Synthesis of Hexagonal CaB3C3 with a Zeolite-Type Framework.

We report the synthesis of a calcium-intercalated boron-carbon framework with the "NPO" zeolite topology. Hexagonal CaB3C3 crystallizes in space group P62m (No. 189), under high-pressure, high-temperature conditions, in a structure predicted for hypothetical metastable carbon allotropes. The structure, which exhibits minimal or no substitutional framework disorder, is comprised of four-coordinated B/C atoms that form channels along the crystallographic Z-axis, trapping calcium ions. Calculations based on density functional theory (DFT) indicate that hexagonal CaB3C3 is thermodynamically stable above 30 GPa, and theoretical/experimental results suggest that the recoverable compound exhibits a superconducting transition near 5 K at ambient pressure. The formation of hexagonal CaB3C3 further expands the range of synthesizable structural topologies for carbon-based tetrahedral nets with versatile physical properties.