Rainer Niewa

On Copper(I) Fluorides, the Cuprophilic Interaction, the Preparation of Copper Nitride at Room Temperature, and the Formation Mechanism at Elevated Temperatures

Our attempts to synthesize the hitherto unknown binary copper(I) fluoride have led to first successes and a serendipitious result: By conproportionation of elemental copper and copper(II) fluoride in anhydrous liquid ammonia, two copper(I) fluorides were obtained as simple NH3 complexes. One of them presents an example of ligand-unsupported cuprophilic interactions in an infinite [Cu2 (NH3 )4 ](2+) chain with alternating Cu-Cu distances. We discovered that both copper(I) fluorides can easily be converted into Cu3 N at room temperature, just by applying a vacuum. Additionally, we investigated the formation mechanism of the classical synthesis route of Cu3 N that starts with CuF2 and flowing NH3 in the temperature range between ambient and 290u2009°C by means of thermal analysis and in situ neutron diffraction. The reaction proceeds at elevated temperatures through the formation of a blue and amorphous ammoniate Cu(NH3 )2 F2 , the reformation of CuF2 , and finally the redox reaction to form Cu3 N.

High-Pressure NiAs-Type Modification of FeN

Abstract The combination of laser‐heated diamond anvil cells and synchrotron Mössbauer source spectroscopy were used to investigate high‐temperature high‐pressure chemical reactions of iron and iron nitride Fe2N with nitrogen. At pressures between 10u2005and 45u2005GPa, significant magnetic hyperfine splitting indicated compound formation after annealing at 1300u2005K. Subsequent inu2005situ X‐ray diffraction reveals a new modification of FeN with NiAs‐type crystal structure, as also rationalized by first‐principles total‐energy and chemical‐bonding studies.

Ternary Amides Containing Transition Metals for Hydrogen Storage: A Case Study with Alkali Metal Amidozincates

The alkali metal amidozincates Li4 [Zn(NH2)4](NH2)2 and K2[Zn(NH2)4] were, to the best of our knowledge, studied for the first time as hydrogen storage media. Compared with the LiNH2-2 LiH system, both Li4 [Zn(NH2)4](NH2)2-12 LiH and K2[Zn(NH2)4]-8 LiH systems showed improved rehydrogenation performance, especially K2[Zn(NH2)4]-8 LiH, which can be fully hydrogenated within 30u2005s at approximately 230u2009°C. The absorption properties are stable upon cycling. This work shows that ternary amides containing transition metals have great potential as hydrogen storage materials.

The surface energies of β-Sn — A new concept for corrosion and whisker mitigation

Abstract Corrosion data provided under high-temperature and high-humidity conditions as well as tin whisker growth studies are explained by differences in the surface energy of lattice planes within the crystal structure of β-tin. For this purpose, electrodeposited tin finishes were investigated regarding their microstructure utilizing X-ray diffraction, cross-sectional SEM and EBSD analyses. The corrosion as well as the tin whisker propensity strongly depends on the preferred orientation of the surface finishes. With an increasing texture along the (211) lattice plane a decreasing corrosion and whisker propensity were observed, on the contrary, the presence of the (101) and (112) textures results in an increased corrosion and whisker propensity. The maximum whisker length was reduced by one order of magnitude by changing the preferred orientation towards the (211) lattice plane of the tin finish. Modified embedded atom method simulations of tin surfaces demonstrate the minimization of the surface energy of (211) surfaces, whereas the surface energies of the (101) and (112) Miller planes are increased. We find a strong connection between the minimization of surface energy and the corrosion and tin whisker propensity of electrodeposited tin finishes. To our best knowledge, this is the first study connecting the influence of the electrodeposition parameters on the corrosion and whisker propensity explained by calculations of the surface energies of the corresponding crystal faces. The applied parameters for electrodeposition influence the grain orientation and thus the surface energy of the tin layers which affects both the corrosion as well as the tin whisker propensity.

Texture of electrodeposited tin layers and its influence on their corrosion behavior

Differences in the degree of corrosion of pure thin tin films electrodeposited on copper substrates were investigated in dependence on the layer thickness and the texture of the tin layers. The change of the preferred orientation of the tin layer deposited by applying different current densities was analyzed using X-ray diffraction. A graphical evaluation was used to determine the degree of corrosion after sample exposition to NaCl contaminations and humidity. Results show that a preferred orientation along the lattice planes (3 2 1) and (2 2 0) enhances the corrosion resistance of the tin layer by about one order of magnitude compared to a non-textured sample based on the corroded area. In contrast, a texture along (1 0 1) and (1 1 2) accelerates the oxidation of tin by a factor of about three to four compared with a randomly orientated specimen. The corrosion dependence on the preferred orientation decreases with increasing layer thickness. Moreover, scanning electron micrographs show no effect to the size of the tin grains on variations of the current density. In summary, changes in the process parameters of the electrodeposition lead to a variation of texture and thus modify the chemical and corrosion properties of the resulting tin layers. Consequently reliability properties like solderability or whisker growth in further applications depend on these parameters.

Transition and Alkali Metal Complex Ternary Amides for Ammonia Synthesis and Decomposition

A new complex ternary amide, Rb2 [Mn(NH2 )4 ], which simultaneously contains both transition and alkali metal catalytic sites, is developed. This is in line with the recently reported TM-LiH composite catalysts, which have been shown to effectively break the scaling relations and achieve ammonia synthesis under mild conditions. Rb2 [Mn(NH2 )4 ] can be facilely synthesized by mechanochemical reaction at room temperature. It exhibits two temperature-dependent polymorphs, that is, a low-temperature orthorhombic and a high-temperature monoclinic structure. Rb2 [Mn(NH2 )4 ] decomposes to N2 , H2 , NH3 , Mn3 N2 , and RbNH2 under inert atmosphere; whereas it releases NH3 at a temperature as low as 80u2009°C under H2 atmosphere. Those unique behaviors enable Rb2 [Mn(NH2 )4 ], and its analogue K2 [Mn(NH2 )4 ], to be excellent catalytic materials for ammonia decomposition and synthesis. Experimental results show both ammonia decomposition onset temperatures and conversion rates over Rb2 [Mn(NH2 )4 ] and K2 [Mn(NH2 )4 ] are similar to those of noble metal Ru-based catalysts. More importantly, these ternary amides exhibit superior capabilities in catalyzing NH3 synthesis, which are more than 3 orders of magnitude higher than that of Mn nitride and twice of that of Ru/MgO. The in situ SR-PXD measurement shows that manganese nitride, synergistic with Rb/KH or Rb/K(NH2 )x H1-x , are likely the active sites. The chemistry of Rb2 /K2 [Mn(NH2 )x ] and Rb/K(NH2 )x H1-x with H2 /N2 and NH3 correlates closely with the catalytic performance.

Trigonal-Bipyramidal Coordination in First Ammoniates of ZnF2: ZnF2(NH3)3 and ZnF2(NH3)2

Single crystals of ZnF2(NH3)3 and ZnF2(NH3)2 were obtained under ammonothermal conditions (250 °C, 196 MPa and 500 °C, 136 MPa). Upon thermal decomposition of both ZnF2(NH3)3 and ZnF2(NH3)2, a microcrystalline powder of ZnF2(NH3) was obtained. ZnF2(NH3)3 and ZnF2(NH3)2 represent probable intermediates in a conceivable ammonothermal synthesis of the semiconductor Zn3N2 and manifest a rare trigonal-bipyramidal coordination of F(-) and NH3 ligands around Zn(2+) according to single-crystal X-ray diffraction. Thermal analysis of all three compounds showed not only ZnF2(NH3) but also ZnF2(NH3)2 to be decomposition intermediates of ZnF2(NH3)3 prior to the formation of ZnF2. All three compounds demonstrate hydrogen bonds, as indicated by the intensities and half-widths of the bands in the vibrational spectra and by short N-H···F distances in the crystal structures of ZnF2(NH3)3 and ZnF2(NH3)2. With ZnF2(NH3)3, ZnF2(NH3)2, and ZnF2(NH3), we present the first ammoniates of ZnF2.

Synthesis and Characterization of Superconducting Ca1−xNaxFFeAs

A representative of the fluoride-containing iron pnictide high-temperature superconductors, namely CaFFeAs, was doped with sodium up to the composition Ca0.86Na0.14FFeAs for the first time. Single crystals with an edge length in the range of 0.1 – 2.0 mm were obtained via solid-state and flux syntheses, respectively. The composition of the crystals was verified by means of single crystal X-ray diffractometry and energy dispersive X-ray spectroscopy (EDX). Measurements of the electrical resistivity, as well as the magnetization on a crystal of Ca0.89Na0.11FFeAs both show a transition to the superconducting state on cooling to 34.5 K. Investigations of the upper critical fields reveal an anisotropy ratio of about five. The lattice parameters and molar volumes increase with rising sodium content. This effect is clearly observable for the c-axis and the volume, whereas the increase of the a-axis is rather minor.

Lithium alkaline earth tetrelides of the type Li2AeTt (Ae=Ca, Ba, Tt=Si, Ge, Sn, Pb): synthesis, crystal structures and physical properties

Abstract Single crystals of the compounds Li2AeTt (Ae=Ca, Ba, Tt=Si, Ge, Sn, Pb) were grown in reactive lithium melts in sealed tantalum ampoules from an equimolar ratio of the alkaline earth metal and the respective group 4 element. All compounds, with the exception of Li2CaSn and Li2CaPb, are isotypic and crystallize in an orthorhombic unit cell (space group Pmmn, no. 59). The crystal structure can be characterized as superimposed corrugated networks of Li2Tt connected by calcium or barium atoms within the third dimension. Li2CaSn and Li2CaPb crystallize in the cubic space group Fm3̅m (no. 225) in a Heusler-type (MnCu2Al) structure. According to magnetic susceptibility and electric resistivity measurements, the compounds Li2BaGe, Li2BaSn, and Li2BaPb represent diamagnetic activated semiconductors.

Superconductivity at Tc = 36.5 K in Na-Substituted SrFe2As2 Single Crystals

We report superconductivity, which is achieved in Sr1-xNaxFe2As2 at Tc = 36.5 K for x = 0.47. Single crystals of Na-substituted SrFe2As2 up to 2 × 2 mm2 were grown from NaF flux. The superconducting transition temperature with a narrow width less than 2 K, confirmed by both magnetic and resistivity measurements, indicates the high quality of our samples. The upper critical fields (0) = 87 T and (0) = 217.5 T are extremely high. The anisotropy ratio γ ~ 2.5 is close to that of other hole-or electron-doped 122 phases but lower than γ ~ 4.3 – 5 of phases from the 1111 series and much lower than γ ~ 7 – 10 of superconducting cuprates. Magnetic relaxation measurements, which are consistent with the collective pinning model, reveal that the flux-creep rate in Na substituted 122 SrFe2As2 is smaller than in 1111 type iron pnictides.