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The vibrational, mechanical, electronic, and optical properties of the ε-O8 phase in the pressure range of 11.4–70 GPa were studied by the first-principle calculation method. The phonon dispersion curves have a tiny virtual frequency at 60 GPa, which indicates that ε-O8 is dynamically unstable at 60 GPa. However, the 3-BM EOS demonstrates that the unit cell is stable up to 70 GPa. It has been shown that ε-O8 remains ductile within the whole applied pressure range. Concurrently, we calculated the variation of the band gap of ε-O8 in the pressure range of 11.4–70 GPa. The results show that the band gap of ε-O8 decreases with increasing pressure. Notably, the band gap disappears within the range of 50–60 GPa, which reveals that the metallic phase transition occurs within this pressure range. [ABSTRACT FROM AUTHOR]

We used the first-principles to study the structural, mechanical, electronic, optical properties and phonon of alkali earth-metal Ba dichalcogenide BaX 2 (X = O, S, Se and Te). First, our calculated lattice constants are in good agreement with the known experimental values. Secondly, the binding and formation energies, bulk modulus, Young's modulus, Poisson's ratio, Debye temperature, sound velocity and phonon dispersion curves of BaX 2 compounds are given. Through basic physical property research, we found that the monoclinic structure of BaX 2 (C2/c) is very likely to exist. The modulation of elastic, electronic and optical properties of this system may be achievable. Furthermore, both elastic and optical properties indicate that BaX 2 is anisotropic. Finally, the electronic properties are studied by density of states and Mulliken population, indicating that all the compounds are mainly ionic properties. [ABSTRACT FROM AUTHOR]