Nanofeather ruthenium nitride electrodes for electrochemical capacitors.

Fast charging is a critical concern for the next generation of electrochemical energy storage devices, driving extensive research on new electrode materials for electrochemical capacitors and micro-supercapacitors. Here we introduce a significant advance in producing thick ruthenium nitride pseudocapacitive films fabricated using a sputter deposition method. These films deliver over 0.8 F cm-2 (~500 F cm-3) with a time constant below 6 s. By utilizing an original electrochemical oxidation process, the volumetric capacitance doubles (1,200 F cm-3) without sacrificing cycling stability. This enables an extended operating potential window up to 0.85 V versus Hg/HgO, resulting in a boost to 3.2 F cm-2 (3,200 F cm-3). Operando X-ray absorption spectroscopy and transmission electron microscopy analyses reveal novel insights into the electrochemical oxidation process. The charge storage mechanism takes advantage of the high electrical conductivity and the morphology of cubic ruthenium nitride and Ru phases in the feather-like core, leading to high electrical conductivity in combination with high capacity. Accordingly, we have developed an analysis that relates capacity to time constant as a means of identifying materials capable of retaining high capacity at high charge/discharge rates.

Antipolar 2D Metallicity with Tunable Valence Wx+ (5 ≤ x ≤ 5.6) in the Layered Monophosphate Tungsten Bronzes [Ba(PO4)2]WmO3m-3.

The newly discovered series of layered monophosphate tungsten bronzes (L-MPTB) [Ba(PO4)2]WmO3m-3 consist of m-layer-thick slabs of WO6 octahedra separated by barium-phosphate spacers. They display a 2D metallic behavior confined in the central part of the perovskite slabs. Here, we report the missing m = 2 member of this series, containing the rather uncommon W5+ oxidation state. We have analyzed its structure-property relationships in relation to the other members of the L-MPTB family. In particular, we have determined its crystal structure by means of single-crystal X-ray and electron diffraction and investigated its physical properties from resistivity, Seebeck-coefficient and heat-capacity measurements combined with first-principles calculations. All the L-MPTB compounds show metallic behavior down to 1.8 K without any clear charge-density-wave (CDW) order. The m = 2 member, however, displays an increased influence of the spacer that translates into anisotropic negative thermal expansion, reversed thermopower and reversed crystal-field splitting of the tungsten t2g orbitals. Our analysis of the full [Ba(PO4)2]WmO3m-3 series reveals a systematic and significant W off-centering in their octahedral coordination. We identify the resulting anti-polar character of these W displacements as the crucial aspect behind the 2D metallicity of these systems: It leads to the presence of bound charges whose screening determines the distribution of mobile charges, tending to accumulate at the center of the [WmO3-m] block. We argue that this mechanism is analogous to enhanced conductivity observed for charged domain walls in ferroelectrics, thus providing a general design rule to promote 2D metallicity in layered systems.

High Capacitance Porous Ruthenium Nitride Films with High Rate Capability for Micro-Supercapacitors.

The demand for high-performance energy storage devices to power Internet of Things applications has driven intensive research on micro-supercapacitors (MSCs). In this study, RuN films made by magnetron sputtering as an efficient electrode material for MSCs are investigated. The sputtering parameters are carefully studied in order to maximize film porosity while maintaining high electrical conductivity, enabling a fast charging process. Using a combination of advanced techniques, the relationships among the morphology, structure, and electrochemical properties of the RuN films are investigated. The films are shown to have a complex structure containing a mixture of crystallized Ru and RuN phases with an amorphous oxide layer. The combination of high electrical conductivity and pseudocapacitive charge storage properties enabled a 16 µm-thick RuN film to achieve a capacitance value of 0.8 F cm-2 in 1 m KOH with ultra-high rate capability.

Ni4Nb1.8W0.1Ti0.1O9: Stabilization of the I-Type Ni4Nb2O9 Structure and First Magnetic Study of This Corundum-like Compound.

For the first time, we report on the structural and magnetic properties of a polycrystalline sample of Ni4Nb2O9 from I-type (Fdd2), obtained by the partial cosubstitution of Nb5+ by Ti4+ and W6+. The crystal structure is investigated by combining synchrotron X-ray, neutron, and electron diffraction at room temperature. This I-type structure is derived from the corundum-like Ni4Nb2O9 II-type (Pbcn) and is noncentrosymmetric and polar. The Ni-lattice is composed of the stacking of distorted honeycomb layers with double zigzag ribbons 60° disoriented from each other in two successive double layers. The connection between layers is ensured by the sharing of octahedra faces building (Nb,W,Ti)2O9, Ni2O9, and Ni3O12 units. This study shows how the disruption of the Nb2O6 units by smaller d0 cations impacts the structure, significantly modifying the Ni network and, thus, the magnetic properties. The latter were studied by dc- and ac-magnetic susceptibility, and the magnetic structure was solved by neutron powder diffraction. A ferrimagnetic behavior occurs below 68 K, followed by a re-entrant spin-glass-like behavior below ≈50 K.

Layered Monophosphate Tungsten Bronzes [Ba(PO4 )2 ]Wm O3m-3 : 2D Metals with Locked Charge-Density-Wave Instabilities.

Phosphate tungsten and molybenum bronzes represent an outstanding class of materials displaying textbook examples of charge-density-wave (CDW) physics among other fundamental properties. Here we report on the existence of a novel structural branch with the general formula [Ba(PO4 )2 ][Wm O3m-3 ] (m=3, 4 and 5) denominated 'layered monophosphate tungsten bronzes' (L-MPTB). It results from thick [Ba(PO4 )2 ]4- spacer layers disrupting the cationic metal-oxide 2D units and enforcing an overall trigonal structure. Their symmetries are preserved down to 1.8 K and the compounds show metallic behaviour with no clear anomaly as a function of temperature. However, their electronic structure displays the characteristic Fermi surface of previous bronzes derived from 5d W states with hidden nesting properties. By analogy with previous bronzes, such a Fermi surface should result into CDW order. Evidence of CDW order was only indirectly observed in the low-temperature specific heat, giving an exotic context at the crossover between stable 2D metals and CDW order.

Origin of Luminescence in La2MoO6 and La2Mo2O9 and Their Bi-Doped Variants.

In this article, lanthanum molybdenum oxides (La2MoO6 and La2Mo2O9) and their Bi-doped derivatives were investigated as potential rare-earth-free phosphors. An X-ray diffraction analysis coupled with an EDX study confirmed the purity of the samples and the insertion of bismuth in a 1 molar % amount. Kubelka-Munk-transformed reflectance spectra clearly indicated that the insertion of Bi induces a shortening of the optical gap in La2MoO6 but has no impact on that of La2Mo2O9. Moreover, excitation and emission spectra evidenced a strong temperature quenching effect in all materials. Also, the CIEx,y parameters at 77 K are almost identical with or without Bi doping for the two host lattices. Clearly, it was shown, by combining experimental data, ab initio calculations, and the empirical positioning of absorption bands that the luminescence of the Bi-doped La2MoO6 sample is mainly related to the host lattice itself and distortions induced by La/Bi substitution. The role of the Bi3+ dopant is indirect, and the luminescence is mainly due to a Mo-O charge transfer rather than an on-site Bi3+ 3P1,0 → 1S0 transition. Concerning La2Mo2O9, there is no effect following the insertion of Bi, implying that the role of Bi is insignificant.

Oxysulfide Ba5(VO2S2)2(S2)2 Combining Disulfide Channels and Mixed-Anion Tetrahedra and Its Third-Harmonic-Generation Properties.

Mixed-anion compounds are among the most promising systems to design functional materials with enhanced properties. In particular, heteroleptic environments around transition metals allow tuning of the polarity or band-gap engineering for instance. We present the original oxysulfide Ba5(VO2S2)2(S2)2, the fifth member in the quaternary system Ba-V-S-O. It exhibits the mixed-anion building units V5+O2S2 and isolated disulfide pairs (S2)2-. The structure is solved by combining single-crystal and powder X-ray diffraction and transmission electron microscopy. First-principles calculations were combined in order to highlight the anion roles. In particular, our density functional theory study shows that the 3p states of the disulfide pairs dictate the band gap. In this study, we point out anionic tools for band-gap engineering that can be useful for the design of phases for numerous applications. Finally, third harmonic generation (THG) was measured and compared to the large THG observed for Cu2O, which reveals the potential for nonlinear-optical properties that should be further investigated.

Pure and RE3+-Doped La7O6(VO4)3 (RE = Eu, Sm): Polymorphism Stability and Luminescence Properties of a New Oxyvanadate Matrix.

Two polytypes of the new oxyvanadate matrix La7O6(VO4)3 were identified and deeply characterized. The crystal structure of the α-polytype was solved using a combination of precession electron diffraction and powder X-ray diffraction (XRD) techniques. It crystallizes in a monoclinic unit cell with space group P21, a = 13.0148(3) Å, b = 19.1566(5) Å, c = 7.0764(17) Å, and ß = 99.87(1)°. Its structure is built upon [La7O6]9+ polycationic units at the origin of a porous 3D network, evidencing rectangular channels filled by isolated VO4 tetrahedra. An in situ high-temperature XRD study highlights a number of complex phase transitions assorted with the existence of a ß-polytype also refined in a monoclinic unit cell, space group P21/n, a = 13.0713(4) Å, b = 18.1835(6) Å, c = 7.1382(2) Å, and ß = 97.31(1)°. Thus, during the transitions, while the polycationic networks are almost identical, the vanadate's geometry is largely modified. The use of Eu3+ and Sm3+ at different concentrations in the host lattice is possible using solid-state techniques. The photoluminescence (PL), PL excitation (PLE) spectra, and luminescence decay times were recorded and discussed. The phosphors present an emission light, being bright and reddish orange after excitation under UV. This is mainly due to the V-O band and f-f transitions. Whatever the studied polytype, the final luminescence properties are retained during the heating/cooling process.

Uranyl Cation Incorporation in the [P8W48O184]40- Macrocycle Phosphopolytungstate.

Association of uranyl nitrate with the macrocycle [P8W48O184]40- in formate buffered aqueous solution leads to the formation of a new compound (K11.3Li8.1Na22)[(UO2)7.2(HCOO)7.8(P8W48O184)Cl8]·89H2O (1). Its characterization by XRD reveals a high disorder of the uranyl cations and the formation of monodimensional chains of anionic [(UO2)7.2(HCOO)7.8(P8W48O184)Cl8]41.4- entities linked through formate ligands. The uranyl species are located either in the coordinating sites of the macrocycle [P8W48O184]40- or at its surface. Further studies of the molecule by SAXS and TEM show that the 1D chain collapses to give rise to the formation of polydisperse spherically aggregated species with an average radius of 129 Å.

Mixed-Valence Iron Dumortierite Fe13.52.22+(As5+O4- x)8(OH)6 and Its Intricate Topotactic Exsolution at Mild Temperatures.

The mixed-valent iron arsenate hydroxide Fe13.52.22+(AsO4- x)8(OH)6, x = 0.25, was prepared using the reaction of iron metal with arsenate in aqueous solution and autogenous pressure. Its crystal structure reveals a dumortierite-like framework with mixed-valent Fe2+/Fe3+ in double chains creating channel walls. Remarkably, hexagonal channels consist of chains of face-sharing Fe2+O6 octahedra, 3/4th occupied, whereas AsO4 tetrahedra occupy triangular ones with a single " up" orientation according to the polar P63 mc symmetry. We have analyzed the transformation of this phase upon heating, in which several chemical processes interact, including dehydroxylation, arsenate to arsenite reduction, and oxidative exsolution of a significant part of iron (ca. 15%) found at the surface as hematite and amorphous Fe-rich surficial layer. It leaves a strongly disordered composite structure between several Fe3+-based subunits, in which ∼80% of them is ordered in a complex supercell. Because of the high degree of disorder, the crystal chemistry of the individual subunits and their plausible imbrication were considered to unravel the most plausible ideal 3D model.

Comprehensive Study of Oxygen Storage in YbFe2O4+x (x ≤ 0.5): Unprecedented Coexistence of FeOn Polyhedra in One Single Phase.

The multiferroic LuFe2.5+2O4 was recently proposed as a promising material for oxygen storage due to its easy reversible oxidation into LuFe3+2O4.5. We have investigated the similar scenario in YbFe2O4+x, leading to a slightly greater oxygen storage (OSC) capacity of 1434 µmol O/g. For the first time, the structural model of LnFe2O4.5 was fully understood by high-resolution microscopy images, and synchrotron and neutron diffraction experiments, as well as maximum entropy method. The oxygen uptake promotes a reconstructive shearing of the [YbO2] sub-units controlled by the adaptive Ln/Fe oxygen coordination and the Fe2/3+ redox. After oxidation, the rearrangement of the Fe coordination polyhedra is unique such that all available FeOn units (n = 6, 5, 4 in octahedra, square pyramids, trigonal bipyramids, tetrahedra) were identified in modulated rows growing in plane. This complex pseudo-ordering gives rise to short-range antiferromagnetic correlation within an insulating state.

Topochemical Reduction of YMnO3 into a Composite Structure.

Topochemical modification methods for solids have shown great potential in generating metastable structures inaccessible through classical synthetic routes. Here, we present the enhanced topotactic reduction of the multiferroic compound YMnO3. At moderate temperature in ammonia flow, the most reduced YMnO3-δ (δ = 0.5) phase could be stabilized. XRD, PND, and HREM results show that phase separation occurs into two intimately intergrown layered sublattices with nominal compositions ∞[YMn2+O2+x](1-2x)+ and ∞[YMn2+O3-x](1-2x)- containing versatile Mn2+ coordinations. The former sublattice shows original AA stacking between Mn layers, while AB stacking in the latter results from oxygen removal from the parent YMnO3 crystal structure.

On the Use of Dynamical Diffraction Theory To Refine Crystal Structure from Electron Diffraction Data: Application to KLa5O5(VO4)2, a Material with Promising Luminescent Properties.

A new lanthanum oxide, KLa5O5(VO4)2, was synthesized using a flux growth technique that involved solid-state reaction under an air atmosphere at 900 °C. The crystal structure was solved and refined using an innovative approach recently established and based on three-dimensional (3D) electron diffraction data, using precession of the electron beam and then validated against Rietveld refinement and denisty functional theory (DFT) calculations. It crystallizes in a monoclinic unit cell with space group C2/m and has unit cell parameters of a = 20.2282(14) Å, b = 5.8639(4) Å, c = 12.6060(9) Å, and ß = 117.64(1)°. Its structure is built on Cresnel-like two-dimensional (2D) units (La5O5) of 4*3 (OLa4) tetrahedra, which run parallel to (001) plane, being surrounded by isolated VO4 tetrahedra. Four isolated vanadate groups create channels that host K(+) ions. Substitution of K(+) cations by another alkali metal is possible, going from lithium to rubidium. Li substitution led to a similar phase with a primitive monoclinic unit cell. A complementary selected area electron diffraction (SAED) study highlighted diffuse streaks associated with stacking faults observed on high-resolution electron microscopy (HREM) images of the lithium compound. Finally, preliminary catalytic tests for ethanol oxidation are reported, as well as luminescence evidence. This paper also describes how solid-state chemists can take advantages of recent progresses in electron crystallography, assisted by DFT calculations and powder X-ray diffraction (PXRD) refinements, to propose new structural types with potential applications to the physicist community.

Selective Metal Exsolution in BaFe(2-y)M(y)(PO4)2 (M = Co(2+), Ni(2+)) Solid Solutions.

The 2D-Ising ferromagnetic phase BaFe(2+)2(PO4)2 shows exsolution of up to one-third of its iron content (giving BaFe(3+)1.33(PO4)2) under mild oxidation conditions, leading to nanosized Fe2O3 exsolved clusters. Here we have prepared BaFe(2-y)M(y)(PO4)2 (M = Co(2+), Ni(2+); y = 0, 0.5, 1, 1.5) solid solutions to investigate the feasibility and selectivity of metal exsolution in these mixed metallic systems. For all the compounds, after 600 °C thermal treatment in air, a complete oxidation of Fe(2+) to Fe(3+) leaves stable M(2+) ions, as verified by (57)Fe Mössbauer spectroscopy, TGA, TEM, microprobe, and XANES. The size of the nanometric α-Fe2O3 clusters coating the main phase strongly depends on the yM metal concentration. For M-rich phases the iron diffusion is hampered so that a significant fraction of superparamagnetic α-Fe2O3 particles (100% for BaFe(0.5-x)Co(1.5)(PO4)2) was detected even at 78 K. Although Ni(2+) and Co(2+) ions tend to block Fe diffusion, the crystal structure of BaFe(0.67)Co1(PO4)2 demonstrates a fully ordered rearrangement of Fe(3+) and Co(2+) ions after Fe exsolution. The magnetic behaviors of the Fe-depleted materials are mostly dominated by antiferromagnetic exchange, while Co(2+)-rich compounds show metamagnetic transitions reminiscent of the BaCo2(PO4)2 soft helicoidal magnet.

Investigation of new alkali bismuth oxosulfates and oxophosphates with original topologies of oxo-centered units.

Two new alkali bismuth oxosulfates, [Bi12O15]Li2(SO4)4 (I) and [Bi7K2O8]K(SO4)4 (II), have been synthesized by heating a mixture of Bi2O3, CuSO45H2O, and A2CO3 (A = Li, K), and characterized by single crystal XRD, transmission electron microscopy, and multiphoton SHG and IR spectroscopy. In the above formula the [BixOy] subunits denote the 3D-porous (I) or 1D-columnar (II) polycationic host-lattice formed of edge-sharing OBi4 or O(Bi,K)4 oxocenterd tetrahedra. The SO4(2-) groups and alkali ions are arranged into channels in the interstices leading to original opened crystal structures for these two first reported alkali oxo-bismuth sulfates. The strong adaptability of the oxocentered framework is demonstrated by the possibility of preparing single crystals of [Bi8.73K0.27O8]K1.54(PO4)4 (III) whose crystal structure is similar to those of II with disorder between OBi4 and O(Bi3,K) tetrahedra and different channel occupancy due to the aliovalent replacement of SO4(2-) for PO4(3-).

Multidimensional open-frameworks: combinations of one-dimensional channels and two-dimensional layers in novel BI/M oxo-chlorides.

Here we discuss the synthesis and characterization of three novel bismuth oxo-chlorides ([Bi6Na0.5O7.5][Na0.5Cl3]channel[Cl]layer; [Bi17PbO22][Cl6]channel[Cl3]layer; [Bi9(Pb0.2Mn0.8)O12][Cl3]channel [Cl2]layer) which all show an original multidimensional crystal structure. It is formed of two-dimensional (2D)-layered blocks separated by Cl(-) layers. The blocks are porous with triangular one-dimensional (1D)-Cl(-) channels with various section sizes. This multidimensional feature is unique in the field of Bi and Pb oxo-halides, while so far only 1D or 2D halides units have been reported. The stability of the framework is allowed by Bi(3+)/M(n+) aliovalent substitution to balance charge neutrality. The channel and tunnel walls are formed by edge-sharing O(Bi,M)4 oxocentered tetrahedra, while the triangular tunnel junctions are achieved by O(Bi,M)5 pyramids. The three compounds are rather stable, but only [Bi6Na0.5O7.5][Na0.5Cl3]tunnel[Cl]layer was obtain as a single-phase material so that its photoluminecence properties have been investigated. It shows an unusual red bright luminescence with a maximum at 14150 cm(-1) at low temperatures due to Bi(3+) transitions that are well explained by the Bi-Cl bonding scheme.

Reversible topochemical exsolution of iron in BaFe(2+)2(PO4)2.

BaFe(2+) 2 (PO4 )2 was recently prepared and identified as the first 2D-Ising ferromagnetic oxide with an original reentrant structural transition driven by high-spin Fe(2+) ions arranged in honeycomb layers. Both long-term air exposure and moderate temperature (T>375 °C) leads to topochemical oxidation into iron-depleted compounds with mixed Fe(2+) /Fe(3+) valence. This process is unique, as the exsolution is effective even from single crystal with preservation of the initial crystallinity, and the structure of the deficient BaFe2-x (PO4 )2 (x

CoVO3 High-Pressure Polymorphs: To Order or Not to Order?

Materials properties are determined by their compositions and structures. In ABO3 oxides different cation orderings lead to mainly perovskite- or corundum like derivatives with exciting physical properties. Sometimes, a material can be stabilized in more than one structural modification, providing a unique opportunity to explore structure-properties relationship. Here, CoVO3 obtained in both ilmenite-(CoVO3 -I) and LiNbO3 -type (CoVO3 -II) polymorphs at moderate (8-12 GPa) and high pressures (22 GPa), respectively are presented. Their distinctive cation distributions affect drastically the magnetic properties as CoVO3 -II shows a cluster-glass behavior while CoVO3 -I hosts a honeycomb zigzag magnetic structure in the cobalt network. First principles calculations show that the influence of vanadium is crucial for CoVO3 -I, although it is previously considered as non-magnetic in a dimerized spin-singlet state. Contrarily, CoVO3 -II shows two independent interpenetrating antiferromagnetic Co- and ferromagnetic V-hcp sublattices, which intrinsically frustrate any possible magnetic order. CoVO3 -II is also remarkable as the first oxide crystallizing with the LiNbO3 -type structure where both metals contain free d electrons. CoVO3 polymorphs pinpoint therefore as well to a much broader phase field of high-pressure A-site Cobaltites.

Degradation mechanisms of organic compounds in molten hydroxide salts: a radical reaction yielding H2 and graphite.

Molten salts are used in various waste treatments, such as recycling, recovery or making inert. Here, we present a study of the degradation mechanisms of organic compounds in molten hydroxide salts. Molten salt oxidation (MSO) using carbonates, hydroxides and chlorides is known for the treatment of hazardous waste, organic material or metal recovery. This process is described as an oxidation reaction due to the consumption of O2 and formation of H2O and CO2. We have treated various organic products, carboxylic acids, polyethylene and neoprene with molten hydroxides at 400 °C. However, the reaction products obtained in these salts, especially carbon graphite and H2 without CO2 emission, challenges the previous mechanisms described for the MSO process. Combining several analyses of the solid residues and the gas produced during the reaction of organic compounds in molten hydroxides (NaOH-KOH), we demonstrate that these mechanisms are radical-based instead of oxidative. We also show that the obtained end products are highly recoverable graphite and H2, which opens a new way of recycling plastic residues.

Novel tailormade Bi4MO4(PO4)2 structural type (M = Mg, Zn).

In the Bi(2)O(3)-MO-P(2)O(5) ternary system, the commonly observed sizable 1D ribbon-like units have been extended to their 2D infinite end member, leading to the novel tailormade Bi(4)MO(4)(PO(4))(2) compounds. It contains planar [Bi(2)O(2)](2+) derivatives, separated by two slabs of PO(4), which create channels hosting the M(2+) cations (M = Mg, Zn). For both compounds, supercell orderings occur comparatively to the predicted ideal crystal structure (V(Mg) = 2V(ideal) and V(Zn) = 8V(ideal)). In the Mg case a transition into the ideal lattice occurs above 450 °C. In spite of the conceptual assembly of 2D motifs, the final architecture is three-dimensional due to strong interbonds. Thus, our work gives new insights on the possibility for versatile organization of original secondary building units (SBUs) able to self-assemble into predicted structural edifices. Single-crystal and powder XRD versus temperature, high-temperature (31)P NMR, as well as transmission electron microscopy were used for structural characterization. Preliminary electric characterization is also reported.