Heterovalent Coexistence‐Driven Symmetry Breaking and Dimensional Evolution: Designing Polar Heart‐Shaped Channels for Exceptional MFIR Nonlinear Optics

ABSTRACT The development of mid‐to‐far‐infrared (MFIR) nonlinear optical (NLO) crystals is perennially restricted by inherent property trade‐offs. We establish a heterovalent coexistence strategy to transcend these limits: by harnessing disparities in ionic charge, radius, and coordination, this approach drives local electronic distortions to simultaneously modulate symmetry, dimensionality, and electronic bands. Although promising, such heterovalent engineering remains rare and underexplored in chalcogenides. We validate this paradigm in Eu II 3 Eu III 2 Sn 3 S 12 , where the heterovalent coexistence triggers a profound transformation relative to the homovalent Eu II 2 SnS 4 : symmetry breaking from the centrosymmetric Pnma to the noncentrosymmetric Pmc 2 1 and dimensional evolution from 0D [SnS 4 ] tetrahedra to 1D chains. This culminates in a 3D Eu–S framework featuring unique polar “heart‐shaped” 10‐membered ring channels. These channels act as structural polar templates, leveraging heterovalent charge disparity to induce constructively aligned dipole moments that boost IR nonlinearity. The crystal exhibits exceptional balanced performance: a robust phase‐matchable second‐harmonic generation (SHG) response (1.1× AgGaS 2 ), broad IR transmission (0.58–16.3 µm), moderate birefringence (0.133) and high thermal stability (>1159 K). First‐principles calculations attribute the SHG response to synergistic contributions of heterovalent [EuS n ] and distorted [SnS n ] polyhedra, establishing the heterovalent coexistence strategy as a powerful paradigm for next‐generation MFIR photonic materials.