First-principles study of structural, electronic, optical and bonding properties of celestine, SrSO4

The first-principles calculations were carried out to investigate the crystal structure, energy band structure, density of states, optical properties, and bonding properties of celestine. The equilibrium lattice parameters of celestine optimized by the generalized gradient approximation (GGA) and the localized density approximation (LDA) are consistent with the experimental data. The calculated results of the band structure and density of states (DOS) are used to study the electronic structures of celestine. The band structure shows that the indirect band gap of celestine is ∼6.0 eV (GGA) or ∼6.4 eV (LDA), which is underestimated by ∼1.6 eV (GGA) or ∼1.2 eV (LDA). Based on the imaginary part of the dielectric function, absorption, reflectivity, refractive index and extinction coefficient of celestine have been obtained. When the scissors approximations are used, the absorption bands start at about 7.6 eV, which are almost the same as the experimental data. Celestine presents an optical isotropy because the optical properties are essentially coincident with each other for polarizing the incoming light along the crystal directions. The density of states and Mulliken population of celestine indicate that the Sr–O bonding is purely ionic whereas S–O bonding is mainly transitional.