Strain-tunable Dirac nodal-line states and electronic phase transitions in two-dimensional boron hydrides

Two-dimensional (2D) boron hydrides (BHs) have recently been proposed as potential Dirac nodal-line (DNL) materials based on density functional theory calculations using the Perdew-Burke-Ernzerhof (PBE) functional. Here, we present a systematic first-principles reinvestigation of these systems using the more accurate HSE06 hybrid functional, together with a comprehensive exploration of possible hydrogenation configurations. Our calculations reveal a previously unreported structural isomer, , which is nearly degenerate in energy with the previously proposed ground state (5-7)-α₂-B₁₆H₁₆. In contrast to earlier predictions, the HSE06 results show that both structures are intrinsic semiconductors rather than nodal-line semimetals at equilibrium, correcting previous interpretations of their electronic topology. Remarkably, we demonstrate that mechanical strain can induce Dirac nodal loops in both systems, enabling reversible transitions between semiconducting and nodal-line semimetal phases. The newly identified structure exhibits significantly lower critical strains for the emergence of DNL states, highlighting its strong strain responsiveness. These findings clarify the intrinsic electronic properties of 2D boron hydrides and establish strain engineering as an effective route for realizing tunable topological electronic states in boron-based 2D materials.