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 Indium ammoniates from ammonothermal synthesis : InAlF6(NH3)2, [In(NH3)6][AlF6], and [In2F(NH3)10]2[SiF6]5 ∙ 2 NH3(2021) Becker, Peter; Cekovski, Toni Boris; Niewa, RainerThe ammonothermal synthesis of three ammoniates of indium, namely InAlF6(NH3)2, [In(NH3)6][AlF6], and [In2F(NH3)10]2[SiF6]5 ∙ 2 NH3 was successful from near-ammononeutral conditions in the presence of fluoride ions. Initially, all these compounds were obtained upon corrosion of the applied liner and crucible material Si3N4, which also contains small amounts of aluminum. The syntheses were performed in supercritical ammonia (T = 753 K, p up to 307 MPa). The crystal structures were solved and refined from single crystal X-ray diffraction intensity data. InAlF6(NH3)2 crystallizes as a typical layer-type structure with corner-sharing [InF4(NH3)2]- and [AlF6]3- octahedra. [In(NH3)6][AlF6] features isolated [In(NH3)6]3+ and [AlF6]3- octahedra. The crystal structure of [In2F(NH3)10]2[SiF6]5 ∙ 2 NH3 contains [(NH3)5In-F-In(NH3)5]5+ octahedra doubles next to [SiF6]2- octahedra and ammonia molecules. All intermediates have strong hydrogen bonding systems. The results from vibrational spectroscopy are reported.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 Metal-rich ternary perovskite nitrides(2019) Niewa, RainerResearch interest in inverse perovskite nitrides, since the early beginnings in the 1940s has considerably intensified in recent years. Within the last decades exploration lead to a wide variety of new compounds, compositions and structural arrangements. Electronic properties of the novel materials span from insulating and semiconducting via semimetallic and metallic, depending on element combination. Similarly, magnetic properties qualify for various applications, according to frequently high Curie temperatures and saturation magnetizations, together with development of delicate magnetic structures and often occurring metamagnetic transitions, to give only few examples. This minireview is intended to give an overview on formation of such metal-rich compounds with focus on chemical systems and crystal chemistry.Item Open Access The quasi-binary acetonitriletriide Sr3[C2N]2(2019) Clark, William P.; Köhn, Andreas; Niewa, RainerThe first quasi-binary acetonitriletriide Sr3[C2N]2 has been synthesised and characterised. The nearly colourless crystals were obtained from the reaction of Sr metal, graphite, and elemental N2, generated by decomposition of Sr(N3)2, in a sealed Ni ampoule with the aid of an alkali metal flux. The structure of this compound was analysed via single-crystal Xray diffraction and the identity of the [C2N]3- anion was confirmed by Raman spectroscopy and further investigated by quantum-chemical methods. Computed interatomic distances within the [C2N]3- anion strikingly match the obtained experimental data.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 Synthesis and characterization of the amidomanganates Rb2[Mn(NH2)4] and Cs2[Mn(NH2)4](2021) Bäucker, Christian; Bauch, Soeren; Niewa, RainerWe report the successful synthesis of Rb2[Mn(NH2)4] and Cs2[Mn(NH2)4] from ammonothermal conditions at 723 K and pressures above 850 bar. Both compounds were obtained single phase according to powder X-ray diffraction. The crystal structures were determined by single crystal X-ray diffraction. For Rb2[Mn(NH2)4] we have obtained the high-temperature phase. The structures are analyzed with respect to the earlier reported alkali metal amidomanganates. Upon heating in inert atmosphere Cs2[Mn(NH2)4] decomposes to manganese nitrides. IR spectroscopic results are reported.
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