Europium-doped LaSi3N5 phosphor synthesized from LaSi/Si/Si3N4/Eu2O3 mixture by direct nitridation at 1390 degrees C and additional annealing at 1630 degrees C for 3 similar to h shows emissions in the green light region. With increasing europium content in the general formula La1-zEuzSi3N5-zO1.5z from z similar to=similar to 0.04 to 0.063 the emission intensity increases. The two maxima of the excitation band for the 6.3 similar to mol% Eu-doped LaSi3N5 are at similar to 4.0 and similar to 3.35 similar to 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 similar to x similar to 1 similar to x similar to 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+/rectangle, La3+/Eu2+ substitutions in cation sites (rectangle=similar to 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 similar to 3.23 similar to eV by the hybrid functional is in reasonable agreement with experimental data.