Global minimum beryllium hydride sheet with novel negative Poisson's ratio: first-principles calculations

As one of the most prominent metal-hydrides, beryllium hydride has received much attention over the past several decades, since 1978, and is considered as an important hydrogen storage material. By reducing the dimensionality from 3 to 2, the beryllium hydride monolayer is isoelectronic with graphene; thus the existence of its two-dimensional (2D) form is theoretically feasible and experimentally expected. However, little is known about its 2D form. In this work, by a global minimum search with the particle swarm optimization method via density functional theory computations, we predicted two new stable structures for the beryllium hydride sheets, named α-BeH₂ and β-BeH₂ monolayers. Both structures have more favorable thermodynamic stability than the recently reported planar square form (Nanoscale, 2017, 9, 8740), due to the forming of multicenter delocalized Be-H bonds. Utilizing the recently developed SSAdNDP method, we revealed that three-center-two-electron (3c-2e) delocalized Be-H bonds are formed in the α-BeH₂ monolayer, while for the β-BeH₂ monolayer, novel four-center-two-electron (4c-2e) delocalized bonds are observed in the 2D system for the first time. These unique multicenter chemical bonds endow both α- and β-BeH₂ with high structural stabilities, which are further confirmed by the absence of imaginary modes in their phonon spectra, the favorable formation energies comparable to bulk and cluster beryllium hydride, and the high mechanical strength. These results indicate the potential for experimental synthesis. Furthermore, both α- and β-BeH₂ are wide-bandgap semiconductors, in which the α-BeH₂ has unusual mechanical properties with a negative Poisson's ratio of -0.19. If synthesized, it would attract interest both in experiment and theory, and be a new member of the 2D family isoelectronic with graphene.