Crystal Structures, Phase Stabilities, Electronic Properties, and Hardness of Yttrium Borides: New Insight from First-Principles Calculations.

Yttrium borides have attracted much attention for their superconducting and high chemical stability. However, the fundamental knowledge of the mechanical properties and hardness of yttrium borides is rather lack. Here, we performed a systematic investigation on yttrium borides with various stoichiometries based on an unbiased CALYPSO structure search method and first-principles calculations. A new YB6 compound with R3m hexagonal structure is observed, which is more stable than the experimental synthesized Pm3m phase. The calculated formation enthalpy, elastic constants, and phonon dispersions distinctly show that the R3m-YB6 is energetically, mechanically, and dynamically stable. The density of states and electronic band structure reveal that all the stable yttrium borides are metallic. Based on a semiempirical method, the Vicker hardness of R3m-YB6 is expectant to be 37.0 GPa, indicating it is a potential ultrahard metal. Our results provide insights for future synthesis and design of new type functional materials.