First‐Principles Prediction of a Metastable Polar Phase for Low Pressure C 3 N 4

Carbon nitride (C 3 N 4 ) exhibits rich polymorphism arising from versatile sp 2 and sp 3 hybridization. Using graph‐space based crystal structure prediction, we identify a previously unreported polar C 3 N 4 polymorph with Pna2 1 symmetry, closely related to the experimentally accessible chiral P4 3 2 1 2 phase. Although constructed from the same ribbon‐like CN fragments, Pna2 1 adopts a different stacking sequence that lowers the symmetry and generates a polar structure. Enthalpy calculations show that Pna2 1 is energetically competitive with P4 3 2 1 2 and becomes more stable than graphitic Pbca above ~4 GPa, indicating a low‐energy metastable mixed sp 2 –sp 3 phase. Simulated X‐ray diffraction patterns suggest that Pna2 1 may coexist with P4 3 2 1 2 in high‐pressure experiments. First‐principles calculations confirm its dynamical and mechanical stability, as well as an indirect bandgap of 3.55 eV. The reduced symmetry further induces spontaneous polarity, enhanced piezoelectricity, and anisotropic optical response. Our results demonstrate how subtle stacking variations within a common fragment‐based framework can generate low‐energy polar semiconductors with distinct functional properties.