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

TL;DR

This study uses first principles calculations to analyze the electronic structures and energy levels of RE3+ and RE2+ doped LaSi3N5 phosphors, revealing how doping affects band gaps and electronic transitions.

Contribution

It provides a detailed first-principles analysis of RE-doped LaSi3N5, including energy level schemes and transition types, validating the use of hybrid functional HSE06 for such systems.

Findings

RE3+ compounds have decreasing band gaps from Ce to Eu.

RE2+ compounds show increasing band gaps from Ce/Pr to Eu.

Energy level schemes match well with experimental data.

Abstract

First principles calculations of rare earth (RE)-doped LaSi3N5 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 RE3+-doped compounds 4f states are shifted by ~6 eV to more negative energies compared with RE2+-compounds. This stabilization causes that RE3+ 4f bands are in a different position relative to the valence band and the conduction band than RE2+ 4f bands and therefore different electronic transitions apply. BG of RE3+-compounds decreases from ~4.6 eV (Ce) to ~0.5 eV (Eu). Except for Ce3+, exhibiting the 4f-5d transition, other RE3+-compounds show the charge transfer of the p - 4f character. BG of RE2+-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.