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Microstructured orthovanadate BaCo3(VO4)2(OH)2 has been prepared by hydrothermal treatment of pyrovanadate Co3V2O7(OH)2·2H2O. The impact of crystallite size on the structural and magnetic properties have been discussed.

International audience; Two new cubic perovskite oxides Ba3CoSb2O9 and Ba2SrCoSb2O9 were prepared and characterized. Ba3CoSb2O9 was prepared in polycrystalline form by high-pressure (HP) high-temperature treatment of the hexagonal 6H polytype of Ba3CoSb2O9. Polycrystalline samples of Ba2SrCoSb2O9 were obtained at ambient pressure by a solidstate reaction method. Combined Rietveld refinements of X-ray and neutron diffraction patterns indicated that HP Ba3CoSb2O9 and Ba2SrCoSb2O9 both crystallize in a Fm (3) over barm cubic double perovskite A(2)BB'O-6 structure with formulae Ba-2(Sb0.83Co0.17)(Co0.5Sb0.5)O-6 and (Ba1.33Sr0.67)Sb(Co0.7Sb0.3)O-6, respectively. Electron diffraction results obtained for Ba2SrCoSb2O9 showed no sign of additional periodicity. Weiss temperatures extracted from high-temperature magnetic susceptibility data were. theta approximate to -113 K and -118 K for HP Ba3CoSb2O9 and Ba2SrCoSb2O9, respectively. HP Ba3CoSb2O9 displays a weak ZFC-FC magnetic irreversibilty below 8 K. Ba2SrCoSb2O9 exhibits a spin freezing at T-f approximate to 6 K, and mu SR and Sb-121 NMR data revealed a magnetic transition towards a disordered ground state.

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 analysi...

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.

International audience; In nano-sized FeWO 4 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. As for any other pseudocapacitive materials, 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 (XAS) has been 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 were obtained in real time upon cycling the electrode, which allowed 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 L 3-edge measurements point out the involvement of the Fe 3+ /Fe 2+ redox couple in the charge storage while W 6+ acts as a spectator cation. The result of this study enables to unambiguously discriminate between the Faradaic and capacitive behavior of FeWO 4. Beside these valuable insights toward the full description of the charge storage mechanism in FeWO 4 , this paper demonstrates the potential of operando X-ray absorption spectroscopy to enable a better material engineering for new high capacitance pseudocapacitive electrode materials.

In nanosized FeWO

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).

In nano-sized 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. As for any other pseudocapacitive materials, 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 (XAS) has been 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 were obtained in real time upon cycling the electrode, which allowed 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 electrode materials.

Single Ni

International audience; 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.

Here, we report a study of white-ochre powders with targeted composition MnWO

International audience; Cool-SPS conditions (low temperature/high pressure) can be used to densify materials with limited thermodynamical stability, due to low temperature phase transition, melting, decomposition… We recently demonstrated the potential of SPS for the stabilization and densification of diversified materials at very low temperatures. Here we report the sintering of a potentially magnetoelectric pyrophosphate with low temperature decomposition. Characterization of its magnetic properties are presented, and dielectric/magnetoelectric characterization could also be performed thanks to the high densification obtained at temperatures as low as 200 °C.

The interest in finding experimental realization of Quantum Liquid Spin phases leads us to elaborate polycrystalline samples of the 6H-B phase of Ba3NiSb2O9 . This structure shows Ni triangular planes and the first magnetic characterizations confirm the absence of any magnetic order as expected for QLS. The samples were synthesized by treatment under HP-HT of the 6H-A phase of Ba3NiSb2O9. The optimized synthesis conditions were 2.5 GPa and 620{\deg}C. By combining several technics as XRD, NPD and TEM we show that even the obtained compound is in fact a new form of Ba3NiSb2O9, that corresponds to a new stacking along c. This preserves the presence of Ni planes but results in a loss of the hexagonal symmetry. The TEM is helpful to conclude to a trigonal symmetry at a scale of 10nm., Comment: 5 figures submitted to Journal of Solid State Chemistry on 9/12/2015_ revised version on 30/01/2016

We show that the type-II multiferroic properties of bulk MnWO4 are kept in nanostructured ceramics of 50 nm grain size prepared via spark plasma sintering (SPS). This means that ferroelectric polarization is robust against downsizing, which is at variance with standard ferroelectrics like BaTiO3. We ascribe this stability to the spin-driven nature of the ferroe-lectric correlations in type-II multiferroics while it is resulting from lattice distortion in other cases. This may open the way for persistent type-II multiferroicity with no need for external stabilization like substrate-generated strain.

In this contribution, based on the analyses of the discharge behavior as well as final properties of the deposited Ni-O films during reactive high power impulse magnetron sputtering discharge, we have demonstrated that monitoring the oxygen flow rate leads to 4 different regimes of discharge. Tuning the oxygen partial pressure allows deposition of a large range of chemical compositions from pure nickel to nickel-deficient NiOx (x > 1) in the poisoned mode. Investigation of the plasma dynamics by time-resolved optical emission spectroscopy suggests that the discharge behavior in the poisoned mode principally comes from the higher contribution of both oxygen and argon ions in the total ionic current, leading to a change in the ion induced secondary electron emission coefficient. Additionally, material characterizations have revealed that optoelectronic properties of NiOx films can be easily tuned by adjusting the O/Ni ratio, which is influenced by the change of the oxygen flow rate. Stoichiometric NiO films...

Inelastic neutron scattering is used to study the low-energy magnetic excitations in the spin-1 triangular lattice of the 6H-B phase of Ba$_3$NiSb$_2$O$_9$. We study two powder samples: Ba$_3$NiSb$_2$O$_9$ synthesized under high pressure and Ba$_{2.5}$Sr$_{0.5}$NiSb$_2$O$_9$ in which chemical pressure stabilizes the 6H-B structure. The measured excitation spectra show broad gapless and nondispersive continua at characteristic wave vectors. Our data rules out most theoretical scenarios that have previously been proposed for this phase, and we find that it is well described by an exotic quantum spin liquid with three flavors of unpaired fermionic spinons, forming a large spinon Fermi surface.

International audience; 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 W6+ are replaced with diamagnetic Mo6+ cations and magnetic couplings among Mn2+ (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.

We present an in-depth study of the magnetic properties of the spin-1 antiferromagnet 6HB-${\mathrm{Ba}}_{3}{\mathrm{NiSb}}_{2}{\mathrm{O}}_{9}$. $\ensuremath{\mu}\mathrm{SR}$ measurements demonstrate that this material shows no static magnetism down to temperatures as low as 20 mK, making it a likely candidate for a quantum spin liquid state. $^{121}\mathrm{Sb}$ NMR shift measurements show that the local, intrinsic susceptibility levels off at temperatures below $\ensuremath{\sim}60$ K. The NMR spin-lattice relaxation rate $1/{T}_{1}$ is essentially constant in temperature and the muon relaxation rate exhibits a low-temperature relaxation plateau, all indications of gapless spin excitations. Our local probe measurements are discussed in the context of several theories proposed for this material.

In the search of an experimental realization of quantum spin liquid phases we have synthesized polycrystalline samples of Ba3NiSb2O9 under high pressure-high temperature conditions. Combining X-ray powder diffraction, neutron powder diffraction and precession electron diffraction we show that the obtained phase isn't hexagonal as reported in the literature, but trigonal. This new structure shows triangular Ni planes, but only in domains of sizes of the order of 10 nm. It therefore is still interesting as a potential candidate for a quantum spin liquid.

International audience; The influence of magnetic field on the dielectric properties of several Mn1-x(Mg,Zn)xWO4 multiferroic ceramics is investigated, and confirms an additional phase transition below the ferroelectric Curie temperature previously detected in this system in zero magnetic field. In highly doped samples an original plateau of the capacitance is observed between T2 and this additional transition. Pyroelectric measurements in zero magnetic field for a Mn0.85Mg0.15WO4 ceramic confirm the existence of this unexpected low temperature phase transition. The magnetic or structural origin of this transition is discussed.

The members of the CuMo1 - xWxO4 series (0 ≤ x < 0.1) undergo a first-order phase transition at normal pressure, which can be induced by temperature. The two allotropic forms exhibit two distinguishable colors, green for the high-temperature form (α) and brownish-red for the low temperature one (γ), which opens up a new market for user-friendly temperature indicators. From X-ray diffraction and microprobe analyses as from optical properties, the tungsten substitution rate for molybdenum is limited to 12%. Beyond, a third, parasitic wolframite-type phase, CuMo0.6W0.4O6, systematically crystallizes besides the α/γCuMo0.9W0.1O4 compounds. Within the CuMo1 - xWxO4 solid-solution domain, the dependence of the transition temperatures was followed by calorimetry, optical reflectivity, and magnetism. On the basis of these measurements, the transition is characterized for all of the chemical compositions by a hysteresis loop of about 90 K in width with a temperature transition strongly dependent on the tungsten co...

Iron tungstate (FeWO4) has been synthesized using two low-temperature synthetic routes and investigated as a new pseudocapacitive electrode material for supercapacitors operating in a neutral aqueous electrolyte. Its electrochemical properties are clearly related to the specific surface area and seem to originate from Fe3+/Fe2+ fast surface reactions. For FeWO4 obtained by polyol-mediated synthesis, a high volumetric capacitance of 210 F·cm−3 (i.e. more than two times higher than that of activated carbon) was measured at 20 mV·s−1 with less than 5% fade over 10,000 cycles. Furthermore, unlike most of the previously investigated iron based electrodes, a unique pseudocapacitive behavior is observed, thus emphasizing the role of the crystallographic structure on the electrochemical signature. Keywords: Iron tungstate, Pseudocapacitive behavior, High density, Neutral aqueous electrolyte, Supercapacitor

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.

International audience; 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.

The low-temperature properties of a diluted antiferromagnetic pyrochlore slab of S = 3/2 spins are investigated through a study of the frustrated oxides Ba 2 Sn 2 Ga 10-7p ZnCr 7p O 22 (p > 0.85). Powder neutron diffraction and 119 Sn Mossbauer absorption show no evidence of long-range magnetic order above 1.5 K. As in SrCr 9q Ga 12-9q O 19 , diffuse magnetic scattering, indicative of short range spin-spin correlations, is observed at low temperature. The dependence of the low-temperature sub-Curie bulk susceptibility to weak site depletion is the inverse of that observed in SrCr 9q Ga 12-9q O 19 .

International audience; 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.

A series of layered double hydroxide materials of composition Co2FeyAl1−y(OH)6Cl·nH2O (0≤y≤1) was prepared via chimie douce. The crystalline parameter related to the cation to cation distance obeys the expected variation, showing that the substitution is effective over the entire range. Local order around Co and Fe cations is studied by X-ray absorption spectroscopy. Moduli of the Fourier transform at the Fe K-edge are superimposable, in agreement with an ordered model, although present in small domains since no superlattice is depicted. The ionic resistivity of the samples is highly dependent on the water molecule content. The conductivity is found to be thermally assisted, and the variation of the slope in the Arrhenius diagram is explained by a Vogel–Tamman–Fulcher-type behavior. Magnetic susceptibility measurements support the proposed cation composition and indicate the onset of local magnetic order at low temperature (below 10 K). The inter-sheet distance influences the magnetic response at low temperature, showing the presence of weak interactions between lamellae.

Two new alkali metal chromium thiophosphates, Na3Cr2P3S12 and K3Cr2P3S12, have been synthesized and their structure determined from single-crystal or powder X-ray diffraction analyses. These isostructural compounds, which exhibit a novel structural arrangement, crystallize in the monoclinic system (space group P21/c) with the cell parameters a=17.4076(10) A, b=11.1723(10) A, c=19.2502(11) A, β=149.731(3)° (V=1887.1(2) A3, Z=4) and a=17.9690(14) A, b=12.0607(5) A, c=19.3109(16) A, β=150.008(3)° (V=2091.98(16) A3, Z=4) for Na3Cr2P3S12 (I) and K3Cr2P3S12 (II), respectively. For (I), the single-crystal refinement led to R/Rw(F2)=0.0417/0.0650 (for 4125 independent reflections and 182 refined parameters). For (II), the Rietveld refinement led to Rp/Rwp=0.0181/0.0233 (for 90 refined parameters). In both materials, the structures consist of 1∞[Cr2P3S12]3− chains built upon two edge-sharing [CrS6] octahedra capped by two [PS4] tetrahedra defining isolated [Cr2P2S12]−8 entities. These entities are linked to each other through an extra [PS4] tetrahedral group. Magnetic measurements evidence antiferromagnetic coupling between Cr3+ cations. The charge balance of the phase is MI3CrIII2PV3S−II12.

The effect of hole doping into the divalent nickel oxide Y 2 BaNiO 5 , which provides a good example of one-dimensional electronic structure in the presence of strong d–d interactions, has been examined by means of several techniques including X-ray absorption and electron energy loss spectroscopies at oxygen K -edges (O K -edges). The crystal structure of this compound shows one-dimensional chains of vertex-sharing NiO 6 octahedra running along the z -axis. Hole doping into this material is realized by substituting Ca 2+ or Sr 2+ ions for the Y 3+ ions. We suggest that the doped holes have antibonding Ni 3d(3z 2 −r 2 )–O2p(z) character.

Experimental bulk susceptibility $\ensuremath{\chi}(T)$ and magnetization $M(H,T)$ of the $S=1$ Haldane chain system doped with nonmagnetic impurities, ${\mathrm{Y}}_{2}{\mathrm{BaNi}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x}{\mathrm{O}}_{5}$ $(xl~0.08),$ are analyzed. A numerical calculation for the low-energy spectrum of noninteracting open segments describes well experimental data above 4 K. Below 4 K, we observe power-law behaviors $\ensuremath{\chi}(T)\ensuremath{\sim}{T}^{\ensuremath{-}\ensuremath{\alpha}}$ and $M(H,T)\ensuremath{\sim}{T}^{1\ensuremath{-}\ensuremath{\alpha}}{f}_{\ensuremath{\alpha}}(H/T),$ with $\ensuremath{\alpha}(l1)$ depending on the doping concentration x. This observation suggests the appearance of a gapless quantum phase due to a broad distribution of effective couplings between the dilution-induced moments.

A thermally activated magnetic relaxation is observed using neutron spin-echo in the pyrochlore slab (kagome bilayer) compound SrCr 9 x Ga 12 - 9 x O 19 ( x = 0.95 ) in a restricted temperature range, 4 K T 7 K , above a cross-over to a low temperature relaxation regime with a weaker temperature dependence. The activation energy of the thermally activated relaxation, of the order of 7 meV, coincides with the energy of a phonon mode observed with neutron and Raman spectroscopy, indicating a phonon-assisted regime. The experimental observation of phonon-assisted process gives additional insight to the importance of spin–phonon coupling in frustrated magnets with regard to the models mostly based on purely magnetic interactions.

Two new compounds Cu4(AsO4)2O (1) and Ba2Cu7As6O24 (2) were obtained from a mixture of CuO and As2O5 · xH2O heated to 1090°C for 72 h in a BaCO3 flux. Compound 1 was obtained as dark green crystals, belonging to the orthorhombic crystal system. Compound 2 was obtained as light gray to colorless crystals belonging to the triclinic crystal system. The structure of 1 contains two copper ions with distorted trigonal bipyramidal geometries and a third with a distorted square pyramidal geometry. One ‚oxide’ ion is surrounded by four copper ions in a distorted tetrahedral arrangement. Compounds 2 consists of two chains with alternating units of trigonal bipyramidal CuO5 and square planar CuO4 units, with the oxygen atoms derived from the AsO4 groups. Magnetic susceptibility measurements of 1 and 2 both show effective magnetic moments, μeff1.90–1.95 B.M./Cu at 25°C, which is consistent with Cu2+ ions having one unpaired electron. Significant antiferromagnetic coupling was revealed by variable temperature measurements. At very low temperatures (15–25 K) samples of both 1 and 2 undergo a transition to a weak ferromagnetic state.

A new mixed metal titanate Ba3Fe24Ti7O53, 1 was obtained by heating stoichiometRic mixtures of BaCO3, Fe3O4 and TiO2 to 1375°C for 48 h. Compound 1 was characterized by single crystal x-ray diffraction analysis. Two of the barium ions lie in a network of channels in the bc crystallographic plane. The iron and titanium atoms were refined in a random distribution in 16 independent MO6 octahedral sites and 2 MO4 tetrahedral sites. Magnetization measurements reveal a strong magnetic ordering up to 600 K. M vs. H loop measurements show no hysteresis effects.

The new mixed metal titanate Ba 2 Fe 2 Ti 4 O 13 ( 1 ) was obtained in crystalline form by heating a mixture of BaCO 3 , Fe 3 O 4 and TiO 2 at 1350°C for 48 h. Compound 1 was characterized by single-crystal X-ray diffraction methods. It is isomorphous to the related known compounds Na 2 Ti 6 O 13 and Ba 2 NiTi 5 O 13 . The X-ray diffraction and Mossbauer measurements of 1 obtained at room temperature indicate that the iron ions are unequally distributed between the three independent sites occupied by the transition metal ions.

The new compound Ba2Mn(AsO4)2 (1) was obtained by heating a mixture of BaCO3, MnO2 and As2O5 in air to 1020°C for 48 h. The manganese ion occupies a distorted octahedral site produced by six oxygen atoms from neighbouring AsO4 groups. Five of the MnO contacts are short, MnO = 2.11(1)−2.18(1) A and one is long MnO = 2.56(2)A. The barium ions occupy channels that run parallel to the a-axis of the crystal. Each MnO6 octahedron is linked to five nearest neighbours via OAsO bridging units. Magnetic measurements at room temperature show an effective magnetic moment, μeff = 6.05 B.M., which is consistent with a high-spin Mn2+ ion with five unpaired electrons. The temperature-dependent magnetic measurements show that 1 obeys the modified Curie-Weiss law, C = 4.58 emu K/mol, with a weak antiferromagnetic coupling between the ions as indicated by the Weiss constant, θp = −11 K.

The α–β phase transition of Mn 2 V 2 O 7 has been studied with the use of single crystals grown from hydrothermal reactions. The structure of the high temperature β-phase (thortveitite) was previously determined from neutron powder diffraction data. Here we report the structure determination of a new phase, α-Mn 2 V 2 O 7 , and the structure refinement of β-Mn 2 V 2 O 7 from single-crystal X-ray diffraction data. α-Mn 2 V 2 O 7 crystallizes in the space group P 1 (no. 2) with a = 6.868(2) A, b = 7.976(2) A, c = 10.927(2) A, α = 87.81(1)°, β = 72.14(1)°, γ = 83.08(1)°, V = 564.5(5) A 3 , Z = 4 at T = 293 K.β-Mn 2 V 2 O 7 crystallizes in the space group C 2/ m (no. 12) with a = 6.7129(6) A, b = 8.7245(5) A, c = 4.9693(4) A, β = 103.591(8)°, V = 282.88(4) A 3 , Z = 2 at T = 323 K.β-Mn 2 V 2 O 7 adopts the thortveitite structure containing edge-sharing MnO 6 octahedra and corner-sharing V 2 O 4− 7 divanadate groups, which have staggered conformation and linear V–O b –V. The structure of α-Mn 2 V 2 O 7 , differs from that of β-Mn 2 V 2 O 7 mainly by the bending of V–O b –V moieties, as observed in the low-temperature forms of other thortveitite-like compounds. Differential scanning calorimetry (DSC) indicated that a reversible first-order phase transition occurs at ∼296 K. The low-temperature phase exhibits a paramagnetic–antiferromagnetic transition at T N =16.0(5) K and follows Curie–Weiss behavior (μ eff = 5.9 μ B /Mn atom) at higher temperature. A change of Weiss constants (θ p = −28 K for α-Mn 2 V 2 O 7 ; −4 K for β-Mn 2 V 2 O 7 ) relevant to the α–β phase transition was observed between 290 and 300 K.

The compounds Ba2CoSi2O7, 1, and BaCo2Si2O7, 2, were obtained by heating mixtures of BaCO3, CoCO3, and SiO2 in the proper stoichiometries to 1300 °C for 48 h. Both materials were characterized by single-crystal X-ray diffraction and magnetization measurements on powder and partially oriented single crystals. Each cobalt ion in 1 is surrounded by four oxygen atoms derived from the [Si2O7]6- groups in the form of a distorted tetrahedron. The cobalt ions are bridged by O−Si−O and O−Si−O−Si−O groups from the [Si2O7]6- ions in an extended two-dimensional arrangement. The barium ions are arranged in layers that run parallel to the Co[Si2O7] layers. In 2 each cobalt ion is also surrounded by an arrangement of four oxygen atoms derived from the Si2O7 groups in a distorted tetrahedral shape. These tetrahedra are linked into chains through the sharing of one of the oxygen atoms from the Si2O7 groups. The CoO4 chains are crosslinked via O−Si−O−Si−O bridges from the Si2O7 groups. The barium ions occupy channels that ...

The title compound features a frustrated magnetic lattice consisting of a pyrochlore slab, kagome planes embedding a triangular plane. The bulk susceptibility is Curie–Weiss-like with Θ CW =−310 K for T⩾150 K , but there is no long-range order down to a temperature of T=1.5 K . Inelastic neutron-scattering data obtained on the thermal TOF-spectrometer IN4 at the ILL reveals the dynamic nature of the broad diffuse scattering that characterises the magnetic response.

The new compound BaMnP2O7 was obtained in two crystallographic modifications, (monoclinic-1) and (triclinic-2), by heating mixtures of BaCO3, P2O5 and MnO2 to 1100°C and to 1000°C for 72 h, respectively. CaMnP2O7 (3) was obtained by heating a mixture of CaCO3, P2O5 and MnO2 to 1050°C for 48 h. Both crystallographic forms of BaMnP2O7 and CaMnP2O7 (3) were investigated by single-crystal X-ray diffraction analysis. The high-temperature monoclinic form of BaMnP2O7 could not be obtained free of the low-temperature triclinic form in the bulk form. In monoclinic-1 the manganese ions exist in a distorted MnO6 octahedron surrounded by five closely and one remotely positioned oxygen atoms. In triclinic-2 and -3 forms the manganese ions are associated in pairs by the formation of Mn2O10 units that share one edge of two adjacent octahedra. The magnetic properties of the triclinic-2 and -3 forms were also investigated. The effective magnetic moments, μeff, are 5.7 B.M. and 5.8 B.M./Mn atom for triclinic-2 and -3, respectively, and are consistent with a high-spin Mn2+ ion in an octahedral environment with five unpaired electrons. The temperature-dependent magnetic measurements of 2 and 3 have revealed a combination of short-range antiferromagnetic coupling, J, between the two Mn ions within the Mn2O10 units and a longer range weaker antiferromagnetic coupling, J′, between the neighbouring Mn2O10 units, |J′/J| = 0.18 and 0.074 for 2 and 3, respectively.

Pseudocapacitive electrodes can be of interest in order to improve the volumetric energy density of electrochemical capacitors, so called supercapacitors, owing to the high density of metal oxides or nitrides as compared to activated carbon [1]. Among all the materials that have been proposed up to now, iron tungstate (FeWO4) represents an interesting candidate both from a fundamental and application point of views. It crystallizes in the wolframite-type structure, characterized by distorted [MO6] and [WO6] octahedra. Different low-temperature methods including polyol-mediated synthesis [2], hydrothermal synthesis [3], ultracentrifugation method [4] have been used in order to obtain high specific surface area nanoparticles which have been characterized and electrochemically tested as supercapacitor electrode materials in aqueous electrolytes [5]. FeWO4 has demonstrated a pseudocapacitive behavior that can be attributed to Fe2+/Fe3+ surface redox reactions and thus is closely related to the specific surface area. To further investigate the charge-storage mechanism of FeWO4, in situ and operando X-ray absorption spectroscopy (XAS) experiments were performed at both Fe K- (7112 eV) and W L-edges (10204 and 12100 eV) on the new ROCK beamline at the SOLEIL synchrotron (France), using a specially designed 3-electrode-cell assembly. The composite working electrodes were composed of 60% FeWO4, 30% carbon black and 10% PTFE with a loading of about 10mg of FeWO4 per cm2. Cyclic voltammograms were obtained between -0.6 and 0 V vs. Ag/AgCl in neutral 5M LiNO3 at room temperature. An acquisition time of 100 ms per spectrum is possible on the ROCK beamline, thanks to its quick-EXAFS monochromator, thus allowing following very accurately the oxidation state evolution of both Fe and W in the electrodes during cycling. The XAS experiments show an evolution of the iron oxidation state (Fe2+/Fe3+) while cycling, thus confirming the Fe-related pseudocapacitive behavior of FeWO4, while no evolution was observed on the tungsten oxidation state. The characterizations and XAS results will be detailed in the presentation. Poor alteration of the crystal structure was also observed which open the way for the use of FeWO4 as possible long term cycling electrode in a supercapacitor. Thus, FeWO4 was tested as negative electrode in an asymmetric electrochemical capacitor using cryptomelane-type MnO2 as the positive in a neutral aqueous electrolyte. Prior to assembling the cell, the electrodes have been individually tested in a 5M LiNO3 electrolyte solution to define both the adequate balance of active material in the supercapacitor and the proper working voltage window. The full asymmetric device has been cycled between 0 and 1.4 V for over 40 000 cycles and subjected to accelerated ageing tests under floating conditions at different voltages, without any significant change on its electrochemical behavior. This remarkable stability shows the interest of developing full oxide-based asymmetric supercapacitors operating in non-toxic aqueous electrolytes. Cell performances will be reported in this communication. References 1) Nicolas Goubard-Bretesché, Olivier Crosnier, Gaëtan Buvat, Frédéric Favier, and Thierry Brousse, submitted to JPS, April 2016. 2) J. Ungelenk, M. Speldrich, R. Dronskowski, C. Feldmann, Solid State Sci. 31 (2014) 62. 3) S.H. Yu, B. Liu, M.S. Mo, J.H. Huang, X.M. Liu, Y.T. Qian, Adv. Funct. Mater. 13 (2003) 639. 4) K. Kisu, M. Iijima, E. Iwama, M. Saito, Y. Orikasa, W. Naoi and K. Naoi, J. Mater. Chem. A, 2014, 2, 13058-13068. 5) N. Goubard-Bretesché, O. Crosnier, C. Payen, F. Favier, T. Brousse, Electrochem. Commun. 57 (2015) 61.

We present local probe results on the honeycomb lattice antiferromagnet Ba(3)CuSb(2)O(9). Muon spin relaxation measurements in a zero field down to 20 mK show unequivocally that there is a total absence of spin freezing in the ground state. Sb NMR measurements allow us to track the intrinsic susceptibility of the lattice, which shows a maximum at around 55 K and drops to zero in the low-temperature limit. The spin-lattice relaxation rate shows two characteristic energy scales, including a field-dependent crossover to exponential low-temperature behavior, implying gapped magnetic excitations.

The new two-dimensional magnetically frustrated title compound is synthesized from an aqueous NH4OH solution containing Ni(OH)2, V2O5, an Ba(OAc)2 (reflux, 4 h).

International audience; Ceramic samples of MnW1-xMoxO4 (x ≤ 0.3) solid solution were prepared by a solid-state route with the goal of increasing the magnitude of the spin-exchange couplings among the Mn2+ ions in the spin spiral multiferroic MnWO4. Samples were characterized by X-ray diffraction, optical spectroscopy, magnetization, and dielectric permittivity measurements. It was observed that the Néel temperature TN, the spin spiral ordering temperature TM2, and the ferroelectric phase-transition temperature TFE2 of MnWO4 increased upon the nonmagnetic substitution of Mo6+ for W6+. Like pure MnWO4, the ferroelectric critical temperature TFE2(x) coincides with the magnetic ordering temperature TM2(x). A density functional analysis of the spin-exchange interactions for a hypothetical MnMoO4 that is isostructural with MnWO4 suggests that Mo substitution increases the strength of the spin-exchange couplings among Mn2+ in the vicinity of a Mo6+ ion. Our study shows that the Mo-doped MnW1-xMoxO4 (x ≤ 0.3) compounds are spin-frustrated materials that have higher magnetic and ferroelectric phase-transition temperatures than does pure MnWO4.

The synthesis and characterization of new divalent metal tert-butylphosphonates are described: MII[(CH3)3CPO3]·xH2O [MII= Co, Mn (x= 1) MII= Zn (x= 2/3)] and Cu1.75O0.75[(CH3)3CPO3]·H2O. Co[(CH3)3CPO3]·H2O crystallizes in the monoclinic space group P21/c with a= 12.256(1)A, b= 17.939(1)A, c= 10.769(1)A, β= 93.57(1)°, V = 2363.1 (4)A3, Z= 12, with R= 0.064 and Rw= 0.075 for 1805 observed reflections, according to the criterion I > 3σ(I). The layer arrangement of this new phase is different from that observed in the case of the n-alkyl MII( RPO3)·H2O analogues owing to the steric effect of the bulky tert-butyl group. The structural relationship between the cobalt, manganese and zinc compounds is discussed. The observed magnetic moments are consistent with the presence of high-spin MnII or CoII and with the existence of both octahedral and tetrahedral environments about these metals.

Y${}_{0.5}$Ca${}_{0.5}$BaCo${}_{4}$O${}_{7}$ was recently introduced as a possible candidate for capturing some of the predicted classical spin kagome ground-state features. Stimulated by this conjecture, we have taken up a more complete study of the spin correlations in this compound with neutron scattering methods on a powder sample characterized with high-resolution neutron diffraction and the temperature dependence of magnetic susceptibility and specific heat. We have found that the frustrated near-neighbor magnetic correlations involve not only the kagome planes but concern the full Co sublattice, as evidenced by the analysis of the wave-vector dependence of the short-range order. We conclude from our results that the magnetic moments are located on the Co sublattice as a whole and that correlations extend beyond the two-dimensional kagome planes. We identify intriguing dynamical properties, observing high-frequency fluctuations with a Lorentzian linewidth $\ensuremath{\Gamma}\ensuremath{\leqslant}20$ meV at ambient temperature. On cooling a low-frequency ($~$1 meV) dynamical component develops alongside the high-frequency fluctuations, which eventually becomes static at temperatures below $T\ensuremath{\approx}50$ K. The high-frequency response with an overall linewidth of $~$10 meV prevails at $T\ensuremath{\leqslant}2$ K, coincident with a fully elastic short-range-ordered contribution.