Geometric frustration in hydrogenated pentahexoctite: a first-principles search for the ground state of alternating...

In this work, we investigate the ground-state configuration of a pentahexoctite monolayer with alternating hydrogenation using first-principles calculations. Due to the presence of pentagonal rings, hydrogenated pentahexoctite inevitably possesses a syn-arrangement, wherein a pair of hydrogens are attached to the same side of the neighboring carbon sites. Thus, a perfectly alternating hydrogenation pattern is geometrically forbidden in the pentahexoctite monolayer. To address this, we focused on searching for a set of "maximally alternating" configurations that exclude sequences of three or more contiguous, same-side hydrogen atoms. Employing a two-stage search, we identified a checkerboard-like structure as the lowest-energy configuration of alternating hydrogenation, possessing the minimum possible number of syn-arrangements. We found that hydrogenation transforms the metallic pristine pentahexoctite into an insulator with a direct band gap. Furthermore, we performed phonon calculations and ab initio molecular dynamics simulations to confirm that this structure is both dynamically and thermally stable. The mechanical properties are also investigated, showing that while hydrogenation reduces the elastic stiffness, the material's overall anisotropic character is maintained. Our results predict a new, stable 2D hydrocarbon and provide fundamental insights into the role of geometric frustration in non-hexagonal 2D materials.