Intermediate Valence Behavior Research Articles

Intermediate valence behavior of the ternary cerium-nickel-phosphide Ce2Ni12P5

The crystal structure of the ternary phosphide Ce2Ni12P5 (La2Ni12P5-type structure) has been determined from X-ray powder diffraction data: full profile refinement, monoclinic symmetry, space group P21/m, a = 10.7809(2) Å, b = 3.6869(1) Å, c = 13.1490(3) Å, β = 107.776(4)º, RI = 0.068, RP = 0.044, RwP = 0.060. Ce2Ni12P5 is a ferromagnet with a Curie temperature of 5.8(5) K. A significant deviation from the linearity of the temperature dependence of the electrical resistance for Ce2Ni12P5 has been found. The low-temperature part of the electrical resistance indicates the presence of magnetic interactions in Ce2Ni12P5. The influence of a magnetic field on the electrical resistance of Ce2Ni12P5 has been studied. The X-ray absorption spectrum at the Ce LIII edge and X-ray emission spectra of Ni and P at the K and LIII edges have been studied experimentally and theoretically using DFT+U calculations. The calculations show good agreement with the experimental measurements. The effective valence of Ce in Ce2Ni12P5 determined based on the Ce LIII absorption spectrum is ϑeff ∼ 3.05.

Tuning the Intermediate Valence Behavior in the Zintl Compound Yb14ZnSb11 by Incorporation of RE3+ [Yb14-xRExZnSb11 (0.2 ≤ x ≤ 0.7), RE = Sc, Y, La, Lu and Gd

The solid solutions of Yb14-xRExZnSb11 (RE = Sc, Y, La, Lu, and Gd; 0.2 ≤ x ≤ 0.7) were prepared to probe the intermediate valency of Yb in Yb14ZnSb11. The substitution of Yb with RE3+ elements should reduce or remove the intermediate valency of the remaining Yb ions. Large crystals are grown from Sn-flux, and the structure and magnetic susceptibility are presented. All compounds crystallize in the Ca14AlSb11 structure type and the RE3+ ions show Yb site substitution preferences that correlate with size. Two compositions of Yb14-xYxZnSb11 were investigated [x = 0.38(3), 0.45(3)] by temperature-dependent magnetic susceptibility and the broad feature in magnetic susceptibility measurements at 85 K in pristine Yb14ZnSb11 attributed to valence fluctuation decreases and is absent for x = 0.45(3). In compounds with nonmagnetic RE3+ substitutions (Sc, Y, La, and Lu), temperature-dependent magnetic susceptibility shows a transition from intermediate valency fluctuation toward temperature-independent (Y, La, and Lu) or Curie-Weiss behavior and possibly low temperature heavy Fermion behavior (Sc). In the example of the magnetic rare earth substitution, RE = Gd, the Curie-Weiss-dependent magnetic moment of Gd3+ is consistent with x. Hall resistivity of Yb14-xYxZnSb11 showed that the carrier concentration decreases with x and the signature of the low-T intermediate valence state seen for x = 0 is suppressed for x = 0.38 and gone for x = 0.45.

A new look at the ground state properties of Ce2Ir3Al9: Coexistence of two competing energy scales

We present an extended research on characterization of the ground state in a polycrystalline compound Ce2Ir3Al9 using powder x-ray diffraction, magnetic susceptibility χ(T), isothermal magnetization M(B), specific heat Cp(T), electrical resistivity ρ(T), magnetoresistivity ρ(B), thermoelectric power S(T) and thermal conductivity κ(T) measurements, together with a theoretical description of the obtained data to develop a more profound understanding of the physics in this compound. Ce2Ir3Al9 crystallizes in the orthorhombic Y2Co3Ga9-type of structure (space group Cmcm, No. 63, oC56, Z = 4). The χ(T) and Cp(T)/T data reveal the signatures of intermediate valence behaviour on Ce ions with the approximate value of the Kondo temperature TK = 96 K and the Sommerfeld coefficient γ = 31 mJ/mol ⋅ K2. In contrast to the magnetometric and thermodynamic measurements, the transport data (ρ(T) and S(T)) indicate the development of the Kondo lattice state. Accordingly, we expect that for Ce2Ir3Al9 the most likely scenario, which develops in a wide T range is the coexistence and competition of two energy scales of interactions between conduction electrons and 4f spins on Ce ions, located in one unique crystallographic site in the unit cell, in consequence carrying out our system from Ce4+Temp.→ Ce3+. At higher temperatures, where the Kondo interaction becomes dominant the influence of the crystal electric field effect seems to play an influential role in the ground state of this compound too.

Intermediate Valence Behavior of Yb2Cu9Al8

Intermediate valence behavior is frequently observed in materials containing Ce, Yb, Eu, Sm, or Tm. In the current work, we report synthesis and characterization of Yb2Cu9Al8 (Th2Zn17 structure type). Its intermediate valence behavior can be described by an excitation energy Eex/kB = 319 K and a spin fluctuation temperature Tsf = 60 K. The valence state of Yb is estimated to be close to 2.04 for the low‐temperature region. The valence gradually evolves to the value of 2.80 at T = 400 K. The specific heat coefficient of γexp = 59 mJ·molYb–1·K–2 indicates a moderate effective mass enhancement, together with finite density of states at the Fermi level. The latter is also confirmed by the band structure calculations.

Pressure-dependent intermediate valence behavior in YbNiGa4 and YbNiIn4

We report a comprehensive structural and valence study of the intermediate valent materials YbNiGa$_{4}$ and YbNiIn$_{4}$ under pressures up to 60 GPa. YbNiGa$_{4}$ undergoes a smooth volume contraction and shows steady increase in Yb-valence with pressure, though the Yb-valence reaches saturation around 25 GPa. In YbNiIn$_{4}$, a change in pressure dependence of the volume and a peak in Yb-valence suggest a pressure induced electronic topological transition occurs around 10-14 GPa. In the pressure region where YbNiIn$_{4}$ and YbNiGa$_{4}$ possess similar Yb-Yb spacings the Yb-valence reveals a precipitous drop. This drop is not captured by density-functional-theory calculations and implies that both the lattice degrees of freedom and the chemical environment play an important role in establishing the valence of Yb.

High thermoelectric power factor of ytterbium silicon-germanium

Metal silicide-based thermoelectric (TE) materials have attracted attention owing to low toxicity and high chemical stability. Here, we demonstrate that ytterbium silicon-germanium, Yb(Si1−xGex)2−δ, shows a large Seebeck coefficient (S) accompanied by metal-like high electrical conductivity (σ) attributed to the intermediate valence behavior of Yb (Yb2+/Yb3+). We revealed that x = 0.5, i.e., YbSiGe, is the best composition with the highest power factor (S2σ) of 3.6 mW m−1 K−2 at room temperature, which is comparable to those of conventional TE materials, such as Bi2Te3.

Revisiting the physical properties of Ce2Ru3Ga9: Intermediate valence, or Kondo lattice system?

In this paper we discuss in details the experimental results regarding to the crystal structure, magnetic, electronic and thermal transport properties of the strongly correlated compound Ce2Ru3Ga9. Based on these data we report the observation of coexistence and competition between intermediate valence state on Ce ions and Kondo-like behaviour in a high-quality polycrystalline alloy. Intermediate valence behaviour manifests itself in the temperature dependence of the magnetic susceptibility (χ), which reveals a broad maximum at around 335 K as is frequently observed for Ce compounds with mixed-valent Ce3+/4+. The electrical resistivity (ρ) behaves in a manner characteristic of systems with strong electronic correlations. First varies in a metallic manner, typical for intermediate valence compounds and than demonstrating a behaviour pointing on the onset of coherence that develops above the room temperature. The thermoelectric power (S) shows a broad maximum with a relatively large value of Smax = 35.7 μV/K, which is typical for the Ce-based systems that exhibit a strong c−f hybridization. Both ρ(T) and S(T) show the −lnT behaviour at high temperatures thereby indicating on the presence of incoherent electron scattering on a local moment of Ce3+. Moreover, a particular attention has been paid to the low-T behaviour in the investigated physical quantities, as it is one of the focuses of this paper.

Coexistence of magnetic order and valence fluctuations in the Kondo lattice systemCe2Rh3Sn5

We report on the electronic band structure, structural, magnetic and thermal properties of Ce$_2$Rh$_3$Sn$_5$. Ce $L_{\mathrm{III}}$-edge XAS spectra give direct evidence for an intermediate valence behaviour. Thermodynamic measurements reveal magnetic transitions at $T_{\mathrm{N1}}\approx$ 2.9 K and $T_{\mathrm{N2}}\approx$ 2.4 K. Electrical resistivity shows behaviour typical for Kondo lattices. The coexistence of magnetic order and valence fluctuations in a Kondo lattice system we attribute to a peculiar crystal structure in which Ce ions occupy two distinct lattice sites. Analysis of the structural features of Ce$_2$Rh$_3$Sn$_5$, together with results of electronic band structure calculations and thermodynamic data indicate that Ce2 ions are in an intermediate valence state with the ground state electronic configuration close to 4$f^0$, whereas Ce1 ions are trivalent (4$f^1$) and contribute to the low temperature magnetic ordering. Thus, our joined experimental and theoretical investigations classify Ce$_2$Rh$_3$Sn$_5$ as a multivalent charge-ordered system.

Intermediate valence to heavy fermion through a quantum phase transition inYb3(Rh1−xTx)4Ge13(T=Co,Ir)single crystals

Single crystals of ${\mathrm{Yb}}_{3}(\mathrm{Rh}{}_{1\ensuremath{-}x}{T}_{x}){}_{4}{\mathrm{Ge}}_{13}$ $(T\phantom{\rule{0.5em}{0ex}}=\phantom{\rule{0.5em}{0ex}}\mathrm{Co},\phantom{\rule{0.16em}{0ex}}\mathrm{Ir})$ have been grown using the self-flux method. Powder x-ray diffraction data on these compounds are consistent with the cubic structure with space group $Pm\overline{3}n$. Intermediate-valence behavior is observed in ${\mathrm{Yb}}_{3}(\mathrm{Rh}{}_{1\ensuremath{-}x}{T}_{x}){}_{4}{\mathrm{Ge}}_{13}$ upon $T$ = Co doping, while $T$ = Ir doping drives the system into a heavy-fermion state. Antiferromagnetic order is observed in the Ir-doped samples ${\mathrm{Yb}}_{3}(\mathrm{Rh}{}_{1\ensuremath{-}x}{T}_{x}){}_{4}{\mathrm{Ge}}_{13}$ for $0.5lx\ensuremath{\le}1$ with ${T}_{N}=0.96$ K for ${\mathrm{Yb}}_{3}\mathrm{Ir}{}_{4}{\mathrm{Ge}}_{13}$. With decreasing $x$, the magnetic order is suppressed towards a quantum critical point around ${x}_{c}$ = 0.5, accompanied by non-Fermi-liquid behavior evidenced by logarithmic divergence of the specific heat and linear temperature dependence of the resistivity. The Fermi-liquid behavior is recovered with the application of large magnetic fields.

Crystal structure and unstable valence in a novel intermetallic phase Ce2Ru2Al

New ternary intermetallic compound Ce2Ru2Al was prepared in polycrystalline form, and its crystal structure was determined from both powder and single crystal X-ray diffraction data. The aluminide was found to crystallize with an orthorhombic structure (space group Cmce) with the lattice parameters: a = 5.8914(4) Å, b = 9.9603(6) Å, and c = 6.9551(4) Å. The derived unit cell is an ordered variant of the binary La2Ni3 type, where one of two Ni sites is fully occupied with Ru atoms, while another one accommodates Al atoms. The structure of Ce2Ru2Al can be regarded as a sequence of atomic layers normal to b-axis. The remarkable structural feature is a short distance between Ce and Ru atoms equal to 2.7816(4) Å. Magnetic measurements revealed that Ce ions in Ce2Ru2Al possess unstable 4f shell that leads to intermediate valence behavior of the compound. Similar valence instability was observed before for other Ce intermetallics with short Ce–Ru bonds.

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 behavior of Yb2Ni12P7 studied by using 31P NMR

The Yb-based heavy-fermion compound Yb2Ni12P7 with a hexagonal Zr2Fe12P7-type crystal structure was investigated by using the 31P nuclear magnetic resonance (NMR) technique. The complicated NMR line changes its shape gradually with decreasing temperature, implying the presence of some Knight shift components. The temperature dependences of the Knight shift and the nuclear spin-lattice relaxation rate 1/T 1 suggest the delocalization of 4f electrons.

Intermediate valence behavior in the novel cage compound CeIr2Zn20

Single crystals of the cerium intermetallic compound CeIr2Zn20 were grown by the flux method, and studied by means of x-ray diffraction, magnetic, electrical transport, and thermodynamic measurements. The compound crystallizes with the cubic CeCr2Al20-type crystal structure, in which the Ce and transition metal atoms are located inside cages formed by the Al/Zn atoms. CeIr2Zn20 shows paramagnetic behavior and metallic-like electrical conductivity. The bulk physical data conjointly indicate its intermediate valence character due to an unstable 4f shell. Accordingly, the charge carriers in this material exhibit moderate mass enhancement. The observed physical behavior in CeIr2Zn20 is in concert with the electronic structure of the compound, calculated within the local spin density approximation.

Observation of the Quantum Critical Behavior in the Evolution from Spin-Glass to Intermediate-Valence Behavior in Ce2Cu1−x Co x Si3 Series

In this study, we report the results of electrical resistivity, magnetic susceptibility, and Ce-LIII-edge X-ray absorption spectrum measurements for a solid solution series Ce2Cu1−xCoxSi3. In the series, Ce2CuSi3 exhibits spin-glass behavior, whereas Ce2CoSi3 displays intermediate-valence behavior. We report the direct observation of the evolution from the RKKY interaction prevailing regime to the non-magnetic intermediate-valence regime in this solid solution series. In addition, we find non-Fermi-liquid behavior from the ρ(T) and χ(T) measurements at x=0.4. Based on the data, it is inferred that at this content, the system is located at the vicinity of the quantum critical point of the evolution.

Thermoelectric properties of the Kondo insulator FeSb2

Recently it was shown [A. Bentien, S. Johnsen, G.K.H. Madsen, B.B. Iversen, F. Steglich, Europhys. Lett. 80, 17008 (2007)], that the strongly correlated Kondo insulator FeSb2 exhibits a colossal Seebeck coefficient; however, due to its large lattice thermal conductivity, the dimensionless thermoelectric figure of merit (zT) is only 0.005 at 12 K. This experimental result motivate us to perform a theoretical study of the thermoelectric properties of Kondo insulators, within the framework of the X-boson approach [R. Franco, M.S. Figueira, M.E. Foglio, Phys. Rev. B 66, 045112 (2002)] for the periodic Anderson model. We consider a set of parameters adequate for describing the compound FeSb2, and we calculate the resistivity, the thermoelectric power (Seebeck coefficient), the charge carrier thermal conductance, the thermoelectric factor power (zT), the Wiedemann-Franz law, the specific heat, and the Sommerfeld’s γ coefficient. The result of the colossal maximum of the Seebeck coefficient, at low temperatures, has the same order of magnitude as the experimental result [A. Bentien, S. Johnsen, G.K.H. Madsen, B.B. Iversen, F. Steglich, Europhys. Lett. 80, 17008 (2007)]. We also show that those temperature region presents an intermediate valence behavior, characterized by a moderate increase of the Sommerfeld’s γ coefficient.

Intermediate-valent Cerium in Ce2Ru4Mg17 and a Group-Subgroup Scheme for La9Ru4In5 and Ce9Ru4Ga5

Ce2Ru4Mg17 was synthesized by high-frequency melting of the elements in a sealed tantalum ampoule. This magnesium-rich compound crystallizes with a new tetragonal structure type: I4̄2m, a = 986.75(8), c = 1008.7(1) pm, wR2 = 0.0513, 909 F2 values and 34 variables. The striking structural motifs in the Ce2Ru4Mg17 structure are slightly bent CeRu2 units with short Ce-Ru distances of 231 pm and additionally a short Ce-Ce distance of 307 pm. These features are a direct consequence of the cerium valence. The CeRu2 units are embedded in a magnesium-rich matrix with a broad range of Mg-Mg distances (291 - 361 pm). Temperature-dependent magnetic susceptibility data show intermediate-valent behavior of the cerium atoms (0.23(5) μB per Ce atom) and no magnetic ordering down to 3 K, indicative of almost tetravalent cerium in Ce2Ru4Mg17. The ceriumrich gallide Ce9Ru4Ga5 shows an unusually short Ce-Ru distance of 237 pm for the Ce2 position as a result of an intermediate cerium valence. The structural distortions are discussed on the basis of a group-subgroup scheme for La9Ru4In5 (space group I4/mmm) and the superstructure variant Ce9Ru4Ga5 (space group I4mm).

Evolution from a non-Fermi liquid Kondo lattice to intermediate valence behaviour in CeRhSn1−xInx

We present investigations of the magnetic and electric transport properties, specific heat, and electronic structure of the intermetallic and strongly correlated system of CeRhSn1−xInx compounds. The main goal of this paper is to determine the hybridization energy between the f electron and conduction electron states, Vcf, and its influence on the ground state properties of the system. The complementary experimental data are discussed on the basis of the Anderson model for a periodic Kondo lattice. CeRhSn is known as a non-Fermi liquid, while CeRhIn is a valence fluctuating system. We discuss the ground state properties of CeRhSn1−xInx and compare the results with those obtained for the doped Ce-based Kondo insulators.

Magnetic order in the filled skutterudites RPt4Ge12 (R = Nd, Eu)

Rare-earth metal filled skutterudites RPt4Ge12 with R=La-Nd, and Eu exhibit a variety of different ground states, e.g., conventional and unconventional superconductivity in LaPt4Ge12 and PrPt4Ge12, respectively, and intermediate valence behavior in CePt4Ge12. In this work we investigate the magnetic state of NdPt4Ge12 and EuPt4Ge12 by specific heat, dc-susceptibility and magnetization. NdPt4Ge12 shows two magnetic phase transitions at TN1 = 0.67 K and TN2 = 0.58 K, while EuPt4Ge12 displays a complex magnetic phase diagram below the magnetic ordering temperature of 1.78 K. The specific heat indicates that in NdPt4Ge12 the crystalline electric field (CEF) ground state of the Nd3+ is a quartet and that, as expected, in EuPt4Ge12 the Eu2+ state is fully degenerate.

Intermediate-valent Cerium in CeRu2Mg5

The magnesium-rich compound CeRu2Mg5 was synthesized by high-frequency melting of the elements in a sealed tantalum ampoule. CeRu2Mg5 crystallizes with a new tetragonal structure type: P42/ncm, a = 961.1(1), c = 723.2(1) pm, wR2 = 0.0284, 481 F2 values and 25 variables. The striking structural motifs in CeRu2Mg5 are short Ce-Ru distances of 232 pm. Each cerium atom is connected to two ruthenium atoms within a three-dimensional [Ru2Mg5] network. CeRu2Mg5 has a pronounced magnesium substructure with short Mg-Mg distances in the range 302 - 341 pm. The short Ce-Ru distances are a consequence of the almost tetravalent character of the cerium atoms. Temperature-dependent magnetic susceptibility data show intermediate-valent behavior of the cerium atoms (0.9(1) μB per formula unit) and no magnetic ordering down to 3 K.

Intermediate-valence behavior of the transition-metal oxideCaCu3Ru4O12

The transition-metal oxide ${\text{CaCu}}_{3}{\text{Ru}}_{4}{\text{O}}_{12}$ with perovskite-type structure shows characteristic properties of an intermediate-valence system. The temperature-dependent susceptibility exhibits a broad maximum around 150--160 K. At this temperature, neutron powder diffraction reveals a small but significant volume change whereby the crystal structure is preserved. Moreover, the temperature-dependent resistivity changes its slope. NMR Knight shift measurements of Ru reveal a cross-over from high temperature paramagnetic behavior of localized moments to itinerant band states at low temperatures. Additional density-functional theory calculations can relate the structural anomaly with the $d$-electron number. The different experimental and calculational methods result in a mutually consistent description of ${\text{CaCu}}_{3}{\text{Ru}}_{4}{\text{O}}_{12}$ as an intermediate-valent system in the classical sense of having low-energy charge fluctuations.