Pseudo Jahn-Teller Effects Control the Trade-off between a Flat and Buckled Structure for Two-Dimensional Materials.

Two-dimensional atomically thin materials have significant applications in catalysis and energy-storage. Structural analyses for 4072 available 2D-systems show that those with heavy atoms prefer buckled structures, while the lighter ones have more planar forms. The origin is traced to the pseudo Jahn-Teller (PJT) effect, where distortions are driven by vibronic interactions between electronic states. Cases involving both Brillouin zone-center (Γ = 0) and non-zone-center phonon condensation reveal that PJT effect-driven symmetry breaking becomes favorable for heavier 2D systems. A heuristic approach involving machine-learning-assisted data analyses also indicates that boron-, carbon-, and nitrogen-containing atomic monolayers are less susceptible toward displacive type-distortions. Chemical control of the PJT effect through atomic constituents can guide the formation of perfectly planar 2D systems.