LiMn(IO₃)₃: A Noncentrosymmetric Iodate Synthesized by Polyhedral Volume Ratio Modulation

Directionally modifying a suitable parent framework is a simple and effective strategy for the targeted design of noncentrosymmetric (NCS) materials. The α-LiIO₃-type compounds A_nM(IO₃)_m feature a robust yet tunable framework composed of A-site polyhedra, MO₆ octahedra, and bridging IO₃ groups, providing an ideal system for structural design. Systematic structural analysis shows that the volume ratio of A-site to M-site coordination polyhedra (V_AOx/V_MO6) influences both the macroscopic symmetry and the dimensionality of the anionic framework. Based on this criterion, by rationally selecting the cation sizes in a way that favored the volume ratio, we achieved the successful synthesis of a new NCS iodate LiMn(IO₃)₃. It crystallizes in the NCS and polar P6₃ space group, with a highly consistent alignment of IO₃ groups. Notably, LiMn(IO₃)₃ shows a strong second-harmonic generation response (4.7 × KH₂PO₄), a wide band gap (3.17 eV), and a large birefringence (0.26 @1064 nm), making it an excellent nonlinear optical crystal. This work also presents its synthesis, crystal structure, and nonlinear optical properties, as well as the results of a theoretical investigation.