Ab initio, crystal field and experimental spectroscopic studies of pure and Ni ²⁺-doped KZnF ₃ crystals

Pure and Ni ²⁺ doped KZnF ₃ single crystals were studied using the combination of the DFT-based ab initio methods, crystal field theory and experimental spectroscopic techniques. The electronic, optical and elastic properties have been calculated and compared with available experimental data and good agreement was achieved. Elastic anisotropy of pure KZnF ₃ was modeled; calculations of the sound velocity, Debye temperature, Grüneisen parameter and specific heat capacity were performed. Comparison of the calculated results for the pure and doped material, which is reported for the first time for the considered material, enabled to identify the changes in the optical and electronic properties, which are due to the introduced nickel impurity ions. In particular, it was shown that the lowest Ni 3d states appear in the host's band gap at about 1.0 eV above the valence band. The changes of the electron density distribution after doping were also shown. Microscopic analysis of the crystal field effects based on the performed ab initio calculations of the Ni ²⁺ density of states at different external pressures enabled to estimate the constants of the electron-vibrational interaction, Huang-Rhys factor, Stokes shift and local bulk modulus around impurity ions. The crystal field calculations of the Ni ²⁺ energy levels were performed to analyze and assign the experimental absorption spectrum. Such a combination of the ab initio and semi-empirical calculating techniques leads to a complementary picture of the physical properties of KZnF ₃:Ni ²⁺ and can be applied to other doped crystals. Highlights: Detailed ab initio calculations of physical properties of pure and Ni ²⁺-doped KZnF ₃ were performed. Microscopic crystal field effects on the Ni ²⁺ 3d states were studied. Crystal field calculations of the Ni ²⁺ energy levels in KZnF ₃ were presented.