New Boron-Rich Materials:  Cubic Carbaborides of Sodium and Potassium

Abstract

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, NaB 5 C and KB 5 C, 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. KB 5 C is both isotypic and isoelectronic to CaB 6 . Insertion of exactly one carbon per octahedron provides an electronic stabilization of the framework, which is reflected by the electrical properties of KB 5 C. An energy band gap shown by “extended Hückel” calculations supports the observation of KB 5 C being a semiconductor.