FeB6 Monolayers: The Graphene-like Material with Hypercoordinate Transition Metal.

By means of density functional theory (DFT) computations and global minimum search using particle-swarm optimization (PSO) method, we predicted three FeB6 monolayers, namely α-FeB6, β-FeB6 and γ-FeB6, which consist of the Fe©Bx (x = 6, 8) wheels with planar hypercoordinate Fe atoms locating at the center of six- or eight-membered boron rings. In particular, the α-FeB6 sheet constructed by Fe©B8 motifs is the global minimum due to completely shared and well delocalized electrons. The two-dimensional (2D) boron networks are dramatically stabilized by the electron transfer from Fe atoms, and the FeB6 monolayers have pronounced stabilities. The α-FeB6 monolayer is metallic, while the β-FeB6 and γ-FeB6 sheets are semiconductors with indirect band gaps and significant visible-light absorptions. Besides the novel chemical bonding, the high feasibility for experimental realization, and unique electronic and optical properties, render them very welcome new members to the graphene-like materials family.