Peroxide bond-driven stability and multifunctional properties of BaO2 in different crystal systems: A first-principles study using GGA and HSE06

This study explores the stability, electronic structure, thermal, and optical properties of barium peroxide (BaO${}_2$) in three different crystal systems: Orthorhombic (BaO${}_2$-Ortho), Tetragonal (BaO${}_2$-Tetra), and Monoclinic (BaO${}_2$-Mono). Density functional theory (DFT) calculations are performed using both the GGA and HSE06 functionals. The energetic, thermal, and dynamical stabilities are evaluated through formation energy calculations, ab-initio molecular dynamics (AIMD) simulations, and phonon band structure analyses, respectively. A strong peroxide bond is found in BaO${}_2$-Mono, resulting in more localized peroxide orbitals, which typically lead to a larger band gap. In contrast, the relatively weaker peroxide bond in BaO${}_2$-Ortho leads to slightly delocalized peroxide orbitals, thereby reducing the band gap. The heat capacity of the most thermally stable BaO${}_2$-Mono structure is significantly higher than that of the other two structures, due to its higher phonon density of states at both low and high frequency ranges. The optical responses, including dielectric functions, refractive index, and optical conductivity, are also investigated. It is found that for BaO${}_2$-Mono, the indirect electronic band gap is very close to the optical band gap due to the flat band nature of the structure. In contrast, the optical band gaps of the other two structures are larger than their respective indirect electronic band gaps. All three BaO${}_2$ structures exhibit very low static dielectric constants, indicating weak electronic screening consistent with their insulating nature. These results show that BaO${}_2$ could be useful for UV optoelectronic applications, particularly in high-energy devices like UV photodetectors and sensors.