Browsing by Author "Engel, Katja"

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    Chronoamperometric investigations on electrochemical synthesis of iron nitrides in molten salt system
    (2024) Thiebes, Yannick M.; Engel, Katja; Niewa, Rainer
    A systematic investigation of the electrochemical iron nitride synthesis in a LiCl/KCl salt melt at 723 K shows an optimum of ϵ-Fe 3N 1+xformation in the range of 2.2 to 2.3 V and for γ′-Fe 4N between 2.4 and 2.5 V enabling the control of the desired iron nitride phase by setting the supplied terminal voltage. The product formation of iron nitrides starts when the electrochemical window is reached, which could be verified by linear sweep voltammetry. Hence, a maximum of nitrogen content in the formed iron nitride phases is observed for 2.27 V. During elongated synthesis periods, convection emerges as the predominant transport mechanism hindering an accelerated reaction rate with higher overpotential applied. Real-time analysis of the background current allows conclusions about the remaining nitride concentration. Additionally, there is concurrent iron nitride formation at the electrode surface through nitride adsorption and autonucleation-induced precipitation of iron and nitride ions. The analysis of the amount of sediment in comparison to the layer thickness of the nitrided working electrode suggests that the autonucleation mechanism dominates over the adsorption mechanism with increasing overpotential and can be further enhanced by this feature.
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    RbEr2AsS7 : a rubidium-containing erbium sulfide thioarsenate(III) with (S2)2- Ligands According to RbEr2S(S2)[AsS2(S2)]
    (2023) Engel, Katja; Schleid, Thomas
    The new rubidium-containing erbium sulfide thioarsenate(III) with the structured formula RbEr2S(S2)[AsS2(S2)] was obtained from the syntheses of elemental erbium (Er), arsenic sesquisulfide (As2S3) and rubidium sesquisulfide (Rb2S3) with elemental sulfur (S) at 773 K as transparent, orange, needle-shaped crystals. RbEr2AsS7 crystallizes monoclinically in the space group C2/c with a = 2339.86(12) pm, b = 541.78(3) pm, c = 1686.71(9) pm and β = 93.109(3) ° for Z = 8. The crystal structure features complex [AsS2(S2)]3- anions with two S2- anions and a (S2)2- disulfide dumbbell coordinating end-on as ligands for each As3+ cation. Even outside the ligand sphere of As3+, S2- and (S2)2- can be found as sulfide anions. Two distinct Er3+ cations are surrounded by either nine or seven sulfur atoms. The [ErS9] polyhedra are corner- and face-connected, while the [ErS7] units share common edges, both building chains along [010]. These different chains undergo edge connectivity with each other, resulting in the formation of corrugated layers, which are held together by Rb+ in chains of condensed [RbS9] polyhedra. So, a three-dimensional network is generated, offering empty channels along [010] apt to take up the As3+ lone-pair cations. Wavelength-dispersive X-ray spectroscopy verified a molar Rb:Er:As:S ratio of approximately 1:2:1:7 and diffuse reflectance spectroscopy showed the typical f-f transitions of Er3+, while the optical band gap was found to be 2.42 eV.