M n+1 AX n phases are a large family of compounds that have been limited, so far, to carbides and nitrides. Here we report the prediction of a compound, Ti 2 InB 2 , a stable boron-based ternary phase in the Ti-In-B system, using a computational structure search strategy. This predicted Ti 2 InB 2 compound is successfully synthesized using a solid-state reaction route and its space group is confirmed as P m2 (No. 187), which is in fact a hexagonal subgroup of P6 3 /mmc (No. 194), the symmetry group of conventional M n+1 AX n phases. Moreover, a strategy for the synthesis of MXenes from M n+1 AX n phases is applied, and a layered boride, TiB, is obtained by the removal of the indium layer through dealloying of the parent Ti 2 InB 2 at high temperature under a high vacuum. We theoretically demonstrate that the TiB single layer exhibits superior potential as an anode material for Li/Na ion batteries than conventional carbide MXenes such as Ti 3 C 2 .

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 P6̅m2, 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 (M n+1AX n , space group P 63/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.

Intrinsic two-dimensional (2-D) magnets are promising materials for developing advanced spintronic devices. Few have already been synthesized from the exfoliation of the van der Waals magnetic materials. In this work,...