Maintain Charge Neutrality Research Articles

Spectroscopic Visualization of a Robust Electronic Response of Semiconducting Nanowires to Deposition of Superconducting Islands

Following significant progress in the visualization and characterization of Majorana end modes in hybrid systems of semiconducting nanowires and superconducting islands, much attention is devoted to the investigation of the electronic structure at the buried interface between the semiconductor and the superconductor. The properties of that interface and the structure of the electronic wavefunctions that occupy it determine the functionality and the topological nature of the superconducting state induced therein. Here we study this buried interface by performing spectroscopic mappings of superconducting aluminum islands epitaxially grown in-situ on indium arsenide nanowires. We find unexpected robustness of the hybrid system as the direct contact with the aluminum islands does not lead to any change in the chemical potential of the nanowires, nor does it induce a significant band bending in their vicinity. We attribute this to the presence of surface states bound to the facets of the nanowire. Such surface states, that are present also in bare nanowires prior to aluminum deposition, pin the Fermi-level thus rendering the nanowires resilient to surface perturbations. The aluminum islands further display Coulomb blockade gaps and peaks that signify the formation of a resistive tunneling barrier at the InAs-Al interface. At low energies we identify a potential energy barrier that further suppresses the transmittance through the interface. A corresponding barrier exists in bare semiconductors between surface states and the accumulation layer, induced to maintain charge neutrality. Our observations elucidate the delicate interplay between the resistive nature of the InAs-Al interface and the ability to proximitize superconductivity and tune the chemical potential in semiconductor-superconductor hybrid nanowires.

Improving the efficiency of Monte Carlo simulations of ions using expanded grand canonical ensembles.

While ionic liquids have promising applications as industrial solvents, predicting their fluid phase properties and coexistence remains a challenge. Grand canonical Monte Carlo simulation is an effective method for such predictions, but equilibration is hampered by the apparent requirement to insert and delete neutral sets of ions simultaneously in order to maintain charge neutrality. For relatively high densities and low temperatures, previously developed methods have been shown to be essential in improving equilibration by gradual insertion and deletion of these neutral sets of ions. We introduce an expanded ensemble approach which may be used in conjunction with these existing methods to further improve efficiency. Individual ions are inserted or deleted in one Monte Carlo trial rather than simultaneous insertion/deletion of neutral sets. We show how charge neutrality is maintained and show rigorous quantitative agreement between the conventional and the proposed expanded ensemble approaches, but with up to an order of magnitude increase in efficiency at high densities. The expanded ensemble approach is also more straightforward to implement than simultaneous insertion/deletion of neutral sets, and its implementation is demonstrated within open source software.

Particle heating and energy partition in low-β guide field reconnection with kinetic Riemann simulations

Kinetic Riemann simulations have been completed to explore particle heating during guide field reconnection in the low-β environment of the inner heliosphere and the solar corona. The reconnection exhaust is bounded by two rotational discontinuities (RDs), and two slow shocks (SSs) form within the exhaust as in magnetohydrodynamic (MHD) models. At the RDs, ions are accelerated by the magnetic field tension to drive the reconnection outflow as well as flows in the out-of-plane direction. The out-of-plane flows stream toward the midplane and meet to drive the SSs. The SSs differ greatly from those in the MHD model. The turbulence at the shock fronts and both upstream and downstream is weak, and so the shocks are laminar and produce little dissipation. Downstream of the SSs, the counterstreaming ion beams lead to higher density, which leads to a positive potential between the SSs which acts to confine the downstream electrons to maintain charge neutrality. The potential accelerates electrons from upstream of the SSs to the downstream region and traps a small fraction but only modestly increases the downstream electron temperature above the upstream value. In the low-β limit, the released magnetic energy is split between bulk flow and ion heating with little energy going to electrons. That the model produces neither strong electron heating nor an energetic electron component suggests that other mechanisms, such as multiple x-line reconnection, are required to explain energetic electron production in large flares. The model can be tested with the expected data from the Parker Solar Probe.

Real-Time Insight into the Doping Mechanism of Redox-Active Organic Radical Polymers

In recent years, organic radical polymers have received great attention as active materials for fast-charging battery electrodes. Organic radical polymers are electrochemically active owing to the reversible reduction-oxidation (redox) reaction of pendant radical groups and offer a vast synthetic landscape for customization. Interest in these polymers as battery electrodes has grown due to its high theoretical capacities, fast electron transfer kinetics, and long cyclability. One commonly studied stable nitroxide radical polymer is poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA). This electroactive material consists of stable nitroxyl radical groups attached to a polymethylacrylate backbone. Upon oxidation or charging, the neutral nitroxide radical transfers an electron to the current collector, resulting in an oxoammonium cation. Simultaneously, anions dope the polymer cathode to maintain charge neutrality. Upon reduction or discharging, the polymer is de-doped and the neutral nitroxide radical is retrieved. Electronic charge transfer within the organic radical polymer occurs by an electron hopping mechanism with Brownian motion of the redox centers. Although there is much more understood about electron transfer in organic radical polymers, there is significantly less understood regarding mass transfer and the doping mechanism, which is equally important for understanding the overall redox mechanism. Here, we specifically examine ion transport in PTMA cathodes for nonaqueous batteries. Using in situ electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D), we quantitatively observe the ion transport (or doping) process in organic radical polymers during the redox process for the first time. EQCM-D monitors changes in frequency and dissipation of a PTMA-coated quartz crystal during cyclic voltammetry. The change of mass and shear viscosity can be further obtained from viscoelastic modeling of the raw data, leading to a quantitative view of mass transport associated with the doping process. This work is reported in Wang, S., Li, F., Easley, A.D., & Lutkenhaus, J.L., Nature Materials 18 (1), 69-75 (2019). Here, we quantify the real-time mass transfer of anions and solvent during the reduction and oxidation of PTMA, which allows for the first-ever separation of lithium expulsion and anion uptake in an organic radical polymer. This is accomplished using in situ electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) during cyclic voltammetry, a method that yields the instantaneous electrode mass change and shear modulus via viscoelastic modeling. We present an analysis method for PTMA by which the mass exchanged per electron transferred is quantified, and, in some cases, the number of solvent molecules transferred can also be calculated. Our results show that two doing modes dominate in oxidation: doping by lithium-ion expulsion and doping by anion uptake. The effect of dopant anion type (i.e. CF3SO3 -, ClO4 -, and BF4 -), the degree of solvent participation, and the effect on charge storage behavior with mass transport is correlated to polymer-dopant and dopant-solvent interactions. Understanding the detailed mass and charge transport process is a leap forward in describing the redox reaction doping mechanism in organic radical polymers and adds a new facet of discussion in the nature of electron transport, as the two are intimately coupled. In turn, this may impact the future design of organic radical polymers for any application in which the polymer is in contact with electrolyte (organic batteries, electrochromics, sensors).

Chromous capacities of ferrochromium and matte smelting slags

Purpose: of this paper is to extend the slag capacity concept to sparingly soluble cationic species encountered for example in matte smelting and ferrochromium processing in terms of chromous capacities. Design/methodology/approach: In this context in order to develop the chromous capacity the experimental data on slag-metal equilibrium for ferrochromium smelting and on chromium distribution between slag and matte phases encountered in sulphide smelting of PGM (Platinum Group Metals) and chromium containing copper nickel concentrates were used. Findings: The calculated chromous capacities were in the 10-8 range for ferrochrome smelting slags and 10-4 range for matte smelting slags. These values indicate the very low and moderate chromous ion dissolving ability of ferrochromium smelting slags and matte smelting slags respectively. Research limitations/implications: Due to highly reducing conditions in ferrochromium smelting and the imposed low oxygen partial pressures in sulphide smelting of chromium and PGM containing concentrates chromium in the slag was assumed to be in its divalent state. The slag-metal/matte reaction for dissolution of chromium into the slag as a cation under reducing conditions -in contrast to reactions for dissolution of species such as sulphur and phosphorus into the slag as anions under oxidizing conditions – required the release of electrons. Thus to maintain charge neutrality the dissolution of chromium from the metal or the matte phases into the slag as Cr2+ ions was accompanied by consumption of these electrons by reduction of Fe2+ ions in the slag into metallic Fe which dissolved in the metal/ matte phase. This was supported by the experimental equilibrium data. Hence the slagmetal/ matte reaction defining the chromous capacity involved both O2- and Fe2+ ions of the slag phase. Moreover for matte smelting the thermodynamic activities of species in the matte phase were assumed to conform to ideal associated solution model due to lack of data. For future work it is advisable to determine such activities. Practical implications: The calculated chromous capacities were in the 10-8 range for ferrochrome smelting slags and 10-4 range for matte smelting slags. The results clearly indicate that in matte smelting lower basicity slags would be preferable to render chromium more soluble in the slag in contrast to ferrochromium smelting where higher basicity slags would be suitable to minimize chromium losses to the slag. Originality/value: This research and hence the paper is probably the first on dissolution of chromium as chromous ion in slags, thus it is novel and original.

Asymmetric-Fluidic-Reservoirs Induced High Rectification Nanofluidic Diode

We demonstrate a novel nanofluidic diode that produces rectification factors in excess of 1000. The nanofluidic diode consists of ion permselective nanopores that connect two reservoirs of different diameters- a micropore reservoir and a macropore reservoir. On the application of +100 V to the micropore, a low OFF state current is observed. The OFF state is caused by formation of the ion depleted zone in the micropore because the anions are prevented from entering the nanopores from the micropore and the cations are depleted in this region to maintain charge neutrality. On the application of −100 V, we observe a high ON state current. The ON state is caused by formation of the ion enriched zone in the microchannel because the anions cannot pass through the nanopores and accumulate in the microchannel. To maintain charge neutrality the cations also become enriched in the microchannel. The ratio of ON state current to the OFF state current gives the rectification of current. Here, plasma oxidation is used to achieve a nanopore with a large wall surface charge density of σn = −55 mC/m2 which yields a rectification of current on the order of 3500 that is nearly two orders of magnitude higher than those reported thus far. In contrast to the other nanofluidic diodes, this nanofluidic diode does not introduce asymmetry to the nanopore, but asymmetry is produced by having the nanopores join a micropore and a macropore. Introduction of asymmetry into the fluidic reservoirs which the nanopores connect is quite simple. Hence, the nanofluidic diode is easy to scale up to industrial level.

Formation of polypyrrole with dexamethasone as a dopant: Its cation and anion exchange properties

There is considerable interest in dexamethasone, a synthetic glucocorticosteroid drug, and a number of reports detailing the controlled release of dexamethasone from polypyrrole have been published. However, polypyrrole-doped with dexamethasone is poorly characterised. In this study, polypyrrole doped with dexamethasone was formed at relatively low applied potentials of 0.70 V or 0.80 V vs SCE by carrying out the electropolymerisation on an initial pre-layer of polypyrrole. These polymers displayed electroactive behaviour with a dexamethasone doping level of 0.30 ± 0.03 and using impedance measurements the charge-transfer resistance was computed as 400 Ω. Mixed ion transport was observed. In addition to the release of dexamethasone, hydrated sodium cations were incorporated to maintain charge neutrality on reduction of the polymer. Approximately 200 μg cm−2 of dexamethasone was released following 60 min on reduction of the polymer at −0.90 V vs SCE. It was possible to re-use the polymer to release the drug by repeated oxidation and reduction steps, where dexamethasone was incorporated during oxidation and released on reduction. During these events, sodium was accumulated within the polymer matrix and this high salt content accompanied with swelling and de-swelling events gave rise to the development of cracks in the polymer matrix.

Fermi Level Determination for Charged Systems via Recursive Density of States Integration

Determining the Fermi level position for a given material is important to understand many of its electronic and chemical properties. Ab initio methods are effective in computing Fermi levels when using charge-neutral supercells. However, in the case where charges are explicitly included, the compensating homogeneous background charge, which is necessary to maintain charge neutrality in periodic models, causes the vacuum potential to be ill-defined - which would otherwise have been a reliable reference potential. Here, we develop a method based on recursively integrating the density of states to determine shifts in the Fermi level upon charging. By introducing incremental charges, one can compute the density of states profile and determine the shift in the Fermi level that corresponds to adding or removing a given increment of charge δq, which allows the evaluation of the Fermi level for any arbitrary charge q. We test this method for a range of materials (graphene, h-BN, C3N4, Cu, and MoS2) and demonstrate that this method can produce a reasonable agreement with models that rely on localized compensating background charges.

Analysis of equilibrium and turbulent fluxes across the separatrix in a gyrokinetic simulation

The SOL width is a parameter of paramount importance in modern tokamaks as it controls the power density deposited at the divertor plates, critical for plasma-facing material survivability. An understanding of the parameters controlling it has consequently long been sought [Connor et al. Nucl. Fusion 39(2), 169 (1999)]. Prior to Chang et al. [Nucl. Fusion 57(11), 116023 (2017)], studies of the tokamak edge have been mostly confined to reduced fluid models and simplified geometries, leaving out important pieces of physics. Here, we analyze the results of a DIII-D simulation performed with the full-f gyrokinetic code XGC1 which includes both turbulence and neoclassical effects in realistic divertor geometry. More specifically, we calculate the particle and heat E × B fluxes along the separatrix, discriminating between equilibrium and turbulent contributions. We find that the density SOL width is impacted almost exclusively by the turbulent electron flux. In this simulation, the level of edge turbulence is regulated by a mechanism that we are only beginning to understand: ∇B-drifts and ion X-point losses at the top and bottom of the machine, along with ion banana orbits at the low field side, result in a complex poloidal potential structure at the separatrix which is the cause of the E × B drift pattern that we observe. Turbulence is being suppressed by the shear flows that this potential generates. At the same time, turbulence, along with increased edge collisionality and electron inertia, can influence the shape of the potential structure by making the electrons non-adiabatic. Moreover, being the only means through which the electrons can lose confinement, it needs to be in a balance with the original direct ion orbit losses to maintain charge neutrality.

Dielectric and optical properties of Ni doped Na0.5Bi0.5TiO3

Polycrystalline Ni doped Na.5Bi0.5TiO3 samples (Na0.5Bi0.5)Ti1-xNixO3, (x = 0.5, 0.10, 0.15) have been prepared by solid state reaction. The appearance of the additional peak in X-ray diffraction pattern indicates the ordering of Ti4+ and Ni2+ ions. Polygonal grains are converted into flakes with an increase of Ni concentration. Replacement of Ti4+ by Ni2+ strongly modifies the relative contribution of two peaks in the Raman bands within 200–400 cm−1. Oxygen vacancy is observed in X-ray photoelectron spectrum to maintain charge neutrality due to aliovalent doping. Broad diffuse phase transition centered at the dielectric constant maximum indicates relaxor behaviour. Comparison between impedance and electric modulus spectrum suggests non-Debye relaxation. The ac conductivity follows the power law with the frequency exponent lies 0.52 − 0.72. The generation of holes by divalent Ni dopant at tetravalent Ti sites enhances optical band gap.

Controlling the B/Ti ratio of TiBx thin films grown by high-power impulse magnetron sputtering

TiBx thin films grown from compound TiB2 targets by magnetron sputter deposition are typically highly over-stoichiometric, with x ranging from 3.5 to 2.4, due to differences in Ti and B preferential-ejection angles and gas-phase scattering during transport from the target to the substrate. Here, the authors demonstrate that stoichiometric TiB2 films can be obtained using high-power impulse magnetron sputtering (HiPIMS) operated in power-controlled mode. The B/Ti ratio x of films sputter-deposited in Ar is controllably varied from 2.08 to 1.83 by adjusting the length of HiPIMS pulses ton between 100 and 30 μs, while maintaining average power and pulse frequency constant. This results in peak current densities JT,peak ranging from 0.27 to 0.88 A/cm2. Energy- and time-resolved mass spectrometry analyses of the ion fluxes incident at the substrate position show that the density of metal ions increases with decreasing ton due to a dramatic increase in JT,peak resulting in the strong gas rarefaction. With ton < 60 μs (JT,peak > 0.4 A/cm2), film growth is increasingly controlled by ions incident at the substrate, rather than neutrals, as a result of the higher plasma dencity and, hence, electron-impact ionization probablity. Thus, since sputter-ejected Ti atoms have a higher probability of being ionized than B atoms, due to their lower first-ionization potential and larger ionization cross-section, the Ti concentration in as-deposited films increases with decreasing ton (increasing JT,peak) as ionized sputtered species are steered to the substrate by the plasma in order to maintain charge neutrality.

Depth dependant element analysis of PbMg1/3Nb2/3O3 using muonic x-rays

The relaxor PbMg1/3Nb2/3O3 (PMN) has received attention due to its potential applications as a piezoelectric when doped with PbTiO3 (PT). Previous results have found that there are two phases existing in the system, one linked to the near-surface regions of the sample, the other in the bulk. However, the exact origin of these two phases is unclear. In this paper, depth dependant analysis results from negative muon implantation experiments are presented. It is shown that the Pb content is constant throughout all depths probed in the sample, but the Mg and Nb content changes in the near-surface region below 100 μm. At an implantation depth of 60 μm, it is found that there is a 25% increase in Mg content, with a simultaneous 5% decrease in Nb content in order to maintain charge neutrality. These results show that the previously observed skin effects in PMN are due to a change in concentration and unit cell.

Defect formation in In2O3and SnO2: a new atomistic approach based on accurate lattice energies

A new consistent interatomic force field for In2O3and SnO2.

Effect of Spin Ordering on Oxygen Reduction Reaction on Af/Pm-NiO Surface

The reduction reaction of diatomic oxygen molecules (O2(g)) to split into monatomic oxide ions (O2-) is attracting attention for the oxide cathode reactivity to reduce cathodic over-potential in solid oxide fuel cells and other electrochemical applications. Various experimental and theoretical approaches have been reported to understand the rate determining step (RDS) of the oxygen reduction reaction (ORR) on the surface of oxide cathodes, mainly composed of 3d transition metal like (La, Sr)CoO3. Although it is generally accepted that RDS of ORR on oxide cathodes is the process for a surface adsorbate of to meet with surface oxygen vacancy (Vö(s)) [i], an atomistic analysis on the adsorption of oxygen molecule and the splitting reaction is still under debated; how the electrons are transferred and how the splitting of oxygen molecule proceeds. On the other hand, the most stable form of O2(g) is so-called “triplet” oxygen, which has a pair of up -spin electrons in two orbitals and the highest molecular orbital of is completely unoccupied. This electron configuration indicates that, the oxygen molecule, during ionization process, initially accept only two down -spin electrons to form bonded to surface Ni ions, and subsequently a pair of up - and down -spin electrons to dissociated into two O2-on surface. One may, then, come up with a simple question whether or not oxygen molecules have such spin preference during ORR. If it is true, ORR on oxide is, to a certain extent, influenced by the spin polarization of donated electrons, and therefore, the spin configuration of the oxide surface. We employed ab initio molecular dynamics (ai-MD) simulation and a nudged elastic band (NEB) density function theory simulation to make precise analysis on the electron transfer process. The material we chose was nickelous oxide (NiO), which is well known to exhibit an anti-ferromagnetic (af) order below Néel temperature around 523 K and becomes the disordered paramagnetic (pm) state above the temperature. The ai-MD simulation was carried out using Ni24O24 and Ni23O23 supercells with and without Vö(s), respectively, connected to a vacuum cavity placed with 8 O2(g) molecules above NiO slab. In the latter case, Vö(s) was introduced as a Schottky pair to maintain charge neutrality. The ai-MD simulation was made with and without spin polarization corresponding to af- and pm-NiO. The simulation results show that, when a O2(g) molecule approaches to the NiO surface, O2(g) is immediately charged with electrons, primarily owing to the increase of the coordination number of Ni2+/3+ ions, from monodentate to bidentate regardless of the presence of Vö(s) and spin polarization [ii]. No splitting reaction was observed on surface without Vö. The adsorbates further dissociated on the surface of pm-NiO with Vö, but no splitting reaction proceeds on the surface of af-NiO even with the presence of Vö. The NEB simulation suggests that almost monotonous decrease of total energy with the increase of coordination number for pm-NiO, but a thermally activated process with its activation energy of 0.7 eV is necessary for the af-NiO case, because of additional spin dipole moment for the latter. Also observed was spin reversal during the electron transfer. The results indicate significant influence of spin polarization on ORR. A strong effect of af-order on the oxygen chemisorption on undoped and Li-doped NiO was discovered in late 60’s, but no reasonable explanation has been given [iii]. A similar strong influence of spin order of oxide cathode is anticipated in other 3d transition metal oxide systems with ferromagnetic and antiferromagnetic configuration, and the extent of “spin buffering capacity” seems a quite important factor that govern the reactivity of ORR on oxide surface. [i] M. Kukja et al, PCCP, 15(2013), 5443–5471: S. Miyoshi et al., Solid State Ionics, 285(2016), 202–208. [ii] S. Sugiura et al., Solid State Ionics, 285(2016), 209–214: S. Sugiura et al., To be submitted. [iii] E.R.S. Winter, J. of Catalysis 6(2016), 35–49: B. De Rosa et al., Reactivity of Solids 4(1987), 53-72.

Improving 800 nm Triggered Upconversion Emission for Lanthanide-Doped CaF2 Nanoparticles through Sodium Ion Doping

Lanthanide ions (Ln3+)-doped CaF2 luminescent nanoparticles exhibit unique optical properties and represent promising candidates for bioapplications; however, their inherent weak luminescence intensity greatly limits their application, especially for Nd3+-sensitized ones. We have doped Ln3+ ions along with Na+ ion into CaF2 lattice to maintain charge neutrality and investigated the influence of Na+ ion on the 800 nm triggered upconversion process. As compared with Na+-free (shell layer) nanoparticles, the crystallinity of Na+-ion-codoped ones has been improved greatly, and the average lifetimes of the excited Nd3+, Yb3+, and Er3+ states have been prolonged after Na+ ion codoping in the sensitizing shell. Thus the energy-transfer efficiency from sensitizers (Nd3+) to activators (Er3+) has been increased by 67%. As a result, the upconversion luminescence intensity of the nanoparticles with suitable Na+ doping reaches 67 times higher than Na+-free nanoparticles upon 800 nm irradiation, although fewer Ln3+ io...

Layered Metal Chalcophosphate (K-MPS-1) for Efficient, Selective, and ppb Level Sequestration of Pb from Water

Sequestration of heavy metal ions from water is an important issue in chemistry. Herein, we report the detailed Pb2+ adsorption and ion exchange kinetics of a potassium intercalated layered metal thiophosphate, K0.48Mn0.76PS3.H2O (K-MPS-1), which is capable of efficient removal of Pb2+ (>99%) from very dilute concentrations (1–100 ppb). K-MPS-1 can selectively capture Pb2+ even in the presence of other monovalent and divalent cations such as Na+, Ca2+, and Mg2+ with high separation factor. K-MPS-1 can operate within a broad pH range of 2–12 effectively with high distribution coefficient (KdPb) of 5.36 × 105 mL/g, following pseudo second order kinetics. Initially, K+ intercalation in MnPS3 (MPS-1) increases the interlayer spacing and subsequently creates Mn2+ vacancy to maintain charge neutrality and destroy the center of symmetry of the P2S6 unit. Finally, Pb2+ kicks out K+ ions from the interlayer and sits into Mn2+ vacant sites in MPS-1 which further helps to regain the center of symmetry of the structure. K-MPS-1 has achieved Pb2+ removal capacity of 393.5 ± 8 mg/g following Langmuir–Freundlich adsorption model. K-MPS-1 is effective to capture Pb2+ far below the mark of tolerance level (15 ppb, USA-EPA) of drinking water.

Increased luminescence and improved decay kinetics in lithium and cerium co-doped yttrium aluminum garnet scintillators grown by the Czochralski method

In this study, four yttrium aluminum garnet single crystals co-doped with cerium and lithium were produced by the Czochralski method and the scintillation and defect properties were investigated. Our results demonstrated an increase in luminescence with Li co-doping as well as elimination of longer decay times. Surprisingly, although Li is monovalent, no oxidation of cerium from Ce3+ to Ce4+ was found as would be expected to maintain charge neutrality. Additionally, thermoluminescence results indicated a reduction in the trapping of charge carriers by shallow and deep traps, and room temperature photoluminescence measurements showed an improvement in the Ce3+ 5d to 4f transition efficiency.

Temperature dependent structural, vibrational and magnetic properties of K3Gd5(PO4)6.

Herein we report the evolution of the crystal structure of K3Gd5(PO4)6 in the temperature range from 20 K to 1073 K, as observed from variable temperature X-ray diffraction and Raman spectroscopic studies. K3Gd5(PO4)6 has an open tunnel containing a three dimensional structure built by [Gd5(PO4)6]3- ions which in turn are formed of PO4 tetrahedra and GdOn (n = 8 and 9) polyhedra. The empty tunnels in the structure are occupied by K+ ions and maintain charge neutrality in the lattice. Evolution of unit cell parameters with temperature shows a systematic increase with temperature. The average axial thermal expansion coefficients between 20 K and 1073 K are: αa = 10.6 × 10-6 K-1, αb = 5.5 × 10-6 K-1 and αc = 16.4 × 10-6 K-1. The evolution of distortion indices of the various coordination polyhedra with temperature indicates a gradual decrease with increasing temperature, while those of Gd2O9 and K2O8 polyhedra show opposite trends. The overall anisotropy of the lattice thermal expansion is found to be controlled largely by the effect of temperature on GdOn polyhedra and their linkages. Temperature dependent Raman spectroscopic studies indicated that the intensities and wavenumbers of most of the Raman modes decrease continuously with increasing temperature. Anharmonic analyses of Raman modes indicated that the lattice, rigid translation and librational modes have larger contributions towards thermal expansion of K3Gd5(PO4)6 compared to high frequency internal modes. The temperature and field dependent magnetic measurements indicated no long range ordering down to 2 K and the observed effective magnetic moment per Gd3+ ion and the Weiss constant are 7.91 μB and 0.38 K, respectively.

The Role of Zn in Chalcopyrite CuFeS2: Enhanced Thermoelectric Properties of Cu1–xZnxFeS2 with In Situ Nanoprecipitates

Chalcopyrite (CuFeS2) is a widespread natural mineral, composed of earth‐abundant and nontoxic elements. It has been considered a promising n‐type material for thermoelectric applications. In this work, a series of Zn‐doped Cu1–xZnxFeS2 (x = 0–0.1) compounds are synthesized by vacuum melting combined with the plasma activated sintering process. The role of Zn in the chalcopyrite and its different effects on thermoelectric properties, depending on its concentration and location in the crystal lattice, are discussed. It is found that Zn is an effective donor which increases the carrier concentration and improves the thermoelectric properties of CuFeS2. When the content of Zn exceeds the solubility limit, Zn partially enters the Cu sites and forms in situ ZnS nanophase. This, in turn, shifts the balance between the anion and cation species which is re‐established by the formation of antisite Fe/Cu defects. Beyond maintaining charge neutrality of the structure, such antisite defects relieve the lattice strain in the matrix and increase the solubility of Zn further. The highest ZT value of 0.26 is achieved at 630 K for Cu0.92Zn0.08FeS2, which represents an enhancement of about 80% over that of the pristine CuFeS2 sample.

Novel highly ordered core–shell nanoparticles

Core–shell nanoparticles have potential for a wide range of applications due to the tunability of their magnetic, catalytic, electronic, optical, and other physicochemical properties. A frequent drawback in the design of core–shell nanoparticles and nanocrystals is the lack of control over an extensive, disordered, and compositionally distinct interface that occurs due to the dissimilarity of structural and compositional phases of the core and shell. In this work, we demonstrate a new hydrothermal nanophase epitaxy (HNE) technique to synthesize highly structurally ordered α-Cr2O3@α-Co0.38Cr1.62O2.92 inverted core–shell nanoparticles (CSNs) with evidence for the nanoscale growth of corundum structure beginning from the core and extending completely into the shell of the CSNs with minimal defects at the interface. The high-resolution TEM results show a sharp interface exhibiting epitaxial atomic registry of shell atoms over highly ordered core atoms. The XPS and Co K-edge XANES analyses indicate the +2 oxidation state of cobalt is incorporated in the shell of the CSNs. Our XPS and EXAFS results are consistent with oxygen vacancy formation in order to maintain charge neutrality upon substitution of the Co2+ ion for the Cr3+ ion in the α-Co0.38Cr1.62O2.92 shell. Furthermore, the CSNs exhibit the magnetic exchange bias effect, which is attributed to the exchange anisotropy at the interface made possible by the nanophase epitaxial growth of the α-Co0.38Cr1.62O2.92 shell on the α-Cr2O3 core of the nanoparticles. The combination of a well-structured, sharp interface and novel nanophase characteristics is highly desirable for nanostructures having enhanced magnetic properties.