First-principles structural design of superhard material of ZrB4.

Using the first-principles calypso algorithm for crystal structure prediction, we have predicted two orthorhombic Cmcm and Amm2 structures of ZrB4, which are energetically much superior to the previously proposed WB4-, CrB4-, and MoB4-type structures and stable against decompression into a mixture of Zr and B at ambient pressure. The two orthorhombic structures consist of a hexagonal B ring and ZrB12 units connected by edges and one hexagonal B ring in Cmcm and Amm2 structure, respectively. The calculated large shear modulus (e.g., 229 GPa) and high hardness (42.8 GPa for Cmcm and 42.6 GPa for Amm2) reveal that they are potentially superhard materials. The high hardness is attributed to a stacking of B-Zr-B "sandwiches" layers linked by strong covalent B-B bonding.