Intermediate Valence Research Articles

Pressure-induced electronic transitions in samarium monochalcogenides

Pressure induced isostructural insulator to metal transition for SmS is characterised by the presence of an intermediate valence state at higher pressure which cannot be captured by the density functional theory. As a direct outcome of including the charge and spin fluctuations incorporated in dynamical mean field theory, we see the emergence of insulating and metallic phases with increasing pressure as a function of changing valence. This is accompanied by significantly improved predictions of the equilibrium lattice constants and bulk moduli for all Sm-monochalcogenides verifying experiments. Nudged Elastic Band analysis reveals the insulating states to have a finite quasiparticle weight, decreasing as the gap closes rendering the transition to be not Mott-like, and classifies these materials as correlated band insulators. The difference between the discontinuous and continuous natures of these transitions can be attributed to the closeness of the sharply resonant Sm-4f peaks to the fermi level in the predicted metallic states in SmS as compared to SmSe and SmTe.

Drastic enhancement of magnetic critical temperature and amorphization in topological magnet EuSn2P2 under pressure

High pressure is an effective tool to induce exotic quantum phenomena in magnetic topological insulators by controlling the interplay of magnetic order and topological state. This work presents a comprehensive high-pressure study of the crystal structure and magnetic ground state up to 62 GPa in an intrinsic topological magnet EuSn2P2. With a combination of high resolution X-ray diffraction, 151Eu synchrotron Mössbauer spectroscopy, X-ray absorption spectroscopy, molecular orbital calculations, and electronic band structure calculations, it has been revealed that pressure drives EuSn2P2 from a rhombohedral crystal to an amorphous phase at 36 GPa accompanied by a fourfold enhancement of magnetic ordering temperature. In the pressure-induced amorphous phase, Eu ions take an intermediate valence state. The drastic enhancement of magnetic ordering temperature from 30 K at ambient pressure to 130 K at 41.2 GPa resulting from Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions likely attributes to the stronger Eu–Sn interaction at high pressure. These rich results demonstrate that EuSn2P2 is an ideal platform to study the correlation of the enhanced RKKY interactions, disordered lattice, intermediate valence, and topological state.

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.

Valence fluctuation inCe2Re3Si5and Ising-type magnetic ordering inPr2Re3Si5single crystals

Single crystals of ${\mathrm{Ce}}_{2}{\mathrm{Re}}_{3}{\mathrm{Si}}_{5}$ and ${\mathrm{Pr}}_{2}{\mathrm{Re}}_{3}{\mathrm{Si}}_{5}$ have been grown by the Czochralski method in a tetra-arc furnace. Powder x-ray diffraction confirmed that these compounds crystallize in the ${\mathrm{U}}_{2}{\mathrm{Mn}}_{3}{\mathrm{Si}}_{5}$-type tetragonal crystal structure with space group $P4/mnc$ (no. 128). The anisotropic physical properties have been studied comprehensively by measuring the magnetic susceptibility, isothermal magnetization, electrical transport, and specific heat. The low value of magnetic susceptibility together with no magnetic transition down to 2 K and the observation of the signature of the Kondo feature in the photoemission spectra provide evidence that the Ce ions are in the intermediate valence state in ${\mathrm{Ce}}_{2}{\mathrm{Re}}_{3}{\mathrm{Si}}_{5}$. On the other hand, ${\mathrm{Pr}}_{2}{\mathrm{Re}}_{3}{\mathrm{Si}}_{5}$ revealed a magnetic ordering at 9 K. The sharp drop in the magnetic susceptibility and a spin-flip-like metamagnetic transition for $H\ensuremath{\parallel}[001]$ in the magnetization plot of ${\mathrm{Pr}}_{2}{\mathrm{Re}}_{3}{\mathrm{Si}}_{5}$ suggest an Ising-type antiferromagnetic ordering. Based on magnetic susceptibility and isothermal magnetization data, a detailed crystal electric field (CEF) analysis shows that the degenerate $\mathit{J}=4$ Hund's rule derived ground state of a ${\mathrm{Pr}}^{3+}$ ion splits into nine singlets with an overall splitting of 1179 K. The magnetic ordering in ${\mathrm{Pr}}_{2}{\mathrm{Re}}_{3}{\mathrm{Si}}_{5}$ is due to the exchange-generated admixture of the lowest-lying CEF energy levels. Heat capacity data reveal a sharp peak at 9 K, which confirms the bulk nature of the magnetic ordering in ${\mathrm{Pr}}_{2}{\mathrm{Re}}_{3}{\mathrm{Si}}_{5}$.

Temperature Dependence and Intermediate Valence on Magnetic Hyperfine Field at a Ce Impurity Diluted in <i>RZ</i>n

Temperature Dependence and Intermediate Valence on Magnetic Hyperfine Field at a Ce Impurity Diluted in <i>RZ</i>n

Hydrobiogechemical interactions in the hyporheic zone of a sulfate-impacted, freshwater stream and riparian wetland ecosystem.

Coupled abiotic and biotic processes in the hyporheic zone, where surface water and groundwater mix, play a critical role in the biogeochemical cycling of carbon, nutrients, and trace elements in streams and wetlands. Dynamic hydrologic conditions and anthropogenic pollution can impact redox gradients and biogeochemical response, although few studies examine the resulting hydrobiogeochemical interactions generated within the hyporheic zone. This study examines the effect of hyporheic flux dynamics and anthropogenic sulfate loading on the biogeochemistry of a riparian wetland and stream system. The hydrologic gradient as well as sediment, surface water, and porewater geochemistry chemistry was characterized at multiple points throughout the 2017 spring-summer-fall season at a sulfate-impacted stream flanked by wetlands in northern Minnesota. Results show that organic-rich sediments largely buffer the geochemical responses to brief or low magnitude changes in hydrologic gradient, but sustained or higher magnitude fluxes may variably alter the redox regime and, ultimately, the environmental geochemistry. This has implications for a changing climate that is expected to dramatically alter the hydrological cycle. Further, increased sulfate loading and dissolved or adsorbed ferric iron complexes in the hyporheic zone may induce a cryptic sulfur cycle linked to iron and carbon cycling, as indicated by the abundance of intermediate valence sulfur compounds (e.g., polysulfide, elemental sulfur, thiosulfate) throughout the anoxic wetland and stream-channel sediment column. The observed deviation from a classical redox tower coupled with potential changes in hydraulic gradient in these organic-rich wetland and stream hyporheic zones has implications for nutrient, trace element, and greenhouse gas fluxes into surface water and groundwater, ultimately influencing water quality and global climate.

Theoretical study of intermediate valence fluctuation in thulium selenide and electrical properties of TmSe1−xTex

The study of valence fluctuation is made on TmSe compound from simulated theoretical x-ray powder diffraction data. The theoretical lattice parameter is in agreement with experimental parameter. The amount of charge transfer is deduced as 0.23ē which proves the presence of intermediate valence fluctuation in the system. Further a theoretical Study is made on TmSe1−xTex (x = 0.5, 0.77, 0.83 and 1.00) to deduce electrical transport properties by using only two parameters, lattice constant and energy gap. The calculated transport properties electrical resistivity, carrier mobility, carrier concentration, carrier effective mass and dielectric constant are compared with the available experimental data which agrees with each other.

Electron fixation of Sn2+/Cu+ bimetallic catalyst on flexible carbon fiber cloth for highly converting gaseous CO2 to formate at moderate potential

Designing highly active and selective electrocatalysts for converting CO2 into value-added chemicals and fuels by electrochemical method is still a challenge. Herein, we have successfully fabricated a lotus-like morphology of Sn/Cu alloy on carbon fiber cloth via a facile electrodeposition method followed by air-dried treatment. Owing to the presence of Sn2+ and Cu+ on the surface of Sn/Cu alloy, this bimetallic Sn/Cu electrode exhibits the outstanding FE of formate with a maximum value of 95.81% at −1.07 V versus RHE, and the total current density at −0.97 V versus RHE keeping unchanged at 33.78 mA cm−2 under the 20 h electrolysis even with the replacement of electrolyte. This work provides an effective strategy for highly boosting the selective production of formate from electrochemical CO2 reduction by stabilizing the intermediate valence of bimetallic alloy.

Evidence of Homogeneous Intermediate Valence Coexisting with the Long-Range Magnetic Order in EuCu2(Si, Ge)2

Evidence of Homogeneous Intermediate Valence Coexisting with the Long-Range Magnetic Order in EuCu2(Si, Ge)2

Structural, Physical, Theoretical and Spectroscopic Investigations of Mixed‐Valent Eu2Ni8Si3 and Its Structural Anti‐Type Sr2Pt3Al8

AbstractEu2Ni8Si3 and its anti‐type representative Sr2Pt3Al8 were synthesized from the elements. They crystallize in the tetragonal crystal system with space group P42/nmc and with lattice parameters of a=997.9(1) and c=747.6(1) pm (Eu2Ni8Si3) as well as a=1082.9(2) and c=823.3(2) pm (Sr2Pt3Al8). Both compounds were investigated via single crystal X‐ray diffraction, indicating slight Si/Ni mixing for the silicide. Sr2Pt3Al8 exhibits a temperature independent magnetic susceptibility, suggesting superimposed dia‐ and Pauli‐paramagnetic contributions. The independent Al and Pt sites of the platinide were further characterized by 27Al and 195Pt solid‐state NMR spectroscopy, which were assigned with the help of electronic structure calculations. ICOHP calculations and Bader charges were used to analyze the bonding situation. Eu2Ni8Si3 in contrast is paramagnetic with a ferromagnetic transition at TC=46.9(2) K and exhibits an effective magnetic moment of μeff=6.61(1) μB per Eu atom. The latter is in line with an intermediate valence that was further proven by 151Eu Mößbauer spectroscopic investigations. At 300 K, the refined Eu2+/Eu3+ ratios are 60 %/40 %, at 78 K 62 % and 38 % (Eu2+/Eu3+) are observed, being in line with the ratio deduced from the magnetic susceptibility. Finally, at 6 K a ratio of 68 % Eu2+ and 32 % Eu3+ was observed. Below the Curie temperature, the Eu2+ signal shows a full magnetic hyperfine splitting, with an internal magnetic field value of B0=28.4 T.

Topology of SmB6 determined by dynamical mean field theory

Whether ${\mathrm{SmB}}_{6}$ is a topological insulator remains hotly debated. Our density functional theory plus dynamical mean field theory calculations are in excellent agreement with a large range of experiments, from the $4{f}^{5.5}$ intermediate valency to x-ray and photoemission spectra. We show that ${\mathrm{SmB}}_{6}$ is a strongly correlated topological insulator, albeit not a Kondo insulator, using a symmetry analysis of the ``pole extended'' Hamiltonian, which fully captures the effect of the self-energy arising from the strong electron-electron correlation within the Sm $4f$ orbitals. The topological surface states are analyzed, addressing conflicting interpretations of photoemission data.

Electronic structure of nitrogen-vacancy doped SmN: Intermediate valence and 4f transport in a ferromagnetic semiconductor

SmN is an intrinsic ferromagnetic semiconductor with electrical transport properties that have been of recent experimental interest. Its conductivity is largely dictated by the concentration of nitrogen vacancies that dope the crystal with electrons. Here, we present a density functional theory based study of SmN, variously doped with nitrogen vacancies. We find that the Sm ions which coordinate a nitrogen vacancy take on an intermediate $2--{3}^{+}$ valence, while the remaining Sm ions in the crystal remain trivalent. Our results show that the localized $4f$ states hybridize with the extended state Sm $5d$ and N $2p$ bands and are involved in electron transport throughout the crystal.

Mixed Valence and Superconductivity in Perovskite Antimonates

Herein, we present results of the 121Sb Mössbauer spectroscopy in perovskite antimonates Ba1–xKxSbO3, sibling compounds of the well-known high-Tc superconductors Ba1–xKxBiO3. Two Sb valence states SbIII and SbV, forming a charge-density-wave (CDW) order, are unambiguously revealed in un- and under-doped phases at x = 0–0.5. As the CDW order is suppressed at x = 0.65, the compound becomes superconducting and all of the Sb sites become equivalent with the intermediate valence of +4.65, consistent with that anticipated from the potassium content. These results provide the direct evidence of metal s electrons participating in the formation of the CDW order and superconductivity in the material, and thus they have important implications for clarifying the underlying mechanism related to mixed valence and potential valence fluctuations of metal ions. The use of Mössbauer active nuclei offers a deeper insight into the structural properties and superconductivity in mixed-valence main-group oxides.

Pressure evolution of the electronic structure of non-centrosymmetric EuRhGe3

Among europium compounds, pressure induced valence transitions and/or intermediate valence states are often observed. In such systems, applying pressure of several GPa can drive a Eu valence from divalent to almost trivalent. Non-centrosymmetric EuRhGe3 possesses magnetic Eu2+ ions and exhibits antiferromagnetic ordering at ∼11 K at ambient pressure. Pressure resistant magnetic ordering and stable divalent Eu state have been reported in EuRhGe3. Here, we study the pressure evolution of the Eu valence of EuRhGe3 by high resolution x-ray absorption spectroscopy using the partial fluorescence yield method. Our study reveals a successive increase of the Eu valence with increasing pressure without any valence transition. The obtained mean Eu valence approaches ∼2.4 around 40 GPa at 300 K. The experimental data are also analyzed by a full multiplet configuration interaction calculation based on the single impurity Anderson model. The analysis reveals a decrease of the Eu 4f orbital occupation by applying pressure. Pressure evolution of the electronic structure studied by density functional theory suggests that the Rh ions have little contribution to the pressure evolution of the Eu valence, while it implies an active involvement of the Ge ions.

The low and high temperature thermoelectric properties of Yb3Si5

Silicides have been of great interest for thermoelectric applications due to their abundant elements as well as thermal and chemical stability. In this paper, we examined the thermoelectric properties of Yb3Si5 polycrystalline samples in a wide temperature range from 10 to 800 K. The temperature dependence of the Seebeck coefficient was successfully analyzed by assuming a narrow 4f quasi-particle band, indicating the intermediate valence state of Yb2+-Yb3+ is responsible for the high power factor. A very large maximum power factor of ∼ 4.70 mWm−1K−2 was observed at 72 K and room temperature value ∼ 1.56 mWm−1K−2 for Yb3Si5. These results shows that Yb-Si compounds have large potential to be used as low temperature TE applications in the future. We also studied the Co-doping effect in Yb3Si5, namely, Yb3Co x Si5−x where x = 0, 0.1, 0.15, 0.20 and investigated their thermoelectric properties. While powder X-ray diffraction analysis confirmed all main peaks indexed to Yb3Si5 phase, SEM and EDX analyses revealed that Co is precipitated as metal particles, forming a composite material with Yb3Si5 phase. Thermoelectric properties of the Co-doped samples are also reported.

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.

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.

High-temperature antiferromagnetism in Yb based heavy fermion systems proximate to a Kondo insulator

Given the parallelism between the physical properties of Ce and Yb based magnets and heavy fermions due to the electron-hole symmetry, it has been rather odd that the transition temperature of the Yb based compounds is normally very small, as low as $\sim$ 1 K or even lower, whereas Ce counterparts may often have the transition temperature well exceeding 10 K. Here, we report our experimental discovery of the transition temperature reaching 20 K for the first time in a Yb based compound at ambient pressure. The Mn substitution at the Al site in an intermediate valence state of $\alpha$-YbAlB$_{4}$ not only induces antiferromagnetic transition at a record high temperature of 20 K but also transforms the heavy fermion liquid state in $\alpha$-YbAlB$_{4}$ into a highly resistive metallic state proximate to a Kondo insulator.

Thermoelectricity and electronic properties ofY1−xCexCrB4

Boron-rich materials combine chemical stability with refractory properties and, consequently, are interesting for high-temperature thermoelectric applications. Therefore, the magnetic, electrical, and thermal transport properties of the ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CrB}}_{4}$ series have been investigated here to employ the concept of correlation-enhanced thermoelectric properties. Combining x-ray diffraction and energy- or wavelength-dispersive spectrometry, we find a rather narrow stability range of ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CrB}}_{4}$, only samples on the Y- and Ce-rich substitution limits $(x=0,\phantom{\rule{0.28em}{0ex}}0.05,\phantom{\rule{0.28em}{0ex}}0.95,\phantom{\rule{0.28em}{0ex}}\mathrm{and}\phantom{\rule{0.28em}{0ex}}1)$ were obtained. Electrical resistivity data show a change from semiconducting $(x=0)$ to metallic behavior upon Ce substitution $(x\ensuremath{\ge}0.95)$. From magnetic susceptibility measurements and x-ray absorption spectroscopy, we find a temperature-dependent intermediate valence state of Ce of about $+3.5$. However, a fit of the magnetic susceptibility data to the Coqblin-Schrieffer model yields a surprisingly high Kondo temperature of about 1100 K. Together with the good thermal conductivity for the studied substitution series this impedes a suitable thermoelectric performance. Electronic structure calculations for ${\mathrm{YCrB}}_{4}$ support its narrow gap semiconducting nature in contrast to previous studies. Surprisingly, its electronic structure is characterized by pronounced van Hove singularities very close to the Fermi-level ${E}_{\mathrm{F}}$. They originate from nearly dispersionless Cr $3{d}_{{z}^{2}\ensuremath{-}{r}^{2}}$-derived bands in a large part of the Brillouin zone, suggesting the appearance of electronic instabilities upon rather small electron doping into these states.

Nonmagnetic-magnetic transition and magnetically ordered structure in SmS

SmS, a prototypical intermediate valence compound, has been studied by performing high-pressure nuclear magnetic resonance measurements on a $^{33}$S-enriched sample. The observation of an additional signal below 15-20 K above a nonmagnetic-magnetic transition pressure $P_{\rm c2} \approx 2$ GPa gives evidence of a magnetic transition. The absence of a Curie-term in the Knight shift near $P_{\rm c2}$ indicates that the localized character of $4f$ electrons is entirely screened and the mechanism of the magnetic ordering is not described within a simple localized model. Simultaneously, the line shape in the magnetically ordered state is incompatible with a spin density wave order. These suggest that the magnetic order in SmS may require an understanding beyond the conventional framework for heavy fermions. The fact that hyperfine fields from the ordered moments cancel out at the S site leads us to a conclusion that the ordered phase has a type II antiferromagnetic structure.