TbNb6Sn6: the first ternary compound from the rare earth–niobium–tin system

The title compound, terbium hexa­niobium hexastannide, TbNb6Sn6, is the first ternary compound from the rare earth–niobium–tin system. It has the HfFe6Ge6 structure type, which can be analysed as an inter­growth of the Zr4Al3 and CaCu5 structures. All the atoms lie on special positions; their coordination geometries and site symmetries are: Tb (dodeca­hedron) 6/mmm; Nb (distorted icosa­hedron) 2mm; Sn (Frank–Caspar polyhedron, CN = 14–15) 6mm and m2; Sn (distorted icosa­hedron) m2. The structure contains a graphite-type Sn network, Kagome nets of Nb atoms, and Tb atoms alternating with Sn2 dumbbells in the channels

Crystalline Copper Selenide as a Reliable Non‐Noble Electro(pre)catalyst for Overall Water Splitting

Electrochemical water splitting remains a frontier research topic in the quest to develop artificial photosynthetic systems by using noble metal‐free and sustainable catalysts. Herein, a highly crystalline CuSe has been employed as active electrodes for overall water splitting (OWS) in alkaline media. The pure‐phase klockmannite CuSe deposited on highly conducting nickel foam (NF) electrodes by electrophoretic deposition (EPD) displayed an overpotential of merely 297 mV for the reaction of oxygen evolution (OER) at a current density of 10 mA cm−2 whereas an overpotential of 162 mV was attained for the hydrogen evolution reaction (HER) at the same current density, superseding the Cu‐based as well as the state‐of‐the‐art RuO2 and IrO2 catalysts. The bifunctional behavior of the catalyst has successfully been utilized to fabricate an overall water‐splitting device, which exhibits a low cell voltage (1.68 V) with long‐term stability. Post‐catalytic analyses of the catalyst by ex‐situ microscopic, spectroscopic, and analytical methods confirm that under both OER and HER conditions, the crystalline and conductive CuSe behaves as an electro(pre)catalyst forming a highly reactive in situ crystalline Cu(OH)2 overlayer (electro(post)catalyst), which facilitates oxygen (O2) evolution, and an amorphous Cu(OH)2/CuOx active surface for hydrogen (H2) evolution. The present study demonstrates a distinct approach to produce highly active copper‐based catalysts starting from copper chalcogenides and could be used as a basis to enhance the performance in durable bifunctional overall water splitting.DFG, 390540038, EXC 2008: Unifying Systems in Catalysis "UniSysCat"TU Berlin, Open-Access-Mittel – 202

Note The Hexagonal Laves Phase MgIr 2

The hexagonal Laves phase MgIr 2 was synthesized from the elements in a sealed tantalum tube in an induction furnace. MgIr 2 was investigated by powder and single crystal X-ray data: P6 3 /mmc, a = 516.9(1), c = 838.5(2) pm, wR2 = 0.0771, 135 F 2 values, and 11 variable parameters. The magnesium atoms have coordination number (CN) 16 (12 Ir + 4 Mg), while the smaller iridium atoms, Ir1 and Ir2, both have CN 12 (6 Ir + 6 Mg). The Ir-Ir distances within the three-dimensional network of face-and corner-sharing Ir 4/2 tetrahedra range from 250 to 267 pm. The magnesium atoms have one shorter (306 pm) and three longer (319 pm) magnesium contacts. The crystal chemistry of MgIr 2 is briefly discussed and compared with the other binary Mg-Ir intermetallics

New Germanium-rich Compounds SrCo₅₋ₓGe₉ (x = 0.39 and 0.28) with Optimized Co-Ge Bonding

New germanium-rich compounds SrCo₅₋ₓGe₉ (x = 0.39 and 0.28) were synthesized by melting mixtures of the three elements by induction heating in niobium crucibles in a water-cooled sample chamber. Both compounds were investigated by X-ray powder and single-crystals diffraction methods; the structures have been refined on the basis of single-crystal data. The compounds crystallize with a new structure type: Pnma, Z = 4, a = 13.932(4), b = 3.957(1), c = 16.838(7) Å, wR2 = 0.064, 1099 F2 values, 93 variables for x = 0.39 and a = 13.951(3), b = 3.964(1), c = 16.869(5) Å, wR2 = 0.046, 1543 F2 values, 93 variables for x = 0.28. The complex crystal structure is built up from slabs of Ge4Co2 octahedra, which consist of five columns of face-sharing octahdra along the b direction. The columns are connected in the ac plane by sharing vertices. These slabs are connected by {Sr@Co5Ge15} polyhedra, which share pentagonal faces of germanium atoms along b. The columns are further condensed by common edges along the a direction. Centering strontium atoms form slightly puckered layers in the ab plane. Along the c direction, the layers of strontium-centered polyhedra are interconnected by the slabs of Ge4Co2 octahedra. The Co5 site of the structure shows defects (39 % and 28 %) leading to the refined composition SrCo₅₋ₓGe₉ for the investigated crystals with x = 0.39 and 0.28. The catalytic properties of the title compound with respect to the selective hydrogenation of ,-unsaturated aldehydes are discussed

Ca4As3 – a new binary calcium arsenide

The crystal structure of the binary compound tetracalcium triarsenide, Ca4As3, was investigated by single-crystal X-ray diffraction. Ca4As3 crystallizes in the Ba4P3 structure type and is thus a homologue of isotypic Sr4As3. The unit cell contains 32 Ca2+ cations, 16 As3− isolated anions and four centrosymmetric [As2]4– dumbbells. The As atoms in each of the dumbbells are connected by a single bond, thus this calcium arsenide is a Zintl phase

Polar Intermetallic Compounds as Catalysts for Hydrogenation Reactions: Synthesis, Structures, Bonding, and Catalytic Properties of Ca₁₋ₓSrₓNi₄Sn₂ (x=0.0, 0.5, 1.0) and Catalytic Properties of Ni₃Sn and Ni₃Sn₂

The potential of polar intermetallic compounds to catalyze hydrogenation reactions was evaluated. The novel compounds CaNi₄Sn₂, SrNi₄Sn₂, and Ca0.5Sr0.5Ni₄Sn₂ were tested as unsupported alloys in the liquid-phase hydrogenation of citral. Depending on the reaction conditions, conversions of up to 21.0 % (253 K and 9.0 MPa hydrogen pressure) were reached. The binary compounds Ni3Sn and Ni₃Sn₂ were also tested in citral hydrogenation under the same conditions. These materials gave conversions of up to 37.5 %. The product mixtures contained mainly geraniol, nerol, citronellal, and citronellol. The isotypic stannides CaNi₄Sn₂, Ca0.5Sr0.5Ni₄Sn₂, and SrNi₄Sn₂ were obtained by melting mixtures of the elements in an arc-furnace under an argon atmosphere. Single crystals were synthesized in tantalum ampoules using special temperature modes. The novel structures were established by single-crystal X-ray diffraction. They crystallize in the tetragonal space group I4/mcm with parameters: a=7.6991(7), c=7.8150(8) Å, wR2=0.034, 162 F2 values, 14 variable parameters for CaNi4Sn2; a=7.7936(2), c=7.7816(3) Å, wR2=0.052, 193 F2 values, 15 variable parameters for Ca0.5Sr0.5Ni4Sn2; and a=7.8916(4), c=7.7485(5) Å, wR2=0.071, 208 F2 values, 14 variable parameters for SrNi4Sn2. The Ca1-ₓSrₓNi₄Sn₂ (x=0.0, 0.5, 1.0) structures can be represented as a stuffed variant of the CuAl₂ type by the formal insertion of one-dimensional infinite Ni-cluster chains [Ni₄] into the Ca(Sr)Sn₂ substructure. The Ni and Sn atoms form a three-dimensional infinite [Ni₄Sn₂] network in which the Ca or Sr atoms fill distorted octagonal channels. The densities of states obtained from TB-LMTO-ASA calculations show metallic character for both compounds

Mg_{1 + x}Ir_{1 - x} (x = 0, 0.037 and 0.054), a binary intermetallic compound with a new orthorhombic structure type determined from powder and single- crystal X-ray diffraction

The new binary compound Mg1 + xIr1 - x (x = 0-0.054) was prepared by melting the elements in the Mg:Ir ratio 2:3 in a sealed tantalum tube under an argon atmosphere in an induction furnace (single crystals) or by annealing coldpressed pellets of the starting composition Mg:Ir 1:1 in an autoclave under an argon atmosphere (powder sample). The structure was independently solved from high-resolution synchrotron powder and single-crystal X-ray data: Pearson symbol oC304, space group Cmca, lattice parameters from synchrotron powder data a = 18.46948 (6), b = 16.17450 (5), c = 16.82131 (5) A Ê . Mg1 + xIr1 - x is a topologically close-packed phase, containing 13 Ir and 12 Mg atoms in the asymmetric unit, and has a narrow homogeneity range. Nearly all the atoms have Frank-Kasper-related coordination polyhedra, with the exception of two Ir atoms, and this compound contains the shortest Ir-Ir distances ever observed. The solution of a rather complex crystal structure from powder diffraction, which was fully confirmed by the single-crystal method, shows the power of powder diffraction in combination with the high-resolution data and the global optimization method