Metastable (Bi, M)₂(Fe, Mn, Bi)₂O_6+<i>x</i> (M = Na or K) Pyrochlores from Hydrothermal Synthesis

The hydrothermal syntheses, structures, and magnetism of two new pyrochlore oxides of compositions (Na_0.60Bi_1.40)­(Fe_1.06Mn_0.17Bi_0.77)­O_6.87 and (K_0.24Bi_1.51)­(Fe_1.07Mn_0.15Bi_0.78)­O_6.86 are described. With preparation at 200 °C for 6 h in solutions of sodium or potassium hydroxide, the alkali metals introduced from these mineralizers are essential to the synthesis of the phases. The average long-range order of the pyrochlore structure, with space group <i>Fd</i>3̅<i>m</i>, was investigated and refined against X-ray and neutron diffraction data, and it was shown that disorder is present in both the metal and coordinating oxygen positions, along with metal-mixing across both the A and B sites of the structure. XANES analysis confirms the presence of Mn⁴⁺, mixed valence Bi³⁺ and Bi⁵⁺, and Fe³⁺, the last also verified by ⁵⁷Fe Mössbauer spectroscopy. Magnetic measurements show a lack of long-range magnetic ordering that is typical of geometrically frustrated pyrochlores. The observed glasslike interactions occur at low temperatures, with the onset temperature depending upon the magnitude of the applied external field. Variable temperature X-ray diffraction shows that these pyrochlores are metastable and collapse on heating at ca. 395 °C, which suggests that their formation by conventional solid-state synthesis would be impossible

Nanocomposite Pyrite–Greigite Reactivity toward Se(IV)/Se(VI)

A nanopyrite/greigite composite was synthesized by reacting FeCl₃ and NaHS in a ratio of 1:2 (Wei et al. 1996). Following this procedure, the obtained solid phases consisted of 30–50 nm sized particles containing 28% of greigite (Fe²⁺Fe³⁺₂S₄) and 72% pyrite (FeS₂). Batch reactor experiments were performed with selenite or selenate by equilibrating suspensions containing the nanosized pyrite–greigite solid phase at different pH-values and with or without the addition of extra Fe²⁺. XANES-EXAFS spectroscopic techniques revealed, for the first time, the formation of ferroselite (FeSe₂) as the predominant reaction product, along with elemental Se. In the present experimental conditions, at pH 6 and in equilibrium with Se⁰, the solution is oversaturated with respect to ferrosilite. Furthermore, thermodynamic computations show that reaction kinetics likely played a significant role in our experimental system

Systematic Study of Exchange Coupling in Core–Shell Fe_3−δO₄@CoO Nanoparticles

Although single magnetic domain nanoparticles are very promising for many applications, size reduction usually results in low magnetic anisotropy and unblocked domain at room temperature, e.g., superparamagnetism. An alternative approach is core–shell nanoparticles featured by exchange bias coupling between ferro­(i)­magnetic [F­(i)­M] and antiferromagnetic (AFM) phases. Although exchange bias coupling has been reported for very diverse core–shell nanoparticles, it is difficult to compare these studies to rationalize the effect of many structural parameters on the magnetic properties. Herein, we report on a systematic study which consists of the modulation of the shell structure and its influence on the exchange bias coupling. A series of Fe_3−δO₄@CoO core–shell nanoparticles has been synthesized by seed-mediated growth based on the thermal decomposition technique. The variation of Co reactant concentration resulted in the modulation of the shell structure for which thickness, crystallinity, and interface with the iron oxide core strongly affect the magnetic properties. The thickest CoO shell and the largest F­(i)­M/AFM interface led to the largest exchange bias coupling. Very high values of coercive field (19 000 Oe) and <i>M</i>_R/<i>M</i>_S ratio (0.86) were obtained. The most stricking results consist of the increase of the coercive field while exchange field vanishes when the CoO thickness decreases: it is ascribed to the diffusion of Co species in the surface layer of iron oxide which generates to some extent cobalt ferrite and induces hard/soft exchange coupling between ferrimagnetic phases

Evidence of New Fluorinated Coordination Compounds in the Composition Space Diagram of FeF₃/ZnF₂–H<i>amtetraz</i>-HF_aq System

The exploration of the composition space diagram of the FeF₃/ZnF₂–H<i>amtetraz</i>-HF_aq system (H<i>amtetraz</i> = 5-aminotetrazole) by solvothermal synthesis at 160 °C for 72 h in dimethylformamide (DMF) has evidenced five new hybrid fluorides (<b>1</b>–<b>5</b>); the structures are characterized from single crystal X-ray diffraction data. [H<i>dma</i>]­·(ZnFe^III(H₂O)₄F₆) (<b>1</b>) and [H<i>dma</i>]­·[H<i>gua</i>]₂­·(Fe^IIIF₆) (<b>2</b>) contain anionic inorganic chains (<b>1</b>) or isolated octahedra (<b>2</b>) weakly hydrogen bonded (Class I hybrids) to dimethylammonium (H<i>dma</i>) and/or guanidinium (H<i>gua</i>) cations which are produced from the tetrazole ligand and solvent decomposition. [H<i>dma</i>]₂­·[H<i>gua</i>]­·[NH₄]­·[ZnFe^IIIF₅(<i>amtetraz</i>)₂]₂ (<b>3</b>), [H<i>dma</i>]₂­·[Zn_1.6Fe^II_0.4Fe^IIIF₆­(<i>amtetraz</i>)₃] (<b>4</b>), and [H<i>dma</i>]­·[Zn₄F₅(<i>amtetraz</i>)₄] (<b>5</b>) are considered as Class II hybrids in which the (<i>amtetraz</i>)⁻ anions are strongly linked to divalent metal cations via N–M bonds. In <b>3</b>, _∞{[NH₄]­·[ZnFe^IIIF₅­(<i>amtetraz</i>)₂]₂} layers are separated by [H<i>dma</i>]⁺ and [H<i>gua</i>]⁺ cations. <b>4</b> and <b>5</b> exhibit three-dimensional (3D) hybrid networks that contain small cavities where [H<i>dma</i>]⁺ cations are inserted. A porous 3D metal–organic framework intermediate is evidenced from the thermogravimetric analysis and X-ray thermodiffraction of <b>5</b>

Bioinspired Iron Sulfide Nanoparticles for Cheap and Long-Lived Electrocatalytic Molecular Hydrogen Evolution in Neutral Water

Alternative materials to platinum-based catalysts are required to produce molecular hydrogen from water at low overpotentials. Transition-metal chalcogenide catalysts have attracted significant interest over the past few years because of their activity toward proton reduction and their relative abundance compared with platinum. We report the synthesis and characterization of a new type of iron sulfide (FeS, pyrrhotite) nanoparticles prepared via a solvothermal route. This material can achieve electrocatalysis for molecular hydrogen evolution with no structural decomposition or activity decrease for at least 6 days at a mild overpotential in neutral water at room temperature

Comparison of Porous Iron Trimesates Basolite F300 and MIL-100(Fe) As Heterogeneous Catalysts for Lewis Acid and Oxidation Reactions: Roles of Structural Defects and Stability

Two porous iron trimesates, namely, commercial Basolite F300 (Fe­(BTC); BTC = 1,3,5-benzenetricarboxylate) with unknown structure and synthetic MIL-100­(Fe) (MIL stands for Material of Institut Lavoisier) of well-defined crystalline structure, have been compared as heterogeneous catalysts for four different reactions. It was found that while for catalytic processes requiring strong Lewis acid sites, Fe­(BTC) performs better, MIL-100­(Fe) is the preferred catalyst for oxidation reactions. These catalytic results have been rationalized by a combined in situ infrared and ⁵⁷Fe Mössbauer spectroscopic characterization. It is proposed that the presence of extra Brønsted acid sites on the Fe­(BTC) and the easier redox behavior of the MIL-100­(Fe) could explain these comparative catalytic performances. The results illustrate the importance of structural defects (presence of weak Brønsted acid sites) and structural stability (MIL-100­(Fe) is stable upon annealing at 280 °C despite Fe³⁺-to-Fe²⁺ reduction) on the catalytic activity of these two solids, depending on the reaction type

Isomorphous Substitution in a Flexible Metal–Organic Framework: Mixed-Metal, Mixed-Valent MIL-53 Type Materials

Mixed-metal iron–vanadium analogues of the 1,4-benzenedicarboxylate (BDC) metal–organic framework MIL-53 have been synthesized solvothermally in <i>N</i>,<i>N</i>′-dimethylformamide (DMF) from metal chlorides using initial Fe:V ratios of 2:1 and 1:1. At 200 °C and short reaction time (1 h), materials (Fe,V)^II/IIIBDC­(DMF_1–<i>x</i>F_<i>x</i>) crystallize directly, whereas the use of longer reaction times (3 days) at 170 °C yields phases of composition [(Fe,V)^III_0.5(Fe,V)_0.5^II(BDC)­(OH,F)]^0.5–·0.5DMA⁺ (DMA = dimethylammonium). The identity of the materials is confirmed using high-resolution powder X-ray diffraction, with refined unit cell parameters compared to known pure iron analogues of the same phases. The oxidation states of iron and vanadium in all samples are verified using X-ray absorption near edge structure (XANES) spectroscopy at the metal K-edges. This shows that in the two sets of materials each of the vanadium and the iron centers are present in both +2 and +3 oxidation states. The local environment and oxidation state of iron is confirmed by ⁵⁷Fe Mössbauer spectrometry. Infrared and Raman spectroscopies as a function of temperature allowed the conditions for removal of extra-framework species to be identified, and the evolution of μ₂-hydroxyls to be monitored. Thus calcination of the mixed-valent, mixed-metal phases [(Fe,V)^III_0.5(Fe,V)_0.5^II(BDC)­(OH,F)]^0.5–·0.5DMA⁺ yields single-phase MIL-53-type materials, (Fe,V)^III(BDC)­(OH,F). The iron-rich, mixed-metal MIL-53 shows structural flexibility that is distinct from either the pure Fe material or the pure V material, with a thermally induced pore opening upon heating that is reversible upon cooling. In contrast, the material with a Fe:V content of 1:1 shows an irreversible expansion upon heating, akin to the pure vanadium analogue, suggesting the presence of some domains of vanadium-rich regions that can be permanently oxidized to V­(IV)

Superparamagnetic MFe₂O₄ (M = Fe, Co, Mn) Nanoparticles: Tuning the Particle Size and Magnetic Properties through a Novel One-Step Coprecipitation Route

Superparamagnetic ferrite nanoparticles (MFe₂O₄, where M = Fe, Co, Mn) were synthesized through a novel one-step aqueous coprecipitation method based on the use of a new type of alkaline agent: the alkanolamines isopropanolamine and diisopropanolamine. The role played by the bases on the particles’ size, chemical composition, and magnetic properties was investigated and compared directly with the effect of the traditional inorganic base NaOH. The novel MFe₂O₄ nanomaterials exhibited high colloidal stability, particle sizes in the range of 4–12 nm, and superparamagnetic properties. More remarkably, they presented smaller particle sizes (up to 6 times) and enhanced saturation magnetization (up to 1.3 times) relative to those prepared with NaOH. Furthermore, the nanomaterials exhibited improved magnetic properties when compared with nanoferrites of similar size synthesized by coprecipitation with other bases or by other methods reported in the literature. The alkanolamines were responsible for these achievements by acting both as alkaline agents and as complexing agents that controlled the particle size during the synthesis process and improved the spin rearrangement at the surface (thinner magnetic “dead” layers). These results open new horizons for the design of water-dispersible MFe₂O₄ nanoparticles with tuned properties through a versatile and easily scalable coprecipitation route

Series of Porous 3-D Coordination Polymers Based on Iron(III) and Porphyrin Derivatives

A new series of 3-D coordination polymers based on iron(III) and nickel(II) tetracarboxylate porphyrin (Ni-TCPP) have been produced using solvothermal conditions. MIL-141(A) solids (MIL stands for Material from Institut Lavoisier), formulated Fe(Ni-TCPP)A•(DMF)_<i>x </i>(A = Li, Na, K, Rb, Cs, DMF = N,N-dimethylformamide, <i>x</i> ∼ 3), are built up from three anionic interpenetrated PtS-type networks charge-balanced by alkali cations (A) entrapped inside the pores. MIL-141(A) thus includes three types of cations, two of which may act as coordinatively unsaturated metal sites (Ni²⁺ and A⁺). These solids all present a permanent porosity with a reasonably high surface area (S_BET = 510–860 m² g^–1) as well as some structural flexibility toward adsorption/desorption processes, modulated in both cases by the nature of A. Thermally Stimulated Current (TSC) measurements indicated that alkali cations are rather homogeneously distributed within the pores, while their interaction with the framework is stronger in MIL-141(A) than in the analogous cation-containing Faujasites X and Y zeolites. Finally, high pressure adsorption isotherms of N₂ and O₂ were measured. Whereas alkali ion-containing zeolites adsorb selectively N₂ toward O₂, the opposite is observed for MIL-141(A). This result is interpreted in light of the TSC data and the possible preferential interaction of the porphyrinic linker with O₂

Series of Porous 3-D Coordination Polymers Based on Iron(III) and Porphyrin Derivatives

A new series of 3-D coordination polymers based on iron(III) and nickel(II) tetracarboxylate porphyrin (Ni-TCPP) have been produced using solvothermal conditions. MIL-141(A) solids (MIL stands for Material from Institut Lavoisier), formulated Fe(Ni-TCPP)A•(DMF)_<i>x </i>(A = Li, Na, K, Rb, Cs, DMF = N,N-dimethylformamide, <i>x</i> ∼ 3), are built up from three anionic interpenetrated PtS-type networks charge-balanced by alkali cations (A) entrapped inside the pores. MIL-141(A) thus includes three types of cations, two of which may act as coordinatively unsaturated metal sites (Ni²⁺ and A⁺). These solids all present a permanent porosity with a reasonably high surface area (S_BET = 510–860 m² g^–1) as well as some structural flexibility toward adsorption/desorption processes, modulated in both cases by the nature of A. Thermally Stimulated Current (TSC) measurements indicated that alkali cations are rather homogeneously distributed within the pores, while their interaction with the framework is stronger in MIL-141(A) than in the analogous cation-containing Faujasites X and Y zeolites. Finally, high pressure adsorption isotherms of N₂ and O₂ were measured. Whereas alkali ion-containing zeolites adsorb selectively N₂ toward O₂, the opposite is observed for MIL-141(A). This result is interpreted in light of the TSC data and the possible preferential interaction of the porphyrinic linker with O₂