Computational prediction of boron-based MAX phases and MXene derivatives

Conventional MAX phases (M is an early transition metal, A represents a p-block element or Cd, and X is carbon or nitrogen) have so far been limited to carbides and/or nitrides. In the present work, a series of stable layered ternary borides were predicted by combining variable-composition evolutionary structure search and first-principles calculations. The predicted Hf2InB2, Hf2SnB2, Zr2TlB2, Zr2PbB2, and Zr2InB2 show a Ti2InB2 type of structure (space group P6¯ m2, No. 187, Nat. Commun. 2019, 10, 2284), and the structures of Hf3PB4 and Zr3CdB4 share the same space group with Ti2InB2 but belong to a new structure type. These two structural prototypes, M2AB2 and M3AB4 (M is Zr or Hf), have the composition and local structures of MAB phases, but inherit a hexagonal symmetry of MAX phases. Moreover, Hf2BiB and Hf2PbB exhibit a typical structure of conventional MAX phases (Mn+1AXn, space group P63/mmc, No. 194). These findings suggest that boron-based ternary compounds may be a new platform of MAX phases. The functionalized two-dimensional (2D) borides derived from the predicted ternary phases are calculated to be with improved mechanical flexibility and adjustable electronic properties relative to the parent ones. In particular, the 2D Hf2B2T2 and Zr2B2T2 (T = F, Cl) can transform from metal to semiconductor or semimetal under appropriate compressive biaxial strains. Moreover, the 2D Zr2B2 exhibits a high theoretical lithium-ion (Li+) storage capacity and low Li+ migration energy barriers. These novel properties render 2D boron-based materials promising candidates for applications in flexible electronic devices and Li+ battery anode materials.