LiMn(IO 3 ) 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 AnM(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 (VAOx/VMO6) 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.