No abstract

Calcium diboride dicarbide, CaB(2)C(2), was synthesized as a crystalline powder and investigated by electron energy loss spectroscopy, X-ray powder diffractometry, conductivity measurements, and LMTO band structure calculations. A new structure model was derived, and the crystal structure was refined by Rietveld methods in the tetragonal space group I4/mcm (No. 140, a = 537.33(1) pm and c = 741.55(2) pm, Z = 4). The boron and carbon atoms are well ordered within layers consisting of four- and eight-membered rings. A convincing coloring scheme is proven by the detection of a superstructure reflection. An earlier assignment of the compound into the LaB(2)C(2) structure family (space group P&fourmacr;2c or P4(2)/mmc, respectively) has been shown to be incorrect. LMTO band structure calculations suggest semiconducting behavior for CaB(2)C(2), which has been confirmed by conductivity measurements.

A new class of compounds has been discovered and identified:  metal borides with a polyhedral three-dimensional anionic framework that is stabilized by carbon insertion. Two representatives, NaB5C and KB5C, were obtained as crystalline powders and characterized by Rietveld refinements of the X-ray data and by electron energy loss spectroscopy. These compounds, which are the first ternary boron/carbon compounds of sodium and potassium, crystallize with the cubic hexaboride structure, with carbon statistically distributed at the boron positions of the octahedral framework. KB5C is both isotypic and isoelectronic to CaB6. Insertion of exactly one carbon per octahedron provides an electronic stabilization of the framework, which is reflected by the electrical properties of KB5C. An energy band gap shown by “extended Hückel” calculations supports the observation of KB5C being a semiconductor.

No abstract

structure structure (solids and liquids) D 2000 -008Crystal Structure of Boron Triiodide, BI 3 .-The air-and light sensitive title compound is obtained as microcrystalline powder by sublimation (vacuum, 23 → -196 • C) of the reaction product of NaBH 4 and I 2 (hexane, reflux, 84 h). BI 3 crystallizes in the hexagonal space group P6 3 /m with Z = 2. The B-I bond length is determined to be 211.2 pm.