Corrugation-Stabilized Layers and Stacking-Selected Ground State in Layered Graphitic C ₃ N ₄

Graphitic C3N4 exhibits rich structural polymorphism due to the flexibility of heptazine-based covalent networks. Although corrugated monolayers are known to be energetically favored over the ideal planar graphitic sheet, the bulk ground state of graphitic C3N4 remains unresolved because the role of interlayer stacking has not been systematically explored. Here we perform a structural search for graphitic C3N4 by combining intralayer corrugation and interlayer stacking within an NX-network-based framework. First-principles optimization of about one hundred candidates shows that the dominant stabilization arises from layer corrugation, while stacking provides an additional energy gain that determines the final bulk ordering. Although the corrugated Pca21 monolayer is lower in energy than P321, stacking of the P321 layer yields a lower-energy P3̅c1 phase that surpasses the commonly assumed Pbca structure. Phonon calculations confirm that P3̅c1 is dynamically stable, and simulated X-ray diffraction patterns reveal characteristic peaks distinguishing it from Pbca. These results demonstrate a two-step stabilization mechanism and identify P3̅c1 as a strong candidate for the lowest-energy graphitic phase.