Browsing by Author "Niewa, Rainer"

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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, Anatoliy
A 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.
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
Open Access
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.
Open Access
Confirmation of siderazot, Fe3N1.33, the only terrestrial nitride mineral
(2021) Bette, Sebastian; Theye, Thomas; Bernhardt, Heinz-Jürgen; Clark, William P.; Niewa, Rainer
Siderazot, 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.
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, Rainer
The 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.
Open Access
Crystal structure and XPS study of titanium-substituted M-type hexaferrite BaFe12−xTixO19
(2023) Mehnert, Kim-Isabelle; Häßner, Manuel; Dreer, Yanina Mariella; Biswas, Indro; Niewa, Rainer
The M-type barium hexaferrite substituted with titanium, BaFe12−xTixO19, was synthesized from sodium carbonate flux and the obtained single crystals with a maximum degree of substitution of up to about x = 0.9 were characterized. XPS measurements were carried out for the identification of side products and in particular in order to assign the valence states of the transition-metal constituents. Due to the aliovalent exchange of iron(III) with titanium(IV), an additional charge balance needs to occur. No titanium(III) was detected, while the amount of iron(II) increased in the same order of magnitude as the amount of titanium(IV); thus, the major charge balancing is attributed to the reduction of iron(III) to iron(II). According to the XPS data, the amount of titanium(IV) typically is slightly higher than that of iron(II). This is in line with a tendency to a minor formation of vacancies on the transition-metal sites becoming more important at higher substitution levels according to PXRD and WDS measurements, completing the picture of the charge-balance mechanism. XRD taken on single crystals indicates the distribution of titanium and vacancies over three of the five transition-metal sites.
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, Martin
With 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.
Open Access
Diversity of strontium nitridogermanates(IV) : novel Sr4[GeN4], Sr8Ge2[GeN4], and Sr17Ge2[GeN3]2[GeN4]2
(2020) Link, Lukas; Niewa, Rainer
Single crystals of three new strontium nitridogermanates(IV) were grown in sealed niobium ampules from sodium flux. Dark red Sr4[GeN4] crystallizes in space group P21/c with a = 9.7923(2) Å, b = 6.3990(1) Å, c = 11.6924(3) Å and β = 115.966(1)°. Black Sr8Ge2[GeN4] contains Ge4- anions coexisting with [GeIVN4]8- tetrahedra and adopts space group Cc with a = 10.1117(4) Å, b = 17.1073(7) Å, c = 10.0473(4) Å and β = 115.966(1)°. Black Sr17Ge6N14 features the same anions alongside trigonal planar [GeIVN3]5- units. It crystallizes in P1 with a = 7.5392(1) Å, b = 9.7502(2) Å, c = 11.6761(2) Å, α = 103.308(1)°, β = 94.651(1)° and γ = 110.248(1)°.
Open Access
Exchange of ammine‐ and fluorido‐ligands in complex salts : the series [Cr(NH3)6][AlF6], [Cr(NH3)5F][SiF6] and K2[Cr(NH3)4F2][Si(NH3)0.5F5.5]2
(2022) Bäucker, Christian; Niewa, Rainer
The three amminechromium(III) complex compounds [Cr(NH3)6][AlF6], [Cr(NH3)5F][SiF6] and K2[Cr(NH3)4F2][Si(NH3)0.5F5.5]2 were obtained from ammonothermal synthesis at Tmax=724 K and pmax=2120 bar. Crystal structures were determined from single crystal X‐ray diffraction. The gradual exchange of ammine ligands by fluoride ligands in the amminechromium(III) ions is reflected by an intriguing change in color. The mutual ligand exchange between ammonia ligands and fluoride ions occurs in both the cations and the anions, indicating the various dissolved species present under the synthetic conditions.
Open Access
Garnet-type Mn3Cr2(GeO4)3
(2012) Lipp, Christian; Strobel, Sabine; Lissner, Falk; Niewa, Rainer
Obtained by a chemical transport reaction, the manganese(II) chromium(III) germanate Mn3Cr2(GeO4)3 crystallizes isotypically with garnet.
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, Rainer
The 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.
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, Rainer
Large 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.
Open Access
Li5Sn, the most lithium-rich binary stannide : a combined experimental and computational study
(2022) Stelzer, Robert U.; Ikeda, Yuji; Srinivasan, Prashanth; Lehmann, Tanja S.; Grabowski, Blazej; Niewa, Rainer
From reaction of excess lithium with tin, we isolate well-crystallized Li5Sn and solve the crystal structure from single-crystal X-ray diffraction data. The orthorhombic structure (space group Cmcm) features the same coordination polyhedra around tin and lithium as previously predicted by electronic structure calculations for this composition, however differently arranged. An extensive ab initio analysis, including thermodynamic integration using Langevin dynamics in combination with a machine-learning potential (moment tensor potential), is conducted to understand the thermodynamic stability of this Cmcm Li5Sn structure observed in our experiments. Among the 108 Li5Sn structures systematically derived using the structure enumeration algorithm, including the experimental Cmcm structure and those obtained in previous ab initio studies, another new structure with the space group Immm is found to be energetically most stable at 0 K. This computationally discovered Immm structure is also found to be thermodynamically more stable than the Cmcm structure at finite temperatures, indicating that the Cmcm Li5Sn structure observed in our experiments is favored likely due to kinetic reasons rather than thermodynamics.
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, Rainer
Barium 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.
Open Access
Metal-rich ternary perovskite nitrides
(2019) Niewa, Rainer
Research 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.
Open Access
A new modification of Rb[Al(NH2)4] and condensation in solid state
(2020) Bäucker, Christian; Niewa, Rainer
A new modification of Rb[Al(NH2)4] in space group C2/c, which differs from the known structural modification in the way the [Al(NH2)4]−-tetrahedra are arranged in the surrounding area of the rubidium cation, was obtained from ammonothermal synthesis at 673 K and 680 bar. The crystal structure was determined by Rietveld refinements and further investigated by infrared and Raman spectroscopy. Thermal gravimetric investigations indicate two decomposition steps up to 450 °C, which can be assigned to ammonia leaving the material while the sample liquefies. During the third and final step, volatile rubidium amide is released, leaving nano-scaled cubic AlN behind. Investigating differently aged samples implies decomposition and condensation of amidoaluminate ions already at ambient temperature, which is supported by refinements of single crystal X-ray diffraction data, revealing lower nitrogen amounts than expected. The observed single crystal also exhibits a significantly smaller volume than the reported structures, further supporting the decomposition-condensation mechanism.
Open Access
Novel fluoridoaluminates from ammonothermal synthesis : two modifications of K2AlF5 and the elpasolite Rb2KAlF6
(2022) Bäucker, Christian; Becker, Peter; Morell, Keshia J.; Niewa, Rainer
Two new modifications of the pentafluoridoaluminate K2AlF5 were obtained from ammonothermal synthesis at 753 K, 224 MPa and 773 K, 220 MPa, respectively. Both crystallize in the orthorhombic space group type Pbcn, with close metric relations and feature kinked chains of cis-vertex-connected AlF6 octahedra resulting in the Niggli formula ∞1{[AlF2/2eF4/1t]2-}. The differences lie in the number of octahedra necessary for repetition within the chains, which for K2AlF5-2 is realized after four and for K2AlF5-3 after eight octahedra. As a result, the orthorhombic unit cell for K2AlF5-3 is doubled in chain prolongation direction [001] as compared to K2AlF5-2 (1971.18(4) pm versus 988.45(3) pm, respectively), while the unit cell parameters within the other two directions are virtually identical. Moreover, the new elpasolite Rb2KAlF6 is reported, crystallizing in the cubic space group Fm3-m with a = 868.9(1) pm and obtained under ammonothermal conditions at 723 K and 152 MPa.
Open Access
Polynator : a tool to identify and quantitatively evaluate polyhedra and other shapes in crystal structures
(2023) Link, Lukas; Niewa, Rainer
Polynator is a Python program capable of identifying coordination polyhedra, molecules and other shapes in crystal structures and evaluating their distortions. Distortions are quantified by fitting the vertices of a model to a selected set of atoms. In contrast to earlier programs, models can be deformable, which allows them to represent a point group or a range of shapes such as the set of all trigonal prisms, rather than a specific, rigid shape such as the equilateral trigonal prism. The program comes with a graphical user interface and is freely available. This paper discusses its working principle and illustrates a number of applications.
Open Access
The quasi-binary acetonitriletriide Sr3[C2N]2
(2019) Clark, William P.; Köhn, Andreas; Niewa, Rainer
The 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.
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, Rainer
The 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].