Ab initio calculations of the electronic structure and specific optical features of β-LiNH4SO4 single crystals

Highlights: • Complex experimental and theoretical studies of electronic and optical properties for β-LiNH₄SO₄ crystals. • X-ray photoelectron spectroscopy and X-ray emission spectroscopy measurements. • Quantum-chemical calculations in the density functional theory (DFT) framework. • Refractive indices dispersions along the principal crystallographic directions as well as birefringence dispersion in the visible spectral range are presented. - Abstract: In the present work complex experimental and theoretical studies of electronic and optical properties for β-lithium-ammonium sulfate crystals of good optical quality are performed using the X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES). Standard immersion and spectroscopic techniques accompanied by the theoretical quantum-chemical calculations in the density functional theory (DFT) framework were applied. Calculations of band structure and related properties were carried out within a framework of local density and generalized gradient approximations as well as hybrid B3LYP functionals. The energy levels features and their origin are established from the DFT calculations and they were ferified by XPS and XES measurements. Theoretical and experimental refractive indices dispersions along the principal crystallographic directions (n_x, n_y and n_z) as well as birefringence dispersion (Δn_x, Δn_y and Δn_z) in the visible spectral range are obtained. It was found a closeness of n_x and n_y curves for the titled crystals. More precise birefringence examining predicts their intersection at λ ≈ 190 nm.