Quantum-mechanical fluctuations between competing phases induce exotic collective excitations that exhibit anomalous behavior in transport and thermodynamic properties, and are often intimately linked to the appearance of unconventional Cooper pairing. High-temperature superconductivity, however, makes it difficult to assess the role of quantum-critical fluctuations in shaping anomalous finite-temperature physical properties. Here we report temperature-field scale invariance of non-Fermi liquid thermodynamic, transport, and Hall quantities in a non-superconducting iron-pnictide, Ba(Fe1/3Co1/3Ni1/3)(2)As-2, indicative of quantum criticality at zero temperature and applied magnetic field. Beyond a linear-in-temperature resistivity, the hallmark signature of strong quasiparticle scattering, we find a scattering rate that obeys a universal scaling relation between temperature and applied magnetic fields down to the lowest energy scales. Together with the dominance of hole-like carriers close to the zero-temperature and zero-field limits, the scale invariance, isotropic field response, and lack of applied pressure sensitivity suggests a unique quantum critical system unhindered by a pairing instability. Extensive theoretical and experimental efforts have been devoted to the effect of quantum criticality in our understanding of the physics of high-temperature superconductors and strongly correlated electron materials, yet it remains a puzzle in condensed matter physics. The authors report observations of a quantum criticality by investigating the non-Fermi liquid thermodynamics and transport behaviour in a non-superconducting iron pnictide.

Quantum-mechanical fluctuations between competing phases at $T=0$ induce exotic finite-temperature collective excitations that are not described by the standard Landau Fermi liquid framework. These excitations exhibit anomalous temperature dependences, or non-Fermi liquid behavior, in the transport and thermodynamic properties in the vicinity of a quantum critical point, and are often intimately linked to the appearance of unconventional Cooper pairing as observed in strongly correlated systems including the high-$T_c$ cuprate and iron pnictide superconductors. The presence of superconductivity, however, precludes direct access to the quantum critical point, and makes it difficult to assess the role of quantum-critical fluctuations in shaping anomalous finite-temperature physical properties, such as Planckian dissipation $\hbar/\tau_{p} =k_{B}T$. Here we report temperature-field scale invariance of non-Fermi liquid thermodynamic, transport and Hall quantities in a non-superconducting iron-pnictide, Ba(Fe$_{1/3}$Co$_{1/3}$Ni$_{1/3}$)$_{2}$As$_{2}$, indicative of quantum criticality at zero temperature and zero applied magnetic field. Beyond a linear in temperature resistivity, the hallmark signature of strong quasiparticle scattering, we find a more universal Planck-limited scattering rate that obeys a scaling relation between temperature and applied magnetic fields down to the lowest energy scales. Together with the emergence of hole-like carriers close to the zero-temperature and zero-field limit, the scale invariance, isotropic field response and lack of applied pressure sensitivity point to the realization of a novel quantum fluid predicted by the holographic correspondence and born out of a unique quantum critical system that does not drive a pairing instability.

The formation of conductive metallic silver upon electrochemical reduction and lithiation of Ag7Fe3(P2O7)(4) is investigated. Alternating current impedance spectroscopy measurements show a 34% decrease in charge transfer resistance upon one electron equivalent (ee) of reduction, which is coincident with the formation of a Ag metal conductive network evidenced by both ex situ and operando X-ray diffraction. Quantitative assessment of Ag metal formation derived from operando XRD shows that only Ag+ ions are reduced during the first 3ee, followed by simultaneous reduction of Ag+ and Fe3+ reduction for the next See (3ee to 8ee), culminating in reduction of the remaining Ag+. Scanning electron microscopy images show smaller Ag metal crystallite size and shorter nearest neighbor distance between and among Ag particles with higher depth of discharge. A high rate intermittent pulsatile discharge test is conducted where the cell delivers 12 total pulses during full discharge to probe the effect of Ag metal formation on the Li/Ag7Fe3(P2O7)(4) cell electrochemistry. The Ohmic resistance is derived from the voltage drop of each pulse. The resistance is 65 Omega initially, reaches its minimum of 26 Omega at 4.5 ee discharge, and levels off at 35 Omega after 7.0 ee reduction. The initial Ag reduction is more significant for the conductive network formation indicated by the decrease of both R-ct and Ohmic resistance, which facilitates the high power output of the cell.

A paradigm for the synthetic manipulation of composition and crystallite size of bimetallic composites via a low-temperature, aqueous co-precipitation technique employing non-stoichiometric ratios of starting materials, AgNO3 and Fe(NO3 3, is introduced. In-situ formation of composites containing crystalline silver ferrite, AgFeO2, and nanocrystalline maghemite, γ-Fe2O3 is demonstrated and established by Raman spectroscopic and X-ray absorption analyses. As a cathode, the lowest silver content composites exhibited profoundly improved electrochemical performance, with reversible capacities approximately 100% higher relative to stoichiometric AgFeO2, and demonstrate the lowest capacity fade.

Ag7Fe3(P2O7)(4) is an example of an electrochemical displacement material which contains two different electrochemically active metal cations, where one cation (Ag+) forms metallic silver nanoparticles external to the crystals of Ag7Fe3(P2O7)(4) via an electrochemical reduction displacement reaction, while the other cation (Fe3+) is electrochemically reduced with the retention of iron cations within the anion structural framework concomitant with lithium insertion. These contrasting redox chemistries within one pure cathode material enable high rate capability and reversibility when Ag7Fe3(P2O7)(4) is employed as cathode material in a lithium ion battery (LIB). Further, pyrophosphate materials are thermally and electrically stable, desirable attributes for cathode materials in LIBs. In this paper, a bimetallic pyrophosphate material Ag7Fe3(P2O7)(4) is synthesized and confirmed to be a single phase by Rietveld refinement. Electrochemistry of Ag7Fe3(P2O7)(4) is reported for the first time in the context of lithium based batteries using cyclic voltammetry and galvanostatic discharge-charge cycling. The reduction displacement reaction and the lithium (de)insertion processes are investigated using ex situ X-ray absorption spectroscopy and X-ray diffraction of electrochemically reduced and oxidized Ag7Fe3(P2O7)(4). Ag7Fe3(P2O7)(4) exhibits good reversibility at the iron centers indicated by similar to 80% capacity retention over 100 cycles following the initial formation cycle and excellent rate capability exhibited by similar to 70% capacity retention upon a 4-fold increase in current.

As the demand for large scale batteries has grown, considerations such as earth abundance, cost, and toxicity have assumed a greater significance. Metal oxides such as magnetite (Fe3O4) are worthy of evaluation as active materials for electrochemical energy storage due to its earth abundance, low cost, environmentally benign iron metal centers and high theoretical capacity, 926 mAh/g. Implementation in the future will require understanding of the fundamental electrochemical reduction-oxidation mechanisms of the electroactive materials is required, coupled with characterization at the mesoscale to understand the underlying contributors to localized resistance which must be addressed in order to achieve significant improvements to current capability and reversibility. An emerging paradigm for the implementation of close-packed materials in higher current applications is the tuning of the materials crystallite dimensions, where the reduction of crystallite size should minimize the path length for ion transport upon discharge, resulting in a reduction of both internal cell resistance and the resultant structural strain associated with lithium insertion. The electrochemical impact of controlled crystallite size of magnetite will be described. In addition to the crystallite size of the electroactive material, a complete study of an electroactive nanomaterial in the complex mesoscale environment of a composite battery electrode should also consider the level of electroactive particle agglomeration, and agglomerate distribution within the battery electrode. Nanocrystalline magnetite (Fe3O4) powders and composite electrodes with different crystallite sizes were prepared, characterized, and electrochemically evaluated. Transmission electron microscopy (TEM) examination of cross-sectioned electrodes was used to quantify the aggregate size of the Fe3O4 active material. Notably, although the crystallite sizes of two magnetite samples Fe3O4 are different (28 and 9 nm), the observed sizes of the aggregates and aggregate distribution within the electrodes were similar, see TEM images of sectioned electrodes fabricated with the 28 nm (A,B) and 9 nm (C,D) sized Fe3O4 in the Figure. Transmission x-ray microscopy (TXM) combined with x-ray absorption near edge spectroscopy (XANES) at the National Synchrotron Light Source (NSLS) were used to determine the distribution of the iron oxidation states within the electrodes before and after electrochemical testing. The impact of both crystallite size and agglomeration on electrochemical performance were assessed. Figure 1

A paradigm for the synthetic manipulation of composition and crystallite size of bimetallic composites via a low-temperature, aqueous co-precipitation technique employing non-stoichiometric ratios of starting materials, AgNO 3 and Fe(NO 3 3 , is introduced. In-situ formation of composites containing crystalline silver ferrite, AgFeO 2 , and nanocrystalline maghemite, γ-Fe 2 O 3 is demonstrated and established by Raman spectroscopic and X-ray absorption analyses. As a cathode, the lowest silver content composites exhibited profoundly improved electrochemical performance, with reversible capacities approximately 100% higher relative to stoichiometric AgFeO 2 , and demonstrate the lowest capacity fade.

Silver ion conducting electrolytes are being explored as a material to be used in solid state batteries. This chapter provides a review of the advances in silver ion conducting solid electrolytes over the last 20 years. We discuss the advances that have been made in the synthesis of new electrolyte materials and provide conductivity values for new electrolytes reported. We then turn to a summary of the models of the origin and enhancement of silver ion conductivity in solid electrolytes with focus on those that have been proposed more recently. Finally, we review the progress of solid state batteries and include values for the reported energy density and voltage of the cells.

The functional capacity of a battery is observed to decrease, often quite dramatically, as discharge rate demands increase. These capacity losses have been attributed to limited ion access and low electrical conductivity, resulting in incomplete electrode use. A strategy to improve electronic conductivity is the design of bimetallic materials that generate a silver matrix in situ during cathode reduction. Ex situ x-ray absorption spectroscopy coupled with in situ energy-dispersive x-ray diffraction measurements on intact lithium/silver vanadium diphosphate (Li/Ag2VP2O8) electrochemical cells demonstrate that the metal center preferentially reduced and its location in the bimetallic cathode are rate-dependent, affecting cell impedance. This work illustrates that spatial imaging as a function of discharge rate can provide needed insights toward improving realizable capacity of bimetallic cathode systems.

The powerful synchrotron analysis technique energy dispersive x-ray diffraction (EDXRD) has been utilized to achieve greater insight into the distribution of silver nanoparticles formed on initial reduction of lithium / silver vanadium phosphorous oxide, Ag2VO2PO4, and silver vanadium diphosphate, Ag2VP2O8, (SVOP family of materials) batteries. By combining the results of EDXRD measurements with ex-situ methods, we are able to effectively probe the discharge mechanism of the bimetallic materials to understand the reduction displacement discharge process.

The electrochemical reduction of Ag0.48VOPO4 center dot 1.9H(2)O is accompanied by vanadium and silver oxidation state changes, characterized with X-ray absorption spectroscopy (XAS), and by structural changes, characterized with X-ray powder diffraction (XRD). The XAS data suggest that the initial reduction process, involving 0 to 0.5 electron equivalents, involved primarily the reduction of vanadium cations, while most of the silver cations are reduced between 0.5 to 1.0 electron equivalents. The XRD data display significant intensity decreases of absorbances associated with the 004 and 006 planes upon electrochemical reduction, consistent with a reduction-displacement of Ag+ with insertion of Li+. Retention of intensity of the absorbance associated with the 002 plane, with only minor decrease in interlayer spacing, indicates retention of the VOPO4 sublattice structure. Uncovering the details of the discharge mechanism of bimetallic cathode materials such as Ag0.48VOPO4 should enable the design of future high current cathodes for secondary batteries displaying an enhanced current capacity based on a reduction-displacement strategy. (C) The Author(s) 2015. Published by ECS. All rights reserved.

When electroactive nanomaterials are fully incorporated into an electrode structure, characterization of the crystallite sizes, agglomerate sizes, and dispersion of the electroactive materials can lend insight into the complex electrochemistry associated with composite electrodes. In this study, composite magnetite electrodes were sectioned using ultramicrotome techniques, which facilitated the direct observation of crystallites and agglomerates of magnetite (Fe3O4) as well as their dispersal patterns in large representative sections of electrode, via 2D cross sectional analysis by Transmission Electron Microscopy (TEM). Further, the electrochemistry of these electrodes were recorded, and Transmission X-ray Microscopy (TXM) was used to determine the distribution of oxidation states of the reduced magnetite. Unexpectedly, while two crystallite sizes of magnetite were employed in the production of the composite electrodes, the magnetite agglomerate sizes and degrees of dispersion in the two composite electrodes were similar to each other. This observation illustrates the necessity for careful characterization of composite electrodes, in order to understand the effects of crystallite size, agglomerate size, and level of dispersion on electrochemistry.

Thermal conductivity, point contact spectroscopy, angle-resolved photoemission and Raman spectroscopy measurements were performed on BaFe1.9Pt0.1As2 single crystals obtained from the same synthesis batch in order to investigate the superconducting energy gap structure using multiple techniques. Low temperature thermal conductivity was measured in the superconducting state as a function of temperature and magnetic field, revealing an absence of quasiparticle excitations in the T → 0 ?> limit up to 15 T applied magnetic fields. Point-contact Andreev reflection spectroscopy measurements were performed as a function of temperature using the needle-anvil technique, yielding features in the conductance spectra at both 2.5 meV and 7.0 meV scales consistent with a multi-gap scenario. Angle-resolved photoemission spectroscopy probed the electronic band structure above and below the superconducting transition temperature of Tc = 23 K, revealing an isotropic gap of magnitude ∼ 3 ?> meV on both electron and hole pockets. Finally, Raman spectroscopy was used to probe quasiparticle excitations in multiple channels, showing a threshold energy scale of 3 meV below Tc. Overall, we find strong evidence for an isotropic gap structure with no nodes or deep minima in this system, with a 3 meV magnitude gap consistently observed and a second, larger gap suggested by point-contact spectroscopy measurements. We discuss the implications that the combination of these results reveal about the superconducting order parameter in the BaFe2−xPtxAs2 doping system and how this relates to similar substituted iron pnictides.

A paradigm for concomitant control of crystallite size and composition of bimetallic composites via co-precipitation is introduced. Direct preparation of composites of silver ferrite and amorphous maghemite via nonstoichiometric synthesis was demonstrated. Notable impact on electrochemistry was observed, with ∼200% increase in reversible capacity for the small crystallite material.

We report superconductivity and magnetism in a new family of topological semimetals, the ternary half-Heusler compound RPdBi (R: rare earth). In this series, tuning of the rare earth f-electron component allows for simultaneous control of both lattice density via lanthanide contraction and the strength of magnetic interaction via de Gennes scaling, allowing for a unique tuning of the normal-state band inversion strength, superconducting pairing, and magnetically ordered ground states. Antiferromagnetism with ordering vector (½,½,½) occurs below a Néel temperature that scales with de Gennes factor dG, whereas a superconducting transition is simultaneously supressed with increasing dG. With superconductivity appearing in a system with noncentrosymmetric crystallographic symmetry, the possibility of spin-triplet Cooper pairing with nontrivial topology analogous to that predicted for the normal-state electronic structure provides a unique and rich opportunity to realize both predicted and new exotic excitations in topological materials.

Previously, we reported that electrodes containing silver vanadium phosphate (Ag2VO2PO4) powder exhibit a 15000 fold increase in conductivity after discharge, concurrent with the formation of silver metal. In this study, in order to disentangle the complex nature of electrodes composed of electroactive powders, an electrochemical reduction of individual particles of Ag2VO2PO4 was conducted, to more directly probe the intrinsic materials properties of Ag2VO2PO4. Specifically, individual particle conductivity data from a nanoprobe system combined with SEM and optical imaging results revealed that the depth of discharge within an Ag2VO2PO4 particle is closely linked to the conductivity increase. Notably, the formation of silver metal may affect both inter-and intraparticle conductivity of the Ag2VO2PO4 material.

EDXRD of Li/C-Ag2VP2O8revealed that higher-rate initial discharge (B) generated a more effective conductive matrixvia in situreduction-displacement-deposition of Ag0.