Constituent Cations Research Articles

Defect Rocksalt Oxides As Cathodes for Rechargeable Magnesium Batteries

Oxide cathode materials are promising candidates for high-energy-density rechargeable magnesium batteries (RMBs) due to their high electrode potential. However, it is still challenging to reversibly insert/extract Mg into/from oxide cathodes since Mg insertion generally induces a partially irreversible phase transformation to a rocksalt structure (rocksalt transformation). This rocksalt transformation generally degrades the cyclability because the densely packed cations in the rocksalt structure impede the Mg migration. We reported that a defect spinel oxide of ZnMnO3 successfully suppressed the rocksalt transformation within a limited capacity of ~70 mAh g-1 [1]. However, the excessive Mg insertion resulted in the rocksalt transformation, limiting the available capacity. Elucidating the activation mechanism of the rocksalt oxide cathodes for RMBs is of great importance in circumventing this issue and ultimately utilizing rocksalt oxides themselves as promising cathode materials for RMBs.To tackle this problem, we designed a disordered-rocksalt oxide of Mg0.35Li0.3Cr0.1Mn0.05Fe0.05Zn0.05Mo0.1O (M7O), in which initially contained Li is extracted in the first charging to yield substantial cation vacancies that facilitate the Mg migration in the structure, eventually activating the reversible Mg insertion/extraction in the subsequent cycles [2]. High configurational entropy derived from many constituent cations also plays a pivotal role. For example, high valence cations of Cr(III) and Mo(IV) do not solely form the rocksalt oxides. However, these elements were successfully incorporated into the structure. Furthermore, the formed vacancies were expected to be stabilized by high configurational entropy during the battery operation.The single-phase oxide of the disordered rocksalt structure was successfully synthesized using the Pechini method. The reversible discharge capacity of ~80 mAh g-1 was constantly observed at a current density of 10.4 mA g-1 at 90 °C. Inductively coupled plasma optical emission spectroscopy confirmed the Li extraction in the 1st charging and subsequent reversible Mg insertion/extraction via the cation vacancies formed by the Li extraction, which was further corroborated by X-ray diffraction. X-ray absorption spectroscopy indicated that Mn, Fe, and Mo compensate for the charge upon charging and discharging by changing their valence states, while those of Cr and Zn hardly changed. The ab initio calculations demonstrated that adjacent two cation vacancies significantly decrease the activation energy of the Mg migration in the rocksalt structure from ~2000 to 800 meV. The composition range to ensure that such a fast diffusion path is thoroughly connected in the particle was evaluated using the percolation theory. The present study opens a new frontier of the oxide cathodes for RMBs.[1] K. Shimokawa et al., Advanced Materials 33, 2007539 (2021). https://onlinelibrary.wiley.com/doi/10.1002/adma.202007539[2] T. Kawaguchi et al., J. Mater. Chem. A (2024). https://pubs.rsc.org/en/content/articlelanding/2024/ta/d3ta07942bImage caption: (Left) Schematic of the reversible Mg insertion/extraction mechanism in the present rocksalt oxide cathode. (Top right) Energy dispersive spectroscopy images of the constituent elements. (Bottom right) Charge/discharge curves of the present cathode material. Figure 1

New ion radii for oxides and oxysalts, fluorides, chlorides and nitrides.

Ion radii are derived here from the characteristic (grand mean) bond lengths for (i) 135 ions bonded to oxygen in 459 configurations (on the basis of coordination number) using 177 143 bond lengths extracted from 30 805 ordered coordination polyhedra from 9210 crystal structures; and (ii) 76 ions bonded to nitrogen in 137 configurations using 4048 bond lengths extracted from 875 ordered coordination polyhedra from 434 crystal structures. There are two broad categories of use for ion radii: (1) those methods which use the relative sizes of cation and anion radii to predict local atomic arrangements; (2) those methods which compare the radii of different cations (or the radii of different anions) to predict local atomic arrangements. There is much uncertainty with regard to the relative sizes of cations and anions, giving rise to the common failure of type (1) methods, e.g. Pauling's first rule which purports to relate the coordination adopted by cations to the radius ratio of the constituent cation and anion. Conversely, type (2) methods, which involve comparing the sizes of different cations with each other (or different anions with each other), can give very accurate predictions of site occupancies, physical properties etc. Methods belonging to type (2) can equally well use the characteristic bond lengths themselves (from which the radii are derived) in place of radii to develop correlations and predict crystal properties. Extensive quantum-mechanical calculations of electron density in crystals in the literature indicate that the radii of both cations and anions are quite variable with local arrangement, suggesting significant problems with any use of ion radii. However, the dichotomy between the experimentally derived ion radii and the quantum-mechanical calculations of electron density in crystals is removed by the recognition that ion radii are proxy variables for characteristic bond lengths in type (2) relations.

Brønsted acidity of bis(trifluoromethanesulfonyl)amide and trifluoromethanesulfonic acid in ionic liquids of ternary ammonium

Bis(trifluoromethanesulfonyl)amide (Tf2N−) and triflate (TfO−) are commonly used anions of protic ionic liquids (PILs). They are the conjugate anions of strong acids, Tf2NH and TfOH, whose Brønsted acidities crucially influence the performance of the PILs. In this study, the Brønsted acidity of Tf2NH and TfOH was compared directly through potentiometric titrations carried out in the ionic liquids of ternary ammonium salts. As a result, Tf2NH exhibits stronger Brønsted acidity than TfOH by approximately 2 pH units. Furthermore, calorimetric titrations revealed that this difference originates from differences in their entropy states. This suggests that the solvation state, rather than the properties in their isolated state, has a more significant impact on the acidities of Tf2NH and TfOH. Besides, ionization thermodynamics turned out to be governed by both the constituent cations and anions of the PILs. Therefore, once the acid-base natures of the constituent ions are characterized, the overall acid-base behavior of the PIL may become predictable.

Nano-Plasticity of an Electrified Ionic Liquid/Electrode Interface: Uncovering Hard-Soft Structuring via Controlled Metal Fill Factor.

Ionic liquids (ILs) nanostructuring at electrified interfaces is of both fundamental and practical interest as these materials are increasingly gaining prominence in energy storage and conversion processes. However, much remains unresolved about IL potential-controlled (re)organization under highly polarized interfaces, mostly due to the difficulty of selectively probing both the distal and proximal surface layers of adsorbed ions. In this work, the structural dynamics of the innermost layer (<10 nm from the surface) were independently interrogated from that of the ionic layers in the sub-surface region (>100 nm from the surface), using an infrared (IR) spectroscopy approach. By tuning the metal fill factor of gold films deposited on conductive metal oxide-modified IR internal reflection elements, the charge-driven (re)structuring of the inner and distal layers of 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate is unveiled. Within a relatively wide potential region (∼±1 V) bounding the potential of zero charges, the ionic liquid is shown to undergo a reversible (i.e., soft) reorganization whereby the innermost layer of anions (cations) is exchanged by a layer of cations (anions). Kinetically unhindered changes in the number density of constituent cations and anions largely follow electrostatic expectations in the subsurface region, whereas the innermost layer exhibits a pronounced hysteresis and very slow relaxation. Under larger negative potential bias, IL restructuring is characterized by a highly irreversible (i.e., hard) and intense interfacial densification of the BMPy+ cations, consistent with the formation of nanoscale segregated liquids. The outcomes of this work reveal a plastic IL nanostructuring under a strong electric field.

Electrocatalytic Activities of High-Entropy Oxides for the Oxygen Evolution Reaction

Electrocatalytic water-splitting hydrogen generation consists of the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER), where the four-electron-relevant OER is the rate-determining step. So far, there have been many efforts to substitute for the highly expensive noble-metal electrocatalysts (platinum, ruthenium or rhodium oxides, etc.). Transition-metal oxides based on Co, Ni, Mn, and V have been suggested as such alternatives, due to their low cost, high efficiency, and high stability. Recently, since the compositional diversity can provide a new breakthrough in that area, a high-entropy oxide (HEO) with five transition-metal cations has been suggested as a promising electrocatalyst toward the OER. In our studies, two kinds of HEOs were prepared and their OER activities were investigated. To begin with, for the (Mg0.2Fe0.2Co0.2Ni0.2Cu0.2)O, the effect of constituent cations on the OER activity was unveiled. Furthermore, a core cation driving the high OER activity was found. For it, the medium-entropy oxides (MEOs) with four cations are prepared by subtracting each cation (Mg, Fe, Co, Ni, or Cu) from the HEO, exhibiting homogeneous morphology, equiatomic composition, and single-phase rocksalt structure. As a result, it is found that the highest concentration of Co3+ in the MEO (w/o Cu) leads to the best OER activity, and thus Co3+ is the core ion driving the high OER activity. Furthermore, it is regarded that Cu2+ ions prevent the conversion of Co or Fe cations from 2+ to 3+ in the HEO and MEOs. Accordingly, maximizing the concentration of Co3+ within electrocatalysts is suggested as an effective design strategy for the high-efficiency electrocatalysts based on high or medium entropy materials. Secondly, the relationship between structure and OER activity was elucidated for the (Mg0.2Fe0.2Co0.2Zn0.2Cu0.2)O with a temperature-dependent rocksalt-to-spinel transition.

In situ synthesis of extremely small, thermally stable perovskite nanocatalysts for high-temperature electrochemical energy devices

High-temperature solid oxide cells (SOCs) offer one of the most efficient and versatile routes for producing electric power and H2. However, the practical use of nanomaterials in SOCs has been limited by their lack of thermal stability. In this study, we present an infiltration technique that enables the in situ synthesis of extremely small, thermally stable perovskite (Sm0.5Sr0.5)CoO3 nanocatalysts on the inner surface of porous SOC electrodes. We identified certain impurity phases, such as SrCO3, that cause fatal degradation and eliminated them using a rational complexation strategy optimized for individual constituent cations. Consequently, we fabricated ∼ 20 nm diameter, highly pure, single-phase nanocatalysts that achieved more than double the performance of a cell with standard (La,Sr)(Co,Fe)O3– and (La,Sr)CoO3-based air electrode. The cells stably operated during long-term tests in both the power generation and H2 production modes, with negligible degradation. Furthermore, we successfully scaled up this process to fabricate large-scale commercial cells using a fully automated process. The key findings of this study will resolve critical barriers with high-temperature nanomaterials and accelerate the commercialization of SOC technology.

Enhancing Water Oxidation Catalysis by Controlling Metal Cation Distribution in Layered Double Hydroxides

AbstractThe sluggish kinetics of the oxygen evolution reaction (OER), the limiting step of the electrochemical water splitting process, hinders the eventual commercialization of this important renewable energy strategy. Hence, the development of efficient electrocatalysts for this reaction is crucial. Multi‐metal‐based (hydr)oxides are promising OER electrocatalysts because the electronic interactions between multiple constituent metal cations can potentially enhance electrochemical performances. However, complex compositions may not always lead to positive synergistic effects. The appropriate distribution of the cations is also critical. However, the high dispersibility of cations in these hydroxides renders the control of their distribution challenging. Herein, an approach is reported to control the metal cation distribution in layered double hydroxides (LDHs) to improve their OER performances. Restacking of exfoliated NiFe and CoAl LDH nanosheets leads to electrochemical synergistic effects between different nanosheets. As far as it is known, the restacked LDH described herein exhibits the lowest overpotential (224 mV) and Tafel slope (34.26 mV dec−1) among reported powder‐type (hydr)oxide and alloy OER electrocatalysts with more than three different metal cations. Thus, a new design approach is suggested to enhance the electrochemical performances of LDHs.

Extractive Spectrophotometric Determination and Theoretical Investigations of Two New Vanadium(V) Complexes.

Two new vanadium (V) complexes involving 6-hexyl-4-(2-thiazolylazo)resorcinol (HTAR) and tetrazolium cation were studied. The following commercially available tetrazolium salts were used as the cation source: tetrazolium red (2,3,5-triphenyltetrazol-2-ium;chloride, TTC) and neotetrazolium chloride (2-[4-[4-(3,5-diphenyltetrazol-2-ium-2-yl)phenyl]phenyl]-3,5-diphenyltetrazol-2-ium;dichloride, NTC). The cations (abbreviated as TT+ and NTC+) impart high hydrophobicity to the ternary complexes, allowing vanadium to be easily extracted and preconcentrated in one step. The complexes have different stoichiometry. The V(V)-HTAR-TTC complex dimerizes in the organic phase (chloroform) and can be represented by the formula [(TT+)[VO2(HTAR)]]2. The other complex is monomeric (NTC+)[VO2(HTAR)]. The cation has a +1 charge because one of the two chloride ions remains undissociated: NTC+ = (NT2+Cl-)+. The ground-state equilibrium geometries of the constituent cations and final complexes were optimized at the B3LYP and HF levels of theory. The dimer [(TT+)[VO2(HTAR)]]2 is more suitable for practical applications due to its better extraction characteristics and wider pH interval of formation and extraction. It was used for cheap and reliable extraction-spectrophotometric determination of V(V) traces in real samples. The absorption maximum, molar absorptivity coefficient, limit of detection, and linear working range were 549 nm, 5.2 × 104 L mol-1 cm-1, 4.6 ng mL-1, and 0.015-2.0 μg mL-1, respectively.

Effects of Physicochemical Properties of Constituent Ions of Ionic Liquid on Its Permeation through a Silicone Membrane.

Ionic liquids (ILs), defined as liquid salts composed of anions and cations, have the advantage of allowing constituent ions to be stably absorbed through biological membranes, such as skin. However, limited information is currently available on the effects of the physicochemical properties of constituent ions on the membrane permeation of ILs. Therefore, we herein investigated the effects of the polarity of constituent cations on the membrane permeation of each constituent ion from IL. Various ILs were prepared by selecting lidocaine (LID) as a cation and a series of p-alkylbenzoic acids with different n-octanol/water partition coefficients (Ko/w) as anions. These ILs were applied to a skin model, a silicone membrane, and membrane permeability was investigated. The membrane permeabilities of p-alkylbenzoic acids from their single aqueous suspensions were also measured for comparison. The membrane permeability of p-alkylbenzoic acid from the aqueous suspension increased at higher Ko/w. However, the membrane permeability of ILs was similar regardless of the Ko/w of the constituent p-alkylbenzoic acid. Furthermore, the membrane permeability of the counterion LID remained unchanged regardless of the constituent p-alkylbenzoic acid. These results suggest that even when the Ko/w of IL constituents markedly differs, the resulting IL does not affect membrane permeability.

Effect of constituent cations on the electrocatalytic oxygen evolution reaction in high-entropy oxide (Mg0.2Fe0.2Co0.2Ni0.2Cu0.2)O

To find core cations driving a high electrocatalytic activity in high-entropy oxide (HEO), the medium-entropy oxides (MEOs) subtracting each cation from the HEO are prepared and evaluated toward the alkaline water-splitting oxygen evolution reaction (OER) activity. • The core cations driving a high electrocatalytic activity in the high-entropy oxide are identified. • The high and medium-entropy oxides by subtracting each cation from the HEO are prepared. • It is found that Co 3+ is the core ion driving the high OER activity. • It is regarded that Cu 2+ ions prevent the conversion of Co or Fe cations from 2 + to 3 + . • Maximizing the concentration of Co 3+ within electrocatalysts is an effective design strategy. The electrocatalytic electrode kinetics should be controlled to overcome overpotentials for the hydrogen generation via water electrolysis, which consists of the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER). Although transition metal catalysts that can replace noble metal catalysts are presented, more studies are still required in terms of stability and activity. Since the compositional diversity can provide a new breakthrough in that area, a high-entropy oxide (HEO) with five cations, (Mg 0.2 Fe 0.2 Co 0.2 Ni 0.2 Cu 0.2 )O, is applied as a promising electrocatalyst toward the OER in this study. For the HEO electrocatalysts, unveiling the effect of constituent cations on the OER activity and finding core cations driving the high OER activity come as a major issue. For it, in this work, the medium-entropy oxides (MEOs) with four cations are prepared by subtracting each cation (Mg, Fe, Co, Ni, or Cu) from the HEO, exhibiting homogeneous morphology, equiatomic composition, and single-phase rocksalt structure. As a result, it is found that the highest concentration of Co 3+ in the MEO (w/o Cu) leads to the best OER activity, and thus Co 3+ is the core ion driving the high OER activity. Furthermore, it is regarded that Cu 2+ ions prevent the conversion of Co or Fe cations from 2 + to 3 + in the HEO and MEOs. Accordingly, maximizing the concentration of Co 3+ within electrocatalysts is suggested as an effective design strategy for the high-efficiency electrocatalysts based on high or medium entropy materials.

Peculiarities of the dependences of the dielectric properties of solid solutions of multicomponent systems on the electronegativity of their constituent cations

Solid solutions (SS) of 3- and 4-component systems based on lead titanate-zirconate were prepared by the method of solid-phase reactions and uniaxial hot pressing. The dependences of the relative permittivity of polarized samples on the electronegativity (EN) of their constituent cations have been studied. The ferro-hardness of the SS (the stability of the domain structure to external influences) is shown to be directly dependent on the EN of elements B in the corresponding oxidation states, i.e., the degree of covalence of the B–O bond. The deviation from this dependence in SS with Ni and Cd is explained by their individual features, which result in changes in the degree of bond covalence in both cationic sublattices. The conducted crystal-chemical analysis made it possible to choose promising SS when creating ferroelectric materials, including textured piezoelectric ceramic materials for piezoelectric transducers for various purposes: Piezotransformers, piezoelectric motors, ultrasonic emitters, filter devices, ultrasonic flaw detectors, accelerometers, etc.

Ionic Dynamics and Vibrational Spectral Diffusion of a Protic Alkylammonium Ionic Salt through Intrinsic Cationic N-H Vibrational Probe from FPMD Simulations.

We employed density functional theory (DFT)-based molecular dynamics simulations to explore the structure, dynamics, and spectral properties of the protic ionic entity trimethylammonium chloride (TMACl). Structural investigations include calculating the site-site radial distribution functions (RDFs), the distribution of constituent cations and anions in three-dimensional space, and combined distribution functions of the hydrogen-bonded pair RDF versus angle, revealing the structural characteristics of the ionic solvation and the intermolecular interactions within ions. Further, we determined the instantaneous vibrational stretching frequencies of the intrinsic N-H stretch probe modes by applying the time-series wavelet method. The associated ionic dynamics within the protic ionic compound were investigated by calculating the time-evolution of the fluctuating frequencies and the frequency-time correlation functions (FFCFs). The time scale related to the local structural relaxation process and the average hydrogen bond lifetime, ion cage dynamics, and mean squared displacement were investigated. The faster decay component of the FFCFs, depicting the intermolecular motion of intact hydrogen bonds in TMACl, is 0.07 ps for the Perdew-Burke-Ernzerhof (PBE)-based simulation and 0.06 ps for the PBE-D2 representation. The slower time scale of the longer picosecond decay time component of PBE and PBE-D2 representations are 3.13 and 2.87 ps, respectively. These picosecond time scales represent more significant fluctuations of the hydrogen-bonding partners in the ionic entity and hydrogen-bond jump events accompanied by large angular jumps. The longest picosecond time scales represent structural relaxation, including large angular jumps and ion-pair dynamics. Also, ion cage lifetimes correlate with the slowest time scale of the associated dynamics of vibrational spectral diffusion despite the type of DFT functional. This study benchmarks DFT treatments of the exchange-correlation functional with and without the van der Waals (vdW) dispersion correction scheme. The inclusion of vdW interactions to the PBE functional represents a less structured state of the ionic entity and faster dynamics of the molecular motions relative to the one predicted by the PBE system. All the results illustrate the necessity of accurately describing the Coulomb interactions, vdW dispersive interactive forces, and localized hydrogen bonds required to sustain the energetic balance in this ionic salt.

Fabrication of ionic liquid-functionalized polystyrene nanospheres via subsurface-initiated atom transfer radical polymerization for anti-fouling application

The chemical and biological characteristics of ionic liquids (IL) can be engineered via a suitable selection of their constituent anions and cations, which is instrumental in the research and the development of anti-biofouling materials. The ring-opening reaction and subsurface-initiated atom transfer radical polymerization (sSI-ATRP) were employed to prepare IL-functionalized polystyrene nanospheres effectively. The sSI-ATRP was used to graft the poly(glycidyl methacrylate) (PGMA) brushes onto crosslinked polystyrene (CPS) nanospheres while they were fully swollen. Subsequently, the imidazolium-based IL was bound onto PGMA brushes through ring-opening reaction between the epoxide groups of PGMA and the amine groups of IL. Furthermore, their anti-wear, antibacterial, and antifouling properties were comprehensively evaluated. Experimental results reveal that the as-prepared IL-functionalized PS nanospheres exhibited good antibacterial properties against both E. coli and S. aureus, attributable to the imidazole groups of IL, in addition to resisting adhesion of Porphyridium and Dunaliella. Moreover, the IL-functionalized CPS nanospheres manifested satisfactory wear resistance as against the traditional surface-initiated atom transfer radical polymerization (SI-ATRP), because the PGMA can graft on the surface and subsurface of the polystyrene nanospheres via sSI-ATRP. The as-obtained IL-functionalized polystyrene nanospheres can be mixed effectively with the self-polishing resin to acquire a novel self-polishing nanocomposite coating, the nanocomposite coating exhibit good antibacterial characteristics against E. coli and S. aureus, as well as a substantial antifouling effect against microalgae (83 % Porphyridium and 85 % Dunaliella were removed).

New insights into Mn2+ and Mg2+ inhibition of calcite growth

Impurity ion and isotope partitioning into carbonate minerals provide a window into the molecular processes occurring at the fluid-mineral interface during crystal growth. Here, we employ calcium isotope fractionation together with process-based modeling to elucidate the mechanisms by which two divalent cations with starkly contrasting compatibility, magnesium and manganese, inhibit calcite growth and incorporate into the mineral lattice. Calcite growth inhibition by Mg2+ is log-linear and KMg is on the order of 0.02–0.03 throughout the range of {Mg2+}/{Ca2+} studied here (0.01–2.6). Mn2+ exhibits much stronger log-linear growth rate inhibition at low Mn2+ concentrations (fluid {Mn2+}/{Ca2+} = 0.001–0.02). Mn2+ is readily incorporated into the calcite lattice to form a calcite-rhodochrosite solid solution, with large partition coefficients (KMn 4.6–15.6) inversely correlated to growth rate. For both Mn2+ and Mg2+, calcium isotope fractionation is found to be invariant with {Me2+}/{Ca2+} despite more than an order of magnitude decline in growth rate. This invariant Δ44/40Ca suggests that the presence of Mn2+ or Mg2+ does not significantly change the relative rates of Ca2+ attachment and detachment at kink sites during growth, indicative of a dominantly kink blocking inhibition mechanism. Because the partitioning behavior dictates that Mn2+ must attach to the surface significantly faster than Ca2+, attachment of Mn2+ is likely to be as a non-monomer species such as an ion pair or possibly a larger polynuclear cluster. We propose that calcite growth rate inhibition by Mn is determined by the kinetics of carbonate attachment at Mn-occupied kink sites, potentially due to slow re-orientation kinetics of carbonate ions that have formed an inner-sphere complex with Mn2+ at the surface but must reorient to incorporate into the lattice. We demonstrate that patterns in Mg2+ partitioning and inhibition behavior are broadly consistent with growth inhibition driven by slow Mg2+-aquo complex dehydration relative to Ca2+ but argue that this mechanism likely represents one endmember scenario, seen in Mg-calcite growth at low supersaturations and net precipitation rates. During growth at faster net precipitation rates, some portion of Mg2+ is likely incorporated as a partially hydrated or otherwise complexed species, but calcite growth remains significantly inhibited by the kinetics of CO32− attachment at Mg2+ kink sites. These findings suggest a hybrid classical/nonclassical growth mechanism whereby Ca2+ incorporates largely as a free ion at kink sites while Mn2+ and some portion of Mg2+ are incorporated via non-monomer attachment. This pattern may be generalizable; trace constituent cations with aquo-complex desolvation rates significantly slower than the mineral growth rate preferentially incorporate as a non-monomer species during otherwise classical crystal growth.

An Observation Related to the Pressure Dependence of Ionic Radii

Here it is shown that the crystal radii of ions are represented by a simple relation rcryst = rB3√(10 m)/N, where m and N are small integer numbers determined by the principal and orbital quantum numbers and valence, and rB is the Bohr radius. The relation holds to within 5%. This finding elucidates that despite their original definition crystal- and ionic radii are not classical but represent the limiting case of spherically symmetric spatial averages of the valence electron states and, therefore, are able to reflect changes in the valence electron configuration with pressure and temperature. The relation is used to show general pressure-effects on the radii, in particular the increase of bond coordination with pressure and metallization as limiting state. The pressure-effect is exemplified for the elements Mg and Si as major constituent cations in the Earth’s mantle, and for Ba as a large ionic lithophile element. It is found that at least to about 140 GPa the radii depend linearly on pressure. Further, if a generalization is permitted for just three elements, the pressure-dependence is lesser the higher the charge of the ion. The three elements exhibit a much weaker pressure-dependence than previously calculated non-bonding radii. For mantle geochemistry this finding implies that elements incompatible in the upper mantle remain so for the main lower mantle minerals bridgmanite and periclase and are hosted by davemaoite.

Ammonia gas sensing and magnetic permeability of enhanced surface area and high porosity lanthanum substituted Co–Zn nano ferrites

This work reports magnetic permeability and ammonia gas sensing characteristics of La3+ substituted Co–Zn nano ferrites possessing chemical formula Co0.7Zn0.3LaxFe2-2xO4 (x = 0–0.1) synthesized by a sol-gel route. Refinement of X-ray diffraction (XRD) patterns of the ferrite powders by the Rietveld technique has revealed the creation of single-phase spinel structure. The tenancy of constituent cations at tetrahedral/octahedral sites was obtained from the refinement of XRD. The crystallite sizes calculated from the W–H method vary from 20 to 24 nm. The scanning electron microscope (SEM) profiles of the ferrite samples were analyzed for the morphological details. The energy dispersive X-ray analysis (EDAX) patterns of the samples were obtained to test the elemental purity of the ferrites within their stoichiometry. The transmission electron microscope (TEM) image of the ferrite (x = 0.1) exhibits the spherical and oval shaped particles with a mean size of 20 nm. Fourier transform infra-red (FTIR) spectra were analyzed to confirm the superseding of La3+ cations at octahedral sites. The Brunauer-Emmett-Teller (BET) analysis of nitrogen adsorption-desorption isotherms of the ferrites was performed to investigate the porous structure and to determine the surface area of the nanocrystalline ferrites. The oxidation states of the constituent ions were confirmed by means of X-ray photoelectron spectroscopy (XPS). The complex permeability as a function of frequency was studied to explore the effects of structural parameters on the magnetic behaviour of the ferrites. Analysis of gas sensing properties of the ferrites have proved that the Co–Zn–La ferrite with controlled La composition can be utilized as an effective ammonia gas sensing material in commercial gas sensors.

Copper-based kesterite thin films for photoelectrochemical water splitting

Abstract Copper kesterite Cu2ZnSnS4 is a promising photoabsorber material for solar cells and photoelectrochemical (PEC) water splitting. In this article, we will first review the crystallographic/energetic structures of Cu2ZnSnS4 in view of its applications to sunlight conversion devices. Then, historical progress in photovoltaic properties of Cu2ZnSnS4-based solar cells is introduced. Finally, studies on PEC H2 evolution over Cu2ZnSnS4-based photocathodes are reviewed in detail. For realizing efficient PEC H2 evolution, surface modifications with an n-type buffer layer (such as CdS) and a catalytic site (such as Pt nanoparticles) were found to be indispensable. Since these surface-modified photocathodes had poor resistances under an operating bias due to the occurrence of oxidative photocorrosion of the CdS layer and elimination of the Pt catalysts, coverage with a protection layer was required to improve the long-term durability. Moreover, partial or complete substitution of the constituent cations with some cations was proved to be effective for improving PEC properties. Although recent studies showed a rapid increase in PEC properties, there is room for further development of PEC properties by using effective combinations among surface protection(s), defect engineering(s), and band engineering(s).

Aggregation of Halloysite Nanotubes in the Presence of Multivalent Ions and Ionic Liquids.

Colloidal stability was investigated in two types of particle systems, namely, with bare (h-HNT) and polyimidazolium-functionalized (h-HNT–IP-2) alkali-treated halloysite nanotubes in solutions of metal salts and ionic liquids (ILs). The valence of the metal ions and the number of carbon atoms in the hydrocarbon chain of the IL cations (1-methylimidazolium (MIM+), 1-ethyl-3-methylimidazolium (EMIM+), 1-butyl-3-methylimidazolium (BMIM+), and 1-hexyl-3-methylimidazolium (HMIM+)) were altered in the measurements. For the bare h-HNT with a negative surface charge, multivalent counterions destabilized the dispersions at low values of critical coagulation concentration (CCC) in line with the Schulze–Hardy rule. In the presence of ILs, significant adsorption of HMIM+ took place on the h-HNT surface, leading to charge neutralization and overcharging at appropriate concentrations. A weaker affinity was observed for MIM+, EMIM+, and BMIM+, while they adsorbed on the particles to different extents. The order HMIM+ < BMIM+ < EMIM+ < MIM+ was obtained for the CCCs of h-HNT, indicating that HMIM+ was the most effective in the destabilization of the colloids. For h-HNT–IP-2 with a positive surface charge, no specific interaction was observed between the salt and the IL constituent cations and the particles, i.e., the determined charge and aggregation parameters were the same within experimental error, irrespective of the type of co-ions. These results clearly indicate the relevance of ion adsorption in the colloidal stability of the nanotubes and thus provide useful information for further design of processable h-HNT dispersions.

Effect of chemical composition on mass transfer in Y&lt;sub&gt;2&lt;/sub&gt;Ti&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;7&lt;/sub&gt; under oxygen potential gradient at high temperatures

The oxygen shielding properties and structural stability of polycrystalline Y2Ti2O7 (YT) solid solution wafers, which were cut from a sintered body and served as models for environmental barrier coatings, were evaluated by the oxygen permeation technique at high temperatures. Here, we examined the effect of compositions by using two samples with slightly different Y/Ti molar ratios. The oxygen permeability constants for YT increased dramatically with a slight increment in the Y/Ti molar ratio, and the activation energy for oxygen permeation markedly decreased. Oxygen permeation in the YT was controlled by the diffusion of oxide ions in the crystal. Almost no decomposition of the YT phase occurred due to migration of the constituent cations at high temperatures, indicating high structural stability under an oxygen potential gradient. The crystal structure was identified as the YT-based pyrochlore structure from Rietveld refinements of synchrotron radiation X-ray powder diffraction data, based on information on the composition dependence of the defect concentrations in the crystals estimated by first principles calculations. The diffusion path for oxide ions in the crystal at high temperature was also investigated based on a spatial distribution map described by the bond valence sum method. Finally, the mechanisms for the uptake of oxygen molecules on the YT surface exposed to a high oxygen partial pressure and the subsequent diffusion of oxide ions in the YT crystals were clarified.

Role of ferrite phase on the structural, ferroelectric and magnetic properties of (1-x) BCT-x CZFO composites

Polycrystalline composite samples of (1-x) (Ba0.8Ca0.2TiO3)–x(Co0.6Zn0.4Fe2O4) (with x = 0.00, 0.01, 0.02, 0.03, 0.04, and 1.00) hereby designated as BZT-CZFO were synthesized via wet chemical method. Room temperature X-ray diffraction confirmed the bi-phase nature of the composites. This was further confirmed by Raman scattering recorded at room temperature. No intermediate or secondary phases were observed for all the compositions. Increasing ferrite concentration decreases the ferroelectric properties of the compounds and the leakage current measurement value was found to obey Ohmic conduction mechanism for all the samples. X-ray photoelectron spectroscopy measurement was carried out to investigate the chemical state of the constituent cations present and it was observed that Ti/Co/Ba retained their characteristic oxidation states, while mixed state was observed for Fe-ions. With increasing ferrite concentration saturation magnetization (Ms) were enhanced and a maximum value achieved for x = 0.04 concentration sample, but remained lower than pure CZFO sample.