Photoconversion Mechanisms and the Origin of Second-Harmonic Generation in Metal Iodates with Wide Transparency, NaLn(IO3)4 (Ln = La, Ce, Sm, and Eu) and NaLa(IO3)4:Ln3+ (Ln = Sm and Eu)

Four new metal iodates, namely, NaLn(IO3)4 (Ln = La, Ce, Sm, and Eu), and a series of NaLa(IO3)4:Ln3+ (Ln = Sm and Eu) solid solutions were synthesized through hydrothermal reactions. The structures of the title compounds are similar to that of NaY(IO3)4 crystallizing in the acentric monoclinic space group Cc. The iodate materials reveal layered structures composed of LnO8 square antiprisms and IO3 polyhedra, in which each layer is connected by the I···O interactions. NaLa(IO3)4 suggests a great potential as a matrix for optical source attributed to its acentricity and broad transparency from visible to mid-IR region. The photoluminescence properties depending on the concentration of Sm3+ reveal that NaLa(IO3)4:Sm3+ undergoes a self-quenching relaxation over 7 mol % of Sm3+ by dipole-quadrupole interactions. Attributable to the asymmetric coordination environment of Ln3+, stronger electric dipole transitions compared to magnetic dipole transitions were observed for both compounds. In addition, the materials exhibit strong second-harmonic generation (SHG) responses and are type I phase-matchable. The structural origin of the SHG properties for the reported iodates is elucidated.