Valence Transition Research Articles

Oxidative degradation of p-chlorophenol by the persulfate-doped Fe-Mn bimetallic hydroxide, the parametrical significance, and systematical optimization.

Aqueous Fe(II)-catalyzed activation commonly deteriorates the oxidation performance of persulfate (PS) to the treatment of organic contaminants. In this study, a PS-doped layered bimetallic hydroxide (Fe-Mn hydroxide) was synthesized to construct a heterogeneously catalytic system to solve the issue brought by homogeneity. The molar ratio of Fe(II) to Mn(II) and the mass ratio of PS to Fe-Mn hydroxide both had a significant impact on the catalytic degradation of p-CP. Reaction temperatures engaged in the most essential role in influencing the degradation and removal of p-chlorophenol (p-CP). The optimal combination of factors for the preparation of PS-hydroxide and the treatment of p-CP was finally determined by significance analysis. The degradation process was appropriately fitted by the pseudo-first-order kinetic model. The benzene ring in p-CP was broken by PS-hydroxide during the adsorption. The surface modification of PS-hydroxide caused by the valence transition of Mn was beneficial to the adsorption and catalytic degradation of p-CP.

A cryostat designed for x-ray fluorescence holography experiments down to 4 K.

A cryostat was designed for x-ray fluorescence holography (XFH) experiments at low temperatures down to 4K, where many functional materials show characteristic transitions, such as high-temperature superconducting, magnetic, dielectric, and valence transitions. For XFH measurements, changes in two angles of samples with respect to the incident synchrotron x-ray beam, i.e., incident angle θ and azimuthal angle ϕ, are necessary. A low-temperature specialized small piezoelectric motor is installed at the cryostat head for ϕ, and the cryostat itself is rotated by a stepping motor for θ. The heat from the piezoelectric motor for ϕ and the cryostat power determining the total cryostat mass were optimized for the limited working spaces and beamtimes of synchrotron experiments. Some examples of the XFH results at low temperatures, such as a Pb crystal and an YbInCu4 valence transition material, are presented to show the feasibility of this low-temperature equipment.

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.

Magnetic constitution of topologically trivial thermoelectric PbTe:Cr

In this paper we report on detailed temperature and magnetic field dependence of magnetization of IV-VI semiconductor PbTe doped with mixed valence transition metal Cr2+/3+. The material is studied solely by a SQUID magnetometer in order to quantitatively determine the contribution of single substitutional Cr2+/3+ as well as of various Cr-Te magnetic nanocrystals, including their identification. The applied experimental procedure reveals the presence of about 1019 cm−3 paramagnetic Cr ions, of which 2/3 are the Cr3+ ions formed via self-ionization of Cr2+ resonant donors. These are known to improve the thermoelectric figure of merit parameter zT of this semiconductor. The magnetic finding agrees with previous Hall effect studies thus providing a new experimental support for the proposed electronic structure model of PbTe:Cr system with resonant Cr2+/3+ state located (at low temperatures) about 100 meV above the bottom of the conduction band. Below room temperature a ferromagnetic-like signal points to the presence of Cr-rich nanocrystalline precipitates. Two most likely candidates, namely: Cr2Te3 and Cr5Te8 are identified upon dedicated temperature cycling of the sample at the remnant state. As an ensemble, the nanocrystals exhibit (blocked) superparamagnetic properties. The magnetic susceptibility of both n- and p-type PbTe in the temperature range 100<T<400 K has been established. These magnitudes are essential to properly account for the high temperature magnetic susceptibility of PbTe:Cr.

Negative thermal expansion in YbMn2Ge2 induced by the dual effect of magnetism and valence transition

Negative thermal expansion (NTE) is an intriguing property, which is generally triggered by a single NTE mechanism. In this work, an enhanced NTE (αv = −32.9 × 10−6 K−1, ΔT = 175 K) is achieved in YbMn2Ge2 intermetallic compound to be caused by a dual effect of magnetism and valence transition. In YbMn2Ge2, the Mn sublattice that forms the antiferromagnetic structure induces the magnetovolume effect, which contributes to the NTE below the Néel temperature (525 K). Concomitantly, the valence state of Yb increases from 2.40 to 2.82 in the temperature range of 300–700 K, which simultaneously causes the contraction of the unit cell volume due to smaller volume of Yb3+ than that of Yb2+. As a result, such combined effect gives rise to an enhanced NTE. The present study not only sheds light on the peculiar NTE mechanism of YbMn2Ge2, but also indicates the dual effect as a possible promising method to produce enhanced NTE materials.

Unraveling the valence state and phase transformation of iron-based electrocatalysts towards oxygen reduction reaction

The commercialization of fuel cells relies largely upon the development of highly active non-precious metal electrocatalysts. Insufficient understanding of the reaction mechanism of Fe-based electrocatalysts towards oxygen reduction reaction (ORR) limits the further improving of its catalytic performance. Herein the valence state and phase transformation of three Fe-based electrocatalysts comprising different valences of iron during the ORR in alkaline medium are investigated by in situ synchrotron radiation X-ray absorption and diffraction studies in direct borohydride fuel cell. There is a dynamic structure associated with the ‘Fe2+/3+’ redox transition confirmed by in situ X-ray absorption near edge structure spectra in Fe-PPy-C and FeS-PPy-C. The metallic Fe, FeS, and Fe2O3 of the three Fe-based electrocatalysts are all converted to FeOOH during ORR. The local structure of Fe atoms and valence transition barrier contribute to most of the ORR activities of the three Fe-based electrocatalysts.

Antiferromagnetic ordering and possible lattice response to dynamic uranium valence in U3O8

Determining the correct electronic structure of ${\mathrm{U}}_{3}{\mathrm{O}}_{8}$ remains a formidable experimental and theoretical challenge. In the low-temperature phase, two crystallographic U sites are separated into a distinct 2U(V)$+1\mathrm{U}$(VI) oxidation configuration. At low temperatures, the U(V) sites form a distorted honeycomb lattice, but the U(VI) sit on a triangular sublattice, suggesting potential for magnetic frustration effects. The spin configuration of the unpaired $f$ electrons on the U(V) sites is likely antiferromagnetic (AFM) from susceptibility measurements, but this has not been confirmed. Here, we present a neutron scattering investigation of the structure and dynamics of ${\mathrm{U}}_{3}{\mathrm{O}}_{8}$ from 1.7 to 600 K. We confirm static AFM ordering onset at between 22 and 25 K, which is present down to at least 1.7 K with AFM peaks corresponding to [0.5 1 1] and [0.5 2 2] in the orthorhombic phase. These measurements rule out static AFM order along the $a$ axis of the $Amm2$ phase, a configuration previously suggested by theory. Above 100 K a quasielastic scattering channel opens that we speculate arises from a lattice relaxation response to thermally activated electron hopping. This term does not conform to a magnetic form factor, so it is not related to spin relaxations. If correct, this mechanism stabilizes a continuous valence transition from $2\mathrm{U}(\mathrm{V})+1\mathrm{U}(\mathrm{VI})$ in the low-temperature ($T&lt;600$ K) orthorhombic phase to the hexagonal phase that contains only one degenerate U site, wherein the U valence can be dynamically stabilized between $\mathrm{U}(\mathrm{V})\ensuremath{\leftrightarrow}\mathrm{U}(\mathrm{VI})$ by phonon-assisted electron hopping.

Synergy between indigenous bacteria and extracellular electron shuttles enhances transformation and mobilization of Fe(III)/As(V)

The reduction of Fe(III) by metal-reducing bacteria through extracellular electron transfer (EET) is a critical link in the biogeochemical cycle of As/Fe, and humic substances are believed to play a role in this process. In this study, the indigenous As-resistant bacterium Bacillus D2201 isolated from the Datong Basin was responsible for the valence transition of Fe and As in the groundwater environment. The bacterium has both the arsC gene for intracellular arsenate reduction and an EET pathway for transferring electrons to an electrode or Fe(III). Chronoamperometry showed that 3.0- and 10.2-fold increases in the output current density were achieved by injecting 0.05 and 0.5 mM AQDS with an inoculation of Bacillus D2201. Interestingly, Fe(III) bio-reduction is not only regulated by AQDS, but also by As(V) stimulation. The increase in pyruvate consumption and levels of intracellular glutathione (GSH) suggest that As pressure promotes cell metabolism and the consumption of electron donors for Fe(III) reduction with strain D2201. The reduction and dissolution of Fe(III) mineral regulated by AQDS dominated the release and mobilization of As. Compared with the AQDS-free treatment, 5.5-, 6.6-, and 7.2-fold increases in the amounts of Fe(II) were released with the addition of 0.1, 0.5, and 1 mM AQDS, respectively, and approximately 2.6-, 2.8-, and 3.2-fold increases in the As(V) levels were observed under the same conditions. These insights have profound environmental implications with respect to the effect of AQDS and As stress on EET and Fe(III) reduction in arsenic-resistant bacteria.

Magnetic field and pressure induced valence and metal-insulator transitions in the spin-one-half Falicov–Kimball model

A combination of exact diagonalization calculations and a well-controlled approximate method is used to study valence and metal-insulator transitions in the spin-one-half Falicov–Kimball model (FKM) in the external magnetic field. It is shown that there exists a critical value of the magnetic field $$h_c$$ that depends on the f-level position $$E_f$$ , above which the spin-one-half FKM maps onto the spinless FKM. In this limit the spin-one-half FKM recovers all physics of the spinless model, including the picture of valence and metal-insulator transitions induced by changes of $$E_f$$ and opens the way to its experimental verification in real systems, like rare-earth compounds in the external magnetic field. In addition, we have found that the spin-one-half FKM in the external magnetic field exhibits a wide spectrum of continuous, as well as discontinuous valence, magnetic and metal-insulator transitions that can be induced by tuning directly the external magnetic field or indirectly the external pressure (via the f-level position). The experimental verification of these transitions represent also an interesting challenge, the solution of which can shed more light on the physics of valence and metal-insulator transitions in correlated d-f electron systems. Most of presented results are calculated for the one-dimensional spin-one-half FKM, but to establish connection with the behavior of real rare-earth materials we discuss also effects of the increasing system dimension and the anisotropic spin-dependent d-f interaction, which can play an important role in these materials.

Thermodynamic evaluation and experimental investigation of CaO-assisted Fe-based chemical looping reforming process for syngas production

Many works have focused on the reactivity and valence transition of Fe-based oxygen carriers in chemical looping partial oxidation reforming process. However, the reforming performance and impacts of operating factors are unclear with the co-feeding of CaO being CO2 carrier to realize simultaneous CO2 capture and utilization. In this work, thermodynamic equilibrium calculation, reactivity test and material characterization are carried out to optimize the process characteristics. The impacts of feeding conditions (Fe/Al weight ratio, methane amount, Ca/Fe mole ratio and packing manner) on the reforming reactivity are obtained to reveal the interaction between CaO and Fe2O3. The gas product distribution indicates that the carbon deposition and oxidation degree is closely related to CH4 amount. The spatial configuration between CaCO3 and Fe2O3 reforming is essential to obtain the desirable reforming products. The difference of FeO existence on the theoretical and experimental phase components illustrates that reaction degree is affected by the reduction agent amount. By comparing the methane temperature-programmed reaction of CaCO3, Fe2O3 and their mixture, it is suggested that the existence of CaCO3 is beneficial for accelerating iron-based methane reforming. This study provides a way to improve reaction degree and optimize process characteristic for chemical looping reforming process.

Interplay of Valence and Semimetal-to-Insulator Transitions in SmS

Recent discoveries of a new type of quantum criticality arising from Yb-valence fluctuations in Yb-based metal in periodic crystal and quasicrystal have opened a new class of quantum critical phenomena in correlated electron systems. To clarify whether this new concept can be generalized to other rare-earth-based semimetal and insulator, we study SmS which exhibits golden-black phase transition under pressure. By constructing the model for SmS, we show that Coulomb repulsion between 4f and 5d orbitals at Sm drives first-order valence transition (FOVT) and semimetal-to-insulator transition (MIT) simultaneously, which explains the golden-black phase transition. We clarify the ground-state phase diagram for the FOVT and MIT by identifying the quantum critical point of the FOVT. We find that exciton condensates in both semimetal and insulator phases. Our result explains measured peak anomalies in the specific heat and compressibility in pressurized golden SmS and provides a cue to clarify recently-observed anomalies in black SmS.

Eu–Mn Charge Transfer and the Strong Charge–Spin–Electronic Coupling Behavior in EuMnO3

Based on first-principles calculations with the DFT + U method, the couplings of lattice, charge, spin, and electronic behaviors underlying the Eu-Mn charge transfer in a strongly correlated system of EuMnO3 were investigated. The potential valence transition from Eu3+/Mn3+ to Eu2+/Mn4+ was observed in a compressed lattice with little distortions, which is achieved under hydrostatic pressure and external strain. The intraplane antiferromagnetism (AFM) of Mn is proved to be instrumental in the emergence of Eu2+. Furthermore, we calculated the magnetic exchange interactions within two equilibrium structures of Eu3+Mn3+O3 and Eu2+Mn4+O3. Mn-Mn ferromagnetic exchange in the ab-plane is enhanced strongly in the Eu2+Mn4+O3 structure, contributing to the existence of mixed states. The versatile electronic structures were obtained within the Eu2+Mn4+O3 phase by imposing different magnetic configurations on the Eu and Mn sublattice, attributed to the coupling of charge transfer and magnetic orderings. It is found that the intraplane ferromagnetic ordering of Mn leads to a metallic electronic structure with the coexistence of Eu2+ and Eu3+, while the intraplane AFM Mn spin ordering leads to insulating states only with Eu2+. Notably, a half-metallic characteristic emerges at the magnetic ground state of CF ordering (C-type AFM for the Eu sublattice and ferromagnetic for the Mn sublattice), which makes such a supposed phase more intriguing than the conventional experimental phase. Additionally, the mixture of delocalized 4f with 5d states of Eu in the background of Mn 3d and O 2p orbitals implies a pathway of Eu 4f 5d ↔ O 2p ↔ Mn 3d for charge transfer between Eu and Mn. Our calculation shows that the Eu-Mn charge transfer could be expected in compressed EuMnO3 and the introduction of Eu2+ 4f states near the Fermi level plays an important role in manipulating the interlinks of charge and spin together with electronic behaviors.

Высоковалентные кластерные соединения переходных металлов, содержащие внедренные гетероатомы: особенности геометрии, электронного строения и физико-химических свойств

Рассмотрено современное состояние химии высоковалентных кластерных соединений переходных металлов 3—7 групп периодической системы, содержащих внедренные гетероатомы. Обсуждены методы синтеза, реакционная способность, основные структурные типы и электронная структура образующихся соединений.

Thermodynamic model of the coupled valence and spin state transition in cobaltates

A class of cobalt-based oxides exhibits a peculiar type of transition, entangling valence and spin state degrees of freedom of 4f and 3d elements. It constitutes one of the most spectacular illustrations of the interplay between charge, spin and lattice degrees of freedom in strongly correlated materials. In this work, we present a thermodynamic model capable to reproduce the main features of this transition. Our approach is based on the minimization of a free energy combining the contributions of two sublattices and the interaction between them. The coupling energies introduced in the model are related to well-known chemical pressure effects in the perovskite structure. The results of this model are compared to experimental data derived from x-ray absorption spectroscopy.

Self-assembled CoOOH on TiO2 for enhanced photoelectrochemical water oxidation

The as-obtained 6% CoOOH-TiO 2 photoelectrode manifests enhanced PEC performance owing to the fast carrier migration across TiO 2 to CoOOH. And the valence transition of surface amorphous CoOOH plays a dominating role in the PEC process. Providing efficient charge transfer through the interface between the semiconductor and co-catalyst is greatly desired in photoelectrocatalytic (PEC) energy conversion. Herein, we excogitate a novel and facile means, via electrochemical activation, to successfully load the amorphous CoOOH layer architecture onto the surface of TiO 2 . Intriguingly, the as-obtained 6% CoOOH-TiO 2 photoelectrode manifests optimal PEC performance with a high photocurrent density of 1.3 mA/cm 2 , 3.5 times higher than that of pristine TiO 2 . Electrochemical impedance spectroscopy (EIS), Tafel analysis and cyclic voltammetry (CV) methods show that the carrier transfer barrier within the electrode and the transition of Co 3+ OOH to Co 4+ OOH have the dominating effects on the PEC performance. Theoretical calculation reveals that the interface between the CoOOH and TiO 2 improves the homogeneity of effective d-orbital electronic-transfer ability among Co sites. This research sheds light on the water oxidation reaction and the design of more favorable PEC co-catalysts.

Chemical tuning of samarium valence in mixed valence (Sm1-xCax)2.75C60 fullerides

Rare-earth (RE) fullerides are an intriguing family of materials in which electronic instabilities on the RE sublattice couple to the electronic and lattice degrees-of-freedom of the strongly-correlated fulleride sublattice. In particular, insulating Sm2.75C60 adopts an orthorhombic superstructure, arising from long-range ordering of Sm partial vacancies, and can be driven to a lattice-collapsed metallic state due to a valence transition towards Sm3+ upon pressurization. Here we use synchrotron X-ray absorption spectroscopy (XAS) at the Sm L3 edge at ambient conditions to authenticate the mixed valence character of the material by identifying two features due to distinct Sm2+ and Sm3+ components – their relative intensity allows a direct measure of the average Sm valence in Sm2.75C60 at +2.07(3). We then attempt to mimic the physical pressure effect on the electronic properties by co-intercalation of the smaller-size valence-precise Ca2+ ion to form the series of ternary solid solutions, (Sm1-xCax)2.75C60 (0 ≤ x ≤ ⅔). XAS measurements in their high-resolution partial fluorescence yield (PFY) variance find that chemical pressure leads to an increase of the Sm3+ contribution to the average valence by >10% in the most contracted member, (Sm⅓Ca⅔)2.75C60 of the present study with the average valence reaching a value of +2.33(2). Assuming full charge transfer between the metal ions and C60, the charge on the C60 units remains invariant throughout at approximately −5.78. This opens the possibility that the system can be tuned further towards the higher average valences needed for the high-pressure insulator-to-metal transition in Sm2.75C60 to be shifted to ambient pressure with a single-valence Sm3+ state accessible at a Ca content of x ~0.9.

Vacuum-Ultraviolet Absorption Spectrum of 3-Methoxyacrylonitrile

The high-resolution absorption spectrum of 3-methoxyacrylonitrile (3MAN) was measured between 5.27 and 12.59 eV using a synchrotron-based Fourier-transform spectrometer. It was related to an absolute absorption cross-section scale. Complementary calculations at the DFT-MRCI/aug-cc-pVTZ level of theory document the vertical transition energies and oscillator strengths toward the first 19 states of both the E and Z geometrical isomers of 3MAN. Comparisons with the experimental absorption spectrum reveal the similarities and differences between 3MAN, a bifunctional molecule, with acrylonitrile and methylvinylether, where only one functional group is present. As in acrylonitrile, several broad valence transitions were observed up to the ionization limit. They are likely associated with the extended π-system induced by the nitrile group but might also involve σσ* transitions close to the ionization limit. As in methylvinylether, Rydberg series converging to the ionization limit are absent. This is attributed to a difference in neutral and cationic geometry due to a 60° rotation of the methyl group.

Insight into Kondo screening in the intermediate-valence compound SmOs4Sb12 uncovered by soft x-ray magnetic circular dichroism

Through a high-precision soft x-ray magnetic circular dichroism (XMCD) study of the intermediate-valence compound $\mathrm{Sm}{\mathrm{Os}}_{4}{\mathrm{Sb}}_{12}$, we show our successful approach of revealing unprecedented details of Kondo screening below a characteristic temperature of ${T}_{0}\ensuremath{\sim}20$ K in the paramagnetic phase. The multiplet XMCD structure at the Sm ${M}_{5}$ edge sensitive to the $4f$ configuration enabled us to observe a clear difference between the temperature evolution above and below ${T}_{0}$ with one- and two-component behavior, respectively. Our findings are in strong contrast to the conventional Kondo crossover that coincides with the valence transition, but are qualitatively accounted for by theoretical XMCD predictions combined with the two-fluid phenomenology recently proposed. This work contributes to the large context of Kondo physics that is closely related to quantum criticality in heavy fermion systems.

Valence Transition of EuRh2Si2 Studied by Synchrotron Mössbauer Spectroscopy

We have studied 151Eu Mossbauer effects on EuRh2Si2 at a high pressure of 1.0 GPa as well as at ambient pressure. The compound shows a temperature-induced valence transition from the Eu2+ state to ...

Towards a Single Chemical Model for Understanding Lanthanide Hexaborides

AbstractLanthanide hexaborides (LnB6) have disparate and often anomalous properties, from structurally homogeneous mixed valency, to superconductivity, spectral anomalies, and unexplained phase transitions. It is unclear how such a diversity of properties may arise in the solids of identical crystal structures and seemingly very similar electronic structures. Building on our previous model for SmB6 (mixed valent, with a peak in specific heat, and pressure induced magnetic phase transitions), we present a unifying dynamic bonding model for LnB6 that explains simultaneously EuB6 (possessing an anomalous peak in specific heat at low T, magnetic phase transitions, and no mixed valency), YbB6 (mixed valent topological insulator), and rather ordinary LaB6. We show that Ln can engage in covalent bonding with boron, and, in some members of the LnB6 family, also easily access alternative bonding states through the electron–phonon coupling. The accessibility, relative energetics, and bonding nature of the states involved dictate the properties.