Charge-driven structural transformation and valence versatility of boron sheets in magnesium borides

Based on density-functional theory simulations, we have predicted a series of stable magnesium borides, MgB(x), with a broad range of stoichiometries, 2 < x <= 16, by removing magnesium atoms from MgB(2). The layered boron structures are preserved through an in-plane topological transformation of the hexagonal lattice domains to triangular lattice domains. The process can be reversibly switched as the charge transfer changes with Mg insertion and extraction. The mechanism of such a charge-driven transformation originates from the versatile valence state of boron in its planar form. The discovery of these new physical phenomena suggests the design of a high-capacity magnesium boron battery.