Europium-Doped LaSi3N5 Ternary Nitride: Synthesis, Spectroscopy, Computed Electronic Structure and Band Gaps

Abstract

Europium-doped LaSi3N5 phosphor synthesized from LaSi/Si/Si3N4/Eu2O3 mixture by direct nitridation at 1390°C and additional annealing at 1630°C for 3 h shows emissions in the green light region. With increasing europium content in the general formula La1-zEuzSi3N5-zO1.5z from z = 0.04 to 0.063 the emission intensity increases. The two maxima of the excitation band for the 6.3 mol% Eu-doped LaSi3N5 are at ~4.0 and ~3.35 eV. First-principles density-functional theory (DFT) calculations are performed to enhance the understanding of the electronic structure of the stoichiometric LaSi3N5 and La/Eu and N/O substituted ternary nitrides. To mimic the realistic concentration of Eu and O the cell volume is expanded to the 2 × 1 × 2 super-cell with 144 atoms. Electronic structure and band gaps are calculated using both the plain DFT and the hybrid functional. Three different doping schemes are investigated combining the La3+/Eu3+, La3+/□, La3+/Eu2+ substitutions in cation sites (□ = cation vacancy) and the N3−/O2− substitution in the framework. The La3+/Eu3+ substitution does not lead to a significant change of the gap. The La3+/Eu2+ substitution introduces a narrow band of Eu 4f-states in the gap thus narrowing the band gap. The calculated value of ~3.23 eV by the hybrid functional is in reasonable agreement with experimental data.