Harald Hillebrecht

Boron: Elementary Challenge for Experimenters and Theoreticians

Many of the fundamental questions regarding the solid-state chemistry of boron are still unsolved, more than 200 years after its discovery. Recently, theoretical work on the existence and stability of known and new modifications of the element combined with high-pressure and high-temperature experiments have revealed new aspects. A lot has also happened over the last few years in the field of reactions between boron and main group elements. Binary compounds such as B(6)O, MgB(2), LiB(1-x), Na(3)B(20), and CaB(6) have caused much excitement, but the electron-precise, colorless boride carbides Li(2)B(12)C(2), LiB(13)C(2), and MgB(12)C(2) as well as the graphite analogue BeB(2)C(2) also deserve special attention. Physical properties such as hardness, superconductivity, neutron scattering length, and thermoelectricity have also made boron-rich compounds attractive to materials research and for applications. The greatest challenges to boron chemistry, however, are still the synthesis of monophasic products in macroscopic quantities and in the form of single crystals, the unequivocal identification and determination of crystal structures, and a thorough understanding of their electronic situation. Linked polyhedra are the dominating structural elements of the boron-rich compounds of the main group elements. In many cases, their structures can be derived from those that have been assigned to modifications of the element. Again, even these require a critical revision and discussion.

Synthesis, Single‐Crystal Structure and Characterization of (CH3NH3)2Pb(SCN)2I2

The perovskite phase (CH3 NH3 )2 Pb(SCN)2 I2 with a structure closely related to the K2 NiF4 -type was identified as the product of the reaction of CH3 NH3 I and Pb(SCN)2 by single-crystal X-ray analysis. This extends the range of suitable dyes for solar cell applications to a class of perovskite-related structures of the general composition (AMX3 )n (AX)m .

Synthesis and crystal structure of Al3BC, the first boridecarbide of aluminium

Abstract Single crystals of the aluminium-rich boridecarbide Al 3 BC with isolated boron and carbon are synthesized from the elements. The crystal structure can be described as a closest packing of Al (sequence ABACBC) with alternating layers of edge-sharing BAl 6 octahedra and trigonal bipyramids CAl 5 , linked by common corners. Al 3 BC plays an important role in the processing of B 4 C/Al cermets.

B4 Tetrahedra for Aluminum Atoms—A Surprising Substitution inτ-Borides Ni20Al3B6 and Ni20AlB14

Stabilized by the tailor-made coordination in the crystal, B4 tetrahedra were characterized for the first time in a solid-state structure (shown on the right). Some of the Al atoms are replaced by B4 tetrahedra in the cubic τ-boride Ni20 AlB14 . The B-B distances of 1.681(15) Å are similar to those in the molecular structures of B4 Cl4 and B4 (tBu)4 , which contain tetrahedral B4 units.

The First Borosulfate K5[B(SO4)4]

Sulfate anions SO4 2 do not show a strong tendency to form condensed oligoanions owing to the high formal charge of sulfur. A recent very interesting example for an oligosulfate is ReO2Cl(S2O7). [1] Disulfate tetrahedra share common corners with an adjacent ReO6 octahedron forming a cyclic moiety. Regarding oligosulfates, only chainlike anions, typically S3O10 2 or even S5O16 2 , were described. In K5[B(SO4)4], sulfate tetrahedra avoid direct condensation but form the unprecedented anion [B(SO4)4] 5 shown in Figure 1. In

Structural and scanning microscopy studies of layered compounds MCl3 (M = Mo, Ru, Cr) and MOCl2 (M = V, Nb, Mo, Ru, Os)

Abstract Structural investigations on trichlorides (α- and β-MoCl3, solid solutions Ru1−5CrxCl3) and oxide dichlorides MOCl2 (M = V, Nb, Mo, Ru, Os) show that layer structures with stacking faults can be solved without evaluation of the diffuse parts of scattering. The combination of X-ray methods and atomic scale STM/AFM imaging yields information about the pattern of the surface atoms and the metal sublattice between the halide layers. Metal-metal bonds, structural distortions of coordination polyhedra and cation distribution in solid solutions have been determined. The correct layer symmetry, obtained by STM/AFM methods, was used in the X-ray structure analysis by single crystal and powder diffraction. Disorder problems due to stacking faults of the layers could be treated in that way.

Li2B12Si2: The First Ternary Compound in the System Li/B/Si: Synthesis, Crystal Structure, Hardness, Spectroscopic Investigations, and Electronic Structure

We present the synthesis, crystal structure, hardness, IR/Raman and UV/Vis spectra, and FP-LAPW calculations of the electronic structure of Li(2)B(12)Si(2), the first ternary compound in the system Li/B/Si. Yellow, transparent single crystals were synthesized from the elements in tin as solvent at 1500 degrees C in h-BN crucibles in arc-welded Ta ampoules. Li(2)B(12)Si(2) crystallizes orthorhombic in the space group Cmce (no. 64) with a=6.1060(6), b=10.9794(14), c=8.4050(8) A, and Z=4. The crystal structure is characterized by a covalent network of B(12) icosahedra connected by Si atoms and Li atoms located in interstitial spaces. The structure is closely related to that of MgB(12)Si(2) and fulfils the electron-counting rules of Wade and Longuet-Higgins. Measurements of Vickers (H(V)=20.3 GPa) and Knoop microhardness (H(K)=20.4 GPa) revealed that Li(2)B(12)Si(2) is a hard material. The band gap was determined experimentally and calculated by theoretical means. UV/Vis spectra revealed a band gap of 2.27 eV, with which the calculated value of 2.1 eV agrees well. The IR and Raman spectra show the expected oscillations of icosahedral networks. Theoretical investigations of bonding in this structure were carried out with the FP-LAPW method. The results confirm the applicability of simple electron-counting rules and enable some structural specialties to be explained in more detail.

The Borosulfate Story Goes on—From Alkali and Oxonium Salts to Polyacids

The structural principles of borosulfates derived from the B/S ratio are confirmed and extended to new representatives of this class showing novel motifs. According to the composition, Na[B(S2O7)2] (P2(1)/c; a=10.949(6), b=8.491(14), c=12.701(8) Å; β=110.227(1)°; Z=4) and K[B(S2O7)2] (Cc; a=11.3368(6), b=14.662(14), c=13.6650(8) Å; β=94.235(1)°; Z=8) contain isolated [B(S2O7)2](-) ions, in which the central BO4 tetrahedron is coordinated by two disulfate units. The alkali cations have coordination numbers of 7 (Na) and 8 (K), respectively. The structure of Cs[B(S2O7)(SO4)] (P2(1)/c; a=10.4525(6), b=11.3191(14), c=8.2760(8) Å; β=103.206(1); Z=4) combines, for the first time, sulfate and disulfate units into a chain structure. Cs has a coordination number of 12. The same structural units were found in H[B(S2O7)(SO4)] (P2(1)/c; a=15.6974(6), b=11.4362(14), c=8.5557(8) Å; β=90.334(3)°; Z=8). This compound represents the first example of a polyacid. The hydrogen atoms were located and connect the chains to form layers through hydrogen-bonding bridges. H3O[B(SO4)2] (P4/ncc; a=9.1377(6), c=7.3423(8) Å; Z=4) is the first oxonium compound of this type to be found. The BO4 tetrahedra are linked by SO4 tetrahedra to form linear chains similar to those in SiS2. The chains form a tetragonal rod packing structure with H3O(+) between the rods. The structures of borosulfates can be classified following the concept described by Liebau for silicates, which was extended to borophosphates by Kniep et al. In contrast to these structures, borosulfates do not comprise B-O-B bonds but instead contain S-O-S connections. All compounds were obtained as colourless, moisture-sensitive single crystals by reaction of B2O3 and the appropriate alkali salt in oleum.

The Influence of the Precipitation/Ageing Temperature on a Cu/ZnO/ZrO2 Catalyst for Methanol Synthesis from H2 and CO2

For heterogeneous catalysts, the constitution of the precursor is an important parameter to adjust the properties of the active catalyst. Therefore, we examined the influence of the temperature during the precipitation process and during the ageing time in the mother liquor for a Cu/ZnO/ZrO2 catalyst system obtained through a coprecipitation route. The variation of the temperature affects the ratio and crystallinity of the precursor phases zincian malachite and aurichalcite, as detected by powder XRD (phase and line width) and FTIR spectroscopy (characteristic asymmetric CO stretching modes of the carbonate anions at

Binary Boron-Rich Borides of Magnesium: Single-Crystal Investigations and Properties of MgB7 and the New Boride Mg~5B44

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