Forschen

The cubic structure of Li₃As stabilized by pressure or configurational entropy via the solid solution Li₃As–Li₂Se (externer Link, öffnet neues Fenster)

The hexagonal to cubic phase transition of Li₃As was investigated at high pressure and temperature, revealing a cubic high-pressure polymorph in the Li₃Bi structure type. This cubic structure type is preserved in the solid solution of Li₃As–Li₂Se synthesized via mechanochemical ball milling. The solid solutions were investigated via X-ray powder diffraction, showing a linear dependency of the lattice parameter a on the mole fraction of the boundary phases Li₃As and Li₂Se, according to Vegard's law. Configurational entropy is generated by mixed anion lattice occupation between arsenide and selenide and therefore stabilizes the cubic structure of the solid solution. At elevated temperatures, the solid solution of Li₃As–Li₂Se reveals an exsolution process by forming the boundary phases Li₃As and Li₂Se, proving the metastable character of the system. Impedance spectroscopy was used to determine the lithium-ion conductivities in the Li₃As–Li₂Se system, showing significantly higher conductivity values (∼10⁻⁴ to 10⁻⁶ S cm⁻¹ at 50 °C) compared to the pure end members Li₃As (∼10⁻⁷ S cm⁻¹ at 50 °C) and Li₂Se (∼10⁻⁷ S cm⁻¹ at 175 °C).

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The cubic structure of Li₃As stabilized by substitution – Li₈TtAs₄ (Tt = Si, Ge) and Li₁₄TtAs₆ (Tt = Si, Ge, Sn) and their lithium ion conductivity (externer Link, öffnet neues Fenster)

The new lithium arsenidotetrelates Li₈SiAs₄, Li₈GeAs₄, Li₁₄SiAs₆, Li₁₄GeAs₆ and Li₁₄SnAs₆ were synthesized via ball milling and structurally characterized by Rietveld analysis of X-ray powder diffraction data. The aliovalent substitution of lithium in hexagonal Li₃As by introducing a tetravalent tetrel cation stabilizes cubic structures for Li₈TtAs₄ (Tt = Si, Ge) in the space group Pa and for the lithium richer compound Li₁₄TtAs₆ (Tt = Si, Ge, Sn) in the higher symmetrical space group Fmm (no. 225). Thermal properties of the arsenidotetrelates were investigated via high temperature powder diffraction and differential thermal analysis revealing a decomposition process of the lithium richer arsenidotetrelate (Li₁₄TtAs₆ → Li₈TtAs₄ + 2Li₃As) into the lithium poorer arsenidotetrelates and lithium arsenide at moderate temperatures. Impedance spectroscopy shows moderate to good lithium ion conductivity for the lithium arsenidotetrelates.

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Li₃As and Li₃P revisited: DFT modelling on phase stability and ion conductivity (externer Link, öffnet neues Fenster)

Phase pure Li₃As and Li₃P were synthesized from the elements by a high temperature route. Crystal structures were refined from powder X-ray diffraction data. The title compounds were further characterized by difference thermal analysis, temperature dependent X-ray powder diffraction and impedance spectroscopy, proving unexpected Li ion conductivity for Li₃As. High pressure behaviour of the title compounds was modeled via density functional theory, confirming the experimentally reported cubic modifications of Li₃P and Li₃As.

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Polyoxometalates in the Hofmeister series (externer Link, öffnet neues Fenster)

We propose a simple experimental procedure based on the cloud point measurement of a non-ionic surfactant as a tool for (i) estimating the super-chaotropic behaviour of polyoxometalates (POMs) and for (ii) establishing a classification of POMs according to their affinity towards polar surfaces.

Polyoxometalates (POMs) are discrete nanometer sized anionic oxo-metal clusters that consist of early transition metals, especially V, Mo, and W and often incorporate a heteroatom.^1,2 (externer Link, öffnet neues Fenster) Due to their unique structural and electronic versatility, POMs find many applications in biology,^3–6 (externer Link, öffnet neues Fenster) as a phasing tool in protein crystallography,⁷ (externer Link, öffnet neues Fenster) as (photo)-catalytically active oxidants^8,9 (externer Link, öffnet neues Fenster) and in molecular materials.^10–13 (externer Link, öffnet neues Fenster)

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Polyoxometalate/Polyethylene Glycol Interactions in Water: From Nanoassemblies in Water to Crystal Formation by Electrostatic Screening (externer Link, öffnet neues Fenster)

In the last decade organic–inorganic hybrid materials have become essential in materials science as they combine properties of both building blocks. Nowadays the main routes for their synthesis involve electrostatic coupling, covalent grafting, and/or solvent effects. In this field, polyoxometalates (POMs) have emerged as interesting inorganic functional building blocks due to their outstanding properties. In the present work the well-known α-Keggin polyoxometalate, α-PW₁₂O₄₀³⁻ (PW), is shown to form hybrid crystalline materials with industrial (neutral) polyethylene glycol oligomers (PEG) under mild conditions, that is, in aqueous medium and at room temperature. The formation of these materials originates from the spontaneous self-assembly of PW with EO_x, (EO=ethylene oxide) with at least four EO units (x>4). The PW–PEG nanoassemblies, made of a POM surrounded by about two PEG oligomers, are stabilized by electrostatic repulsions between the negatively charged PW anions. Addition of NaCl, aimed at screening the inter-nanoassembly repulsions, induces aggregation and formation of hybrid crystalline materials. Single-crystal analysis showed a high selectivity of PW towards EO₅–EO₆ oligomers from PEG200, which is made of a mixture of EO_3–8. Therefore, a general “soft” route to produce POM–organic composites is proposed here through the control of electrostatic repulsions between spontaneously formed nanoassemblies in water. However, this rational design of new POM hybrid (crystalline) materials with hydrophilic blocks, using such a simple mixing procedure of the components, requires a deep understanding of the molecular interactions.

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Preparation of Magnesium, Cobalt and Nickel Ferrite Nanoparticles from Metal Oxides using Deep Eutectic Solvents (externer Link, öffnet neues Fenster)

Natural deep eutectic solvents (DESs) dissolve simple metal oxides and are used as a reaction medium to synthesize spinel-type ferrite nanoparticles MFe₂O₄ (M=Mg, Zn, Co, Ni). The best results for phase-pure spinel ferrites are obtained with the DES consisting of choline chloride (ChCl) and maleic acid. By employing DESs, the reactions proceed at much lower temperatures than usual for the respective solid-phase reactions of the metal oxides and at the same temperatures as synthesis with comparable calcination processes using metal salts. The method therefore reduces the overall required energy for the nanoparticle synthesis. Thermogravimetric analysis shows that the thermolysis process of the eutectic melts in air occurs in one major step. The phase-pure spinel-type ferrite particles are thoroughly characterized by X-ray diffraction, diffuse-reflectance UV/Vis spectroscopy, and scanning electron microscopy. The properties of the obtained nanoparticles are shown to be comparable to those obtained by other methods, illustrating the potential of natural DESs for processing metal oxides.

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