Micro- and nanostructured layered-kagome zinc orthovanadate BaZn3(VO4)2(OH)2.

Pure micro- and nanocrystalline powders of the layered-kagome zinc orthovanadate BaZn3(VO4)2(OH)2 have been successfully prepared and thoroughly characterised. Microstructured samples (BaZn3-MPs) have been produced by hydrothermal reaction using synthetic martyite Zn3V2O7(OH)2·2H2O as the starting reagent. Nanoparticles (NPs) with an average size of ≈ 60 nm (BaZn3-NPs-7h) or ≈ 50 nm (BaZn3-NPs-25min) have been obtained by using a coprecipitation method at ambient pressure, and by varying the stirring time. Rietveld refinements of X-ray diffraction data indicate that micro- and nanostructured BaZn3(VO4)2(OH)2 both crystallize in a R3̄m structure very similar to that of the known layered-kagome compound BaCo3(VO4)2(OH)2. Transmission electron microscopy observation of BaZn3-NPs-7h and BaZn3-NPs-25min reveals crystallized NPs with homogenous distributions of Ba, Zn, and V elements. FT-IR and Raman spectra show subtle differences between micro- and nanostructured samples which cannot be linked to any differences in the average crystal structures. The high resolution 51V MAS NMR spectrum of BaZn3-MPs shows a single isotropic line attributed to VO43- groups with C3v point group. The spectra of the nanostructured samples reveal the presence of a weak additional signal which decreases in intensity with increasing the NPs size, and which has been tentatively assigned to the presence at the surface of the NPs of a small amount of V5+ ions in a different chemical environment. Nanostructuring also impacts the optical properties of BaZn3(VO4)2(OH)2. The UV-vis absorption spectra of NPs exhibit an additional weak transition in the visible domain which is not observed for the microstructured sample.

Microstructured layered-kagome BaCo3(VO4)2(OH)2 with variable crystallite size: alternative synthetic route and comparison with nanostructured samples.

Microplatelets of the layered-kagome compound BaCo3(VO4)2(OH)2, which is the Co2+ analogue of mineral vesignieite BaCu3(VO4)2(OH)2, have been prepared with very high yield by hydrothermal reaction using synthetic karpenkoite Co3V2O7(OH)2·2H2O as starting reagent. The Rietveld refinement of X-ray diffraction data indicates that Co3V2O7(OH)2·2H2O is isostructural with martyite Zn3V2O7(OH)2·2H2O. Two single-phased samples of microstructured BaCo3(VO4)2(OH)2 have been characterized using powder X-ray diffraction, FT-IR and Raman spectroscopies, thermal analyses, scanning electron microscopy, energy-dispersive X-ray spectroscopy and magnetisation measurements. Their crystallite sizes perpendicular to the c-axis are in the range of 92(3) to 146(6) nm and depend on the synthesis conditions. Results have been compared to those previously obtained for quasi-spherical nanoparticles having a crystallite size of the order of 20 nm, to explore the effect of the crystallite size on the properties of BaCo3(VO4)2(OH)2. This study highlights that the magnetic properties depend on the crystallite sizes only at low temperatures.

Composites between Perovskite and Layered Co-Based Oxides for Modification of the Thermoelectric Efficiency.

The common approach to modify the thermoelectric activity of oxides is based on the concept of selective metal substitution. Herein, we demonstrate an alternative approach based on the formation of multiphase composites, at which the individual components have distinctions in the electric and thermal conductivities. The proof-of-concept includes the formation of multiphase composites between well-defined thermoelectric Co-based oxides: Ni, Fe co-substituted perovskite, LaCo0.8Ni0.1Fe0.1O3 (LCO), and misfit layered Ca3Co4O9. The interfacial chemical and electrical properties of composites are probed with the means of SEM, PEEM/XAS, and XPS tools, as well as the magnetic susceptibility measurements. The thermoelectric power of the multiphase composites is evaluated by the dimensionless figure of merit, ZT, calculated from the independently measured electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (λ). It has been demonstrated that the magnitude's electric and thermal conductivities depend more significantly on the composite interfaces than the Seebeck coefficient values. As a result, the highest thermoelectric activity is observed at the composite richer on the perovskite (i.e., ZT = 0.34 at 298 K).

Investigating the Cycling Stability of Fe2WO6 Pseudocapacitive Electrode Materials.

The stability upon cycling of Fe2WO6 used as a negative electrode material for electrochemical capacitors was investigated. The material was synthesized using low temperature conditions for the first time (220 °C). The electrochemical study of Fe2WO6 in a 5 M LiNO3 aqueous electrolyte led to a specific and volumetric capacitance of 38 F g-1 and 240 F cm-3 when cycled at 2 mV·s-1, respectively, associated with a minor capacitance loss after 10,000 cycles. In order to investigate this very good cycling stability, both surface and bulk characterization techniques (such as Transmission Electron Microscopy, Mössbauer spectroscopy, and magnetization measurements) were used. Only a slight disordering of the Fe3+ cations was observed in the structure, explaining the good stability of the Fe2WO6 upon cycling. This study adds another pseudocapacitive material to the short list of compounds that exhibit such a behavior up to now.

Unveiling Pseudocapacitive Charge Storage Behavior in FeWO4 Electrode Material by Operando X-Ray Absorption Spectroscopy.

In nanosized FeWO4 electrode material, both Fe and W metal cations are suspected to be involved in the fast and reversible Faradaic surface reactions giving rise to its pseudocapacitive signature. In order to fully understand the charge storage mechanism, a deeper insight into the involvement of the electroactive cations still has to be provided. The present paper illustrates how operando X-ray absorption spectroscopy is successfully used to collect data of unprecedented quality allowing to elucidate the complex electrochemical behavior of this multicationic pseudocapacitive material. Moreover, these in-depth experiments are obtained in real time upon cycling the electrode, which allows investigating the reactions occurring in the material within a realistic timescale, which is compatible with electrochemical capacitors practical operation. Both Fe K-edge and W L3 -edge measurements point out the involvement of the Fe3+ /Fe2+ redox couple in the charge storage while W6+ acts as a spectator cation. The result of this study enables to unambiguously discriminate between the Faradaic and capacitive behavior of FeWO4 . Beside these valuable insights toward the full description of the charge storage mechanism in FeWO4 , this paper demonstrates the potential of operando X-ray absorption spectroscopy to enable a better material engineering for new high capacitance pseudocapacitive materials.

Copper-Substituted NiTiO3 Ilmenite-Type Materials for Oxygen Evolution Reaction.

Single Ni1-xCuxTiO3 (0.05 ≤ x ≤ 0.2) Ilmenite-type phases were successfully prepared through a solid-state reaction route using divalent metal nitrates as precursors and characterized. Their electrocatalytic performance for oxygen evolution reaction (OER) in alkaline media is presented. The Cu content was determined (0.05 ≤ x ≤ 0.2) by X-ray diffraction. A thorough powder neutron diffraction study was carried out to identify the subtle changes caused by copper substitution in the structure of NiTiO3. The evolution of the optical and magnetic properties with the Cu content was also investigated on the raw micrometer-sized particles. A reduction in particle size down to ≈15 nm was achieved by ball-milling the raw powder prepared by the solid-state reaction. The best catalytic activity for OER was obtained for nanometer-sized particles of Ni0.8Cu0.2TiO3 drop-casted on the Cu plate. For this electrode, a current density of 10 mA cm-2 for oxygen production was generated at 345 and 470 mV applied overpotentials with 1 and 0.1 M NaOH solutions as electrolytes, respectively. The catalyst retained this OER activity at 10 mA cm-2 for long-term electrolysis with a faradic efficiency of 90% for O2 production in a 0.1 M NaOH electrolyte.

Evidence of Wolframite-Type Structure in Ultrasmall Nanocrystals with a Targeted Composition MnWO4.

Here, we report a study of white-ochre powders with targeted composition MnWO4 prepared via a coprecipitation method. Through X-ray total scattering combined with pair distribution function analysis and Rietveld refinement of X-ray diffraction data, we find that their crystal structure is similar to that of bulk-MnWO4, despite a mean crystallite size of 1.0-1.6 nm and a significant deviation of the average chemical composition from MnWO4. The chemical formula derived from elemental and thermogravimetric analyses is Mn0.8WO3.6(OH)0.4·3H2O. X-ray absorption and magnetic susceptibility measurements show that Mn and W have the same oxidation states as in MnWO4. No magnetic ordering or spin glass or superparamagnetic behavior is observed above 2 K, unlike in the case of MnWO4 nanocrystals having a mean size higher than 10 nm.

Phase transitions and magnetic structures in MnW1-x Mo x O4 compounds (x ⩽ 0.2).

Temperature-dependent specific heat, magnetization and neutron diffraction data have been collected in zero magnetic field for polycrystalline samples of MnW1-x Mo x O4 (x ⩽ 0.2) solid solution whose end-member MnWO4 exhibits a magnetoelectric multiferroic phase (AF2 phase) between T 1 ≈ 8 K and T 2 = 12.5 K. In MnW1-x Mo x O4, diamagnetic W(6+) are replaced with diamagnetic Mo(6+) cations and magnetic couplings among Mn(2+) (3d (5), S = 5/2) ions are modified due the doping-induced tuning of the orbital hybridization between Mn 3d and O 2p states. It was observed that magnetic phase transition temperatures which are associated with the second-order AF3-to-paramagnetic (T N) and AF2-to-AF3 (T 2) transitions in pure MnWO4 slightly increase with the Mo content x. Magnetic specific heat data also indicate that the first-order AF1-to-AF2 phase transition at T 1 survives a weak doping x ⩽ 0.05. This latter phase transition becomes invisible above the base temperature 2 K for higher level of doping x ⩾ 0.10. Neutron powder diffraction datasets collected above 1.5 K for a sample of MnW0.8Mo0.2O4 were analyzed using the Rietveld method. The magnetic structure below ≈ 14 K is a helical incommensurate spin order with a temperature-independent propagation vector k = (-0.217(6), 0.5, 0.466(4)). This cycloidal magnetic structure is similar to the polar AF2 structure observed in MnWO4. The AF1 up-up-down-down collinear spin arrangement observed in MnWO4 is absent in our MnW0.8Mo0.2O4 sample.

Incorporation of Jahn-Teller Cu(2+) Ions into Magnetoelectric Multiferroic MnWO4: Structural, Magnetic, and Dielectric Permittivity Properties of Mn1-xCuxWO4 (x ≤ 0.25).

Polycrystalline samples of Mn1-xCuxWO4 (x ≤ 0.5) have been prepared by a solid-state synthesis as well as from a citrate synthesis at moderate temperature (850 °C). The goal is to study changes in the structural, magnetic, and dielectric properties of magnetoelectric type-II multiferroic MnWO4 caused by replacing Jahn-Teller-inactive Mn(2+) (d(5), S = 5/2) ions with Jahn-Teller-active Cu(2+) (d(9), S = 1/2) ions. Combination of techniques including scanning electron microscopy, powder X-ray and neutron diffraction, and Raman spectroscopy demonstrates that the polycrystalline samples with low copper content 0 ≤ x ≤ 0.25 are solid solution that forms in the monoclinic P2/c space group. Rietveld analyses indicate that Cu atoms substitutes for Mn atoms at the Mn crystallographic site of the MnWO4 structure and suggest random distributions of Jahn-Teller-distorted CuO6 octahedra in the solid solution. Magnetic susceptibility reveals that only 5% of Cu substitution suppresses the nonpolar collinear AF1 antiferromagnetic structure observed in pure MnWO4. Type-II multiferroicity survives a weak Cu substitution rate (x < 0.15). Multiferroic transition temperature and Néel temperature increase as the amount of Cu increases. New trends in some of the magnetic properties and in dielectric behaviors are observed for x = 0.20 and 0.25. Careful analysis of the magnetic susceptibility reveals that the incorporation of Cu into MnWO4 strengthens the overall antiferromagnetic interaction and reduces the magnetic frustration.

Pair distribution function and density functional theory analyses of hydrogen trapping by γ-MnO2.

In the presence of "Ag2O" as a promoter, γ-MnO2 traps dihydrogen in its (2 × 1) and (1 × 1) tunnels. The course of this reaction was examined by analyzing the X-ray diffraction patterns of the HxMnO2/"Ag2O" system (0 ≤ x < 1) on the basis of pair distribution function and density functional theory (DFT) analyses. Hydrogen trapping occurs preferentially in the (2 × 1) tunnels of γ-MnO2, which is then followed by that in the (1 × 1) tunnels. Our DFT analysis shows that this process is thermodynamically favorable.

Frustrated magnetism in the S = 1 kagomé lattice BaNi3(OH)2(VO4)2.

Frustrated magnets with integer spin are predicted to have exotic physical properties including spin nematicity, yet few are known to exist. We report a new, frustrated S = 1 magnet, BaNi(3)(OH)(2)(VO(4))(2), which is the structural analogue of the mineral vesignieite. Magnetic frustration arises from a competition between ferromagnetic and antiferromagnetic ordering leading to a glassy transition at 19 K.

DFT-NMR Investigation and (51)V 3QMAS experiments for probing surface oh ligands and the hydrogen-bond network in a polyoxovanadate cluster: the case of Cs(4)[H(2)V(10)O(28)].4H(2)O.

This work shows that the combination of first-principles calculations and (51)V NMR experiments is a powerful tool to elucidate the location of surface hydroxyl groups and to precisely describe the hydrogen bond network in the complex decavanadate cluster Cs(4)[H(2)V(10)O(28)].4H(2)O, enhancing the strength of NMR crystallography. The detailed characterization of H-bond networks for these kinds of inorganic compounds is of primary importance and should benefit from the DFT-NMR predictions by considering explicitly the periodic boundary conditions. The determination of the Cs(4)[H(2)V(10)O(28)].4H(2)O structure by single-crystal X-ray diffraction was not sufficiently accurate to provide the location of protons. From available diffraction data, five different protonated model structures have been built and optimized using DFT-based methods. The possible interconversion of two decavanadate isomers through a proton exchange is evaluated by calculating the energy barrier and recording variable-temperature (1)H MAS NMR spectra. First-principles calculations of (51)V NMR parameters clearly indicate that these parameters are very sensitive to the local intermolecular hydrogen-bonding interactions. Considering the DFT error limits, the fairly good agreement between calculated and experimental NMR parameters arising from the statistical modeling of the data allows the unambiguous assignment of the five (51)V NMR signals and, thus, the location of OH surface ligands in the decavanadate cluster. In particular, first-principles calculations accurately reproduce the (51)V quadrupolar parameters. These results are fully consistent with (51)V 3QMAS NMR spectra recorded with and without (1)H decoupling. Finally, correlations are established between local octahedral VO(6) deformations and (51)V NMR parameters (C(q) and Deltadelta), which will be useful for the characterization of a wide range of chemical species containing vanadium(V).