New boron-rich materials: Cubic carbaborides of sodium and potassium

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 cr ystalline powders and characterized by Rietveld refinements of the X-ray da ta 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 a nd isoelectronic to CaB6. Insertion of exactly one carbon per octahedron pr ovides an electronic stabilization of the framework, which is reflected by the electrical properties of KB5C. An energy band gap shown by "extended Hu ckel" calculations supports the observation of KB5C being a semiconductor.