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 Pm3̅m 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.

Structures, Mobilities, and Electronic Properties of Functionalized Silicene: Superhalogen BO2 Adsorption

The narrow band gap of silicene severely hinders its application in nanoelectronic devices. Therefore, it is significant to open the band gap of silicene and maintain its high carrier mobility. And for that, the adsorption of different coverage superhalogens BO₂ on the silicene surface have been investigated based on density functional theory and the CALYPSO method. The results show that BO₂ unit prefers to adsorb on silicene with adjacent mode irrespective of the size of substrate. The electronic structure analysis indicates that the density of states near the Fermi level are mainly contributed by Si-p and BO₂-p orbitals. (BO₂)_n-silicene exhibits metallic character with the exception of (BO₂)₂ adsorbed on 4 × 4 supercell. As for (BO₂)₂-silicene, silicene transforms from a gapless direct semiconductor to an indirect semiconductor. Furthermore, the effective electron mass of two BO₂ superhalogens on 4 × 4 silicene is estimated and found to be smaller than that of graphene. It is expected to result in higher electron mobility.