Biuret—A Crucial Reaction Intermediate for Understanding Urea Pyrolysis To Form Carbon Nitrides: Crystal-Structure Elucidation and In Situ Diffractometric,Vibrational and Thermal Characterisation

The crystal structure of biuret was elucidated by means of XRD analysis of single crystals grown through slow evaporation from a solution in ethanol. It crystallises in its own structure type in space group C2/c (a=15.4135(8) Å, b=6.6042(3) Å, c=9.3055(4) Å, Z=8). Biuret decomposition was studied in situ by means of temperature-programmed powder XRD and FTIR spectroscopy, to identify a co-crystalline biuret–cyanuric acid phase as a previously unrecognised reaction intermediate. Extensive thermogravimetric studies of varying crucible geometry, heating rate and initial sample mass reveal that the concentration of reactive gases at the interface to the condensed sample residues is a crucial parameter for the prevailing decomposition pathway. Taking these findings into consideration, a study on the optimisation of carbon nitride synthesis from urea on the gram scale, with standard solid-state laboratory techniques, is presented. Finally, a serendipitously encountered self-coating of the crucible inner walls by graphite during repeated synthetic cycles, which prove to be highly beneficial for the obtained yields, is reported.     

Freitod in Dahlem (1915): Unveröffentlichte Briefe von Edith Hahn und Lise Meitner über Dr. Clara Haber geb. Immerwahr

The present contribution contains previously unpublished letters by Clara Haber's contemporaries Edith Hahn and Lise Meitner that were written within two weeks of Clara Haber′s tragic suicide. It also contains a commentary about the circumstances of obtaining these letters as well as notes concerning their interpretation.     

Regioisomeric 4‐nitroindazole N1‐ and N2‐(β‐d‐ribonucleosides)

The structures of the isomeric nucleosides 4‐nitro‐1‐(β‐d ‐ribo­furan­osyl)‐1H‐indazole, C₁₂H₁₃N₃O₆, (I), and 4‐nitro‐2‐(β‐d ‐ribo­furan­osyl)‐2H‐indazole, C₁₂H₁₃N₃O₆, (II), have been determined. For compound (I), the conformation of the gly­cosylic bond is anti [χ = −93.6 (6)°] and the sugar puckering is C2′‐exo–C3′‐endo. Compound (II) shows two conformations in the crystalline state which differ mainly in the sugar pucker; type 1 adopts the C2′‐endo–C3′‐exo sugar puckering associated with a syn base orientation [χ = 43.7 (6)°] and type 2 shows C2′‐exo–C3′‐endo sugar puckering accompanied by a somewhat different syn base orientation [χ = 13.8 (6)°].     

Square-wave voltammetry and impedance spectroscopy in tin doped melts with the compositions xNa2O · 15Al2O3 · (85 − x)SiO2 (x = 8.5, 11 and 16)

Glasses with the base compositions xNa₂O · 15Al₂O₃ · (85 ? x)SiO₂ (x = 8.5, 11 and 16) doped with 0.5 mol% SnO₂ were investigated by both square-wave voltammetry and impedance spectroscopy in the temperature range from 1300 to 1600 °C. Each recorded square-wave voltammogram exhibits a well pronounced peak attributed to the Sn²⁺/Sn⁴⁺-redox pair. Impedance spectra were measured in a frequency range from 0.1 to 10⁵ s^?1 as a function of the superimposed dc-potential and were simulated using an equivalent circuit taking into account the resistivity of the melt, the electrochemical double layer, a resistor attributed to a kinetically hindered electron transfer and a Warburg parameter which accounts for the diffusion process of Sn⁴⁺ and Sn²⁺ to and from the electrode. Additionally, two impedance elements, a resistor and a capacitance both attributed to adsorption processes were necessary to fit the impedance spectra.     

Eu3F4S2: Synthesis, crystal structure, and magnetic properties of the mixed-valent europium(II,III) fluoride sulfide EuF2·(EuFS)2

Using the method to synthesize rare-earth metal(III) fluoride sulfides MFS (M=Y, La, Ce–Lu), in some cases we were able to obtain mixed-valent compounds such as Yb₃F₄S₂ instead. With Eu₃F₄S₂ another isotypic representative has now been synthesized. Eu₃F₄S₂ (tetragonal, I4/mmm, a=400.34(2), c=1928.17(9) pm, Z=2) is obtained from the reaction of metallic europium, elemental sulfur, and europium trifluoride in a molar ratio of 5:6:4 within seven days at 850 °C in silica-jacketed gas-tightly sealed platinum ampoules. The single-phase product consists of black plate-shaped single crystals with a square cross section, which can be obtained from a flux using equimolar amounts of NaCl as fluxing agent. The crystal structure is best described as an intergrowth structure, in which one layer of CaF₂-type EuF₂ is followed by two layers of PbFCl-type EuFS when sheeted parallel to the (001) plane. Accordingly there are two chemically and crystallographically different europium cations present. One of them (Eu²⁺) is coordinated by eight fluoride anions in a cubic fashion, the other one (Eu³⁺) exhibits a monocapped square antiprismatic coordination sphere with four F⁻ and five S²⁻ anions. Although the structural ordering of the different charged europium cations is plausible, a certain amount of charge delocalization with some polaron activity has to take place, which is suggested by the black color of the title compound. Temperature dependent magnetic susceptibility measurements of Eu₃F₄S₂ show Curie–Weiss behavior with an experimental magnetic moment of 8.19(5) μ_B per formula unit and a paramagnetic Curie temperature of 0.3(2) K. No magnetic ordering is observed down to 4.2 K. In accordance with an ionic formula splitting like (Eu^II)(Eu^III)₂F₄S₂ only one third of the europium centers in Eu₃F₄S₂ carry permanent magnetic moments. ¹⁵¹Eu-Mössbauer spectroscopic experiments at 4.2 K show one signal at an isomer shift of −12.4(1) mm/s and a second one at 0.42(4) mm/s. These signals occur in a ratio of 1:2 and correspond to Eu²⁺ and Eu³⁺, respectively. The spectra at 78 and 298 K are similar, thus no change in the Eu²⁺/Eu³⁺ fraction can be detected.     

Micro-solid oxide fuel cells: status, challenges, and chances

Abstract  

Micro-solid oxide fuel cells (micro-SOFC) are predicted to be of high energy density and are potential power sources for portable electronic devices. A micro-SOFC system consists of a fuel cell comprising a positive electrode-electrolyte-negative electrode (i.e. PEN) element, a gas-processing unit, and a thermal system where processing is based on micro-electro-mechanical-systems fabrication techniques. A possible system approach is presented. The critical properties of the thin film materials used in the PEN membrane are discussed, and the unsolved subtasks related to micro-SOFC membrane development are pointed out. Such a micro-SOFC system approach seems feasible and offers a promising alternative to state-of-the-art batteries in portable electronics.     

Independent domination in subcubic graphs of girth at least six

A set $S$ of vertices in a graph $G$ is an independent dominating set of $G$ if $S$ is an independent set and every vertex not in $S$ is adjacent to a vertex in $S$. The independent domination number, $i\left(G\right)$, of $G$ is the minimum cardinality of an independent dominating set. In this paper, we extend the work of Henning, Löwenstein, and Rautenbach (2014) who proved that if $G$ is a bipartite, cubic graph of order $n$ and of girth at least $6$, then $i\left(G\right)\le \frac{4}{11}n$. We show that the bipartite condition can be relaxed, and prove that if $G$ is a cubic graph of order $n$ and of girth at least $6$, then $i\left(G\right)\le \frac{4}{11}n$.