Effect of photo irradiation to hydrogen impurity in b-rhombohedral boron

β-rhombohedral boron (β-boron) has recently shown an interesting photo-response, which could be interpreted as change of state of hydrogen impurity [1]. Yet, the exact locations and bonding environments of hydrogen within its complex structure remain uncertain. Using density functional theory, we explored how hydrogen interacts with the intrinsic defects that stabilize β-boron. We find that hydrogen preferentially occupies interstitial sites forming bent B–H–B bridges, similar to those in diborane, and binds to dangling bonds associated with boron vacancies. The predicted lattice expansion upon hydrogen incorporation agrees closely with experimental observations. These results provide atomistic insight into how hydrogen is accommodated in β-boron and suggest pathways for tuning its behavior under light or thermal excitation.

[1] T. Hiroto et al., Solid State Sci. 163, 107907 (2025).