Existence of BeCN and Its First-Principles Phase Diagram: Be and C Introducing Structural Diversity.

The existence and structure of BeCN, the lightest representative of II-IV-V compounds, have for long remained unsolved, although previous theoretical studies have relied on assuming chemical similarity toward the known wurtzite-type BeSiN. To solve the BeCN puzzle, we have now explored its potential-energy surface and here predict two additional polymorphs with space groups 2 (porous phase) and 2 (graphitic phase) in addition to another 4̅2 type (carbodiimide-like), which is only slightly higher in energy than the wurtzite type. The phase diagram constructed from density-functional theory shows the 2-type to be the ground state, stable in terms of the Gibbs energy under standard conditions, whereas the 2- and 4̅2-types are high-temperature phases; the wurtzite type, however, is the high-pressure phase. The kinetic barrier between the porous and graphitic phases is small, about 4 kJ mol, but larger toward the carbodiimide type, 25 kJ mol, and the wurtzite type, 28 kJ mol. Chemical-bonding analysis further reveals how beryllium and carbon induce structural diversity. As regards the second-lowest 2-type, a monolayer of such graphitic BeCN shows the potential of photoelectrochemical water splitting, while a bilayer configuration should exhibit ferroelectricity with a polarization of 0.75 pC m. Further electronic-structure data of the four polymorphs indicate their potential for nonlinear optics.