Structure prediction and first-principles investigation of layered Group-2 triphosphides: Polymorphism, interlayer coupling, and optoelectronic properties

Group-2 triphosphides XP 3 differ markedly from the honeycomb-like XP 3 monolayers reported for many Group 13–15 elements, as they form black-phosphorus-related sandwich-like layered structures with Group-2 metal atoms arranged around phosphorus substructures. Here, we combine MatterSim-accelerated USPEX crystal-structure prediction with validation at the density functional theory (DFT) level to investigate Group-2 XP 3 compounds with X = Mg, Ca, Sr, and Ba. The experimentally reported CaP 3 , SrP 3 , and BaP 3 phases are recovered, and additional low-energy polymorphs are identified. The selected structures are classified into four polymorph families, α , D- α , mS, and mP, showing both structural transferability and composition-dependent selectivity across the series. Charge analysis, electron-density differences, exfoliation energies, and Non-Covalent Interactions (NCI) results indicate that these layered phosphides exhibit substantially stronger interlayer coupling than typical elemental van der Waals materials, mainly associated with metal-to-phosphorus charge redistribution. Except for bulk α -MgP 3 , the studied structures are semiconductors with a broad range of band gaps, and their band-edge states are mainly governed by the phosphorus substructure. Dimensional reduction from bulk to monolayer induces a direct-to-indirect band-gap crossover in α -SrP 3 and an indirect-to-quasi-direct crossover in α -BaP 3 . Time-Dependent DFT (TDDFT) calculations within the Self-Consistent Ewald electrostatic embedding framework further show that studied monolayers, especially α -CaP 3 , α -BaP 3 , mS-SrP 3 , and mS-BaP 3 , exhibit pronounced visible-region absorption. These results provide a systematic structure–property picture of layered Group-2 triphosphides and might therefore serve as a reference for future studies of their polymorphs and heterostructures.