Kondo temperature of Ce2Ni3Si5 TK = 140 К obtained in the present work is in a very good agreement with the earlier estimates based on the temperature dependences of the macroscopic magnetic and thermodynamic properties.

Valence fluctuations in Ce2Rh3+xGe1-x

Ternary intermetallic alloys Ce2Rh3+xGe1-x (0 ≤ x ≤ 0.33) have been found to crystallize in a rhombohedral structure of the Mg2Ni3Si-type (space group R-3m). With varying the stoichiometry, the lattice parameters significantly vary from a = 5.5887(2) Å, c = 11.9557(5) Å (hexagonal setting) for x = 0 to a = 5.5127(2) Å, c = 12.2790(5) Å for x = 0.33, while the unit cell volume remains nearly unchanged. By means of magnetic susceptibility, heat capacity and electrical resistivity measurements, the Ce2Rh3+xGe1-x alloys have been shown to be intermediate valence systems with highly unstable 4f shell of Ce ions. Notably, the ferromagnetic ordering, claimed in the literature to coexist with the valence fluctuations in Ce2Rh3Ge, has been revealed to be an extrinsic effect due to impurity contribution.

Optical study on intermediate-valence compounds Yb1−xLuxAl3**Project supported by the National Natural Science Foundation of China (Grant Nos. 11327806 and GZ1123) and the National Key Research and Development Program of China (Grant Nos. 2016YFA0300902 and 2017YFA0302904).

We report an optical spectroscopy study on intermediate valence system Yb1−xLuxAl3 with x = 0, 0.25, 0.5, 0.75, and 1. The Kondo temperature in the system is known to increase with increasing Lu concentration. Therefore, it is expected that the energy scale of the hybridization gap should increase with increasing Lu concentration based on the periodic Anderson model. On the contrary, we find that the spectral structure associated with the hybridization effect shifts monotonically to lower energy. Furthermore, the Lu substitution results in a substantial increase of the free carrier spectral weight and less pronounced plasma frequency reduction upon lowering temperature. We attribute the effect to the disruption of the Kondo lattice periodicity by the random substitution of Yb by Lu. The work highlights the importance of the lattice periodicity of the rare earth element for understanding the Kondo lattice phenomena.

Kondo temperature and Heavy Fermion behavior in Yb1−xYxCuAl series of alloys

Results on x-ray diffraction, electrical resistivity, specific heat and magnetization on the Yb1−xYxCuAl series of compounds are reported. The analysis of the x-ray data shows the increase of the unit cell volume with the Y dilution. The electrical resistivity shows an evolution from Kondo lattice regime for x≤0.6 to single impurity behavior for x = 0.8 and 0.94. The electronic coefficient γ shows values of Heavy Fermion systems along the series for 0 ≤x<1. On the other hand, dc magnetic susceptibility measurements show typical curves of intermediate valence systems with a maximum around 25K. Below this maximum, the values of low temperature susceptibility (χ(0)) decrease with the increase of Y content. From the dependence of χ(0) and γ upon Y substitution, an increase of 12% of the Kondo temperature (TK) for x = 0.8 alloy respect to the reference YbCuAl (x = 0) is estimated. This is further supported by the evolution of the temperature of the maximum in the magnetic contribution of the specific heat. The overall results can be explained by the increase of the hybridization as consequence of negative pressure effects obtained by the chemical substitution of Yb by Y, thus leading to the increase of TK, in agreement with the Doniach's diagram.

Peculiar hydrogenation process of Ce2Ni2Ga

Hydrogenation studies were carried out on the compound Ce2Ni2Ga that was described in the literature as an intermediate valence system. The alloy was found to absorb hydrogen very easily already at room temperature under pressure less than 0.1MPa forming hydrides Ce2Ni2GaHx with x up 5.9, which are however multiphase and contain CeH2+δ and some other undisclosed phases. Analyses of the X-ray diffraction and magnetic data revealed that decomposition of the parent compound occurs immediately after its exposure to hydrogen. Synthesis under hydrogen pressure of 3MPa at 470K resulted in amorphous material with x=7.3. The Ce2Ni2GaHx system was found to exhibit a HDDR (hydrogenation, disproportionation, desorption, recombination) process.

Thermoelectric power as a probe of density of states in correlated actinide materials: The case ofPuCoGa5superconductor

We present measurements of the thermoelectric power of the plutonium-based unconventional superconductor ${\mathrm{PuCoGa}}_{5}$. The data is interpreted within a phenomenological model for the quasiparticle density of states of intermediate valence systems, and the results are compared with results obtained from photoemission spectroscopy. The results are consistent with the intermediate valence nature of $5f$ electrons; furthermore, we propose that measurements of the Seebeck coefficient can be used as a probe of density of states in this material, thereby providing a link between transport measurements and photoemission in strongly correlated materials. We discuss these results and their implications for the electronic structure determination of other strongly correlated systems, especially actinide materials.

Racah materials: role of atomic multiplets in intermediate valence systems.

We address the long-standing mystery of the nonmagnetic insulating state of the intermediate valence compound SmB6. Within a combination of the local density approximation (LDA) and an exact diagonalization (ED) of an effective discrete Anderson impurity model, the intermediate valence ground state with the f-shell occupation 〈n4f〉 = 5.6 is found for the Sm atom in SmB6. This ground state is a singlet, and the first excited triplet state ~3 meV higher in the energy. SmB6 is a narrow band insulator already in LDA, with the direct band gap of ~10 meV. The electron correlations increase the band gap which now becomes indirect. Thus, the many-body effects are relevant to form the indirect band gap, crucial for the idea of “topological Kondo insulator" in SmB6. Also, an actinide analog PuB6 is considered, and the intermediate valence singlet ground state is found for the Pu atom. We propose that [Sm, Pu]B6 belong to a new class of the intermediate valence materials with the multi-orbital “Kondo-like" singlet ground-state. Crucial role of complex spin-orbital f n–f n+1 multiplet structure differently hybridized with ligand states in such Racah materials is discussed.

Interplay of electronic structure and unusual development in crystal structure of YbAuIn and Yb3AuGe2In3

First-principles calculations within the DFT are employed to investigate the relationship between the electronic structure and the unexpected features of the hexagonal cell parameters of YbAuIn and . Calculations indicate that YbAuIn is an intermediate valent system with one Yb 4f state pinned to the Fermi level, while is closer to integer valency with all Yb 4f states occupied. Structural relaxations performed on LaAuIn and LuAuIn analogues reveal that expansion of the c-parameter in is attributable to larger size of the divalent Yb compared with intermediate valent Yb.

Antiferromagnetic Kondo lattice to intermediate valence transition in Ce(Au1−xNix)2Si2 ([formula omitted

Transition effects from magnetically ordered Kondo lattice (KL) behaviour in CeAu2Si2 to intermediate valence (IV) behaviour in CeNi2Si2 have been investigated by substitution of Au with Ni. These investigations were done through measurements of the lattice parameters, electrical resistivity (ρ(T)), magnetisation (M(μ0H)) and magnetic susceptibility (χ(T)) on the Ce(Au1−xNix)2Si2 (0≤x≤1) series. Lattice parameters as derived from XRD measurements deviate from Vegard's rule around x=0.6–0.8. ρ(T) data indicate KL behaviour in the presence of a crystal field for x≤0.6, the occurrence of magnetic ordering for x≤0.2 and valence fluctuation for x≥0.8. χ(T) data at high temperatures, follow the Curie–Weiss relation for alloys in the concentration 0≤x≤0.6 (KL region) and give effective magnetic moment values μeff close to that expected for free Ce3+ ion. The low temperature χ(T) data indicate the onset of antiferromagnetic ordering for x≤0.2. For alloys in the concentration range 0.8≤x≤1, the χ(T) behaviour is reminiscent of intermediate valence (IV) systems. M(μ0H) measurements indicate metamagnetic behaviour for alloys in the concentration range 0≤x≤0.1.

Intermediate-valence Yb-based quasicrystals and approximants

We are searching for intermediate-valence (IV) quasicrystals where IV ions are quasi-periodically arranged. Our previous study revealed that an icosahedral Au-Al-Yb quasicrystal forms an IV system [1]. X-ray absorption spectroscopy near the Yb L_3 edge indicated that the Yb ions in the quasicrystal assume a mean valence of 2.61, between a divalent state (4f^14, J = 0) and a trivalent one (4f^13, J = 7/2). Additionally we found that non-Fermi-liquid behaviour appears at low temperatures without doping, pressure, or field tuning. In this study we examined Au-M-Yb system, where M = Sn, Ge, and Ga. X-ray absorption spectroscopy measurements at SPring-8 (BL22XU) showed that each compounds forms IV system. The Yb valence values are respectively 2.27 for a Au-Sn-Yb 2/1 crystalline approximant, 2.24 for a Au-Ge-Yb 1/1 approximant, and 2.36 for a Au-Ga-Yb 1/1 approximant. By following the trend of Yb-based IV compounds, these valence values close to divalent suggest that the Yb 4f character in these compounds would be itinerant rather than localized one.

Magnetic and related properties of Ce5CoGe2, CeCoGe and CeCo2Ge2

Magnetic susceptibility, magnetization, specific heat, electrical resistivity and thermoelectric power of polycrystalline Ce5CoGe2, CeCoGe and CeCo2Ge2 were studied at temperatures down to 2 K and in magnetic fields up to 5 T. The novel phase Ce5CoGe2 was found to be a ferromagnet with the Curie temperature of about 11 K, which exhibits in the paramagnetic region some features of a dense Kondo system with strong crystal field effect. The re-investigated CeCoGe was found to order antiferromagnetically below 4.6 K, while CeCo2Ge2 is an intermediate-valence system, in agreement with previous reports. The thermoelectric power of the latter compound is large (up to 40 μV/K) and positive in the whole temperature range studied, while in the magnetically ordered systems Ce5CoGe2 and CeCoGe it is mostly negative and distinctly smaller (down to −1.5 and −11 μV/K, respectively).

Magnetic order induced by Fe doping in the intermediate valence system β-YbAlB4

We have succeeded in doping Fe at the Al site in the intermediate valence and quantum critical material β-YbAlB4 up to 6%. Our measurements of the magnetization and the specific heat for β-YbAl0.94Fe0.06B4 show magnetic ordering at 10 K. The antiferromagnetic Weiss temperature of 88 K, which is smaller than the 110 K for β-YbAlB4, indicates that Fe doping suppresses the Kondo temperature and induces magnetism, driving the system away from the quantum critical point.

Magnetic, transport and electronic properties of CeAu1−xNixIn intermetallics

Polycrystalline compounds CeAu1−xNixIn (x=0.2, 0.4, 0.6 and 0.8) and their La-based counterparts were investigated by means of X-ray diffraction, magnetic, electrical resistivity and X-ray photoelectron spectroscopy (XPS) measurements. All these materials crystallize in the hexagonal ZrNiAl-type structure. The CeAu1−xNixIn alloys with x⩽0.6 exhibit Curie–Weiss paramagnetism down to the lowest temperatures studied, while that with x=0.8 shows a magnetic behavior characteristic of intermediate valence systems. In all the alloys, the electrical transport is influenced by Kondo effect, which for x⩽0.4 transforms with decreasing temperature from incoherent to coherent scattering, while for larger x exhibits single-ion character in the entire temperature range investigated. The XPS data indicates sizable hybridization of the Ce-4f electrons with conduction band, with the average occupation factor of the 4f1 ground state systematically diminishing with increasing the Ni content.

Synthesis, crystal structure and physical properties of Ce2Ru2Ga3

The novel compound Ce2Ru2Ga3 was synthesized by arc-melting the elemental constituents in argon atmosphere followed by long-term annealing at 873 K. It forms in an orthorhombic system, with a crystal structure of new type (space group P212121, Z = 4, oP28) and the lattice parameters: a = 4.488(2) Å, b = 6.912(4) Å, c = 17.092(9) Å. The gallide is structurally related to intermetallics crystallizing with the NdRh2Sn4-type structure or its derivatives. It represents an unique family of materials with abnormally short Ce–Ru distances, which usually exhibit some instability of the valence state of cerium atoms. Based on the low-temperature magnetic and electrical transport data, as well as the X-ray absorption data, Ce2Ru2Ga3 has been characterised as another intermediate valence system.

Antiferromagnetic ordering in an intermediate valence compound Ce9Ru4Ga5

We report on the low-temperature physical behavior in a novel compound Ce9Ru4Ga5 that crystallizes with its own type of crystal structure containing an anomalously short Ce–Ru distance. From the X-ray absorption spectroscopy data the compound was characterized as an intermediate valence system. Our bulk property (magnetic susceptibility, heat capacity and electrical resistivity) measurements revealed an antiferromagnetic ordering below TN=3.7K. The long-range magnetic order probably arises due to a fraction of Ce ions (nearly one third of cerium in the unit cell of Ce9Ru4Ga5), which possess nearly stable 4f1 configuration with the effective valence close to 3+.

T/B scaling of magnetization in the mixed valent compound β-YbAlB4

Here we provide the first clear evidence of Fermi-liquid breakdown in an intermediate valence system. We employ high precision magnetization measurements of the valence fluctuating superconductor β-YbAlB4 to probe the quantum critical free energy down to temperatures far below the characteristic energy scale of the valence fluctuations. The observed T/B scaling in the magnetization over three decades not only indicates unconventional quantum criticality, but places an upper bound on the critical magnetic field |Bc| < 0.2 mT, a value comparable with the Earth's magnetic field and six orders of magnitude smaller than the valence fluctuation scale. This tiny value of the upper bound on Bc, well inside the superconducting dome, raises the fascinating possibility that valence fluctuating β-YbAlB4 is intrinsically quantum critical, without tuning the magnetic field, pressure, or composition: the first known example of such a phenomenon in a metal.