First-Principles calculations of the interconfigurational transition energies of 4f - 4f-15d of Ln3+ ions in LiYF4 and CaF2

Abstract In studying the design of novel phosphors and solid-state laser materials, luminescence with a characteristically long lifetime originates from f-f emissions (infrared, visible and ultraviolet). However, due to high demand for novel luminescence materials which excite in the ultraviolet (UV) and vacuum ultraviolet (VUV) ranges, the interconfigurational f-d transition has been of great interest. In this work, we investigated the 4fn - 4fn-15d transition energy of trivalent lanthanide ions in LiYF4 and CaF2 host crystals (LiYF4: Ln3+ and CaF2: Ln3+) without referring to any empirical parameters. These crystals are considered to be promising fluoride hosts for solid-state laser crystals. LnF85− atomic clusters were built based on the crystal structures of LiYF4 and CaF2. The relativistic MO calculations were done using the relativistic Discrete Variational Xα (DV-Xα) algorithm. The relativistic version of discrete variational multi-electron method (DVME) algorithm was used to calculate the multiplet energies. We considered the effect of lattice relaxation by a simple estimation using Shannon's crystal radii. Given that the theoretical transition energies are typically underestimated due to the underestimation of the corresponding effects, the absolute transitional energies have received corrections based on one-electron calculations utilizing the Slater's transition-state technique. The in-depth study indicates that the lattice-relaxation effect does not substantially impact LiYF4: Ln3+, but greatly affects the transitional energy of heavy lanthanides doped in CaF2 from 4fn - 4fn-15d. The energy corrections based on Slater's transition-state method are necessary to improve the accuracy of our calculations. By a combination of these two methods, the transition energies of 4fn - 4fn-15d of LiYF4: Ln3+ and CaF2: Ln3+ agree very well with the experimental data. This research is important to providing guidance in the search for new lanthanide-based phosphors in UV or VUV regions.