Existence of BeCN₂ and Its First-Principles Phase Diagram: Be and C Introducing Structural Diversity

The existence and structure of BeCN₂, the lightest representative of II-IV-V₂ compounds, have for long remained unsolved, although previous theoretical studies have relied on assuming chemical similarity toward the known wurtzite-type BeSiN₂. To solve the BeCN₂ puzzle, we have now explored its potential-energy surface and here predict two additional polymorphs with space groups Cmc2₁ (porous phase) and Pmc2₁ (graphitic phase) in addition to another I4̅m2 type (carbodiimide-like), which is only slightly higher in energy than the wurtzite type. The phase diagram constructed from density-functional theory shows the Cmc2₁-type to be the ground state, stable in terms of the Gibbs energy under standard conditions, whereas the Pmc2₁- and I4̅m2-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^-1, but larger toward the carbodiimide type, 25 kJ mol^-1, and the wurtzite type, 28 kJ mol^-1. Chemical-bonding analysis further reveals how beryllium and carbon induce structural diversity. As regards the second-lowest Pmc2₁-type, a monolayer of such graphitic BeCN₂ shows the potential of photoelectrochemical water splitting, while a bilayer configuration should exhibit ferroelectricity with a polarization of 0.75 pC m^-1. Further electronic-structure data of the four polymorphs indicate their potential for nonlinear optics.