Intermediate Valence Research Articles

Structure and compositional trends in alkali-metal containing titanium and vanadium oxide chalcogenides and the new van der Waals phase Ti2Te2O

We report the solid-state synthesis of all eighteen layered oxide chalcogenides in the structural family AM2Q2O (A = K/Rb/Cs; M = Ti/V; Q = S/Se/Te), allowing the determination of trends in composition and reactivity within the series. All materials are isostructural, crystallising in the primitive tetragonal space group P4/mmm as reported previously for five compounds in the series. The titanium or vanadium ions have intermediate valency on a single crystallographic site, leading to temperature-independent paramagnetism and correlated electronic behaviour which is influenced by the compositional variation. Furthermore, the alkali metal ions in ▪ and ▪ can be removed by oxidative deintercalation using ▪ at room temperature to produce a new metastable van der Waals layered phase, ▪. During the deintercalation reaction the oxide chalcogenide layers undergo a relative shift by (0.5,0.5) in the ab plane such that ▪ is body-centered with space group I4/mmm.

Electronic density of state in valence fluctuating kondo lattice systems studied by point-contact spectroscopy (Review article)

This article reviews our recent point-contact spectroscopy (PCS) investigation in valence fluctuating Kondo lattice systems, EuNi2P2 and Ge-substituted EuNi2(P0.8Ge0.2)2, and YbPd. A heavy fermion (HF) compound EuNi2P2 exhibits a significant valence fluctuation owing to the intermediate Eu valence of 2.5 + at low temperatures, leading to a strong hybridization between the conduction and f electrons (c–f hybridization). The electronic density of state measurements on EuNi2P2 by using PCS technique reveal that the HF behavior arises from the formation of the indirect hybridization gap with a double-peak structure near the Fermi level due to c–f hybridization. In EuNi2(P0.8Ge0.2)2, the valence fluctuation is suppressed by the Ge substitution, and the separation of the double-peak structure decreases. These suggest that the HF behavior is induced by the valence fluctuation. In YbPd, where two-dimensional (2D) layers of Yb3+ and Yb2.6+ are alternately aligned, a HF-like behavior with a large Sommerfeld coefficient appears at low temperatures in the specific heat. The PCS spectra represent a zero-bias dip structure with an asymmetric background, reproduced by the summation of two Fano curves with two different characteristic temperatures, meaning the formation of two Kondo resonance states for each Yb3+ and Yb2.6+ site. These suggest that a local coherence causes the large Sommerfeld coefficient in the 2D layers of Yb3+, which are screened by the conduction electrons provided by the valence fluctuating Yb2.6+ sites. The observed results indicate that PCS is a powerful technique for examining the variation of electronic DOS in valence fluctuating Kondo lattice systems.

Magnetic-field-induced valence change in Eu(Co1−xNix)2P2 up to 60 T

The solid solution 122 compounds, $\mathrm{Eu}{({\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{2}{\mathrm{P}}_{2}$, show a valence transition between divalent state and intermediate valence states at Eu, which is firmly correlated to multiple degrees of freedom in the solid such as the isostructural transition between the collapsed tetragonal (cT) and uncollapsed tetragonal (ucT) structures, $3d$ magnetism, and the formation of P-P dimers. To gain insights into the correlated behavior, we investigate the effect of high magnetic fields on the samples of $x=0.4$ and 0.5 using magnetostriction and magnetization measurements up to 60 T. The samples are in the Eu valence-fluctuating regime, where the possible structural transition from cT to ucT may be induced by the Eu valence change under the magnetic fields. For both samples, magnetostriction smoothly increases with increasing magnetic fields. The behavior is in good agreement with the calculated results using the interconfigurational fluctuation (ICF) model that describes the Eu valence change. This indicates that $\mathrm{\ensuremath{\Delta}}L$ represents the change of the Eu valence state in these compounds. Magnetization curves for both compounds show good agreement with the ICF model at high magnetic fields. In contrast, in the low-magnetic-field region, magnetization curves do not agree with the ICF model. These results indicate that the Eu valence changes manifest themselves in the magnetization curves at high magnetic fields and that the magnetism of the $3d$ electrons manifests itself in the magnetization at low magnetic fields. Hence, we conclude that the valence change occurs within the Eu valence fluctuation regime coupled with the cT structure. Thereby, we believe that the transition to ucT structure, which is firmly coupled with the divalent Eu state, does not occur within the magnetic field range of the present study. Even higher magnetic fields or pulsed magnetic fields with slower pulse durations may induce such large state changes in the present material, which is triggered by the Eu valence change under high magnetic fields.

Topological quantum phase transition of nickelocene on Cu(100)

Local quantum phase transitions driven by Kondo correlations have been theoretically proposed in several magnetic nanosystems; however, clear experimental signatures are scant. Modeling a nickelocene molecule on a Cu(100) substrate as a two-orbital Anderson impurity with single-ion anisotropy coupled to two conduction bands, we find that recent scanning tunneling spectra reveal the existence of a topological quantum phase transition from the usual local Fermi liquid with high zero-bias conductance to a non-Landau Fermi liquid, characterized by a non-trivial quantized Luttinger integral, with a small conductance. The effects of intermediate valence, finite temperature, and structural relaxation of the molecule position allow us to explain the different observed behaviors.

Formation of 2/1 and 1/1 approximants in a quaternary Al–Pd–Au–Yb system

In this study, 2/1 and 1/1 approximants in a quaternary Al–Pd–Au–Yb system were synthesized by substituting Au into original Al–Pd–Yb quasicrystals. In the Al50Pd24Au10Yb16 and Al52Pd22Au10Yb16 alloys, 2/1 and 1/1 phases were formed, respectively, and the lattice constants were relatively small—2.314 and 1.431 nm for the 2/1 and 1/1 phases, respectively. Scanning transmission electron microscopy revealed that both approximants comprise Tsai-type atomic clusters. Inverse magnetic susceptibility for the approximants obeyed the Curie–Weiss law, indicating the presence of paramagnetism. The effective magnetic moments were estimated to be 3.20 μB and 2.94 μB for the 2/1 and 1/1 phases, respectively, indicating that Yb in both approximants adopts an intermediate valence state.

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.

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.

Evidence of the Anomalous Fluctuating Magnetic State by Pressure-Driven 4f Valence Change in EuNiGe3.

In rare-earth compounds with valence fluctuation, the proximity of the 4f level to the Fermi energy leads to instabilities of the charge configuration and the magnetic moment. Here, we provide direct experimental evidence for an induced magnetic polarization of the Eu3+ atomic shell with J = 0, due to intra-atomic exchange and spin-orbital coupling interactions with the Eu2+ atomic shell. By applying external pressure, a transition from antiferromagnetic to a fluctuating behavior in EuNiGe3 single crystals is probed. Magnetic polarization is observed for both valence states of Eu2+ and Eu3+ across the entire pressure range. The anomalous magnetism is discussed in terms of a homogeneous intermediate valence state where frustrated Dzyaloshinskii-Moriya couplings are enhanced by the onset of spin-orbital interaction and engender a chiral spin-liquid-like precursor.

Valence and magnetism in EuPd3S4 and (Y,La)xEu1−xPd3S4

The double Dirac candidate EuPd${}_{3}$S${}_{4}$ appears to exhibit a roughly 50:50 mixture of Eu${}^{2+}$ and Eu${}^{3+}$. Substituting yttrium rapidly drives the remaining europium divalent, whereas lanthanum has the opposite effect. Analysis shows that average site volumes play a dominant role in the valence distribution suggesting that local strains matter more than electron count. Remarkably, lanthanum substitution also promotes valence instability leading to the appearance of intermediate valence ``Eu${}^{2.5+}$'', the fraction of which increases with both lanthanum content and temperature. Extrapolation of the onset temperature to zero lanthanum predicts ${T}_{\text{onset}}$ \ensuremath{\sim} 340 K matching recent reports of a structural transition attributed to a loss of valence order.

A giant polyoxomolybdate molecular catalyst with unusual Mo6+/Mo5+ synergistic mechanism for oxidation of hydroxyfurfural under atmospheric pressure

Selective oxidation of 5-hydroxymethylfurfural (HMF) under atmospheric conditions is challenging due to the slow reaction rate and multiple parallel reaction routes. This work reports a green approach for selective oxidation of HMF to 2,5-diformylfuran (DFF) by using a giant Keplerate-type polyoxometalate of [(NH4)42[MoVI72MoV60O372(CH3COO)30(H2O)72] (Mo132). Mo132 features extraordinary catalytic activity under atmospheric and even anaerobic conditions, achieving the conversion of 99.2% with DFF selectivity of 100%. Mo132 also shows outstanding recyclability with the activity decreasing by only 3.4% after five recycle tests. The intermediate valence state content of Mo5+ in Mo132 is about 46%, which displays superior catalytic activities during the reactions. Mechanism exploration firstly indicates that the proposed proton coupled electron transfer occurs on special terminal oxygen atoms in Mo6+ sites being surrounded by Mo5+ species. This unveils a new Mo6+/Mo5+ synergistic effect to enhance the efficiency for catalyzing the oxidation of HMF. This work expands the application of high-nuclear molybdenum clusters in catalysis, and supplies a new strategy for the justified design and synthesis of POMs-based catalysts in biorefinery.

Impact of 1,10-Phenanthroline-Induced Intermediate Valence on the Luminesence of Divalent Europium Halides.

Starting from EuX2 (X = Cl, Br, I), we systematically investigated a variety of divalent europium complexes containing bidentate 1,10-phenanthroline (Phen) ligands. Depending on the Eu/Phen ratio, mono-, di-, and polynuclear complexes are formed, with the latter yielding one-dimensional ∞1[EuBr2(phen)] chains. Seven new divalent europium complexes, [Eu(phen)4(H2O)]Br2·2MeCN, [Eu(phen)4]I2·1.7Tol, [EuBr(phen)3]2Br2·4MeCN, [EuCl2(phen)2]2·2MeCN, [EuBr2(phen)2]2, [EuI2(phen)2]2, and [EuBr2(phen)]x, are presented in this work. All species show remarkable optical properties based on a partial electron transfer from the EuII center to the Phen ligand. The photophysical characterization is further supported by electrochemistry studies in order to describe the intermediate valence of the Eu center.

Intermediate Valence Ion‐Mediated Electrodeposition Process (Small 44/2022)

Microstructure Design In article number 2203229, Shikuan Yang and co-workers propose an avenue to regulate the morphologies of the electrodeposits during the anodic electrodeposition process by introducing the intermediate valence ions (e.g., Pb2+, Mn2+, etc.) into the electrolyte solutions. The intermediate valence ions could react with the electrochemically oxidized Ag2+/Ag3+ ions with strong oxidation abilities, affecting the growth kinetics of the Ag7O8NO3 electrodeposits and giving rise to microcrystals with controllable shapes.

Electrochemical and energy storage properties of layer-by-layer assembled vanadium oxide electrode-based solid-state supercapacitor in n+-SnO2:F/n-V2O5 heterostructure device form using ionic liquid gel electrolyte

Vanadium oxide film electrodes synthesized by layer-by-layer assembly using sol–gel spin casting and variable 3 & 1 h annealing process over SnO2:F film coated glass substrates are investigated for supercapacitor energy storage using ionic liquid gel-electrolyte. The X-ray photoelectron spectroscopy analysis of V2p3/2 core-level and O1s peak show short-term (1 h) annealing forms vanadium as V2O5 alongside multivalent and oxygen deficient phases, whereas synthesis by 3-h annealing forms stochiometric V2O5 film. In stochiometric V2O5, capacitive contributions are dominantly from redox processes with peaks identified from V5+/V4+, and V4+/V3+ valance states change and partially from electrical double layer (EDL) yielding high specific capacitance of 346.9 Fg−1. Supercapacitor with V2O5 in mixed valence and oxygen defects phases shows enhanced EDL contribution alongside Faradaic yielding 316.2 Fg−1 specific capacitance. These mechanistic differences are analyzed for ionic diffusion limitations. The linear charge/discharge curves at 0.04–0.15 A/g current density show 85–90% Coulomb efficiency and steady energy density 6.8–5.5 Whkg−1 as specific power increases from 1.9 to 5.7 kWkg−1. Oxygen-deficient V2O5-based supercapacitor shows higher 19.2 Whkg−1 energy density declining to 8.9 Whkg−1 with specific power change from 1.1 to 4.6 kWkg−1. Raman spectra show during charge/discharge, the V5+- V3+ redox is mediated by V4.67+, V4.57+, and V3.33+ intermediate valence states. The pseudocapacitive energy storage depends on charge transfer across n+-SnO2:F/n-V2O5 heterostructure with different band alignments for stochiometric and oxygen deficient V2O5. The conduction band shift to the flat band position determines the potential range and extent of V5+ -V3+ redox reaction.

In-Situ Characterization of Electroreduced Tantalum and Niobium in Deep Eutectic Solvents

Room temperature ionic solvents are expected to possess electrochemical properties suitable for tantalum and niobium redox analogous to high temperature molten salts, in which successful bulk metal electrodeposition is reported. However, with the combination of perplexing multivalent chemistry of niobium and tantalum and alien electrochemical properties of ionic solvents, when compared to better characterized aqueous environments, the fundamental understanding of the electrochemical behavior of niobium and tantalum in these promising, yet compositionally diverse, solvents is severely underpopulated. Herein we report a new toolbox of coordinating additives that form, in many traditional and ionic solvents, chemically-robust reduced niobium and tantalum complexes with distinct photometric and paramagnetic properties, that serve as metal redox indicators and intermediate valence traps during electroreduction. We showcase the utility of these indicator species for clarifying the electrochemical behavior of niobium and tantalum in chemically varied ionic solvent environments by comparing in-situ electrochemical UV-Vis and electron paramagnetic resonance (EPR) observations in a series of chemically dissimilar deep eutectic solvents.

Intermediate Valence Ion-Mediated Electrodeposition Process.

The assembly of biomolecules and ions (e.g., biomineralization process) generates many intricate structures in nature. However, human beings' control over the assembly processes of ions is in its infant stage compared with nature. Here, it is reported that the intermediate valence metal ions in the electrolyte can influence the growth speed of certain crystal facets and in turn adjust the shape of the electrodeposits created by anodic electrodeposition. This is because the intermediate valence metal ions (e.g., Pb2+ , Mn2+ , etc.) can be oxidized by the electrochemically oxidized high valence ions (e.g., Ag2+ and Ag3+ ). Therefore, the concentration of the electrochemically oxidized high valence ions can be controlled by the intermediate valence ions, affecting the growth kinetics of the electrodeposits. Taking the anodic electrodeposition of Ag7 O8 NO3 as an example, the role of intermediate valence ions in tailoring the shape of the Ag7 O8 NO3 electrodeposits is demonstrated. Moreover, the growth location of the second-order structure can be controlled by the intermediate valence metal ions. Additionally, the designed complex microarchitectures starting from certain crystal facets to form hollow nanoframes can be selectively etched. The control capability over the electrochemical assembly process of metal ions is significantly strengthened by introducing intermediate valence ions into the electrolyte.

Influence of the Pd–Si Ratio on the Valence Transition in EuPd2Si2 Single Crystals

Single crystals of intermediate valent EuPd$_2$Si$_2$ were grown from an Eu-rich melt by the Bridgman as well as the Czochralski technique. The chemical and structural characterization of an extracted single crystalline Czochralski-grown specimen yielded a slight variation of the Si-Pd ratio along the growth direction and confirms the existence of a finite Eu(Pd$_{1-m}$Si$_m$)$_2$ homogeneity range. The thorough physical characterization carried out on the same crystal showed that this tiny variation in the composition strongly affects the temperature T$_v$ at which the valence transition occurs. These experiments demonstrate a strong coupling between structural and physical properties in the prototypical valence-fluctuating system EuPd$_2$Si$_2$ and explain the different reported values of T$_v$.

Pressure effect on magnetism and valence in ferromagnetic superconductor Eu(Fe0.75Ru0.25)2As2

Eu(Fe0.75Ru0.25)2As2 is an intriguing system with unusual coexistence of superconductivity and ferromagnetism, providing a unique platform to study the nature of such coexistence. To establish a magnetic phase diagram, time-domain synchrotron Mössbauer experiments in 151Eu have been performed on a single crystalline Eu(Fe0.75Ru0.25)2As2 sample under hydrostatic pressures and at low temperatures. Upon compression the magnetic ordering temperature increases sharply from 20 K at ambient pressure, reaching ∼49 K at 10.1 GPa. With further compression, the magnetic order is suppressed and eventually collapses. Isomer shift values from Mössbauer measurements and x-ray absorption spectroscopy data at Eu L 3 edge show that pressure drives Eu ions to a homogeneous intermediate valence state with mean valence of ∼2.4 at 27.4 GPa, possibly responsible for the suppression of magnetism. Synchrotron powder x-ray diffraction experiment reveals a tetragonal to collapsed-tetragonal structural transition around 5 GPa, a lower transition pressure than in the parent compound. These results provide guidance to further work investigating the interplay of superconductivity and magnetism.

Temperature dependence and intermediate valence on magnetic hyperfine field at a Ce impurity diluted in RZn

In this work, we theoretically study the influence, at finite temperature, of the intermediate valence on the local moment formation and the magnetic hyperfine field at a Ce impurity diluted in RZn (R = Gd, Tb, Dy). The temperature dependence of the local magnetic moments and related hyperfine magnetic interaction is calculated by a functional integral approach in the static approximation, thereby extracting the 4f Ce valence at the range of temperature T ≤ TC (TC = critical temperature). Our self-consistent calculation shows that the intermediate valence model, focusing on the temperature dependence, explains the experimentally observed magnetic hyperfine fields at Ce impurity in these compounds, in particular exhibiting the two regimes of the magnetic hyperfine fields (and local magnetic moments) with temperature.

Intermediate valence state in YbB4 revealed by resonant x-ray emission spectroscopy

We report the temperature dependence of the Yb valence in the geometrically frustrated compound from 12 to 300 K using resonant x-ray emission spectroscopy at the Yb transition. We find that the Yb valence, v, is hybridized between the v = 2 and v = 3 valence states, increasing from at 12 K to at 300 K, confirming that is a Kondo system in the intermediate valence regime. This result indicates that the Kondo interaction in is substantial, and is likely to be the reason why does not order magnetically at low temperature, rather than this being an effect of geometric frustration. Furthermore, the zero-point valence of the system is extracted from our data and compared with other Kondo lattice systems. The zero-point valence seems to be weakly dependent on the Kondo temperature scale, but not on the valence change temperature scale T v .

Hole conductivity through a defect band in ZnGa2O4

Semiconductors with a wide band gap ($>3.0$ eV), high dielectric constant ($>10$), good thermal dissipation, and capable of $n$- and $p$-type doping are highly desirable for high-energy power electronic devices. Recent studies indicate that ${\mathrm{ZnGa}}_{2}{\mathrm{O}}_{4}$ may be suitable for these applications, standing out as an alternative to ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$. The simple face-centered-cubic spinel structure of ${\mathrm{ZnGa}}_{2}{\mathrm{O}}_{4}$ results in isotropic electronic and optical properties, in contrast to the large anisotropic properties of the $\ensuremath{\beta}$-monoclinic ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$. In addition, ${\mathrm{ZnGa}}_{2}{\mathrm{O}}_{4}$ has shown, on average, better thermal dissipation and potential for $n$- and $p$-type conductivity. Here we use density functional theory and hybrid functional calculations to investigate the electronic, optical, and point defect properties of ${\mathrm{ZnGa}}_{2}{\mathrm{O}}_{4}$, focusing on the possibility for $p$- and $n$-type conductivity. We find that the cation antisite ${\mathrm{Ga}}_{\mathrm{Zn}}$ is the lowest-energy donor defect that can lead to unintentional $n$-type conductivity. The stability of self-trapped holes (small hole polarons) and the high formation energy of acceptor defects make it difficult to achieve $p$-type conductivity. However, with an excess of Zn, forming ${\mathrm{Zn}}_{(1+2x)}{\mathrm{Ga}}_{2(1\ensuremath{-}x)}{\mathrm{O}}_{4}$ alloys, this compound can display an intermediate valence band, facilitating $p$-type conductivity. Due to the localized nature of this intermediate valence band, $p$-type conductivity by polaron hopping is expected, explaining the low mobility and low hole density observed in recent experiments.