High-Pressure Synthesis of Two Pb₃(C₃N₆) Polymorphs Featuring Fully Deprotonated [C₃N₆]⁶– Melaminate Anions

Graphitic carbon nitride (g-C3N4), melamine, and their derivatives are of significant interest due to their diverse structural properties and potential applications in catalysis, energy storage, and materials science. In this study, two new compounds, both with the Pb3(C3N6) chemical formula, were synthesized at pressures of 40(2) to 48(2) GPa through a direct reaction between Pb, C and N2, or Pb and C6N4, in laser-heated diamond anvil cells. Their crystal structures were solved and refined using synchrotron single-crystal X-ray diffraction. The two Pb3(C3N6) polymorphs, namely tP48-Pb3(C3N6) and hP72-Pb3(C3N6), crystallize in the noncentrosymmetric space groups P4̅21m and P6122, respectively. Both solids contain the previously unknown hydrogen-free [C3N6]6– melaminate anion and were experimentally found recoverable to ambient conditions. Density functional theory calculations provide further insights into the structural and electronic properties, and reveal that both compounds are direct narrow band gap semiconductors.