Barbara Albert

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.

Possible Superhardness of CrB4

Chromium tetraboride [orthorhombic, space group Pnnm (No. 58), a = 474.65(9) pm, b = 548.0(1) pm, c = 286.81(5) pm, and R value (all data) = 0.041], formerly described in space group Immm, was found not to be superhard, despite several theory-based prognoses. CrB(4) shows an almost temperature-independent paramagnetism, consistent with low-spin Cr(I) in a metallic compound. Conductivity measurements confirm the metallic character.

Crystal structure refinement and bonding patterns of CrB4: a boron-rich boride with a framework of tetrahedrally coordinated B atoms.

Crystals of chromium tetraboride, a recently proposed candidate superhard material, have been grown for the first time to allow for a first structure refinement of the compound [orthorhombic, space group Immm (No. 71), a = 474.82(8) pm, b = 548.56(8) pm, and c = 287.17(4) pm, R value (all data) = 0.018]. The previously proposed structure model is confirmed, and accurate interatomic distances are presented for the first time. First-principles electronic structure calculations emphasize the unique framework of three-dimensionally linked B atoms that are tetrahedrally coordinated and carry a slightly negative charge. All B-B bonding is of the 2-center 2-electron type. CrB(4) is metallic with a pseudogap at the Fermi level.

Crystal and electronic structure of BaB6 in comparison with CaB6 and molecular [B6H6]2−

Abstract Barium hexaboride BaB 6 has been obtained in the form of single crystals. Its crystal structure was refined in space group Pm 3 m (no. 221, a =426.15 (7) pm). The electronic situation of BaB 6 is discussed on the basis of different band structure calculations performed within the density functional theory (LMTO, plane wave). The comparison with CaB 6 and the molecular anion [B 6 H 6 ] 2− shows a similar band ordering. The different orbital contributions are strongly mixed and the inter-octahedral bonds are lower in energy than some of the intra-octahedral framework interactions.

Room-temperature synthesis of metal borides

Abstract Black amorphous precipitates of unknown composition have been obtained from aqueous solutions of sodium tetrahydridoborate and nickel salts. These precipitates have been analyzed by means of X-ray diffraction, electron microscopy, and electron energy loss spectroscopy, especially focussing on the energy-loss near-edge structure. By comparing the fine structure of the BK ionization edge of the nano-scalic product with that of amorphous boron and crystalline nickel boride, we show that the primary precipitate contains a metal boride formed at room temperature. The precipitate exhibits catalytic activity, e.g. for arylations of arylhalogenides. When annealed, the primary product transforms into crystalline Ni3B. An earlier crystal structure determination of Ni3B was confirmed by Rietveld refinement of highly resolved synchrotron data.

Wet‐Chemical Synthesis of Nanoscale Iron Boride, XAFS Analysis and Crystallisation to α‐FeB

The reaction of lithium tetrahydridoborate and iron bromide in high boiling ether as reaction medium produces an ultrafine, pyrophoric and magnetic precipitate. X-ray and electron diffraction proved the product to be amorphous. According to X-ray absorption fine structure spectroscopy (XAFS) the precipitate has FeB structure up to nearly two coordination spheres around an iron absorber atom. Transmission electron microscopy (TEM) confirms the ultrafine powder to be nanoscale. Subsequent annealing at 450 °C causes the atoms to arrange in a more distinct FeB structure, and further thermal treatment to 1050 °C extends the local structure to the α-modification of FeB. Between 1050 °C and 1500 °C α-FeB is transformed into β-FeB.

Structure refinements of iron borides Fe2B and FeB

Abstract Crystals of the iron borides FeB and Fe2B were obtained from two different high temperature synthesis routes, viz. in combined copper/gallium flux and under high pressure conditions. Their crystal structures were investigated at room and low temperatures. Structure models from the literature were confirmed. The position of the boron atoms was unambiguously determined. FeB (Pnma, no. 62) contains zig-zag chains of boron atoms in which the boron atoms are coordinated by seven iron atoms in the form of a mono-capped trigonal prism. Fe2B (I4/mcm, no. 140) contains single boron atoms in a square antiprismatic iron atom coordination.

Probing for Structural Features of Boron-rich Solids with EELS†

Crystal structures of boron-rich solids are characterized by boron atom arrangements that are quite diverse: chains, sheets, and a variety of polyhedra like octahedra, pentagonal bipyramids, cuboctahedra, and icosahedra are observed. Probing by electron energy-loss spectroscopy (EELS), these different structural features are mirrored by a pronounced variation of the energy loss near-edge fine structure (ELNES) of the BK ionization edges. For identification, characteristics of these fine structures can be used as so-called “coordination fingerprints”, which is shown for solids like MgB2, TaB2, ZrB2, CaB6, SrB6, BaB6, NaB5C, KB5C, Na3B20, Na2B29, UB12, ZrB12, LaB2C2, CeB2C2, and CaB2C2. In addition, theoretical calculations of ELNES based on the density functional theory (FLAPW method) are presented for an example of boron-rich solids. Bestimmung struktureller Charakteristika in borreichen Festkorpern durch Elektronenenergieverlustspektroskopie Die Kristallstrukturen von borreichen Festkorpern zeigen stark variierende Boratom-Anordnungen: Man findet Ketten, Schichten und eine Vielzahl verschiedener Polyeder wie Oktaeder, pentagonale Bipyramiden, Kuboktaeder und Ikosaeder. Mittels der Elektronenenergieverlustspektroskopie (EELS) konnen diese strukturellen Charakteristika abgebildet werden, denn die Nahkanten-Feinstruktur (ELNES) der BK-Ionisierungskanten variiert signifikant mit der Boratom-Anordnung. Fur Verbindungen wie MgB2, TaB2, ZrB2, CaB6, SrB6, BaB6, NaB5C, KB5C, Na3B20, Na2B29, UB12, ZrB12, LaB2C2, CeB2C2 und CaB2C2 wird gezeigt, das die unterschiedlichen Feinstrukturen als “Fingerabdruck” der Koordination der Boratome und damit der Identifizierung von Verbindungen dienen konnen. Zusatzlich werden fur ein Beispiel borreicher Festkorper theoretische Berechnungen der ELNES, die auf der Dichtefunktionaltheorie basieren (FLAPW-Methode), vorgestellt.

NaB15: a new structural description based on X-ray and neutron diffraction, electron microscopy, and solid-state NMR spectroscopy

A boron-rich sodium boride, formerly known as NaB15, has been subjected to a comprehensive structural reinvestigation using X-ray single-crystal and powder diffraction, low-temperature neutron and electron diffraction, high-resolution transmission electron microscopy, and 23Na solid-state NMR spectroscopy. The results indicate that the previously published orthorhombic space group is incorrect. Consistent with all of the experimental results a modified structural description is developed in the monoclinic space group Ilml (a = 585.92(3), b= 1039.92(6), c = 833.17(5) pin, beta = 90.373(5) from powder data). Because one of the interstitial boron atom positions remains unoccupied, the accurate compositional formula is NaB145 or Na2B29.

Solid solution between lithium-rich yttrium and europium molybdate as new efficient red-emitting phosphors

Li3.5Y1.5(MoO4)4 was synthesized in the form of phase-pure crystals. It forms a solid solution with Li3.5Eu1.5(MoO4)4. Single crystals obtained from Li2MoO4 fluxes had edge lengths of up to 0.5 mm. The crystal structures of Li3.5(Y1−xEux)1.5(MoO4)4 (x = 0, 0.1, 0.25, 0.5, 0.75 and 1) were determined in the triclinic crystal system (e.g. P, no. 2, Z = 1; Li3.5Y1.5(MoO4)4: a = 5.1875(2) A, b = 6.6380(2) A, c = 10.2731(4) A, α = 100.082(3)°, β = 100.257(2)° and γ = 111.943(2)°). All the compounds are isostructural and crystallize with a structure related to the scheelite-type structure with mixed occupancy of one cation position, while the two others are occupied by Li ions exclusively. Thermal analyses reveal stability in air up to 995 K for Li3.5Y1.5(MoO4)4; the decomposition temperature decreases with an increase in Eu-content. Spectroscopic properties (emission and excitation) were investigated in the context of the search for new thermographic phosphors. Excitation at 395 nm leads to strong red emission at 613 nm. Features of the emission spectra suggest the potential of Li3.5(Y1−xEux)1.5(MoO4)4 as red phosphor candidates for light emitting diodes. Their luminescence properties at high temperatures were also investigated.