Orange-Emitting Li₄Sr₄[Si₄O₄N₆]O:Eu²⁺-a Layered Lithium Oxonitridosilicate Oxide

We report on the structure and properties of the lithium oxonitridosilicate oxide Li₄Sr₄[Si₄O₄N₆]O:Eu²⁺ obtained from solid-state metathesis. The crystal structure was solved and refined from single-crystal X-ray data in the space group P4₂/ nmc (No. 137) [ Z = 2, a = 7.4833(6), c = 9.8365(9) Å, and R1(obs) = 0.0477]. The structure of Li₄Sr₄[Si₄O₄N₆]O:Eu²⁺ is built up from a layered 2D network of SiN₃O tetrahedra and exhibits stacking disorder. The results are supported by transmission electron microscopy and energy-dispersive X-ray spectroscopy as well as lattice energy, charge distribution, and density functional theory (DFT) calculations. Optical measurements suggest an indirect band gap of about 3.6 eV, while DFT calculations on a model free of stacking faults yield a theoretical electronic band gap of 4.4 eV. Samples doped with Eu²⁺ exhibit luminescence in the orange spectral range (λ_em ≈ 625 nm; full width at half-maximum ≈ 4164 cm^-1; internal quantum efficiency at room temperature = 24%), extending the broad field of phosphor materials research toward the sparsely investigated materials class of lithium oxonitridosilicate oxides.