Fundamental View of Electronic Structures of β-NaYF4, β-NaGdF4, and β-NaLuF4

Abstract

We discussed the electronic structures of β-NaLnF4 (Ln = Y, Gd, and Lu). We found the band gap keeps nearly constant (8–9 eV). However, the difference of the experimentally observed band gap arises from the different positions of 4f orbital levels relative to the valence band maximum. The 4f empty state of Gd falls into the band gap, led to a decreased band gap for β-NaGdF4, and is spin-polarized. In contrast, both filled and empty 4f levels of Lu widely separated below and above the valence and conduction band edges, respectively, which means they do not influence the optical transitions in the band gap of the host lattice. By projecting the components of self-energy and wave function relaxation in 4f orbitals, we indicated a hidden level of Gd and Lu ions in the β-lattices, giving three Gd/Lu ions in the lattice split into two different types of electronic levels. This analysis helped us understand the essential mechanism and modified the energy migration mediated upconversion (EMU) model. Different 4f ...