Materials properties of magnesium and calcium hydroxides from first-principles calculations

Recent experiments have revealed a good potential of applicability of multifunctional hydroxides X(OH)₂ (X = Mg and Ca) in optoelectronic devices and emphasized the lack of fundamental understanding of their materials properties. This work presents systematic study of ground state properties, electronic structure, dielectric and optical properties of the materials by first-principles calculations within PBE-GGA and range-separated hybrid functional schemes as well as by using GW approximation. The relevant HSE06 hybrid functional mixing parameters were determined from a self-consistent adjustment to the electronic dielectric constant ε_∞. We show that the overall performance of our adaptation of the HSE06 functional via implementation of the modified amount of the Fock exchange is nearly best for the ground state properties as compared to the other relevant methods of Hartree-Fock and density-functional theory. Structural stability of the crystalline X(OH)₂ hydroxides has been considered in static and dynamic aspects. The most important factors describing the bonding situation have been investigated, and a crystal-chemical integrity of the hydroxides has been analyzed. From electronic structure studies it was found that both materials are direct band gap insulators. Predictions for the fundamental band gaps were shown to be in the range of 7.7-8.3 eV for Mg(OH)₂ and 7.3-7.6 eV for Ca(OH)₂. The origin of the conduction and valence band states near the band edges has been studied in terms of orbital and site projected density of states as well as by comparison with the X-ray photoelectron spectroscopy measurements. It was shown that effective masses of carriers at the Γ-point in vicinity of the band extreme are strongly anisotropic and for the electrons are similar to those in the ZnO crystal. Optical properties of the bulk X(OH)₂ hydroxides have been investigated in terms of the real and imaginary parts of the optical dielectric function calculated in GW approximation. Electronic character of anisotropy of optical properties has been clarified.