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.