First-principles investigation of structural, mechanical, electronic, and Raman properties of Pmna-B6X

A type of B₁₂-based borides, named as Pmna -B 6 X (X = S, Se, O, and As) is proposed and explored via first-principles calculations. They possess three-dimensional covalent B-X framework and exhibit layer-like atomic arrangements. All four compounds are verified to be thermodynamically, mechanically, and dynamically stable at ambient pressure. These compounds are semiconductors with great hardness, exhibiting band gaps of 1.62–4.15 eV and Vickers hardness up to 29.84 GPa, and show distinct tensile anisotropy. The inherent structural anisotropy of Pmna -B 6 S and Pmna -B 6 Se accelerates the pressure-driven evolution of band gaps. Within 0–30 GPa, band edges in Pmna -B 6 S and Pmna -B 6 Se exhibit pressure-induced decoupling: the conduction band minimum remains fixed, while the valence band maximum shifts from U -point to the Γ -point. The calculated Raman spectra of Pmna -B 6 S and Pmna -B 6 Se are in good agreement with those of the experimental data. This work clarifies the intrinsic physical properties of hard semiconducting Pmna -B 6 X phases. Furthermore, the pressure-dependent electronic structures of Pmna -B 6 S and Pmna -B 6 Se offer valuable insights into pressure-tunable electronic behaviors of boron-rich boride semiconductors.