Additive-Triggered Polar Polymorph Formation: β-Sc(IO₃)₃, a Promising Next-Generation Mid-Infrared Nonlinear Optical Material

Noncentrosymmetric (NCS) solids have attracted interest for their potential in ferroelectric, piezoelectric, and nonlinear optical (NLO) devices, but their synthesis remains a major challenge. In this study, the additive Li₂CO₃ triggers formation of an NCS precursor at an early nucleation stage, and plays a crucial role in the successful polymorphism transformation. The resultant metastable β-Sc(IO₃)₃ is a promising mid-infrared NLO crystal, with the strongest second-harmonic generation responses (2.2×KTiOPO₄ @ 2100 nm, 16×KH₂PO₄ @ 1064 nm) and the largest optical band gap (4.52 eV) for a rare-earth iodate, as well as sufficient birefringence (Δn=0.219 @ 546 nm) for type I phase-matching, and wide optical transparency, which are induced by optimal alignment of the iodate anions. This study reveals the key role of additives in the growth of polar NCS solids, a discovery with implications for the strategic design of new NCS polymorphism materials with exceptional NLO properties.