Anion-Stabilized Incommensurate Modulation Enables Nonlinear Optical Response in Eu2SiO3.89N0.11

Silicates exhibit remarkable structure plasticity, but their complex aperiodic states are often thermally restricted. In this work, we synthesized a nitrogen-doped Eu2SiO4 phase. Multimodal structure analysis reveals that nitrogen incorporation stabilizes an incommensurately modulated structure at ambient conditions (q = 0.284(3)a *). Nitrogen preferentially occupies one of the oxygen sites, adjusting local bond valence requirements and bond length distribution. This triggers distortions in [SiO3.89N0.11] tetrahedra that accommodate internal strain. Consequently, doping stabilizes the modulated structure previously observed only at high temperature in undoped Eu2SiO4. Crucially, the anion-induced polar displacements generate an observable second-harmonic generation response and pronounced birefringence. This structural transformation demonstrates that subtle anion engineering is a potent strategy for stabilizing complex aperiodic structures and provides a new route for designing mixed-anion functional materials.