Universität Stuttgart
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Item Open Access Confirmation of siderazot, Fe3N1.33, the only terrestrial nitride mineral(2021) Bette, Sebastian; Theye, Thomas; Bernhardt, Heinz-Jürgen; Clark, William P.; Niewa, RainerSiderazot, the only terrestrial nitride mineral, was reported only once in 1876 to occur as coating on volcanic rocks in a fumarolic environment from Mt. Etna and, to date, has been neither confirmed nor structurally characterized. We have studied the holotype sample from the Natural History Museum, London, UK, originally collected by O. Silvestri in 1874, and present siderazot with epsilon-Fe3N-type crystal structure and composition of Fe3N1.33(7) according to crystal structure Rietveld refinements, in good agreement with electron microprobe analyses. Crystal structure data, chemical composition, and Raman and reflectance measurements are reported. Possible formation conditions are derived from composition and phase stability data according to synthetic samples.Item Open Access The reduced nitridogermanates(III) Ca6[Ge2N6] and Sr6[Ge2N6] with Ge-Ge bonds(2021) Link, Lukas; Pathak, Manisha; Jach, Franziska; Koželj, Primoz; Ormeci, Alim; Höhn, Peter; Niewa, RainerThe first nitridogermanates(III) Ca6[Ge2N6] and Sr6[Ge2N6] were synthesized from sodium flux and structurally characterized by powder and single crystal X‐ray diffraction, respectively. They crystallize isostructurally to each other and homeotypic to Ca6[Cr2N6]H in space group R3‾. They feature unprecedented, mutually isolated, ethane‐like [GeIII2N6]12- anions in a staggered conformation. The compounds are semiconductors according to resistivity measurements and electronic structure calculations, yielding band gaps of 1.1 eV for Ca6[Ge2N6] and 0.2 eV for Sr6[Ge2N6].Item Open Access Magnetic and structural properties of barium hexaferrite BaFe12O19 from various growth techniques(2017) Vinnik, Denis A.; Tarasova, Aleksandra Yu.; Zherebtsov, Dmitry A.; Gudkova, Svetlana A.; Galimov, Damir M.; Zhivulin, Vladimir E.; Trofimov, Evgeny A.; Nemrava, Sandra; Perov, Nikolai S.; Isaenko, Ludmila I.; Niewa, RainerBarium hexaferrite powder samples with grains in the m-range were obtained from solid-state sintering, and crystals with sizes up to 5 mm grown from PbO, Na2CO3, and BaB2O4 fluxes, respectively. Carbonate and borate fluxes provide the largest and structurally best crystals at significantly lower growth temperatures of 1533 K compared to flux-free synthesis (1623 K). The maximum synthesis temperature can be further reduced by the application of PbO-containing fluxes (down to 1223 K upon use of 80 at % PbO), however, Pb-substituted crystals Ba1-xPbxFe12O19 with Pb contents in the range of 0.23(2) x 0.80(2) form, depending on growth temperature and flux PbO content. The degree of Pb-substitution has only a minor influence on unit cell and magnetic parameters, although the values for Curie temperature, saturation magnetization, as well as the coercivity of these samples are significantly reduced in comparison with those from samples obtained from the other fluxes. Due to the lowest level of impurities, the samples from carbonate flux show superior quality compared to materials obtained using other methods.Item Open Access Die reduzierten Nitridogermanate(III) Ca6[Ge2N6] und Sr6[Ge2N6] mit Ge-Ge-Bindungen(2021) Link, Lukas; Pathak, Manisha; Jach, Franziska; Koželj, Primoz; Ormeci, Alim; Höhn, Peter; Niewa, RainerDie ersten Nitridogermanate(III) wurden in Form der Verbindungen Ca6[Ge2N6] und Sr6[Ge2N6] mithilfe von Natrium als Flussmittel synthetisiert und durch Röntgenbeugung am Pulver beziehungsweise am Einkristall charakterisiert. Ca6[Ge2N6] und Sr6[Ge2N6] kristallisieren isostrukturell zueinander und homöotyp zu Ca6[Cr2N6]H in der Raumgruppe R-3. Sie weisen neuartige, voneinander isolierte, Ethan-analoge [GeIII2N6]12--Anionen in gestaffelter Konformation auf. Widerstandsmessungen und Berechnungen der elektronischen Struktur zufolge handelt es sich um Halbleiter mit Bandlücken von 1.1 eV für Ca6[Ge2N6] und 0.2 eV für Sr6[Ge2N6].Item Open Access Thermal history dependent Al distribution in aluminum substituted strontium hexaferrite(2020) Häßner, Manuel; Vinnik, Denis A.; Niewa, RainerSingle crystals of aluminum substituted strontium hexaferrite SrFe12-xAlxO19 were grown from sodium oxide based flux. The substitution level aimed for was x = 1.2. Annealing experiments performed on single crystals show that the Al distribution on the five iron sites of the hexaferrite structure depends on the annealing time at 900 °C. Single crystal X-ray diffractometry shows that annealing a crystal after the initial synthesis has an impact on the Al content on the octahedrally and tetrahedrally coordinated sites. Furthermore, it was found that heating in a corundum crucible increases the overall Al content. Magnetic measurements show that annealing in a platinum or corundum crucible decreases coercivity and remanence while the saturation magnetization is hardly influenced.Item Open Access Crystal structure and phase stability of Co2N: a combined first-principles and experimental study(2021) Ikeda, Yuji; Lehmann, Tanja S.; Widenmeyer, Marc; Coduri, Mauro; Grabowski, Blazej; Niewa, RainerThe crystal structure and phase stability of Co2N are revisited based on experiments and first-principles calculations. Powder X-ray diffraction (PXRD) measurements and Rietveld refinements clearly confirm that the stable crystal structure of Co2N is an isotype of η-Fe2C and Co2C with the space group Pnnm rather than the closely related ζ-Fe2N with the space group Pbcn. The refined lattice parameters of Co2N in the Pnnm structure are a = 4.6108(1) Å, b = 4.3498(1) Å, c = 2.85592(7) Å, obtained from X-ray diffraction using synchrotron radiation. Furthermore, differential scanning calorimetry (DSC) with subsequent diffraction experiments reveal an endothermal transition to an ε-type order at 398 °C followed by an exothermal decomposition at 446 °C. First-principles density-functional-theory (DFT) calculations including the Hubbard U correction (DFT+U) demonstrate that it is essential for transition metal nitrides to consider strong electron correlation to predict the correct experimental structure and magnetic state. In particular, an effective value of Ueff = 2.75 eV can be utilized to obtain an antiferromagnetic Pnnm phase of Co2N in agreement with experiments.Item Open Access Li16Sr6Ge6N, Li16Sr6Ge6.5 and related lithium alkaline‐earth metal tetrelides : alternative filling of voids by nitride or tetrelide ions(2022) Höhn, Peter; Pathak, Manisha; Prots, Yurii; Ovchinnikov, Alexander; Schmidt, Marcus; Bobnar, Matej; Krnel, Mitja; Ormeci, Alim; Niewa, RainerLarge black single crystals with a metallic luster of Li16Sr6Ge6N and several other representatives of the series Li16Ae6Tt6N and Li16Ae6Tt6.5 (Ae=Ca, Sr; Tt=Si, Ge, Sn, Pb) were grown from mixtures of the respective elements with addition of binary alkaline‐earth metal nitrides or lithium nitride in the case of the nitrides. For the synthesis a modified high‐temperature centrifugation‐aided filtration (HTCAF) technique using reactive lithium melts was employed. These metallic phases crystallize in an ordered defect‐variant of the Sc11Ir4 type with selective occupation of the smaller octahedral voids in the origin (000) with N and the larger rhombic dodecahedral voids in (00 1/2) with Tt. Charge balance assuming the presence of exclusively closed shell ions for all examples accounts for an electronic excess. Diamagnetism despite metallic properties is consistent with results from electronic structure calculations.Item Open Access De-hydrogenation/rehydrogenation properties and reaction mechanism of AmZn(NH2)n-2nLiH systems (A = Li, K, Na, and Rb)(2022) Cao, Hujun; Pistidda, Claudio; Richter, Theresia M. M.; Capurso, Giovanni; Milanese, Chiara; Tseng, Jo-Chi; Shang, Yuanyuan; Niewa, Rainer; Chen, Ping; Klassen, Thomas; Dornheim, MartinWith the aim to find suitable hydrogen storage materials for stationary and mobile applications, multi-cation amide-based systems have attracted considerable attention, due to their unique hydrogenation kinetics. In this work, AmZn(NH2)n (with A = Li, K, Na, and Rb) were synthesized via an ammonothermal method. The synthesized phases were mixed via ball milling with LiH to form the systems AmZn(NH2)n-2nLiH (with m = 2, 4 and n = 4, 6), as well as Na2Zn(NH2)4∙0.5NH3-8LiH. The hydrogen storage properties of the obtained materials were investigated via a combination of calorimetric, spectroscopic, and diffraction methods. As a result of the performed analyses, Rb2Zn(NH2)4-8LiH appears as the most appealing system. This composite, after de-hydrogenation, can be fully rehydrogenated within 30 s at a temperature between 190 °C and 200 °C under a pressure of 50 bar of hydrogen.Item Open Access Aging‐driven composition and distribution changes of electrolyte and graphite anode in 18650‐type Li‐ion batteries(2022) Petz, Dominik; Baran, Volodymyr; Peschel, Christoph; Winter, Martin; Nowak, Sascha; Hofmann, Michael; Kostecki, Robert; Niewa, Rainer; Bauer, Michael; Müller‐Buschbaum, Peter; Senyshyn, AnatoliyA series of low‐temperature studies on LiNi0.80Co0.15Al0.05O2 18650‐type batteries of high‐energy type with different stabilized states of fatigue is carried out using spatially resolved neutron powder diffraction, infrared/thermal imaging, and quasi‐adiabatic calorimetry. In‐plane distribution of lithium in the graphite anode and frozen electrolyte in fully charged state is determined non‐destructively with neutron diffraction and correlated to the introduced state of fatigue. An independent electrolyte characterization is performed via calorimetry studies on variously aged 18650‐type lithium‐ion batteries, where the shape of the thermodynamic signal is evolving with the state of fatigue of the cells. Analyzing the liquid electrolyte extracted/harvested from the studied cells reveals the decomposition of conducting salt to be the main driving factor for fatigue in the electrolyte degradation.Item Open Access Approaching dissolved species in ammonoacidic GaN crystal growth: a combined solution NMR and computational study(2020) Becker, Peter; Wonglakhon, Tanakorn; Zahn, Dirk; Gudat, Dietrich; Niewa, Rainer