Existence of BeCN<sub>2</sub> and Its First-Principles Phase Diagram: Be and C Introducing Structural Diversity
Dong Luo, Ketao Yin, Richard Dronskowski
Abstract
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.