DFT-based investigation of Ca ₂ H ₃ Br and Ba ₂ H ₃ I: electronic, optical, and hydrogen storage properties for energy applications

Abstract The electronic, optical, thermodynamic stability, and hydrogen storage properties of the metal hydride halides Ca 2 H 3 Br and Ba 2 H 3 I were systematically analyzed using density functional theory (DFT). The optimized crystal structures demonstrate that both compounds crystallize in a monoclinic lattice with distinct atomic arrangements. Electronic band structure calculations reveal that both materials are indirect semiconductors, with Ca 2 H 3 Br exhibiting a smaller band gap (1.580 eV) than Ba 2 H 3 I (2.185 eV), indicating stronger visible-light absorption potential. Density of states analysis shows that Ba 2 H 3 I has a higher valence band density, favoring hole transport, while Ca 2 H 3 Br has a larger conduction band density, enhancing electron mobility. Optical properties, including the real and imaginary parts of the dielectric function, refractive index, extinction coefficient, absorption coefficient, and optical conductivity, indicate strong photon–material interactions in both the visible and UV regions. Ca 2 H 3 Br exhibits higher absorption in the visible range, whereas Ba 2 H 3 I is more efficient in the UV region. The calculated formation energies of −1.104 eV/atom for Ca 2 H 3 Br and −1.014 eV/atom for Ba 2 H 3 I confirm their thermodynamic stability. Moreover, hydrogen storage analysis shows gravimetric capacities of 1.86 wt% for Ca 2 H 3 Br and 0.75 wt% for Ba 2 H 3 I, highlighting Ca 2 H 3 Br as the more promising candidate for hydrogen-based energy applications. These results demonstrate that both materials are multifunctional, combining optoelectronic performance with stability and hydrogen storage potential, making them suitable for next-generation energy and photonic devices.