Sonication-assisted liquid phase exfoliation of two-dimensional CrTe3 under inert conditions

Liquid phase exfoliation (LPE) has been used for the successful fabrication of nanosheets from a large number of van der Waals materials. While this allows to study fundamental changes of material properties' associated with reduced dimensions, it also changes the chemistry of many materials due to a significant increase of the effective surface area, often accompanied with enhanced reactivity and accelerated oxidation. To prevent material decomposition, LPE and processing in inert atmosphere have been developed, which enables the preparation of pristine nanomaterials, and to systematically study compositional changes over time for different storage conditions. Here, we demonstrate the inert exfoliation of the oxidation-sensitive van der Waals crystal, CrTe3. The pristine nanomaterial was purified and size-selected by centrifugation, nanosheet dimensions in the fractions quantified by atomic force microscopy and studied by Raman, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX) and photo spectroscopic measurements. We find a dependence of the relative intensities of the CrTe3 Raman modes on the propagation direction of the incident light, which prevents a correlation of the Raman spectral profile to the nanosheet dimensions. XPS and EDX reveal that the contribution of surface oxides to the spectra is reduced after exfoliation compared to the bulk material. Further, the decomposition mechanism of the nanosheets was studied by time-dependent extinction measurements after water titration experiments to initially dry solvents, which suggest that water plays a significant role in the material decomposition.

Decoration of the [Nb6O19]8– cluster shell with six Cu2+-centred complexes generates the [(Cu(cyclen))6Nb6O19]4+ moiety: room temperature synthesis, crystal structure and selected properties

Mixing an aqueous solution of K8[Nb6O19]⋅16H2O with a DMSO/H2O solution of Cu(ClO4)2 · 6 H2O and cyclen at room temperature afforded crystallization of blue crystals of [(Cu(cyclen))6Nb6O19]⋅[ClO4]4·≈4H2O after slow evaporation of the solvents. The crystal structure contains the Lindqvist anion [Nb6O19]8– which is covalently expanded by six symmetry-related [Cu(cyclen)]2+ complexes via Nb-μ 2-O-Cu bridges yielding the positively charged [(Cu(cyclen))6Nb6O19]4+ cluster shell. The ClO4 − anions and crystal water molecules reside in the empty spaces of the packed clusters. The compound shows two electronic d-d transitions at energetic positions explaining the blue color.

Room-Temperature Solid-State Transformation of Na4 SnS4 ⋅ 14H2 O into Na4 Sn2 S6 ⋅ 5H2 O: An Unusual Epitaxial Reaction Including Bond Formation, Mass Transport, and Ionic Conductivity

A highly unusual solid-state epitaxy-induced phase transformation of Na₄ SnS₄  ⋅ 14H₂ O (I) into Na₄ Sn₂ S₆  ⋅ 5H₂ O (II) occurs at room temperature. Ab initio molecular dynamics (AIMD) simulations indicate an internal acid-base reaction to form [SnS₃ SH]^3- which condensates to [Sn₂ S₆ ]^4- . The reaction involves a complex sequence of O-H bond cleavage, S^2- protonation, Sn-S bond formation and diffusion of various species while preserving the crystal morphology. In situ Raman and IR spectroscopy evidence the formation of [Sn₂ S₆ ]^4- . DFT calculations allowed assignment of all bands appearing during the transformation. X-ray diffraction and in situ ¹ H NMR demonstrate a transformation within several days and yield a reaction turnover of ≈0.38 %/h. AIMD and experimental ionic conductivity data closely follow a Vogel-Fulcher-Tammann type T dependence with D(Na)=6×10^-14  m²  s^-1 at T=300 K with values increasing by three orders of magnitude from -20 to +25 °C.

Directed Dehydration as Synthetic Tool for Generation of a New Na 4 SnS 4 Polymorph: Crystal Structure, Na + Conductivity, and Influence of Sb‐Substitution

We present the convenient synthesis and characterization of the new ternary thiostannate Na₄SnS₄ (space group I41/acd ) by directed removal of crystal water molecules from Na₄SnS₄⋅14 H₂O. The compound represents a new kinetically stable polymorph of Na₄SnS₄, which is transformed into the known, thermodynamically stable form (space group P4‾21c ) at elevated temperatures. Thermal co‐decomposition of mixtures with Na₃SbS₄⋅9 H₂O generates solid solution products Na_4−xSn_1−xSb_xS₄ (x=0.01, 0.10) isostructural to the new polymorph (x=0). Incorporation of Sb⁵⁺ affects the bonding and local structural situation noticeably evidenced by X‐ray diffraction, ¹¹⁹Sn and ²³Na NMR, and ¹¹⁹Sn Mössbauer spectroscopy. Electrochemical impedance spectroscopy demonstrates an enormous improvement of the ionic conductivity with increasing Sb content for the solid solution (σ_25°C=2×10⁻³, 2×10⁻², and 0.1 mS cm⁻¹ for x=0, 0.01, and 0.10), being several orders of magnitude higher than for the known Na₄SnS₄ polymorph.

Composition-driven archetype dynamics in polyoxovanadates

Molecular metal oxides often adopt common structural frameworks (i.e. archetypes), many of them boasting impressive structural robustness and stability. However, the ability to adapt and to undergo transformations between different structural archetypes is a desirable material design feature offering applicability in different environments. Using systems thinking approach that integrates synthetic, analytical and computational techniques, we explore the transformations governing the chemistry of polyoxovanadates (POVs) constructed of arsenate and vanadate building units. The water-soluble salt of the low nuclearity polyanion [V₆As₈O₂₆]⁴⁻ can be effectively used for the synthesis of the larger spherical (i.e. kegginoidal) mixed-valent [V₁₂As₈O₄₀]⁴⁻ precipitate, while the novel [V₁₀As₁₂O₄₀]⁸⁻ POVs having tubular cyclic structures are another, well soluble product. Surprisingly, in contrast to the common observation that high-nuclearity polyoxometalate (POM) clusters are fragmented to form smaller moieties in solution, the low nuclearity [V₆As₈O₂₆]⁴⁻ anion is in situ transformed into the higher nuclearity cluster anions. The obtained products support a conceptually new model that is outlined in this article and that describes a continuous evolution between spherical and cyclic POV assemblies. This new model represents a milestone on the way to rational and designable POV self-assemblies.

Systems-based elucidation of the polyoxovanadate speciation reveals that heterogroup substitution can transform spherical kegginoids into tubular architectures in a programmable manner.

Structural characterization of sodium and potassium 3-nitrohydrogenphthalate coordination polymers

The synthesis, crystal structures and properties of two alkali metal 3-nitrohydrogenphthalates obtained by a 1:2 reaction of M2CO3 (M = K or Na) with 3-nitrophthalic acid (LH2) are reported. In the anhydrous potassium coordination polymer [K(LH)] (LH = 2-carboxy-3-nitrobenzoate) 1, the K+ cation is bonded to nine oxygen atoms from six symmetry related (LH)– ligands resulting in a distorted {KO9} coordination polyhedron. Five of the six oxygen atoms including a nitro oxygen atom of the crystallographically unique 2-carboxy-3-nitrobenzoate are involved in metal binding. The μ6-bridging mode of (LH)– places the K+ cations into the layers of the two-dimensional (2D) coordination polymer. Each {KO9} polyhedron in 1 shares edges with two other polyhedra along the b and c axes. A low temperature structure redetermination of [Na(L#H)(H2O)3]·H2O (L#H = 2-carboxy-6-nitrobenzoate) 2 has revealed that the (L#H)− anion is bonded to the Na+ cation in a monodentate fashion via the carbonyl oxygen atom of the –COOH group and two of the three unique aqua ligands exhibit a bridging bidentate mode stabilizing a chain polymer. The structure of compound 2 thus consists of chains of edge-sharing {NaO6} octahedra. Thermal decomposition of 1 or 2 results in the formation of metal carbonate residues.

Crystal structure of μ3-tetrathioantimonato-tris[(cyclam)zinc(II)] tetrathioantimonate acetonitrile disolvate dihydrate showing Zn disorder over the cyclam ring planes (cyclam = 1,4,8,11-tetraazacyclotetradecane)

In the crystal structure of the title compound, [Zn(cyclam)]²⁺ cations and SbS₄^3– anions are present, which are linked to aceto­nitrile and water solvate mol­ecules via inter­molecular hydrogen bonding.

Reaction of Zn(ClO₄)₂·6H₂O with cyclam (cyclam = 1,4,8,11-tetra­aza­cyclo­tetra­decane, C₁₀H₂₄N₄) and Na₃SbS₄ in an aceto­nitrile/water mixture led to the formation of crystals of the title compound, [Zn₃(SbS₄)(C₁₀H₂₄N₄)₃](SbS₄)·2CH₃CN·2H₂O or [(Zn-cyclam)₃(SbS₄)₂](H₂O)₂(aceto­nitrile)₂. The set-up of the crystal structure is similar to that of [(Zn-cyclam)₃(SbS₄)₂]^.8H₂O reported recently [Danker et al. (2021 ▸). Dalton Trans. 50, 18107–18117]. The crystal structure of the title compound consists of three crystallographically independent Zn^II cations (each disordered around centers of inversion), three centrosymmetric cyclam ligands, one SbS₄^3– anion, one water and one aceto­nitrile mol­ecule occupying general positions. The aceto­nitrile mol­ecule is equally disordered over two sets of sites. Each Zn²⁺ cation is bound to four nitro­gen atoms of a cyclam ligand and one sulfur atom of the SbS₄^3– anion within a distorted square-pyramidal coordination. The cation disorder of the [Zn(cyclam)]²⁺ complexes is discussed in detail and is also observed in other compounds, where identical ligands are located above and below the [Zn(cyclam)]²⁺ plane. In the title compound, the building units are arranged in layers parallel to the bc plane forming pores in which the aceto­nitrile solvate mol­ecules are located. Inter­molecular C—H⋯S hydrogen bonding links these units to the SbS₄^3– anions. Between the layers, additional water solvate mol­ecules are present that act as acceptor and donor groups for inter­molecular N—H⋯O and O—H⋯S hydrogen bonding.

Time-dependent investigation of a mechanochemical synthesis of bismuth telluride-based materials and their structural and thermoelectric properties

Here, we report on the time dependence of a synthesis procedure for generation of both n- and p-type bismuth telluride-based materials. To initiate the reaction, the starting materials were first mechanical pre-reacted. The Rietveld refinements of X-ray diffraction (XRD) data collected after different milling times demonstrate that Bi₂Te₃ was formed after only 10 min, and longer milling times do not alter the composition. To complete the phase formation, the powders were treated by field-assisted sintering and heat treatment afterwards. The effect of this fast procedure on the structural and thermoelectric properties was investigated. Samples were obtained with relative densities above 99%. A clear preferred orientation of the crystallites in the samples is evidenced by Rietveld refinements of XRD data. The thermoelectric characteristics demonstrate a good performance despite the short milling time. Further, it was demonstrated for this fast synthesis that the physical transport properties can be varied with well-known n- and p-type dopants like CHI₃ or Pb. For these non-optimized materials, a ZT value of 0.7 (n-type) and 0.9 (p-type) between 400 and 450 K was achieved. The long-term stability is demonstrated by repeated measurements up to 523 K showing no significant alteration of the thermoelectric performance.

Understanding sodium storage properties of ultra-small Fe3S4 nanoparticles - a combined XRD, PDF, XAS and electrokinetic study

Various electrode materials are considered for sodium-ion batteries (SIBs) and one important prerequisite for developments of SIBs is a detailed understanding about charge storage mechanisms. Herein, we present a rigorous study about Na storage properties of ultra-small Fe₃S₄ nanoparticles, synthesized applying a solvothermal route, which exhibit a very good electrochemical performance as anode material for SIBs. A closer look into electrochemical reaction pathways on the nanoscale, utilizing synchrotron-based X-ray diffraction and X-ray absorption techniques, reveals a complicated conversion mechanism. Initially, separation of Fe₃S₄ into nanocrystalline intermediates occurs accompanied by reduction of Fe³⁺ to Fe²⁺ cations. Discharge to 0.1 V leads to formation of strongly disordered Fe⁰ finely dispersed in a nanosized Na₂S matrix. The resulting volume expansion leads to a worse long-term stability in the voltage range 3.0-0.1 V. Adjusting the lower cut-off potential to 0.5 V, crystallization of Na₂S is prevented and a completely amorphous intermediate stage is formed. Thus, the smaller voltage window is favorable for long-term stability, yielding highly reversible capacity retention, e.g., 486 mAh g^-1 after 300 cycles applying 0.5 A g^-1 and superior coulombic efficiencies >99.9%. During charge to 3.0 V, Fe₃S₄ with smaller domains are reversibly generated in the 1^st cycle, but further cycling results in loss of structural long-range order, whereas the local environment resembles that of Fe₃S₄ in subsequent charged states. Electrokinetic analyses reveal high capacitive contributions to the charge storage, indicating shortened diffusion lengths and thus, redox reactions occur predominantly at surfaces of nanosized conversion products.

Synthesis and crystal structure of poly[[di-μ3-tetrathioantimonato-tris[(cyclam)cobalt(II)]] acetonitrile disolvate dihydrate] (cyclam = 1,4,8,11-tetraazacyclotetradecane)

In the crystal structure of the title compound, the [SbS₄]³⁻ anions are linked by the Co(cyclam) complex cations into rings, which are further connected into layers that are linked by inter­molecular hydrogen bonding via the water solvate mol­ecules and are arranged in such a way that cavities are formed, in which the disordered aceto­nitrile solvate mol­ecules are located.

Reaction of Co(ClO₄)₂·6H₂O with cyclam (cyclam = 1,4,8,11-tetra­aza­cyclo­tetra­deca­ne) and Na₃SbS₄·9H₂O (Schlippesches salt) in a mixture of aceto­nitrile and water leads to the formation of crystals of the title compound with the composition {[Co₃(SbS₄)₂(C₁₀H₂₄N₄)₃]·2CH₃CN·2H₂O}_n or {[(Co-cyclam)₃(SbS₄)₂]·2(aceto­nitrile)·2H₂O}_n. The crystal structure of the title compound consists of three crystallographically independent [Co-cyclam]²⁺ cations, which are located on centers of inversion, one [SbS₄]³⁻ anion, one water and one aceto­nitrile mol­ecule that occupy general positions. The aceto­nitrile mol­ecule is disordered over two orientations and was refined using a split model. The Co^II cations are coordinated by four N atoms of the cyclam ligand and two trans-S atoms of the tetra­thio­anti­monate anion within slightly distorted octa­hedra. The unique [SbS₄]³⁻ anion is coordinated to all three crystallographically independent Co^II cations and this unit, with its symmetry-related counterparts, forms rings composed of six Co-cyclam cations and six tetra­thio­anti­monate anions that are further condensed into layers. These layers are perfectly stacked onto each other so that channels are formed in which acetontrile solvate mol­ecules that are hydrogen bonded to the anions are embedded. The water solvate mol­ecules are located between the layers and are connected to the cyclam ligands and the [SbS₄]³⁻ anions via inter­molecular N—H⋯O and O—H⋯S hydrogen bonding.

Synthesis and structural characterization of a new heterometallicmolybdate coordination polymer based on a µ3-bridging amino alcohol

The reaction of Na2MoO4·2H2O with 2-amino-2-(hydroxymethyl)propane-1,3-diol (LH) in water at room temperature results in the formation of the heterometallic coordination polymer [Mo2O6L2(Na2(H2O)4)]·2H2O 1 (L = 2-amino-3-hydroxy-2-(hydroxymethyl)propan-1-olato). The structure of 1 consists of a neutral (Mo2O6) unit located on an inversion center. The Mo atoms exhibit hexa-coordination and are bonded to two terminal and two bridging oxido ligands, an alkoxide oxygen and the amine N atoms of an anionic ligand L– resulting in the formation of an edge-sharing {Mo2O8N2} bioctahedron. The Na+ cations of a centrosymmetric bis(μ2-aqua)-bridged (Na2(H2O)4)2+ unit are penta-coordinated and bonded to two symmetry related L– ligands via the oxygen atoms of their OH groups. The µ3-bridging tetradentate binding mode of L– results in the formation of a two-dimensional heterometallic coordination polymer. The constituents of 1 viz. (Mo2O6), (L)–, (Na2(H2O)4)2+ and lattice water molecules are interlinked with the aid of three varieties of hydrogen bonding interactions. The corresponding tungstate reported recently has been obtained through a similar synthetic protocol and is isostructural.

Synthetically Produced Isocubanite as an Anode Material for Sodium-Ion Batteries: Understanding the Reaction Mechanism During Sodium Uptake and Release

Bulk isocubanite (CuFe₂S₃) was synthesized via a multistep high-temperature synthesis and was investigated as an anode material for sodium-ion batteries. CuFe₂S₃ exhibits an excellent electrochemical performance with a capacity retention of 422 mA h g^-1 for more than 1000 cycles at a current rate of 0.5 A g^-1 (0.85 C). The complex reaction mechanism of the first cycle was investigated via PXRD and X-ray absorption spectroscopy. At the early stages of Na uptake, CuFe₂S₃ is converted to form crystalline CuFeS₂ and nanocrystalline NaFe_1.5S₂ simultaneously. By increasing the Na content, Cu⁺ is reduced to nanocrystalline Cu, followed by the reduction of Fe²⁺ to amorphous Fe⁰ while reflections of nanocrystalline Na₂S appear. During charging up to -5 Na/f.u., the intermediate NaFe_1.5S₂ appears again, which transforms in the last step of charging to a new unknown phase. This unknown phase together with NaFe_1.5S₂ plays a key role in the mechanism for the following cycles, evidenced by the PXRD investigation of the second cycle. Even after 400 cycles, the occurrence of nanocrystalline phases made it possible to gain insights into the alteration of the mechanism, which shows that Cu_xS phases play an important role in the region of constant specific capacity.

The cation and anion bonding modes make a difference: an unprecedented layered structure and a tri(hetero)nuclear moiety in thioantimonates(V)

Mixing solutions of M²⁺ (M = Cu²⁺ or Zn²⁺) salts containing cyclam (cyclam = 1,4,8,11-tetraazacyclotetradecane) as the ligand and an aqueous solution of Na₃SbS₄·9H₂O at room temperature led to the crystallization of two new compounds within minutes: {[Cu(cyclam)]₃[SbS₄]₂}_n·20nH₂O (I) and {[Zn(cyclam)]₃[SbS₄]₂}·8H₂O (II). In the structure of I [SbS₄]^3- anions acting as a tridentate ligand join CuN₄S₂ octahedra generating twelve-membered rings by corner-sharing of SbS₄ and CuN₄S₂ units. The rings are condensed into layers, which are stacked onto each other in a 6R polytype manner. The layers contain large pores with the water molecules located between the layers above and below the pores. In contrast, the structure of II comprises a discrete molecular tri(hetero)nuclear moiety with a bidentate [SbS₄]^3- anion connecting two rectangular pyramidal ZnN₄S polyhedra. The crystal water molecules of I and II can be thermally removed, and I and II are recovered by treatment under a humid atmosphere. The EPR spectrum of I indicates the presence of Cu²⁺ cations, which is unusual in the environment of S^2- anions. The different bonding situations and the preferences for the coordination geometries of Cu²⁺ and Zn²⁺ cations are rationalized by DFT based calculations, demonstrating that Cu²⁺ prefers an octahedral environment while Zn²⁺ adopts the square-pyramidal coordination. The pronounced differences in the vibrational spectra are also analyzed with DFT, showing how the different modes are influenced by the differing bond strengths.

Synthesis and crystal structure of bis-[μ-N,N-bis-(2-amino-eth-yl)ethane-1,2-di-amine]-bis-[N,N-bis-(2-amino-eth-yl)ethane-1,2-di-amine]-μ4-oxido-hexa-μ3-oxido-octa-μ2-oxido-tetra-oxido-tetra-nickel(II)hexa-tantalum(V) nona-deca-hydrate

Reaction of K8{Ta6O19}·16H2O with [Ni(tren)(H2O)Cl]Cl·H2O in different solvents led to the formation of single crystals of the title compound, [Ni4Ta6O19(C6H18N4)4]·19H2O or {[Ni2(κ4-tren)(μ-κ3-tren)]2Ta6O19}·19H2O (tren is N,N-bis-(2-amino-eth-yl)-1,2-ethanediamine, C6H18N4). In its crystal structure, one Lindqvist-type anion {Ta6O19}8- (point group symmetry ) is connected to two NiII cations, with both of them coordinated by one tren ligand into discrete units. Both NiII cations are sixfold coordinated by O atoms of the anion and N atoms of the organic ligand, resulting in slightly distorted [NiON5] octa-hedra for one and [NiO3N3] octa-hedra for the other cation. These clusters are linked by inter-molecular O-H⋯O and N-H⋯O hydrogen bonding involving water mol-ecules into layers parallel to the bc plane. Some of these water mol-ecules are positionally disordered and were refined using a split model. Powder X-ray diffraction revealed that a pure crystalline phase was obtained but that on storage at room-temperature this compound decomposed because of the loss of crystal water mol-ecules.

Long-Term Stable, High-Capacity Anode Material for Sodium-Ion Batteries: Taking a Closer Look at CrPS4 from an Electrochemical and Mechanistic Point of View

Electrochemical performance of the layered compound CrPS₄ for the usage as anode material in sodium-ion batteries (SIBs) was examined and exceptional reversible long-term capacity and capacity retention were found. After 300 cycles, an extraordinary reversible capacity of 687 mAh g^-1 at a current rate of 1 A g^-1 was achieved, while rate capability tests showed an excellent capacity retention of 100%. Detailed evaluation of the data evidence a change of the electrochemical reaction upon cycling leading to the striking long-term performance. Further investigations targeted the reaction mechanism of the first cycle by applying complementary techniques, i.e., powder X-ray diffraction (XRD), pair distribution function (PDF) analysis, X-ray absorption spectroscopy (XAS), and ²³Na/³¹P magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The results indicated an unexpectedly complex reaction pathway including formation of several intercalation compounds, depending on the amount of Na inserted at the early discharge states and subsequent conversion to Na₂S and strongly disordered metallic Cr at the completely discharged state. While XAS measurements suggest no further presence of intermediates after formation of Na intercalation compounds, several different phases are detected via MAS NMR upon continued discharging. Especially the data obtained from the MAS NMR investigations therefore point toward a very complex reaction pathway. Furthermore, solid electrolyte interphase (SEI) formation, resulting in the presence of NaF, was observed. After recharging the anode material, no structural long-range order occurred, but short-range order indeed resembled the local environment of the starting material, to a certain extent.

Superior Sodium Storage Properties in the Anode Material NiCr2 S4 for Sodium-Ion Batteries: An X-ray Diffraction, Pair Distribution Function, and X-ray Absorption Study Reveals a Conversion Mechanism via Nickel Extrusion

The pseudo-layered sulfide NiCr₂ S₄ exhibits outstanding electrochemical performance as anode material in sodium-ion batteries (SIBs). The Na storage mechanism is investigated by synchrotron-based X-ray scattering and absorption techniques as well as by electrochemical measurements. A very high reversible capacity in the 500th cycle of 489 mAh g^-1 is observed at 2.0 A g^-1 in the potential window 3.0-0.1 V. Full discharge includes irreversible generation of Ni⁰ and Cr⁰ nanoparticles embedded in nanocrystalline Na₂ S yielding shortened diffusion lengths and predominantly surface-controlled charge storage. During charge, Ni⁰ and Cr⁰ are oxidized, Na₂ S is consumed, and amorphous Ni and Cr sulfides are formed. Limiting the potential window to 3.0-0.3 V an unusual nickel extrusion sodium insertion mechanism occurs: Ni²⁺ is reduced to nanosized Ni⁰ domains, expelled from the host lattice, and is replaced by Na⁺ cations to form O3-type like NaCrS₂ . Surprisingly, the discharge and charge processes comprise Na⁺ shuttling between highly crystalline NiCr₂ S₄ and NaCrS₂ enabling a superior long-term stability for 3000 cycles. The results not only provide valuable insights for the electrochemistry of conversion materials but also extend the scope of layered electrode materials considering the reversible nickel extrusion sodium insertion reaction as new concept for SIBs.

Synthesis, crystal structure and selected properties of K2[Ni(dien)2]{[Ni(dien)]2Ta6O19}·11 H2O

The new compound K2[Ni(dien)2]{[Ni(dien)]2Ta6O19}·11 H2O crystallized at room temperature applying a diffusion based reaction in a H2O/DMSO mixture using K8{Ta6O19}·16 H2O, Ni(NO3)2·6H2O and dien (diethylenetriamine). In the crystal structure, the Lindqvist-type anion [Ta6O19]8– is structurally expanded by two octahedrally Ni2+-centered complexes via three Ni–µ 2-O–Ta bonds thus generating the new {[Ni(dien)]2Ta6O19}4– anion. Two KO8 polyhedra share a common edge to form a K2O14 moiety, which connects the {[Ni(dien)]2Ta6O19}4– cluster shells into chains. The isolated [Ni(dien)2]2+ complexes are located in voids generated by the structural arrangement of the chains. An extended hydrogen bonding network between the different constituents generates a 3D network. The crystal water molecules can be thermally removed to form a highly crystalline dehydrated compound. Partial water uptake leads to the formation of a crystalline intermediate with a reduced unit cell volume compared to the fully hydrated sample. Water sorption experiments demonstrate that the fully dehydrated sample can be fully reconverted to the hydrated compound. The crystal field splitting parameters for the octahedrally coordinated Ni2+-centered complexes have been evaluated from an UV/Vis spectrum yielding D q = 1056 cm−1 and B = 887 cm−1.

CuFeS2 as a Very Stable High-Capacity Anode Material for Sodium-Ion Batteries: A Multimethod Approach for Elucidation of the Complex Reaction Mechanisms during Discharge and Charge Processes

Highly crystalline CuFeS₂ containing earth-abundant and environmentally friendly elements prepared via a high-temperature synthesis exhibits an excellent electrochemical performance as an anode material in sodium-ion batteries. The initial specific capacity of 460 mAh g^-1 increases to 512 mAh g^-1 in the 150th cycle and then decreases to a still very high value of 444 mAh g^-1 at 0.5 A g^-1 in the remaining 550 cycles. Even for a large current density, a pronounced cycling stability is observed. Here, we demonstrate that combining the results of X-ray powder diffraction experiments, pair distribution function analysis, and ²³Na NMR and Mössbauer spectroscopy investigations performed at different stages of discharging and charging processes allows elucidation of very complex reaction mechanisms. In the first step after uptake of 1 Na/CuFeS₂, nanocrystalline NaCuFeS₂ is formed as an intermediate phase, which surprisingly could be recovered during charging. On increasing the Na content, Cu⁺ is reduced to nanocrystalline Cu, while nanocrystalline Na₂S and nanosized elemental Fe are formed in the discharged state. After charging, the main crystalline phase is NaCuFeS₂. At the 150th cycle, the mechanisms clearly changed, and in the charged state, nanocrystalline Cu_xS phases are observed. At later stages of cycling, the mechanisms are altered again: NaF, Cu₂S, and Cu_7.2S₄ appeared in the discharged state, while NaF and Cu₅FeS₄ are observed in the charged state. In contrast to a typical conversion reaction, nanocrystalline phases play the dominant role, which are responsible for the high reversible capacity and long-term stability.

Mechanochemical Synthesis and Magnetic Characterization of Nanosized Cubic Spinel FeCr2S4 Particles

Nanosized samples of the cubic thiospinel FeCr2S4 were synthesized by ball milling of FeS and Cr2S3 precursors followed by a distinct temperature treatment between 500 and 800 °C. Depending on the applied temperature, volume weighted mean (L vol) particle sizes of 56 nm (500 °C), 86 nm (600 °C), and 123 nm (800 °C) were obtained. All samples show a transition into the ferrimagnetic state at a Curie temperature T C of ∼ 167 K only slightly depending on the annealing temperature. Above T C, ferromagnetic spin clusters survive and Curie-Weiss behavior is observed only at T ≫ T C, with T depending on the heat treatments and the external magnetic field applied. Zero-field-cooled and field-cooled magnetic susceptibilities diverge significantly below T C in contrast to what is observed for conventionally solid-state-prepared polycrystalline samples. In the low-temperature region, all samples show a transition into the orbital ordered state at about 9 K, which is more pronounced for the samples heated to higher temperatures. This observation is a clear indication that the cation disorder is very low because a pronounced disorder would suppress this magnetic transition. The unusual magnetic properties of the samples at low temperatures and different external magnetic fields can be clearly related to different factors like structural microstrain and magnetocrystalline anisotropy.

Three intersecting {V12} rings: {V30Sb8}, an ultra-large polyoxovanadate cluster shell

[VIV30SbIII8O78]12-, currently the largest known antimonato-polyoxovanadate (Sb-POV), features three perpendicular, intersecting 12-membered rings of edge-sharing O4V[double bond, length as m-dash]O square pyramids. While in two rings the apices of all O4V[double bond, length as m-dash]O pyramids point outwards, four apices of the third ring are directed into the cavity of the cluster shell, a concave structural motif not previously observed in polyoxovanadate chemistry. SbIII centers cap the eight niches defined by the octands of the {V30O78} cluster shell, resulting in discrete trigonal pyramidal SbO3 units, a second unprecedented feature. Within the resulting spin topology with numerous local geometrically frustrated motifs, the 30 spin-1/2 sites couple antiferromagnetically via a complex set of exchange pathways.

Hexaniobate anions connected by [Ni(cyclam)]2+ complexes yield two interpenetrating three-dimensional networks

Syntheses were performed at room temperature using Ni(NO3)2·6H2O, cyclam (cyclam = 1,4,8,11-tetraazacyclotetradecane) and the precursors Li8[Nb6O19]· ≈22H2O or Na7[HNb6O19]·15H2O in a DMSO-H2O mixture. Yellow crystals of the new compound {[Ni(cyclam)]2H4Nb6O19}·12H2O could be obtained after one week applying the Li+ or Na+ salt as starting materials. The crystal structure is unique in polyoxoniobate (PONb) chemistry and displays two interpenetrating three-dimensional (3D) networks. The [Nb6O19]8– anion is expanded by four Ni2+ centered complexes via Ni–O bonds to terminal O2− anions of the hexaniobate anion. The 3D networks are generated by further Ni–O bond formation between neighboring [Nb6O19]8− anions. The remaining void space is occupied by H2O molecules which form a water cluster.

Synthesis and crystal structure of (1,4,7,10-tetra-aza-cyclo-dodecane-κ4 N)(tetra-sulfido-κ2 S 1,S 4)manganese(II)

The title compound, [Mn(S4)(C8H20N4)], was accidentally obtained by the hydro-thermal reaction of Mn(ClO4)2·6H2O, cyclen (cyclen = 1,4,7,10-tetra-aza-cyclo-dodeca-ne) and Na3SbS4·9H2O in water at 413 K, indicating that polysulfide anions might represent inter-mediates in the synthesis of thio-metallate compounds using Na3SbS4·9H2O as a reactant. X-ray powder diffraction proves that the sample is slightly contaminated with NaSb(OH)6 and an unknown crystalline phase. The crystal investigated was twinned with a twofold rotation axis as the twin element, and therefore a twin refinement using data in HKLF-5 format was performed. The asymmetric unit of the title compound consists of one MnII cation, one [S4]2- anion and one cyclen ligand in general positions. The MnII cation is sixfold coordinated by two cis-S atoms of the [S4]2- anions, as well as four N atoms of the cyclen ligand within an irregular coordination. The complexes are linked via pairs of N-H⋯S hydrogen bonds into chains, which are further linked into layers by additional N-H⋯S hydrogen bonding. These layers are connected into a three-dimensional network by inter-molecular N-H⋯S and C-H⋯S hydrogen bonding. It is noted that only one similar complex with MnII is reported in the literature.

Synthesis and crystal structure of catena-poly[[bis[(2,2';6',2''-terpyridine)-manganese(II)]-μ4-penta-thio-dianti-monato] tetra-hydrate] showing a 1D MnSbS network

In the crystal structure of the title compound, two [Sb₂S₅ [anions built up of two SbS₃ units sharing common corners with each linked by two [Mn(terpyridine)]²⁺ cations into chains that are further linked into a 3D network by inter­molecular O—H⋯O and O—H⋯S hydrogen bonding.

The asymmetric unit of the title compound, {[Mn₂Sb₂S₅(C₁₅H₁₁N₃)₂]·4H₂O}_n, consists of two crystallographically independent Mn^II ions, two unique terpyridine ligands, one [Sb₂S₅]⁴⁻ anion and four solvent water mol­ecules, all of which are located in general positions. The [Sb₂S₅]⁴⁻ anion consists of two SbS₃ units that share common corners. Each of the Mn^II ions is fivefold coordinated by two symmetry-related S atoms of [Sb₂S₅]⁴⁻ anions and three N atoms of a terpyridine ligand within an irregular coordination. Each two anions are linked by two [Mn(terpyridine)]²⁺ cations into chains along the c-axis direction that consist of eight-membered Mn₂Sb₂S4 rings. These chains are further connected into a three-dimensional network by inter­molecular O—H⋯O and O—H⋯S hydrogen bonds. The crystal investigated was twinned and therefore, a twin refinement using data in HKLF-5 [Sheldrick (2015 ▸). Acta Cryst. C71, 3–8] format was performed.

New pronounced progress in the synthesis of group 5 polyoxometalates

This highlight summarizes new developments made in group 5 polyoxometalate science of high nuclearity clusters with focus on synthetic approaches.