Electronic structure and energy level schemes of RE3+:LaSi3N5 and RE2+:LaSi3N5−xOx phosphors (RE=Ce, Pr, ND, Pm, Sm, Eu) from first principles

Abstract

First principles calculations of rare earth (RE)-doped LaSi 3 N 5 host lattice are performed to obtain the electronic structure, the band gap (BG), and the character of electronic transitions.Doping with both trivalent and bivalent RE cations is inspected. RE 4f states form two bands of occupied and unoccupied states separated by ~5 eV. In RE 3+ -doped compounds 4f states are shifted by ~6 eV to more negative energies compared with RE 2+ -compounds. This stabilization causes that RE 3+ 4f bands are in a different position relative to the valence band and the conduction band than RE 2+ 4f bands and therefore different electronic transitions apply. BG of RE 3+ -compounds decreases from ~4.6 eV (Ce) to ~0.5 eV (Eu). Except for Ce 3+ , exhibiting the 4f→5d transition, other RE 3+ -compounds show the charge transfer of the p → 4f character. BG of RE 2+ -compounds increases from ~0.80 eV (Ce, Pr) to ~0.95 eV (Nd, Pm), ~1.43 eV (Sm), and ~3.28 eV (Eu) and the electronic transition is of the 4f→5d character. The energy level scheme constructed from ab initio calculated electronic structures agrees well with the experimental energy level diagram. The agreement demonstrates the reliability of the hybrid functional HSE06 to describe correctly bands of nonbonding RE 4f electrons. Keywords:4f states in lanthanides, LaSi3N5 host lattice, ab initio calculation, 4f energy level scheme, band gaps Pm, Sm, Eu, Gd, Tb, Dy, Ho) in good agreement with experiments. Another way to improve the standard DFT is to modify the intra-atomic Coulomb interaction through the LDA+U approach [13][14][15]. It was shown that this approach allowed for a correct treatment of electronic states in Ce 2 O 3 [16] and in CeO 2 [17]. The physical idea behind LDA+U or GGA+U scheme comes from Hubbard Hamiltonian. In the practical implementations, the onsite two-electron integrals are expressed in terms of two parameters. These are the Hubbard

Electronic structure, magnetic properties, and mixed valence character of Ce2Ni3Si5 from first principles calculations

“…From the partial (projected) density of states (PDOS) shown in Figure , the splitting of Ce 3+ 4f states to occupied and unoccupied bands increase with increasing the U eff value. The U eff value of 5.4 eV reproduces the splitting of the 4f band of Ce 3+ calculated by the hybrid functional . The occupied and unoccupied bands of Ce 3+ 4f states are separated by 5 eV.…”

“…To address strongly correlated f electron of Ce and d electrons of Ni, the PBE + U functional was used as described by Dudarev et al The U eff value has been optimized to 5.4 eV for Ce, to reproduce the splitting of the 4f band of Ce 3+ calculated by the hybrid functional . Other U eff values around 5.4 eV were applied to check the influence on the splitting of the 4f band and on the magnetic properties.…”

“…One way to improve the DFT methods is the hybrid functional which reduces the self‐interaction error by mixing of the DFT exchange energy with the non‐local HF exchange energy in a certain proportion. Recently, it is reported that the hybrid functional predicts accurate band gap and describes correctly the 4f bands of RE elements . However, the hybrid functionals are computationally expensive.…”

First-principles study ofCe3+-dopedlanthanum silicate nitride phosphors: Neutral excitation, Stokes shift, and luminescent center identification

“…As an example, a recent first-principle study of LaSi 3 N 5 , doped with Ce as well as some other dopant ions, has been published. 24 The hybrid functional HSE06 has been used. They point out that, in their scheme, simply based on the Kohn-Sham band structure, the Ce 5d states are in the conduction band, at variance with experimental data.…”

“…30 The U value has been optimized to 4.6 eV (J = 0.5 eV), in order to reproduce the location of the 4f inside the gap, following the hybrid functional study of LaSi 3 N 5 :Ce 3+ . 24 Because of the similar composition of the two nitrides, the same U (J) value was also used in the study of La 3 Si 6 N 11 :Ce 3+ .…”