Mixed-valence Materials Research Articles

Emerging Mixed-Valence Porous Materials.

The development and applications of porous materials have been revolutionized over the past decades. To fine-tune their fascinating behaviors such as charge-transport, magnetic, and catalytic properties, one of the most effective strategies is incorporating mixed valency into the structures. This Concept provides recent progress on emerging crystalline porous materials, namely, metal-organic frameworks, metal-organic polyhedra, covalent organic frameworks, and hydrogen-bonded organic frameworks, where their mixed valency is either intrinsic or postsynthetically induced via an external stimulus. We further highlight the investigation of mixed valency and potential applications, which sheds light on the design and exploitation of mixed-valence crystalline porous materials.

Charge-lattice coupling and the dynamic structure of the U–O distribution in UO2+x

The different structures and behaviors of UO2+x observed in crystallographic and local structure measurements were examined by extended X-ray absorption fine structure (EXAFS) measurements of pristine UO2.0, p+ and He2+ irradiated UO2.0, and, at multiple temperatures, bulk U4O9 and U3O7 and thin film U4O9-δ on an epitaxial substrate. The disorder caused by irradiation is mostly limited to increased widths of the existing U–O/U pair distributions, with any new neighbor shells being minor. As has been previously reported, the disorder caused by oxidative addition to U4O9 and U3O7 is much more extensive, resulting in multisite U–O distributions and greater reduction of the U–U amplitude with different distributions in bulk and thin-film U4O9. This includes the significant spectral feature near R = 1.2 Å for all U4O9 and U3O7 samples fit with a U-oxo type moiety with a U–O distance around 1.7 Å. In addition to indicating that these anomalies only occur in mixed valence materials, this work confirms the continuous rearrangement of the U–O distributions from 10 to 250 K. Although these variations of the structure are not observed in crystallography, their prominence in the EXAFS indicates that the dynamic structure underlying these effects is an essential factor of these materials.

Migdal–Eliashberg superconductivity in a Kondo lattice

We apply the Migdal–Eliashberg theory of superconductivity to heavy-fermion and mixed valence materials. Specifically, we extend the Anderson lattice model to a case when there exists a strong coupling between itinerant electrons and lattice vibrations. Using the saddle-point approximation, we derive a set of coupled nonlinear equations which describe competition between the crossover to a heavy-fermion or mixed-valence regimes and conventional superconductivity. We find that superconductivity at strong coupling emerges on par with the development of the many-body coherence in a Kondo lattice. Superconductivity is gradually suppressed with the onset of the Kondo screening and for strong electron-phonon coupling the Kondo screening exhibits a characteristic re-entrant behavior. Even though for both weak and strong coupling limits the suppression of superconductivity is weaker in the mixed-valence regime compared to the local moment one, superconducting critical temperature still remains nonzero. In the weak coupling limit the onset of the many body coherence develops gradually, in the strong coupling limit it emerges abruptly in the mixed valence regime while in the local moment regime the f-electrons remain effectively decoupled from the conduction electrons. Possibility of experimental realization of these effects in Ce-based compounds is also discussed.

A mixed-valence polyoxometalate-based 3D inorganic framework cathode material for high-efficiency rechargeable AZIBs

A mixed-valence polyoxometalate-based 3D inorganic framework cathode material for high-efficiency rechargeable AZIBs

Assembly of a trapped valent CeIII/IV-TCNQ complex through metal-ligand redox cooperativity.

Complex (Cp3CeIV)2(TCNQ)(CeIIICp3)2 (1) was prepared by reducing neutral TCNQ0 (tetracyanoquinodimethane) with Cp3Ce(THF). Two types of cerium centres with a dianionic TCNQ2- moiety are present in 1, wherein each of the four cyano-groups are bound by a cation. Formation of this trapped-valent organocerium compound 1 is facilitated by metal-ligand redox cooperativity. Characterization of 1 was carried out using structural-, magnetometry-, and IR-spectroscopic analyses. Photophysical studies on this compound reveal CeIII luminescence, and opens up avenues for promising multifunctional, mixed-valent lanthanide materials.

Benzene-1,4-Di(dithiocarboxylate) Linker-Based Coordination Polymers of Mn2+, Zn2+, and Mixed-Valence Fe2+/3.

Three new coordination polymers (CPs) constructed from the linker 1,4-di(dithiocarboxylate) (BDDTC2-)─the sulfur-analog of 1,4-benzenedicarboxylate (BDC2-)─together with Mn-, Zn-, and Fe-based inorganic SBUs are reported with description of their structural and electronic properties. Single-crystal X-ray diffraction revealed structural diversity ranging from one-dimensional chains in [Mn(BDDTC)(DMF)2] (1) to two-dimensional (2D) honeycomb sheets observed for [Zn2(BDDTC)3][Zn(DMF)5(H2O)] (2). Gas adsorption experiments confirmed a 3D porous structure for the mixed-valent material [Fe2(BDDTC)2(OH)] (3). 3 contains a 1:1 ratio of Fe2+/3+ ions, as evidenced by 57Fe Mössbauer, X-band EPR, and X-ray absorption spectroscopy. Its empirical formula was established by elemental analysis, thermal gravimetric analysis, infrared vibrational spectroscopy, and X-ray absorption spectroscopy in lieu of elusive single-crystal X-ray diffraction data. In contrast to the Mn- and Zn-based compounds 1 and 2, the Fe2+/3+ CP 3 showed remarkably high electrical conductivity of 5 × 10-3 S cm-1 (according to van der Pauw measurements), which is within the range of semiconducting materials. Overall, our study confirms that sulfur derivatives of typical carboxylate linkers (e.g., BDC) are suitable for the construction of electrically conducting CPs, due to acceptedly higher covalency in metal-ligand bonding compared to the electrically insulating carboxylate CPs or metal-organic frameworks. At the same time, the direct comparison between insulating CPs 1 and 2 with CP 3 emphasizes that the electronic structure of the metal is likewise a crucial aspect to construct electrically conductive materials.

(Invited) Microelectrode-Based Diagnosis of Charge Propagation and Redox Transitions in Concentrated Polyoxometallate Electrolyte of Potential Utility for Redox Flow Battery

Concentrated solutions of Keggin-type silicotungstic acid, as well as the system’s single crystals (H4SiW12O40*31H2O) and their colloidal suspensions have been tested using the microelectrode methodology to determine mass-transport, electron self-exchange and apparent (effective) diffusion-type coefficients for charge propagation and homogeneous (electron self-exchange) rates of electron transfers. Silicotungstic acid facilitates proton conductivity, and undergoes fast, reversible, multi-electron electron transfers leading to the formation of highly conducting, mixed-valence (tungsten(VI,V) heteropoly blue) compounds. To develop useful electroanalytical diagnostic criteria, electroanalytical approaches utilizing microdisk electrodes have been adapted to characterize redox transitions of the system and to determine kinetic parameters. Combination of micoroelectrode-based experiments performed in two distinct diffusional regimes: radial (long-term experiment; e.g., slow scan rate voltammetry or long-pulse chronoamperometry) and linear (short-term experiment; e.g., fast scan rate voltammetry or short-pulse chronocoulometry) permits absolute determination of such parameters as effective concentration of redox centers (C0 ) and apparent transport (diffusion) coefficient (Dapp ). The knowledge of these parameters, in particular of [Dapp 1/2 C0 ] seems to be of importance to the evaluation of utility of redox electrolytes for charge storage.While current densities which reflect dynamics of electrochemical processes have an influence on the systems’ current densities, the viscosity of the electrolyte and the mass transport dynamics are also affected by the nature of the redox-active material and its concentration. Trying to develop useful electroanalytical diagnostic approaches, we have successfully utilized microelectrode-based probes, as well as the historical concepts of charge propagation in semi-solid or semi-liquid systems developed for mixed-valence polynuclear materials in order to characterize concentrated redox electrolytes. Among important parameters are concentration of redox centers (C0 ) and apparent transport (diffusion) coefficient (Dapp ). The knowledge of these parameters and, in particular of [Dapp 1/2 C0 ], are crucial when it comes to evaluation of the diffusional-type charge propagation dynamics in the concentrated electrolyte which may reflect both physical mass transport and electron self-exchange (electron-hopping) contributions. Both potential-step (chronocoulometry, chronoamperometry) and cyclic voltammetric experiments utilizing microdisk electrodes have been adapted to characterization (identification of redox transitions and determination of kinetic parameters) of model inorganic redox electrolytes, namely highly-concentrated solutions or colloidal suspensions of Keggin-type polyoxometallate, silicotungstic acid.

A Multi-Technique Analysis of Surface Materials From Blood Falls, Antarctica

Aperiodic discharge of brine at Blood Falls forms a red-tinged fan at the terminus of Taylor Glacier, Antarctica. Samples from this discharge provide an opportunity for mineralogical study at a Martian analogue study site. Environmental samples were collected in the field and analyzed in the laboratory using Fourier transform infrared, Raman, visible to near-infrared, and Mössbauer spectroscopies. Samples were further characterized using microprobe and inductively coupled plasma optical emission spectroscopy for chemistry, and x-ray diffraction, scanning electron microscopy, and transmission electron microscopy for mineralogy, crystallography, and chemistry. The mineralogy of these samples is dominated by the carbonate minerals calcite and aragonite, accompanied by quartz, feldspar, halide, and clay minerals. There is no strong evidence for crystalline iron oxide/hydroxide phases, but compositionally and morphologically diverse iron- and chlorine-rich amorphous nanospheres are found in many of the samples. These results showcase the strengths and weaknesses of different analytical methods and underscore the need for multiple complementary techniques to inform the complicated mineralogy at this locale. These analyses suggest that the red color at Blood Falls arises from oxidation of dissolved Fe2+ in the subglacial fluid that transforms upon exposure to air to form nanospheres of amorphous hydroxylated mixed-valent iron-containing material, with color also influenced by other ions in those structures. Finally, the results provide a comprehensive mineralogical analysis previously missing from the literature for an analogue site with a well-studied sub-ice microbial community. Thus, this mineral assemblage could indicate a habitable environment if found elsewhere in the Solar System.

Molecular beam epitaxy of TmTe thin films on SrF2 (111)

The odd parity nature of 4f states characterized by strong spin–orbit coupling and electronic correlations has led to a search for novel topological phases among rare earth compounds, such as Kondo systems, heavy Fermions, and homogeneous mixed-valent materials. Our target system is thulium telluride thin films whose bandgap is expected to be tuned as a function of lattice parameter. We systematically investigate the growth conditions of TmxTey thin films on SrF2 (111) substrates by molecular beam epitaxy. The ratio between Te and Tm supply was precisely tuned, resulting in two different crystalline phases, which were confirmed by x-ray diffraction and x-ray photoemission spectroscopy. By investigating the crystalline quality as a function of the substrate temperature, the optimal growth conditions were identified for the desired Tm1Te1 phase. Additional low energy electron diffraction and reflective high energy electron diffraction measurements confirm the epitaxial growth of TmTe layers. X-ray reflectivity measurements demonstrate that homogeneous samples with sharp interfaces can be obtained for varied thicknesses. Our results provide a reliable guidance to prepare homogeneous high-quality TmTe thin films and thus serve as a basis for further electronic investigations.

Valence-selective local atomic structures in inorganic materials by X-ray fluorescence holography

X-ray fluorescence holography (XFH) can be used to conduct atom-resolved structural characterization of materials around a specific element, and has been applied to various functional materials. Recently, a valence-selective function has been found by this technique by employing incident X-ray energies near an absorption edge of a specified element. In this article, the principle and experimental procedure of a valence-selective XFH and subsequent data analysis procedure using the sparse modeling approach of L1 regression are introduced. Then, the excellent XFH results with valence-selective studies are reviewed, such as Y oxide thin film, YbInCu4 valence transition material, and Fe3O4 mixed valence material.

Microelectrode Based Devices for Diagnosis of Electron Transport and Electrochemical Behavior of Redox-Conducting Materials

We are going to address practical issues related to the principles and analytical aspects of electrochemical studies (typically done at room temperature) of solid, rigid or semi-rigid (nonfluid) systems in the absence of a liquid solution phase. Representative examples include redox and conducting organic polymers, melts and solid solutions of redox centers in solid ionic conductors, mixed-valence polynuclear inorganic materials, transition metal salts, oxides, and zeolites. The emphasis is on the elements of dynamics for the efficient delivery of charge and on reactivity of the ‘redox conducting’ materials. The effective (apparent) diffusional mechanism is critical to the success of most analytical measurements in solid-state. Historically, electrochemistry in the absence of a bulk liquid phase has focused on the problems of energy storage and production. Recently, it has become possible to apply conventional electrochemical methods to characterize solid-state type redox electrolytes with respect to analytical characterization of materials, measurement of electron transfer rates, and determination of analytes using amperometric sensing. The characterization of materials by solid-state voltammetry primarily has focused on mixed-valence, ionically conducting compounds and on diverse organic ionomers hosting identifiable molecular (redox) sites. Potential materials for solid-state electrochemical measurements are expected to contain three-dimensionally distributed highly concentrated redox centers between which fast electron self-exchange (hopping) is feasible. Among inorganic systems, single crystals of heteropolyacids of molybdenum and tungsten, as well thin films of Prussian Blue type metal hexacyanoferrates will be considered. These redox centers are fixed and, although they may have short range mobility about an equilibrium position, they classically are macroscopically immobile. The applicable materials also must host mobile counter-ions that are capable of providing charge balance during electron transfers, thereby serving the same purpose as the supporting electrolytes in conventional electrochemistry. The population of these ions must be sufficient to support diffusive mass transport of electrons and to minimize ohmic effects. We will describe typical electrochemical cells and experimental tactics to overcome the relatively slow dynamics of transport in solid (non-fluid) systems. Application of microdimensional electrodes leads to an improvement in the quality of solid-state electrochemical data and provides new diagnostic and analytical possibilities. Basic concepts of mechanistic studies, which are relevant to the development of novel analytical methods, together with trends towards possible future applications will also be addressed.

Exposing the inadequacy of redox formalisms by resolving redox inequivalence within isovalent clusters

In this report we examine a family of trinuclear iron complexes by multiple-wavelength, anomalous diffraction (MAD) to explore the redox load distribution within cluster materials by the free refinement of atomic scattering factors. Several effects were explored that can impact atomic scattering factors within clusters, including 1) metal atom primary coordination sphere, 2) M-M bonding, and 3) redox delocalization in formally mixed-valent species. Complexes were investigated which vary from highly symmetric to fully asymmetric by 57Fe Mössbauer and X-ray diffraction to explore the relationship between MAD-derived data and the data available from these widely used characterization techniques. The compounds examined include the all-ferrous clusters [ n Bu4N][(tbsL)Fe3(μ3-Cl)] (1) ([tbsL]6- = [1,3,5-C6H9(NC6H4-o-NSi t BuMe2)3]6-]), (tbsL)Fe3(py) (2), [K(C222)]2[(tbsL)Fe3(μ3-NPh)] (4) (C222 = 2,2,2-cryptand), and the mixed-valent (tbsL)Fe3(μ3-NPh) (3). Redox delocalization in mixed-valent 3 was explored with cyclic voltammetry (CV), zero-field 57Fe Mössbauer, near-infrared (NIR) spectroscopy, and X-ray crystallography techniques. We find that the MAD results show an excellent correspondence to 57Fe Mössbauer data; yet also can distinguish between subtle changes in local coordination geometries where Mössbauer cannot. Differences within aggregate oxidation levels are evident by systematic shifts of scattering factor envelopes to increasingly higher energies. However, distinguishing local oxidation levels in iso- or mixed-valent materials can be dramatically obscured by the degree of covalent intracore bonding. MAD-derived atomic scattering factor data emphasize in-edge features that are often difficult to analyze by X-ray absorption near edge spectroscopy (XANES). Thus, relative oxidation levels within the cluster were most reliably ascertained from comparing the entire envelope of the atomic scattering factor data.

Charge Propagation in Redox-Conducting Materials: Diagnosis of Mechanisms and Dynamics with Microelectrochemical Devices

Historically, electrochemistry in the absence of a bulk liquid phase has focused on the problems of energy storage and production. Recently, it has become possible to apply conventional electrochemical methods to characterize solid-state type redox electrolytes with respect to analytical characterization of materials, measurement of electron transfer rates, and determination of analytes using amperometric sensing. The characterization of materials by solid-state voltammetry primarily has focused on mixed-valence, ionically conducting compounds and on diverse organic ionomers hosting identifiable molecular (redox) sites. We are going to address practical issues related to the principles and analytical aspects of electrochemical studies (typically done at room temperature) of solid, rigid or semi-rigid (nonfluid) systems in the absence of a liquid solution phase. Representative examples include redox and conducting organic polymers, melts and solid solutions of redox centers in solid ionic conductors, mixed-valence polynuclear inorganic materials, transition metal salts, oxides, and zeolites. The emphasis is on the elements of dynamics for the efficient delivery of charge and on reactivity of the ‘redox conducting’ materials. The effective (apparent) diffusional mechanism is critical to the success of most analytical measurements in solid-state. The potential materials for solid-state electrochemical measurements are expected to contain three-dimensionally distributed highly concentrated redox centers between which fast electron self-exchange (hopping) is feasible. These redox centers are fixed and, although they may have short range mobility about an equilibrium position, they classically are macroscopically immobile. The applicable materials also must host mobile counter-ions that are capable of providing charge balance during electron transfers, thereby serving the same purpose as the supporting electrolytes in conventional electrochemistry. The population of these ions must be sufficient to support diffusive mass transport of electrons and to minimize ohmic effects. We will describe typical electrochemical cells and experimental tactics to overcome the relatively slow dynamics of transport in solid (non-fluid) systems. Application of microdimensional electrodes leads to an improvement in the quality of solid-state electrochemical data and provides new diagnostic and analytical possibilities. Basic concepts of mechanistic studies, which are relevant to the development of novel analytical methods, together with trends towards possible future applications will also be addressed.

Two successive magnetic transitions observed in a new mixed valence compound PbNi3(PO4)3 with channels structure

A new mixed valence material PbNi3(PO4)3 was synthesized by high-temperature solid state reaction. The structure consists of 3D framework constructed by NiO6 octahedra and PO4 tetrahedra, forming 2D channels in which Pb ions are located inside. Magnetic measurements are consistent with the structural feature, confirming the presence of mixed valence Ni2+/Ni3+ ions with the ratio of 2:1 in the expected spin-lattice. Two successive magnetic transitions are observed at low-temperature, in which a collinear antiferromagnetic ordering occurs at ∼15 K and ferrimagnetic-like ordering occurs at ∼4 K.

Tunable Mixed-Valence Doping toward Record Electrical Conductivity in a Three-Dimensional Metal-Organic Framework.

Partial oxidation of an iron-tetrazolate metal-organic framework (MOF) upon exposure to ambient atmosphere yields a mixed-valence material with single-crystal conductivities tunable over 5 orders of magnitude and exceeding 1 S/cm, the highest for a three-dimensionally connected MOF. Variable-temperature conductivity measurements reveal a small activation energy of 160 meV. Electronic spectroscopy indicates the population of midgap states upon air exposure and corroborates intervalence charge transfer between Fe2+ and Fe3+ centers. These findings are consistent with low-lying Fe3+ defect states predicted by electronic band structure calculations and demonstrate that inducing metal-based mixed valency is a powerful strategy toward realizing high and systematically tunable electrical conductivity in MOFs.

Possible charge order structure of stoichiometric YbFe2O4

ABSTRACTWe report a possible charge ordering model of triangular mixed valence material YbFe2O4. It was found that a crystal synthesized with high stoichiometry on iron vacancy shows new and simple extinction rule in the superlattice reflections. The Laue class of the diffraction signal is expressed as 2/m. With the consideration of the charge neutrality and the superlattice signal of (1/3 1/3 0), we derived 5 possible charge ordering models of this material.

Incipient class II mixed valency in a plutonium solid-state compound.

Electron transfer in mixed-valent transition-metal complexes, clusters and materials is ubiquitous in both natural and synthetic systems. The degree to which intervalence charge transfer (IVCT) occurs, dependent on the degree of delocalization, places these within class II or III of the Robin-Day system. In contrast to the d-block, compounds of f-block elements typically exhibit class I behaviour (no IVCT) because of localization of the valence electrons and poor spatial overlap between metal and ligand orbitals. Here, we report experimental and computational evidence for delocalization of 5f electrons in the mixed-valent PuIII/PuIV solid-state compound, Pu3(DPA)5(H2O)2 (DPA = 2,6-pyridinedicarboxylate). The properties of this compound are benchmarked by the pure PuIII and PuIV dipicolinate complexes, [PuIII(DPA)(H2O)4]Br and PuIV(DPA)2(H2O)3·3H2O, as well as by a second mixed-valent compound, PuIII[PuIV(DPA)3H0.5]2, that falls into class I instead. Metal-to-ligand charge transfer is involved in both the formation of Pu3(DPA)5(H2O)2 and in the IVCT.

Surface-dominated conduction up to 240 K in the Kondo insulator SmB6 under strain.

SmB6 is a strongly correlated mixed-valence Kondo insulator with a newly discovered surface state, proposed to be of non-trivial topological origin. However, the surface state dominates electrical conduction only below T∗ ≈ 4 K (ref.), limiting its scientific investigation and device application. Here, we report the enhancement of T∗ in SmB6 under the application of tensile strain. With 0.7% tensile strain we report surface-dominated conduction at up to a temperature of 240 K, persisting even after the strain has been removed. This can be explained in the framework of strain-tuned temporal and spatial fluctuations of f-electron configurations, which might be generally applied to other mixed-valence materials. We note that this amount of strain can be induced in epitaxial SmB6 films via substrate in potential device applications.

Rational Integration of Inbuilt Aperture with Mesoporous Framework in Unusual Asymmetrical Yolk-Shell Structures for Energy Storage and Conversion.

Despite the attractive benefits of hollow structures as electrodes for advanced energy storage-conversion capabilities, one prevailing shortcoming is their compromised structural integrity and volumetric energy density due to the introduction of an ultrathin shell with an excessively underutilized large hollow cavity. Herein, we report a facile and template-free synthetic route to realize unusual asymmetrical yolk-shell (AYs) structures composed of mixed-valence NiCo2O4 material. Explicitly, this work highlights the unusual off-central core, an AYs structure that encompasses a hemispherical hollow interior, and a mesoporous solid counterpart. As such, it retains desirable hollow structural characteristics while favorably precludes the excessive unexploited hollow interior space for increased active material packing. Unlike the conventional symmetrical yolk-shell (SYs) which is composed of a porous shell framework radially throughout the structure, the mesoporous solid constitution of the AYs structure offers an inbuilt reinforced framework to support the partial porous shell and concurrently leaves sufficient void for volumetric buffering. Another unique structural feature of the AYs structure is the formation of a submicron aperture or opening on the shell that enhances accessibility of electrolyte diffusion. All of these synergistic structural features of NiCo2O4 AYs structures enhance the pseudocapacitive and electrocatalytic properties.

Detection of the Spontaneous Action Potentials of HEK 293 Cells by Prussian Blue thin Films

The gold standard for investigating neuronal electrical activities has been electrodes. Modern electrophysiology methods boast high sensitivity and temporal resolution with the ability to measure single ion channels and networks of neurons. However, the spatial resolution and robustness are limited by the size and geometry of the electrode. In this era, optical methods have picked up momentum as powerful tools to measure the electrical activities of neurons by using an optical probe that transduces the electrical signal into an optical signal. Prussian blue (PB) is a mixed-valence inorganic material composed of alternating ferric and ferrous ions in a cubic, cyano-bridged lattice with open sites for alkaline metal intercalation. This material is known for its biocompatibility, insolubility in water, activities toward alkaline metal ions and electrochromicity. It is the latter property that makes Prussian blue an excellent candidate to measure the electrical potentials of excitable membranes. The color of this material depends on the applied electrical potential which in turn controls the redox state of the irons. Its blue color is turned to colorless by reduction and is recovered by oxidation. We hypothesize that the electrical potential of excitable membranes will modulate the spectral properties of the material which will be detected through a differential photodiode detector. A PB thin film was electrodeposited onto an ITO-coated glass slide and its optical properties were characterized. As a model for electrophysiology measurements, a clone line of modified HEK 293 cells that stably express Nav 1.3 and KIR 2.1 and generate spontaneous electrical action potentials were used. Herein, we demonstrate the ability to detect the extracellular action potentials of HEK 293 cells and evaluate the film's potential for imaging networks of neuronal cells.