Compression-induced hexa-to-tetra phase transition of confined germanene.

Via molecular dynamics (MD) simulations we find the existence of the new allotrope of two-dimensional (2D) germanene, i.e. 2D tetra-germanene (tetra-Ge) which contains entirely tetragons. We compress 2D hexa-germanene (hexa-Ge) step by step over a broad density range at constant temperature and hexa-tetra Ge phase transition occurs. We find that the compression of hexa-Ge at 2000 K (not far above the melting point of hexa-Ge) leads to the formation of tetra-Ge with the highest quality. Atomic structure of the obtained tetra-Ge at 300 K is analyzed in details. Although fraction of tetragons in the tetra-Ge is very high (larger than 0.99), some defects are found in addition to the skew tetragons. Due to containing almost entirely tetragons, tetra-Ge may exhibit new behaviors unlike those of the hexa-Ge. Subsequent studies in this direction for 2D tetra-Ge. In addition, first-principles calculations under density functional theory confirm the existence of stable tetra-Ge.